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Converting the Heathkit SB-620 for 3395kHz IF Receivers

from Stephen H. Lafferty on February 14, 2009
View comments about this article!

Converting the Heathkit SB-620 for 3395kHz IF Receivers

From Stephen H. Lafferty, WANNABE2 February, 2009 Rev.3a

0x01 graphic

The Heathkit SB-620 “Scanalyzer” is an IF spectrum analyzer. It graphs the amplitude of signals in a range of frequencies around the one that your receiver is looking at. It can show spans of 500kHz, 50kHz and 10kHz, so you can see all of the signals around you at a glance. Problem is, it has to be setup for the particular IF frequency of your receiver. In the original kit, Heath kindly included the various coils and other parts which you would need for a wide range of IF frequencies. You could only install one though and of course, the coils for the others are rare.

I have a classic Heathkit SB-301 receiver which has an IF of 3395kHz. This is also the IF of many other Heath receivers, including: SB-100, SB-101, SB-110, SB-102, SB-104, SB-300, SB-301, SB-303, HW-100, HW-101 and HW-104. The SB-620 I had was setup for 455kHz, though.

Here is how I converted it to work with my receiver. The photo above shows it operating with actual 80m signals. Please refer to the complete schematic of the modified unit at the end of this article. The changes to be made are summarized in the table below.

Test Equipment

I recommend that you have the following test equipment to debug and adjust the modified unit:

  • RF Generator

  • Frequency counter

  • Volt-ohmmeter

  • Oscilloscope

Why the Booster Amp?

My good friend Dave, who has to have a logical explanation for everything, demanded this section. :) The transistor booster amp is optional. I needed it. Maybe my SB-620 was weak. Maybe your receiver supplies more IF signal than my SB-301. You could try it without the amp and add it if necessary (as I did). Still, it remains somewhat of a mystery as to why Heath didn't provide more gain. There might be many answers to that question.

I found that, with just the gain of the RF amp stage ahead of the mixer, the IF tap on the SB-301 produces only about -90dBm for a weak signal. With about 20dB of loss due to the 10pF tap, this seems reasonable. Before adding the amp, my SB-620 could display a pip at -80dBm. Heath specs about -87dBm. Admittedly, a couple tubes in the SB-620 look rather weak on an Eico 667 tester. On the other hand, some people don't consider tube testers to be very accurate at gauging the quality of a tube. I must also confess that my antenna is just an inverted-L, 20-feet high and 60-feet long, without any tuning network. So your situation might be somewhat better than mine.

I did go through the unit pretty well and couldn't find any smoking gun as far is gain is concerned. Could have tried to scratch for a couple dB here and there. Bear in mind though, that on the 40dB log scale displayed, differences of 6dB or so don't look like very much. I like to be able to see the noise floor on a spectrum analyzer. It makes a good reference point. Even in the impressive scope photos in the Heath manual, you don't see the noise floor. It was clear that the only way to get there would be with an extra amplifier. It does show the noise on 80m now, at least in my neighborhood.

Heath might have been satisfied with less gain than I would want. They had to keep the cost reasonable and with its big transformer, CRT and HV power supply, the parts cost was probably getting worrisome. In 1969 they sold it for just $119. Being just an optional accessory, it couldn't command a high price, like the $260 SB-301. So I hope this explains to some extent, why I feel the amplifier is needed and why Heath might not have felt that way. The PIP Gain control has stayed at max, by the way.

Making the Coils

The two coils needed are L3, a tapped oscillator coil and L2, a 1:1 IF transformer of sorts. A quick search convinced me that the original parts are rare. Using information gleaned from the web and from kind members of eHam.net, I constructed new ones. For L3 you can start with:

  • http://www.surplussales.com/inductors/Ind-SlugTu/Ind-SlugTu-1.html
    part number: (IVF) SMB-72905. $5.60 each.

  • The inductance is about right but it needs a tap added as shown in the picture below. Note that it does NOT have the low-side range claimed. Might have been a typo. (Have informed Surplus Sales.) I found that it goes down to about 20uH. That is the reason that C4 was changed to a trimmer.

