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Author Topic: Heathkit SB-102 troubles  (Read 5693 times)
KD0ILM
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Posts: 36




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« on: March 21, 2013, 12:34:35 PM »

I got so much help here working my my LMO I thought I would come back to the well again and hope for the goodwill I received last time.

I had a pretty good functioning SB-102. It had a jumpy LMO which I removed fixed and calibrated.(thanks to the help you guys gave me) Besides that I did not change any other components. I did clean band switches etc. with Deoxit. I now have it operating again sort of...

1. Slight buzz on the audio when volume is turned down. sounds like 60 cycle hum to me. Not bothersome but there.

2. The RF gain control seems inoperative. I believe that it is full on. This did work correctly prior to my interventions,

3. When I tune up and check plate current with the mic/CW control counterclockwise as suggested in the book the needle goes between the 50 and the 100 on the meter which I assume is 50Ma. The new behavior is that when I switch back from tune to LSB or USB the needle returns to zero for a second and then I hear the relay in the finals area click and the plate current goes back up closer to the 100 mark and there is a small amount of RF being transmitted (about 2 watts) and of course the receiver is dead. When I first fired the rig up after my work it worked fine except for the buzz and the inoperative RE gain control. When I tried the mike nothing happened and I got a bit of audio feedback through the mike. I then noticed that I had soldered the wires to the female connector back together in reverse. I fixed that so the wire with the resistor was in plug #1 and the mike works fine but I have my new problem.

Any ideas?
Kevin
KD0ILM/XV2KM
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AB1MN
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Posts: 42




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« Reply #1 on: March 21, 2013, 04:45:37 PM »

Hi Kevin,

In my consulting business, I always tell clients that "the first question is free" after that . . . Wink

1. Slight buzz on the audio when volume is turned down. sounds like 60 cycle hum to me. Not bothersome but there.

If this is in receive mode, I would consider the filter caps in the power supply, or perhaps more likely, a grounding or shielding problem on the audio leads.

By the way, you did tighten up ALL of the screws that hold the circuit boards in place, didn't you?

2. The RF gain control seems inoperative. I believe that it is full on. This did work correctly prior to my interventions,

When you turn the RF gain down, does the S-Meter go up? When the RF gain is all the way down, the S-Meter should read full scale.

If nothing seems to happen when adjusting the RF gain, measure the voltage on the control itself. I think the wiper (center terminal) should go from about -20v to 0 volts as the control is advanced from full CCW to full CW.

3. . . . .

In USB/LSB with the mic gain all the way down, the meter should read 50 (ma). This is the resting bias on the 6146s. If not, adjust the bias pot (center rear of the chassis) so that it does. From your description, it sounds like you should also adjust the balanced modulator (capacitor and pot) to null out the carrier. The adjustment is a compromise between USB and LSB. The easiest way I have found to do this is to listen to the signal on a separate receiver while adjusting the controls (on the pcb) for minimum carrier. You may need to go back and forth between USB and LSB to find the best compromise.

You can also consult the manual and follow the procedure there for adjusting the balanced modulator.

Report back what you find if you need more ideas,

Bob  AB1MN
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KD0ILM
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Posts: 36




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« Reply #2 on: March 21, 2013, 05:20:50 PM »

Bob,
The RF gain does not affect the S-Meter which is one of the things that led me to think it was out (other than the obvious). I will check the pot, the value you gave me was helpful. Perhaps I cleaned it to death.
Do you think that adjusting the bias and null would explain why my transmit relay is tripping when I switch from "tune" to LSB or  USB?
Kevin
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AB1MN
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Posts: 42




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« Reply #3 on: March 21, 2013, 05:32:48 PM »

Hi Again,

The transmit/receive relays (two of them, one in the center rear of the chassis and one in the finals cage for switching the antenna) are activated while in Tune and not activated while in USB/LSB unless you have pushed the PTT switch on the mic, or tripped the VOX.
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KD0ILM
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Posts: 36




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« Reply #4 on: March 21, 2013, 05:53:53 PM »

Yes, it will trip after a few seconds or sometimes after a minute or so with no microphone connected, the rig in the PTT position (not VOX) in both LSB and USB.
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K8AXW
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Posts: 3900




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« Reply #5 on: March 21, 2013, 07:25:50 PM »

Kevin:  It has been too many years since I've owned the 102 so I'm afraid that I can't offer much specific help.

However, considering the 102 is over 30 years old, the next step I'd take would be to replace the power supply filter caps as suggested by a previous poster.  I also seem to recall an electrolytic cap in the cathode circuit of the audio amplifier.  If there is one, replace it. 

It has been pointed out here in the past that the resistors Heath used are notorious for changing value over time.  Rather than shotgun this possibility I'd suggest going back and checking all tubes again and then start checking the voltages as Heath was famous for providing.

