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Author Topic: Is it possible to repair a het oscillator crystal?  (Read 4730 times)
KE2KB
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Posts: 633




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« on: October 16, 2017, 06:10:11 PM »

Hi;
I have an old HW-101 which I have "re-capped". The rig is basically operational, but on the 7Mhz band I get only 1/2 the expected output into a 50 ohm dummy load (should be 100W). On 3.5Mhz I get the full 100W, and on all other bands (except 7Mhz) the output is around 75W to 100W.

I did some experimentation, and found that the receiver sensitivity on 7Mhz is also lower than on the other bands.
When I set up a signal source to produce an S-9 reading on the rig's meter, I get S-9 on all bands at the respective frequencies - except on 7Mhz. For the same signal level I get only S-6.

So this has led me away from a driver problem to a heterodyne oscillator problem. It appears that the 7Mhz crystal is a bit "soft".

Is there a way to open this crystal and repair it?

Thanks

Frank [KE2KB]
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WB6BYU
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« Reply #1 on: October 16, 2017, 08:25:23 PM »

I don't think that's the problem.  Usually a crystal will fail entirely - they don't
usually "go soft" with time.  (Though they may shift in frequency.)

Is there an oscillator coil adjustment for each band?  That would be the first
place to check.  There are also some tuned circuits in the driver / RF amp area that
are band-switched, and could be out of alignment.  It's possible that that crystal
is a bit more sluggish than the others and the gain of the tube has dropped, but
that would best be fixed by adjusting the feedback capacitors.

One thing you can try doing is to use your signal generator in place of the
heterodyne crystal oscillator on each band and see if that equalizes the output
power.

But I suspect that the HW-101 uses hermetically sealed crystals, and opening them
up to make adjustments should be a very last resort, as the frequency may shift,
and the crystal may not age as gracefully in the future.  I might consider it if the
crystal was totally dead and I was going to throw it away anyway, but since it seems
to be working somewhat I'd check the other bandswitched circuits first, then play
with the oscillator and see if it needs additional feedback first.
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VK6HP
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Posts: 157




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« Reply #2 on: October 17, 2017, 03:47:09 AM »

I would definitely check the alignment as a first action, as Dale suggests.  However, I have seen crystals exhibit progressively lower levels of activity, possibly due to contamination via an imperfect seal.  I'm told that real enthusiasts can clean even small, fairly modern crystals but, personally, I wouldn't bother - replacements are pretty cheap, especially if you can get a serviceable unit for the HW-101.

My first experience of a dying crystal was in an early hybrid (tube/solid state) aircraft radio, fitted to a not-quite-as-old Cessna.  After pulling every trick in the book to keep it functional, it eventually got to the point where my (friendly) local air traffic controller suggested that the random dropouts and re-start attempts were trying his patience. I think I spent about $20 on a custom-made replacement, in the days when suppliers were plentiful.
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KE4OH
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Posts: 131




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« Reply #3 on: October 17, 2017, 05:43:24 AM »

Low output on 40m is a common ailment with HW-100/101 and SB-100/101/102 transceivers. It's not usually a problem with hetrodyne crystals. There are a couple of well-documented fixes. Use The Google to search for HW-101 Bulletins and similar. There are a couple of different versions of the factory service notes out there.

From memory, one of the main fixes has to do with grounding of the vertical circuit boards and shields that surround the band switch.

The service bulletins have a slight change to the alignment procedure for 40m.

And believe it or not, make sure that your pilot lights are #47. The factory usually supplied #44. These draw the wrong amount of current and can contrubute to low output.
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73 de Steve KE4OH
K6BRN
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Posts: 462




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« Reply #4 on: October 17, 2017, 07:05:58 AM »

Frank:

No, it is not practical to repair a damaged crystal, for many reasons.  Your problem is not likely to be the crystal, anyway.  It sounds like the 40M local oscillator frequency MAY have drifted, causing the RX and low level TX signal to be attenuated by the IF filter, because the signal is no longer centered in the passband.  Or it could be that the bandswitch contact is simply eroded or dirty, attenuating the signal.

So...

1.  Test the tubes in the signal path and carefully reseat them, just as a start.  Then...
2.  Inspect and clean the bandswitch wafer contacts.  I had a similar problem with a HW-101 I rebuilt some time ago and this was the cause.
3.  If 1 and 2 above do not work, inspect and replace or tune the frequency padding components in the 40M LO signal path.  I do not have the schematic in front of me and so cannot be more specific.