  • The terminal on the right was added using #12 wire and 5-minute epoxy. If you wrap it all the way around as I did, be sure to cut the circle with a rotary tool or such after it hardens to avoid a shorted-turn effect.

0x01 graphic

  • To connect the tap: (Refer to figure above.)

    • Disconnect the outer end of the coil wire from its terminal and unwind 10-turns. By “outer end,” I mean the end which is on the outside of the windings, so you can unwind some turns.

    • Route the wire to the new lug and wrap a couple turns around it so you can solder.

    • Route the rest of the wire back to the coil and wind 6-turns in the direction it originally had, on top of the existing turns.

    • Wrap the end around the disconnected old lug and cut off the remaining wire.

    • Solder the old lug and the new lug. 750C iron temperature and extra rosin flux will help get through the insulation.

    • Secure any loose wire with adhesive or clear fingernail polish. I used heatshrink around the coil wire for protection.

To make the L2 transformer, you can construct it as shown below:

0x01 graphic

  • Order the two toroids needed from the same website mentioned above. Part number (ICH) MM-T37-2 Iron Powder Toroid, Red, $0.45 each.

  • I used a 10-32x2” nylon screw as a form with seven nuts: One holds the top toroid, two hold the other toroid, two hold the terminal board and two secure the assembly to the chassis.

  • Wind 62-turns of #34 magnet wire around each of two toroids. As you can see in the photo below, it's a bit of a dexterity test but you get better at it quickly and it goes a lot faster than you might imagine at first. Try to keep tension on the turns as you wind. Avoid crossing over previous turns. At the end, don't worry if they rearrange themselves rather haphazardly around the core. Just try to keep tension on it. Twist the two leads on each finished coil a couple of times to secure the windings.

0x01 graphic

  • Position the toroid coils on the nylon screw and adjust their separation to exactly 5/8”. Measure that between the closest edges of the toroid cores.

  • Make a terminal board from a square of perf board cut on 0.6” rows. Drill a 7/32” hole in the center for the screw.

  • I use press-fit wire wrap stakes for terminals, cutting them off underneath at about 1/8.”

  • Polarity of the primary and secondary does not seem to matter but you will want to keep the Heath terminal pattern (including the unused #1 lug) for ease of installation, as shown below.

0x01 graphic

  • Mount the terminal board as shown in the photo. Bring the leads from the toroids down the column in wide spirals, to keep them snugly against the post. Wrap a couple of tight turns around the bottom of the terminals under the board and solder.

  • Two more nuts at the end of the screw are used to mount L2 to the chassis.

  • Sources:

10-32x2” nylon screws #67347161 $10.35 per 100.

10-32 nylon nut #67352005 $8.16 per 100.

34 Awg Copper Magnet Wire 1/4 lb $8.79.

574-T68/C Wire Wrap Post $10.71 per 100.

Removing the Old Parts

My SB-620 was setup for 455kHz. If yours is something different, the details of this section may not apply. Rather than attempt to salvage resistors and some caps which are attached to the coils, I removed them with the coils and provided replacements, where appropriate. I would urge you to remove the original coils carefully so that they might be used in another SB-620 whose owner needs the old frequency.

The figure below shows the main cuts used to remove the old parts. One part that you DO need to salvage is the varactor diode which is mounted on L3. The Web suggests that the diode might be an MV1638. It is shown roughly sketched in green since Heath didn't show it. A missing cap is also roughly shown in green. Use a heatsink such as a hemostat while you remove the diode. I know stainless steel isn't the best heatsink but its tight grip and small jaws work well in a crowded situation like this. Minimize the heat stress.

0x01 graphic

Installing the New Parts

  • Mount oscillator coil L3 with the included nut and a 1-post terminal strip nearest the front panel. Since the existing hole is larger than the threaded shaft, I drilled-out a suitable fiber washer to make a ring to fill the gap. The next photo shows L3 before the variable cap C4 was added.

