Grundig AM/FM portable "Transistor 305" troubleshooting

klem kedidelhopper · 2012-03-05T03:47:09+00:00

I would like to ask for help once again with the repair of my ownpersonal old radio. I started this project almost a year ago, and it'sbeen on the back burner since then. But I had a little free time andso I put it back on the bench. I would really like to see this radioworking again.This is a Grundig "transistor 305", circa 1970 I think. The radio isintermittent. I have been working on this set on and off for almostthe past year. I recently took some voltage measurements and there aresome serious differences in base and emitter voltages in the last IFstage and the detector stage when this radio fails. I have tried manythings, heat and cold, including substituting all the transistors inthe RF and IF sections one at a time and have studied the schematicuntil I felt a migraine coming on. l think that by now I could almostdraw it from memory, and I'm still coming up empty.I considered trying to inject a signal and/or signal tracing, howeverwith the circuit voltages so far out of whack when it fails I thinkthat approach would be futile. The base and emitter voltages in thesethree stages increase substantially when the radio quits. Somevoltages are jumping from for example 1.2V up to 4.0 V. Withconditions like this you would think that the cause would be obvious,and perhaps I've been staring at this this too long and it is. Howeverthe remedy has stubbornly eluded me thus far. The intermittent natureof the problem is what makes it so difficult.This is typical of what is going on: If the radio has been off for along period of time it will typically work for several hours and thenquit. In the beginning of this Quitting sequence, just after it firstquits it will try to come back on intermittently for a short period oftime, crackling etc, (as though something was intermittently breakingdown). Eventually it will just remain silent. There was anelectrolytic in the audio driver section, C57 a 100uf/3.0 volt withslightly high ESR. Replacing it improved the low frequency...

Mar 05, 2012 Last reply: 4 years ago 65 Replies

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Winston 14 years ago

Heh! Good one!

Especially Sylvia and Franc that got you pointed in the proper direction.

You are now the World Expert on the Grundig Transistor 305. Congratulations!

Hoo Boy. Cannot wait for that.

--Winston :)

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Michael A. Terrell 14 years ago

They show up on news:rec.antiques.radio+phono from time to time.

You can't have a sense of humor, if you have no sense.

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josephkk 14 years ago

when

with

start

with

you

there

Please to explain just why. As i don't think any possible intermittent failure of C38 will explain the voltages seen.

?-)

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Sylvia Else 14 years ago

Such a crack would certainly stop the radio from working, but there would be nothing left to pull the emitters positive. They'd go to ground, pulled there through R26. That's not what the OP observed.

The loss of the emitter-base currents (those junctions now being reverse biased) would cause the base voltages to drop somewhat, but not by anything like the amounts seen.

I have my own doubts about C38, notwithstanding the OP's apparently successful repair, but that's a separate issue.

Sylvia.

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Lloyd Carmel 4 years ago

responding to

formatting link

, Lloyd Carmel wrote: Lenny, you are my hero.

I recently had the same problem with my Grundig Transistor 305.

The same solution worked like a charm.

Lloyd

klem kedidelh> I never thought of myself as being obsessive compulsive. In fact I've

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Lloyd Carmel 4 years ago

responding to

formatting link

Lloyd Carmel wrote: Lenny, you are my hero.

I recently had the same problem with my Grundig Transistor 305.

The same solution worked like a charm.

Lloyd

klem kedidelh> I never thought of myself as being obsessive compulsive. In fact I've

Vote

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