Good luck and Best Regards,

Brian - K6BR

 


First, check the tubes in the oscillator circuit to be sure they are OK, first. 
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KE2KB
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« Reply #5 on: October 17, 2017, 08:44:28 AM »

Thanks guys; You have given me something to work with.
What I have already done:
1) done the alignment procedure for all bands. This never changes the low output on 40m.
2) Pulled and re-seated the tubes. Re-seated and checked the tube shields.
3) Removed the driver board cover, cleaned and re-adjusted the copper tabs that bond the cover to the shields.
4) tightened all circuit board screws.
5) Checked (with an ohmmeter), some of the band switch contacts. None of the ones I checked were bad.

Unfortunately, the "signal generator" I am using is my RigExpert AA170 antenna analyzer. I don't think it is suitable to replace the heterodyne oscillator.

I will do some Google searches.

Thanks again.
Frank [KE2KB]
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G3RZP
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« Reply #6 on: October 17, 2017, 09:02:25 AM »

Solder sealed crystals, such as the ones used in the HW101, can lose activity over a period of time as well as drift in frequency. This is because of contamination getting onto the crystal blank: usually, this was mainly flux fumes condensing over a period of years. Despite this, they had advantages over the unsealed FT243 type holder....This is why for highest stability and minimal aging, crystals were sealed in glass envelopes and there are examples of HC-6/U size crystals that are just that - HP used them in the IF filter of the 141 series spectrum analyser. The expense of glass crystal holders led, back in the 1970s, to the development of both cold and resistance welding of cans for higher stability.

If all else fails, I have had some success in opening the crystal using a soldering iron, washing it in denatured alcohol and then acetone and allowing it to dry before re-soldering the can. Done that on HC-25/U too. It requires a good light and a steady hand. Check all the things the other guys have said first, because it's fairly easy to have an accident.
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KE2KB
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Posts: 633




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« Reply #7 on: October 17, 2017, 09:07:43 AM »

I found the service bulletin for the HW-101.
I also found that I replaced the #47 bulbs (one was already burned out) in my rig with "JKL316" from Mouser. Checking the specs, I find that the JKL316 is 6.3V @ 0.7A, while the #47 is 6.3V @ 0.15A.
So first thing I will do is replace those bulbs with #47's.
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KE2KB
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Posts: 633




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« Reply #8 on: October 17, 2017, 09:18:19 AM »

And believe it or not, make sure that your pilot lights are #47. The factory usually supplied #44. These draw the wrong amount of current and can contrubute to low output.
I checked and found that I had replaced the #47's with JKL316's from Mouser. Checking the specs, I find that the #47 is 6.3V @ 0.15A, while the JKL316 is 6.3V @ 0.7A.
So if the bulbs have a higher current rating, then they would have a lower resistance, right? And that would cause a higher current through the tube filaments, which could cause overheating, or at least the tubes to run out of spec?
I am going to change the bulbs before I try anything else.
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KE2KB
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Posts: 633




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« Reply #9 on: October 17, 2017, 06:21:25 PM »

Note on the HW-101 service bulletin HW-101-1 which I found today:
The date of this bulletin is 2/11/1971.
I purchased my HW-101 in 1977, so I expect that any of the changes listed in the 1971 bulletin were already implemented in my kit.
That said, I will continue looking for more specific information, and possible mods that were not included in the Heathkit parts and manual.
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W6EM
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Posts: 1647




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« Reply #10 on: October 18, 2017, 06:54:48 PM »

And believe it or not, make sure that your pilot lights are #47. The factory usually supplied #44. These draw the wrong amount of current and can contrubute to low output.
I checked and found that I had replaced the #47's with JKL316's from Mouser. Checking the specs, I find that the #47 is 6.3V @ 0.15A, while the JKL316 is 6.3V @ 0.7A.
So if the bulbs have a higher current rating, then they would have a lower resistance, right? And that would cause a higher current through the tube filaments, which could cause overheating, or at least the tubes to run out of spec?
I am going to change the bulbs before I try anything else.
Unless the bulbs are connected in series or series-parallel with tube filaments, no, they would not shift the voltage across tube filaments.  But, drawing over 4 times the current (0.7A vs. 0.15A) might overload the 6.3V filament winding of the transformer, causing excessive voltage drop and resultant low filament voltage.  Either replace with #47s or better yet, LED replacements which should draw no more than 10 to 20mA each; or about 10% of what a #47 would.
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KE2KB
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Posts: 633