0x01 graphic

  • Mount transformer L2 with the two nylon nuts. Use a fiber washer as with L3. Orient pin-1 towards the nearby terminal strip.

  • Install the new resistors and capacitors for L3 as shown in the following diagram. The numbers on the L3 terminals match those of the original schematic. Lug-2 is represented by the terminal strip lug. Notice that the trimmer caps are not shown, as they go in last. Don't forget 1800pF C21 (like I did :) which is partially hidden. Its hidden lead goes to Lug-3 on L2 which is also hidden. (It's on the side of L2 nearest the pots.)

0x01 graphic

  • Solder the trimmer cap to lugs 1 and 3 of L3 as shown in the photo of the booster amp, later in this article.

Below you can see L2 with the trimmer caps. Mount the trimmers across lugs 1-2 and 1-4 of L2. Sharp eyes will notice that the forgotten C21 is missing. This picture was also taken before the booster amp was added. With the booster amp, C23 (above the left trimmer) will go to the amp instead of V2-2.

0x01 graphic

Adding the Transistor Booster Amp

The photo below shows the IF Booster Amp. It provides about 17dB of gain which I found was needed to use the scanalyzer effectively with my SB-301 receiver.

0x01 graphic

  • The schematic of the booster amp is shown below. It connects in the main circuit as triangle A1, between C23 and V2-2.

0x01 graphic

  • You can make the amp on a piece of perf-board cut to 0.8” x 1.2”. Drill a 7/64 hole for the 4-40 mounting screw, 0.1” from the side with the terminals and 0.2” from the side with RA6.

  • The layout of the board is shown below. Wires on the bottom are in green and top side leads are in orange. Notice that CA2 is piggy-backed on RA3. I used a 1W 33K and a 1W 39K in series to make the 75K 2W resistor.

0x01 graphic

  • The 2N5179 RF transistor has a fourth lead tied to the case. I labeled it “g”.

  • If you decide not to use this layout, try to keep leads and parts which are connected to the base, away from those at the collector. Use good RF layout practice.

  • The parts list for the booster amp is below.

  • Include a lug connected to the board ground on top at the location of the screw.

  • To mount the board in the Scanalyzer, remove the screw and nut nearest the front panel which fastens the tube socket for V2. Replace it with a 4-40x1/4” screw and 9/16” standoff, hex size 3/16” and threaded for 4-40 screws.

  • Mount the board, as shown above, to the standoff with another 4-40x1/4” screw.

  • Change C23 to connect from lug-2 of L2 to the input lug of the amp board (instead of V2-2).

  • Connect a wire from the output lug of the amp board to V2-2.

  • Find the +250V terminal marked in the photo below (taken before modifications). Run a red wire from there to the top of RA6. Use heat shrink to cover all of the exposed wire at resistor. Very important for safety. Seeing the transistor, one might not expect HV at that board.

0x01 graphic

Adjusting the Scanalyzer

Now that you've done the hard part, it's time to have some fun! The first thing we need to do is to adjust the local oscillator. It should be at 3.745MHz. Use a scope probe on your frequency counter and clip to V2-2 to read the frequency. Carefully set the PIP Center control for the exact center of its rotation. Set L3 for the lowest frequency (highest inductance). Then adjust trimmer C4 for 3.745MHz.

Next, we will peak the bandpass filter at L2 for 3395MHz. Set an RF generator to that frequency and connect it to the IF input on the rear panel. The switch there should be in the up position for “Ham scan”. To find the signal, set PIP Gain to max, PIP Center to center position, Variable Sweep width to max, Variable Sweep Rate to center, Amplitude Scale to Log and Sweep Width to Variable. Zero-in on it by reducing Variable Sweep Width while adjusting PIP Center if necessary to keep it visible.

Change Amplitude Scale to Lin and adjust PIP Gain and your generator level to get a good showing of amplitude. Adjust trimmers C19A and C22A for max indication. This completes the adjustments for the new circuitry. I recommend that you align the IF, input trap and log tracking.