While you might have started out with a "live one" with the exception of the LMO problem, you should understand that you are actually resurrecting an old radio and patience and a lot of work will probably be needed.

As for the well, it's pretty deep Kevin.  Don't worry about wearing out your welcome.
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KD0ILM
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Posts: 36




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« Reply #6 on: March 22, 2013, 08:57:21 PM »

Checking the voltage across the RE gain pot was a great idea. The voltage was perfect but I noticed too leads that had gotten too close. Problem solved.

As I start to measure and replace resistors should I move from carbon to metal film? If I buy a better grade of resistor is there any need to up the wattage say from 1/2 watt to 1 watt as some have suggested?
Kevin
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G3RZP
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Posts: 4718




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« Reply #7 on: March 23, 2013, 06:54:21 AM »

The same wattage should be OK. With carbon composition resistors, they all change with time, regardless of manufacturer, unless they are high stability ones which, over here at least, have an extra pink band....I use carbon film now, but metal film is OK. Whichever you have or maybe get cheapest.
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KAPT4560
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Posts: 84




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« Reply #8 on: March 23, 2013, 07:39:40 AM »

 There are service bulletins that made the SB-102 a better xceiver:

 http://www.nostalgickitscentral.com/heath/Service_Bulletins/sb102.txt

 More restoration information and personal conclusions about the SB-102 here:
 
 http://www.nostalgickitscentral.com/heath/heathkit.html
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K8AXW
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Posts: 3900




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« Reply #9 on: March 23, 2013, 09:34:41 AM »

ILM:  Since I have a natural affinity for remembering nonsense and little else of value, I thought I would tell you know one thing I've never forgotten about assembling the SB-102.

It has 1,619 parts!  (Nuts, screws, washers, etc., plus components and chassis pieces)


It was considered a "40 hour project" and I spent 80 hours building it.  Much of the time was spent trimming excess wire and routing.  When I finished it was beautiful!  I also had a great deal of fun with it.  It was my first multi-band transceiver.  Sure, it was primitive by today's standards but nevertheless it was a fun radio with which I worked the world.  (Just some more nonsense.)

I hope you get it working.

Al - K8AXW
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KD0ILM
Member

Posts: 36




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« Reply #10 on: March 24, 2013, 09:17:39 AM »

Well the radio is working well now. In fact I could not be happier with it. I let is run for 10 hours yesterday just to make sure everything was stable, and it was.

Besides "rebuilding" and re-calibrating the LMO all I really did was clean, lube, clean contacts (a lot) and check. I get good power out, the receiver works well so I am happy. I will not align it and hope I get that done without screwing anything up.

I believe that "if it ain't broken, don't fix it" but.......have never followed my own advice. I am restraining myself from replacing things in a working radio but wonder if I am being foolish by not replacing electrolytic caps, checking resistors etc.

The encouragement here was most helpful. Now onto a HW-101 that will not transmit. The 101 came from a smoker so I imagine it will have it's own set of issues. I am shocked at how much tar etc. can get inside a radio with no fan to actively draw air into it. I realize that the heat moves the air, but as I am cleaning inside it is unbelievable how much smoking residue there is. Never mind the outside.

I am so happy that I got into this hobby later in life (at 58), it has been the source of many hours of enjoyment. I think setting up the station and fooling with the old equipment is as much fun as operating.
Best and thanks
Kevin
KD0ILM/XV2KM
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KD0REQ
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Posts: 971




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« Reply #11 on: March 24, 2013, 07:11:06 PM »

on the tubed HW/SB transceivers, every 22K resistor going to a tube plate is highly suspect, as are the bias and coupling resistors in the power amp areas.  I have replaced some 22Ks that went up to 80K.  shotgun 'em and uprate to 1 watt.  there is a Heath service note to uprate the 22K at the tube driving the AGC/meter circuit with a 2 watter.

no carbon comps, please, use metal film if it's not carrying RF, carbon film will replace everything.
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K8AXW
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Posts: 3900




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« Reply #12 on: March 24, 2013, 08:58:48 PM »

ILM:  Congrats on getting the 102 working.  As for replacing anything.... wait until it craps out and then fix it.  As for the suggestions on the 22K resistors.....check the voltages on the tubes and see how far they are from normal.  If they're within 10-15%, leave 'em alone.

On the 101 from the house of a smoker.  You might just as well stop where you are and research eHam.com for a product to WASH the rig with.  What I mean by wash is like hose the damn thing down!

Way back when I used to sell and service CB radios I'd get radios owned by smokers and when I opened them up I would almost puke!  Many times the tar and even cobwebs were so bad that components were unidentifiable. 

I would then take them to my place of employment and wash them down with a chemical solvent that would dissolve all of this stuff and evaporate quickly.  I'd put it in a hot place for several hours to make sure it dried out.  Then I'd go to work on it.

You might just as well plan on this method of attack.
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