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Author Topic: HW101 meter drift  (Read 3099 times)
KE4JOY
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Posts: 1384




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« on: October 22, 2012, 03:04:31 PM »

Yea I know what else is new...  Roll Eyes

Got this  HW101 griefkit. It works pretty good but the meter drift is driving me crazy. Ive tried different 6AU6s made sure they werent the 5v filimant kind. Did some other 'cleaning up' in the circuit as well.

But it dirfts, slowly, I will 'zero' it to about +5 to +7db on startup and it slowly sinks to or below zero after an hour.

Whats the most likely culprit here?
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KD0REQ
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Posts: 1006




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« Reply #1 on: October 22, 2012, 03:52:37 PM »

look for 22K half watt resistors from B+ to plate.  replace 'em with 1 or 2 watters in metal oxide.

well, they SAY they're 22K.  I've got some I swapped out in my HW-100 (still a work in progress, everybody needs a hobby) that were over 41K.  and they don't stay on one value in use.

for that matter, the more resistors you lift a lead on, the more you will find that are seriously out of value in the Green Line due to age.  this is one reason why Heathkits didn't make Collins look like a bargain rig.
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AD4U
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« Reply #2 on: October 22, 2012, 06:13:44 PM »

Yep - When restoring my almost mint physical condition SB-102 I had to replace 37 resistors that had gone way high in value.  Apparently in some runs of SB and HW rigs Heathkit used some resistors that did not age very well.  Also many resistors in the SB and HW tube rigs were operating at or in some cases over their rated value. 

As previously posted, pay particular attention to the 22K resistors in the low voltage (300VDC) areas.  They are the ones dissipating more watts than their rated value.

Dick  AD4U
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KE3WD
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Posts: 5689




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« Reply #3 on: October 22, 2012, 07:48:10 PM »

Carbon composition resistors can absorb humidity and that causes clumping and that is the reason they go way out of tolerance.

Replace them with modern equivalents that should last a lot longer.  Metal film.


73
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N4NYY
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Posts: 4818




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« Reply #4 on: October 22, 2012, 08:07:33 PM »

I restored one a while back and the carbons were way out. I also think there may have been a meter service bulletin mod. I cannot recall for sure.
« Last Edit: October 22, 2012, 08:10:10 PM by N4NYY » Logged
N4NYY
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Posts: 4818




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« Reply #5 on: October 22, 2012, 08:09:41 PM »

MARCH 26, 1976

HW-101 BULLETIN NO:

SSB TRANSCEIVER HW-101-10

S-METER DRIFT

To bring the meter drift to an acceptable level, install the following:

CHANGE: R107 from 100K Ohm 1/2 Watt to 100K 1 Watt

[PN 1-28-1]

This makes the voltage divider string more stable with temperature changes caused by internal heating.

This change will be made in future production runs.
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KE4JOY
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Posts: 1384




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« Reply #6 on: October 23, 2012, 01:05:41 PM »

Actually I had made that mod when originally restoring the rig. Put a 1 watt resistor but it was cabrob I believe. I will take a second look at it.

Thanks.
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KE4JOY
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Posts: 1384




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« Reply #7 on: October 23, 2012, 05:13:17 PM »

A quick update. It turned out to be a bad tube. Least likely suspect!  Wink

Had a coupla big 100K resistors, somewhat close to tolerance and I tried them both swapping out, still same results...  Huh

It was frustrating because it would take at least 5 minutes for things to stabilize. But while testing  I noted some noise on the circut. The meter was bouncing as if a signal. I isolated all rf input and it still flicked around at times flicking to S7 or so and staying there for a half a second or so.

Well the long warm up time got me to thinking. I dug into the tube box found 3 6AU6 tubes and played round robbin with three tubes in two slots. It was slow going at first given a min of 15 min warm up but eventually the flakey tube betrayed itself. .  Grin

Anyhow its rock solid now. Thanks all!
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KD0REQ
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« Reply #8 on: October 23, 2012, 07:27:02 PM »

the old-time pencil-tapping trick will get quite a few nutty tubes to show themselves, but if it's gas, you either have to leave 'em on the tube tester and watch for needle bounces, or catch them in the circuit.
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KE4JOY
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Posts: 1384




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« Reply #9 on: October 24, 2012, 12:55:09 PM »

No tube tester here except putting them in the circuit and let em burn  Grin

I did a little more round robbins with tube swaps and as it turns out it wasent the tube at all but rather the tube socket V3....

LOL Goofy radio is haunted. Oh well at least I still have some servicable 6AU6s.
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N8CMQ
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Posts: 387




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« Reply #10 on: October 26, 2012, 10:52:04 PM »

Your failed tube socket is why I am replacing the plastic sockets with ceramic in my Hallicrafters SX-101...
I am also using name brand resistors, metal and carbon film, to replace the old resistors.
Plus, all the caps are getting replaced, so it will be like a new rig when I am done.
Good luck with your project!
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