Installing an IF Tap in the Receiver

You will need to install a tap on the IF of the receiver to be able to use it with the SB-620. Heath has good instructions for this in the manual. Generally, they advise connecting about a 10pF cap to the plate or collector of the mixer stage which is driven by the receiver's variable oscillator. This is routed by a length of shielded cable to a connector. The SB-301 has spare phono connectors for this type of thing. I noticed that the tap drops the level about 20dB. That's the price paid for not loading the mixer heavily. Unfortunately, it also increases the need for the booster amp.

Operating Tips

I won't try to duplicate Heath's excellent operating instructions but here are a few things that are handy to know.

  • If possible, you will want to be able to disable the receiver's AGC. Otherwise the whole spectrum moves up and down when you are receiving an SSB signal. If you can't disable it, use slow AGC mode if it is available.

  • You may find that some signals, particularly CW, seem to disappear as the receiver is tuned to them. In the SB-301, the crystal IF filter which follows the mixer loads the plate heavily in its passband. That causes the level that is tapped there to drop a great deal. Fortunately, the notch that it creates seems to be very narrow, so it isn't really a problem for viewing voice signals. You just need to know that it's there so you can take it into account.

  • During operation, you will want to know where your receiver passband is on the screen. Yet, it can be surprisingly hard to get the SB-620 centered on the receiver if you don't know the tricks. PIP Center must be precisely set to target the receive frequency in the 10kHz and 50kHz modes. A sure way to set that is to put the receiver in 100kHz cal mode. Tune accurately to one of the harmonics.

    Next, set the SB-620 to variable sweep, minimum Var Sweep Rate. Center the Var Sweep Width, PIP Center and Horiz Pos controls. Find the tallest cal signal and adjust PIP Center to bring it to center screen. Reduce Var Sweep Width to minimum and adjust PIP Center to bring it back to center.

    Switch to 50kHz sweep width. Re-center with PIP Center. You will notice that PIP Center is pretty touchy now. Switch to 10kHz sweep width. Re-center with PIP Center. It will be really touchy. You may have to hunt for the signal a little. Notice that the PIP moves in the direction that you turn PIP Center.

    Now switch back to variable sweep mode. The position of the PIP is the true receiver frequency. Adjust Horiz Pos to put it in the center. The SB-620 is now aligned to the receiver's passband. This may sound a bit complicated but getting the PIP Center and Horiz Pos controls accurately set make it much easier the next time. Just remember that your goal is to get the PIP targeted in the narrowest (10kHz) sweep range. You have to work your way down in range to find it.

General Info about the SB-620

  • It might seem obvious but it should be stated that the SB-620 horizontal scale increases in frequency from left to right. You might be surprised to find though, that as you tune your receiver up the band, IF signals seem to move up in frequency, instead of down. The reason, in the case of the SB-301, is that the receiver uses a high-side LO. That causes the spectrum at IF to be reversed.

  • The local oscillator in the SB-620 is a high-side LO. With the Scanalyzer IF at 350kHz and the receiver IF at 3395kHz, the nominal Scanalyzer LO is at 3745kHz.

  • 3dB IF bandwidth of the SB-620 is fixed by its crystal filter at roughly 200Hz. They specify a usable resolution of 1kHz.

The Var Sweep Width control affects the 10kHz and 50kHz modes, as well as the variable mode. As called for in the Heath alignment procedure, you should find out what setting of Var Sweep Width is needed to get actual 10, 50 and 500kHz sweeps. In my case, they all happened to be at the second mark of the control.

Schematic

The schematic below incorporates all of the modifications described in this article. That includes adding trimmers to the 3395kHz option circuit, replacing C4 in the oscillator with a trimmer and adding the booster amp, A1.

Click the Photo for the fullsize schematic.

Acknowledgments

I would like to thank Allen Ward, KA5N for his very helpful advice regarding the coils.