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« Reply #11 on: October 18, 2017, 08:26:21 PM »

Yes. The two #47 lamps are connected in a weird series/parallel configuration with the tube filaments. Looks like I need to replace those lamps with the #47's.
That said, I am beginning to lose confidence in the possibility that this old rig is worth putting any more efford (and cash) into. If any of the tubes is bad, I have no way of finding out, except by "educated" guesswork, and trial & error in replacing. I am not going to invest in a tube tester.
In addition, there are the carbon comp resistors that are probably out of spec, and I really don't think I have the patience to replace all of those.
Then, there is the grounding issue - maybe need to replace some of those little pieces of wire with braid - and on it goes.

Then, to top it all off; after experiencing what a modern receiver (the SDRPlay) can do, I am not sure I really want to fool around with the 101 anymore.
A year ago when I started this project, it seemed like a good idea. But even then I realized that it may be more than just replacing electrolytic caps. I needed to replace the caps in the rig and power supply just so that I could safely power it up; and I have now done that. But I am unwilling to put any more cash into this rig, and running out of patience as well.

Without proper test equipment (an oscilloscope), I feel like I am taking shots in the dark. I could invest a couple hundred in a PC based scope, but honestly, I would rather forego the test equipment and use that money towards a new(er) rig that will have features that will make hamming more enjoyable.
One thing that really stands out is noise reduction. The 101 has nothing at all in that regard. When I use NR and/or NB on SDRUno with the SDRPlay, I realize how useful it is.

Xmas is approaching - maybe Santa will be good to me... Wink
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K6BRN
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« Reply #12 on: October 19, 2017, 02:37:59 AM »

Eh...? OK.  You bought a 50 year old tube rig not realizing it would probably be a project rather than an "daily driver"?  Well....wow.

You might want to make that a "lesson learned" and avoid buying a 50 year old SB-200 to go with your brand new Icom IC-7300.  The experience will be similar.

And, you'll need an antenna, BTW.  Prepare for more projects.

Brian - K6BRN
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KE2KB
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Posts: 633




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« Reply #13 on: October 19, 2017, 09:38:13 AM »

Eh...? OK.  You bought a 50 year old tube rig not realizing it would probably be a project rather than an "daily driver"?  Well....wow.

You might want to make that a "lesson learned" and avoid buying a 50 year old SB-200 to go with your brand new Icom IC-7300.  The experience will be similar.

And, you'll need an antenna, BTW.  Prepare for more projects.

Brian - K6BRN
I didn't buy the HW-101 used. I bought it when I was a kid, bran-new and assembled it myself. It worked flawlessly for about 10 years until I moved into an apartment, and couldn't work HF anymore. It sat in the attic of my parents' house for 30 years. When I moved back into the same house I grew up in, I thought it might be worthwhile to get the rig working again. I was pretty much interested in doing only what was needed for safety, so I replaced all the electrolytics in the rig and its power supply.

So, it's more time than money I have spent on the rig.

Frank - KE2KB
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KE4OH
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Posts: 131




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« Reply #14 on: October 20, 2017, 06:53:40 AM »

I didn't buy the HW-101 used. I bought it when I was a kid, bran-new and assembled it myself. It worked flawlessly for about 10 years until I moved into an apartment, and couldn't work HF anymore. It sat in the attic of my parents' house for 30 years. When I moved back into the same house I grew up in, I thought it might be worthwhile to get the rig working again. I was pretty much interested in doing only what was needed for safety, so I replaced all the electrolytics in the rig and its power supply.

So, it's more time than money I have spent on the rig.

Frank - KE2KB

I hear you. Look, there are guys like me who get a kick out of knocking these old rigs back into shape. The time and money isn't really the issue.  You really don't need a bunch of test equipment or even money to fix an HW-101. You didn't have either one when you built this as a kid, did you? And you made it work. But, what you do need now is patience.

The rig is basically working. It's probably not a tube. You only have low output on 40m. It's still likely a grounding issue. Resistors?  Maybe, but why only a problem on 40m? You may have a busted core in one of the coils.

So, there are several things to run down. And it may take a while. If this isn't an aspect of the hobby that you enjoy, that's okay. Might be time to let this rig go to somebody who likes fixing them. But maybe give it another shot? If you succeed, you might just find that you like fixing boat anchors after all!
« Last Edit: October 20, 2017, 06:58:06 AM by KE4OH » Logged

73 de Steve KE4OH
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