I also thank Kees Talen, K5BCQ for his excellent Web page on the SB-620, which made this project possible: http://www.qsl.net/k5bcq/620/620.html

Thanks to David Gillespie, for reading the manuscript and making several good suggestions for improvements.

And of course, many thanks to the Heathkit company for this and all the wonder, teaching and joy that they have given us over the years.

Contact Info

I will try to help if you have any questions about this article. Comments and feedback about your experiences with the modification are most welcome. Of course, while it is open, it would be best to correspond using eHam.net's article comment feature. Otherwise, you can reach me at: slafferty@bellsouth.net

Member Comments:
This article has expired. No more comments may be added.
 
Converting the Heathkit SB-620 for 3395kHz IF Rece  
by WA7PRC on February 14, 2009 Mail this to a friend!
That's a pretty good article, Stephen!

I've owned a '620 since the mid 70s, and used it (at first) with my SB-102. I was fortunate in that mine came to me already configured for Heath's 3395 KHz IF. Most of the time, I seemed to have plenty of deflection on the '620.

The '620 can also be used with other IFs w/o modification, as there's a switch in the cathode of the input amplifier to change its operation to that of a mixer. With it wired for a 455 KHz IF, you would then need only a source of 3850 KHz (3395 - 455) or 2940 KHz (3395 - 455) injection signal. I used this scheme when I retired the SB102 in favor of a Kenwood TS120S and later, a TS130S (8830KHz IF). For these rigs, the injection oscillator needed to be 3395 + 8830 = 12225 KHz. I fabricated an oscillator dead-bug style, using a crystal meant for an old 2m rig (146.76 / 12 = 12.23 MHz). The '620s sweep oscillator needed only a slight adjustment.

A 10pF capacitor coupling from the rig's mixer output into 12" of coax (20 to 30pF per foot) reduces the voltage by up to 25%. The attenuation gets worse as the length of the cable is increased along with the shunt capacitance. RG-62/U coax has somewhat less capacitance/foot (about 13pF) but is harder to find.

Having a buffer (with a Lo-Z output) near the mixer gets away from the problem of shunt capacitance in the interconnecting cable. In the case of my Kenwood rigs, I used a buffer amplifier but I put it inside also fabricated dead-bug style). My buffers use the easy-to-find 2N3904, and I get about 18dB gain. A description, schematic, and photos are here: http://tinyurl.com/bcp2e4

73,
Bryan WA7PRC
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WA7PRC on February 14, 2009 Mail this to a friend!
I hate when I have brainfade!

"...you would then need only a source of 3850 KHz (3395 - 455)..."

should be

"...you would then need only a source of 3850 KHz (3395 + 455)..."

Bryan WA7PRC
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by K0BG on February 14, 2009 Mail this to a friend!
Tinkering with older gear can be a lot of fun, but sometimes the end result isn't worth the effort.

I purchased a 630 during the mid 70s, when the local Heath store had their grand opening. If I recall correctly, it cost $99. Assembly wasn't difficult, but it turned out the CRT was defective. It was replaced of course, but even then the utility left a lot to be desired.

Signals even 20 dB out of the noise were difficult to see especially at the edges of the display, and there was a hit on the SB102 it was connected to. As a result, I sold it after just a few weeks for the same price I paid for it.

Nowadays, it doesn't hold a candle to any of the built in SAs, but it does look neat along side the other Heath gear.

Alan, KØBG
www.k0bg.com
 
Converting the Heathkit SB-620 for 3395kHz IF Rece  
by K9JCS on February 14, 2009 Mail this to a friend!
Outstanding Article!! I don't have an SB-620 and probably never will, but this article sets the standard for the way "how to" articles should be done. Thanks

Jim
K9JCS
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WR8Y on February 14, 2009 Mail this to a friend!
K9JCS is right.

Also, EXCELLENT PHOTOGRAPHY! Just what camera and lens did you use?
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WB6DGN on February 14, 2009 Mail this to a friend!
"Also, EXCELLENT PHOTOGRAPHY! Just what camera and lens did you use?"

I'll second that! Also, a little bit about the lighting you used would be helpful. I see the potential for another article here!
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by AC7CW on February 14, 2009 Mail this to a friend!
Great article!! I have collected a complete SB-102 setup with every accessory imaginable including the SB-640 [the second rarest Heathkit} It's all waiting for a refurb. I'll build in the amp to the 620.

The photos are truly well done. It hit me while reading that we have much better articles these days than we did in the past, recall all the Popular Electronics magazines with the corrections to the schematics way back in the day? They never seemed to get a schematic right the first time!! If you could share how you did the photos maybe it would improve articles going forward...
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by N9DG on February 14, 2009 Mail this to a friend!
"Also, EXCELLENT PHOTOGRAPHY! Just what camera and lens did you use?"

If you save the picture local to your comouter and then do a "right click", "properties" on the saved jpg you can find out much about the pictures themselves. For example the picture of the first coil reports this in Ubuntu Linux, (Windows should also be able to display similar data):

Image Type: jpeg (The JPEG image format)
Width: 684 pixels
Height: 419 pixels
Camera Brand: Minolta Co., Ltd.
Camera Model: DiMAGE 7Hi
Date Taken: 2009:01:03 19:41:34
Exposure Time: 1/179 sec.
ISO Speed Rating: 200
Flash Fired: Flash did not fire, compulsory flash mode.
Metering Mode: Pattern
Exposure Program: Normal program
Focal Length: 47.9 mm (35 equivalent: 186 mm)
Software: Adobe Photoshop 7.0
Description: Minolta DSC

 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by KG4TKC on February 14, 2009 Mail this to a friend!
I don't know if I will ever run across an SB-620,but this article none the less is a fine one. Thanks for taking the time to put it together,with the clear details and fine pictures. This is an A-1 job. 73 es GL-KG4TKC
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WR8Y on February 14, 2009 Mail this to a friend!
N9DG,

WOW! I used Microsoft Picture manager and got almost the same you did!

Thanks, I really learned something this time. Questions answered!
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WR8Y on February 14, 2009 Mail this to a friend!
No wonder the pics are so good: they were taken with a $1500 camera!
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by N0YXB on February 14, 2009 Mail this to a friend!
I love articles like this! Aside from the great photos, the drawings are excellent too. Well done. And only one negative comment so far. But no matter, that poster always makes negative comments and the funny thing is I'm not convinced he's aware of it.

Vince
N0YXB
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by AC7CW on February 15, 2009 Mail this to a friend!
The camera is a good one but just 5 mega pixels with the ability to focus up close... photos like that are about the lighting as much as anything.
 
Converting the Heathkit SB-620 for 3395kHz IF Rece  
by G3VGR on February 15, 2009 Mail this to a friend!
I gave away my SB101 station to a collector a few years ago and never owned a SB620 anyway, so the article content isn't useful to me (although very nostalgic!). That said, this is probably the best written technical article I've ever seen on eHam and I'd like to congratulate you on putting together such lucid and well illustrated instructions. It certainly sets the benchmark.
73, Dave
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by KA5N on February 15, 2009 Mail this to a friend!
Great job Steve! Outstanding photos. This should be a great aid to anyone who needs to "refrequency" their SB-620.

Thanks for the kudos, glad to help.
73
Allen KA5N
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WANNABE2 on February 15, 2009 Mail this to a friend!
Hi Bryan,

Thank you for the interesting alternatives. Using the mixer mode of the SB-620 frontend to look at other frequencies with an extra LO is certainly a viable
solution. The only concern I would have is that there is no filtering to reject the frontend mixer's image at 4305MHz or 2485MHz. Perhaps that is not a problem.
I do note however, the SB-301 LO can be in the 5MHz range, though.

The other thing about using the frontend tube in mixer mode is that with conversion loss and the cathode drive, one would not expect as much gain. So that will work against the additional gain desired.

Nevertheless, taken with your other suggestions, the approach certainly has merit. I applaud your bringing this under-appreciated feature of the SB-620 to light.

Thank you for the nice webpage on your buffer amplifier addition to the receiver. Your amp is an excellent solution for receivers which have a 50-ohm impedance point from which to tap. I will have to confess that I did consider adding an amp to the SB-301 after I found the loss from the Heath-recommended tap. The reasons that I went with an amp in the SB-620 are:
- There is a high level of LO present at the tap point in the SB-301. Adding the amp there would have boosted the LO as well as the very low-level desired signal.
- In the SB-620, the amp is after the PIP level control and bandpass filtering, reducing the possibility of an overload or IM from the wideband signals at the receiver tap point.
- In the SB-620, any loading caused by the amp does not affect performance of the receiver. It's true that the 10pF tap could have some effect but that was at least approved by Heath. I could have used a FET amp in the 301 but to get the gm I would want would take several mA of current or more. That would result in a large size dropping resistor. With the 2N5179, I was able to run the transistor at only 2mA and the resistor was kept to a reasonable 2W.

Of course, all of these are soft reasons and there is much to be said for your approach too. It really just amounts to preferences. For me, wanting to keep my painstakingly-restored SB-301 as pristine as possible is as good a reason as any. The poor SB-620 came as an unwanted part of a purchase many years ago and had to be modified anyway, so it was less of a sacred cow. :)

Regarding the choice of the 2N5179 versus a garden variety 2N3904, the high Ft at low current let me keep the current down while keeping the input resistance of the amp above 10K and the gain above 17dB. They are available from Mouser and Jameco for around $1. Besides, surely most good ham experimenters
have a few of those around! :)

I truly appreciate your stimulating comments and valuable contributions to this shadowy corner of the boatanchor zone.

Steve
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WANNABE2 on February 15, 2009 Mail this to a friend!
Many thanks to each of you kind folks for the nice comments about the article and photos! It's appreciated more than you know.

Regarding the questions about the photo techniques, as sharp readers have already uncovered, the camera is my beloved Minolta Dimage 7Hi. Though it is a bit long in the tooth for a digital camera and Minolta has sold out to Sony, I still love it for its effective combination of features and usability. The most significant here is the 14-element 7:1 zoom lens with macro mode at max and min zoom. While I did pay $1200 or so for the camera "way back" in 2003, its 5Mpix resolution and 800 max ISO are an anachronism in today's market. I notice that KEH has used 8Mpix Minolta A2's (a later version) for $205.

I try to shoot at ISO-200 to keep the noise down. I also tend to use aperture priority to increase depth of field for macro shots. Those choices require lots of light. The overhead fluorescents in my lab are quite bright and are 5000K color temp. The custom fluorescent bench lamp is also balanced color temp. I tend to push the bench lamp in as close as possible to maximize the light and get some shadow fill from the longish tube. Shots are made with -0.3EV exposure compensation to avoid highlight clipping.

If there is a secret to the photos, it's PhotoShop. I use it to touch-up the Minolta's already excellent auto color balance. Gamma is usually increased to bring out shadow detail and correct the -0.3EV. Medium to highlight range might need curve adjustment to achieve adequate brightness without compressing or clipping highlights. Finally, the price of the great zoom lens is a small amount of flare. That is typically corrected with 15% or so unsharp masking over a 100-pixel or so radius.

Uh, sorry to bend your ears...you got me started. Photography was once my other hobby. :)

Steve L.

 
Converting the Heathkit SB-620 for 3395kHz IF Rece  
by WD9FUM on February 17, 2009 Mail this to a friend!
Now that's what I call a "how to" article!
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by KC8VWM on February 17, 2009 Mail this to a friend!
I think I spotted a cold solder joint at C23 at lug 2.

j/k

Very nice article. These are the sort of components and technology I feel at home with the most.

My Best, Charles - KC8VWM

 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by KC8VWM on February 17, 2009 Mail this to a friend!
WR8Y

No wonder the pics are so good: they were taken with a $1500 camera!

--------

You mean he should have used a vidicon camera with 17 vacuum tubes inside it instead?

http://www.labguysworld.com/Dage-101AF-S.html

73



 
Replies to WD9FUM and Charles, KC8VWM  
by WANNABE2 on February 18, 2009 Mail this to a friend!
Hi WD9FUM and Charles, KC8VWM,

Thank you both so very much for the kudos! Much of the credit though, should go to those acknowledged at the end of the article. They really made it possible.

And thanks, Charles for the great link to the labguysworld.com site. [Hint to those who get an error with that: leave off the last letter, l.]

Sincerely,

Steve
 
RE: Converting the Heathkit SB-620 for 3395kHz IF  
by WA7PRC on February 21, 2009 Mail this to a friend!
WANNABE2 on February 15, 2009:

"Thank you for the interesting alternatives. Using the mixer mode of the SB-620 frontend to look at other frequencies with an extra LO is certainly a viable solution. The only concern I would have is that there is no filtering to reject the frontend mixer's image at 4305MHz or 2485MHz. Perhaps that is not a problem. I do note however, the SB-301 LO can be in the 5MHz range, though."

I think you meant images at 4305 & 2585 KHz. There should be sufficient rejection by the mixer's 455 KHz output tuning so, images (and LO feedthrough) shouldn't be a problem.

"The other thing about using the frontend tube in mixer mode is that with conversion loss and the cathode drive, one would not expect as much gain. So that will work against the additional gain desired."

True, though a buffer placed at the receiver mixer output can make up the needed gain.

"Nevertheless, taken with your other suggestions, the approach certainly has merit. I applaud your bringing this under-appreciated feature of the SB-620 to light."

It really is a spiffy piece of gear. The only thing I wish I had was a wider CRT. With the preselector of the hollow-state era rigs, you couldn't "see" much beyond +/- 50 KHz. With the bandpass filter on the frontend of my solid-state rigs, I can see the entire band... but it's all jammed into a small CRT. Again, I limit sweep width to +/- 50 KHz. Sigh.

"Thank you for the nice webpage on your buffer amplifier addition to the receiver. Your amp is an excellent solution for receivers which have a 50-ohm impedance point from which to tap."

A simple JFET source-follower input could be added for a high input impedance.

"I will have to confess that I did consider adding an amp to the SB-301 after I found the loss from the Heath-recommended tap."

I noted that as well, when I used my SB-620 with my SB-102. I changed from RG-59/U cable to RG-62/U (as specified in the SB-620 manual) and noticed an improvement due to the difference in shunt capacitance. RG62 has about 13pF/ft and RG59 has about 20pF/ft (RG58 has even more, about 30pF/ft). What kind of cable did you use and how long was it?

"For me, wanting to keep my painstakingly-restored SB-301 as pristine as possible is as good a reason as any. The poor SB-620 came as an unwanted part of a purchase many years ago and had to be modified anyway, so it was less of a sacred cow. :)"

I'm not sure which of these pieces of gear is harder to find. They're both great!

"Regarding the choice of the 2N5179 versus a garden variety 2N3904, the high Ft at low current let me keep the current down while keeping the input resistance of the amp above 10K and the gain above 17dB. They are available from Mouser and Jameco for around $1. Besides, surely most good ham experimenters have a few of those around! :)"

Aw, I just reached into my pocket and the first thing that came out (along w/ some pocket lint) was a 2N3904. hi!

"I truly appreciate your stimulating comments and valuable contributions to this shadowy corner of the boatanchor zone."

You're welcome, and thank you too!

Bryan WA7PRC
 
Converting the Heathkit SB-620 for 3395kHz IF Rece  
by KC1RS on February 24, 2009 Mail this to a friend!
Steve,

Wow, a lot of work doing the conversion. I wish I had known you were at 455 and wanted to get to 3395 because my 620 is at 3395 and I want to convert it over to 455 kHz.....want to trade parts?

Russell
KE4PTM
 
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