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Grundig AM/FM portable "Transistor 305" troubleshooting

I would like to ask for help once again with the repair of my own personal old radio. I started this project almost a year ago, and it's been on the back burner since then. But I had a little free time and so I put it back on the bench. I would really like to see this radio working again.

This is a Grundig "transistor 305", circa 1970 I think. The radio is intermittent. I have been working on this set on and off for almost the past year. I recently took some voltage measurements and there are some serious differences in base and emitter voltages in the last IF stage and the detector stage when this radio fails. I have tried many things, heat and cold, including substituting all the transistors in the RF and IF sections one at a time and have studied the schematic until I felt a migraine coming on. l think that by now I could almost draw it from memory, and I'm still coming up empty.

I considered trying to inject a signal and/or signal tracing, however with the circuit voltages so far out of whack when it fails I think that approach would be futile. The base and emitter voltages in these three stages increase substantially when the radio quits. Some voltages are jumping from for example 1.2V up to 4.0 V. With conditions 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. However the remedy has stubbornly eluded me thus far. The intermittent nature of the problem is what makes it so difficult.

This is typical of what is going on: If the radio has been off for a long period of time it will typically work for several hours and then quit. In the beginning of this Quitting sequence, just after it first quits it will try to come back on intermittently for a short period of time, crackling etc, (as though something was intermittently breaking down). Eventually it will just remain silent. There was an electrolytic in the audio driver section, C57 a 100uf/3.0 volt with slightly high ESR. Replacing it improved the low frequency audio response, however the AF amplifier is not the problem though. You can still get a good audio signal from the volume control out when it fails. So the AF section seems to be unaffected. The voltages to that part of the circuit substantiate that as well. And the RF and oscillator circuit voltages during dead time seem to be fine as well. The problem when it occurs affects both AM and FM.

The strange thing is that when powering this radio up again after a shut down, the time it remains on before it quits once again is directly proportional to the time that it has been off. For instance if you leave it off overnight it might remain on for a half hour or so the next morning before it quits again. Recently after trying it again after months of it sitting idle it played for about 3 hours before it quit. Then five minutes later when trying it again it quit almost immediately. The few other electrolytics in the affected areas check good on ESR and bridging them during dead time yielded no improvement either. I have the schematic and I have uploaded it to the site listed below.

I know that there are technicians out there better at this than I am, and I would really appreciate it if someone could please take a look at the schematic along with the voltage readings I obtained during Go and No Go conditions and give me your opinions. With three stages affected I suspect that I'm looking for a common denominator but I'm just not sure. The initial voltage readings (on turn on with the unit working normally) will be listed in the left hand column. The voltage readings during a failed condition are listed in the right hand column. The major differences are marked with an asterisk*.

I used to strap this radio to my bicycle when I was a kid and ride around The Bronx with it. It has a great sound. I've owned it since I was a teenager. I guess it's just a sentimental thing. Thanks for any assistance. Lenny

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Voltage readings Test point Radio working: Radio not working ------------------- --------------------

------------------------- E. AF178 -1.10V -1.20V B. AF178 -1.50V -1.50V E. AF124 -1.10V -1.20V B. AF124 -1.32V -1.44V E. AF121 -0.85V -0.90V B. AF121 -1.20V -1.30V E. AF126 (1) -1.00V -3.70V * B. AF126 (1) -1.20V -4.00V * E. AF126 (II) -0.80V -3.70V * B. AF126 (II) -1.10V -3.60V * E. AC151 (I) - 0.90V -0.80V C. AC151 (I) --3.05V -2.95V E. AC151 (II) --0.66V -0.62V C. AC151 (II) --3.20V -3.10V

Thanks once again for looking. Lenny

Reply to
klem kedidelhopper
Loading thread data ...

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"A drawing error occurred."

Try cutepdf.

Reply to
spamtrap1888

Hey Lenny,

It looks like a thermal intermittent. Transistor AF126 is being told to 'cut off' when the area around it heats sufficiently. I suspect trim pot R22 (Just below AF126) has a fractured solder joint which pops open under thermal expansion.

Use your plastic spudger tool to gently wiggle R22 and see if you can reproduce the problem at will.

Next available moment, suggest you replace the solder on R22 using plenty of liquid RMA flux, then clean the area with naphtha and an acid brush. Use lots of ventilation and 'exam gloves' on your hands. Naphtha is an excellent cleaner and will suck the oil right out of your hands. DAMHIKT. :)

--Winston

Reply to
Winston

On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper put finger to keyboard and composed:

I suspect that C45 (near pin #8 of 7209-301) may be shorting.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

Voltage readings

working

=A0 =A0 =A0 =A0 =A0-1.20V

=A0 =A0 =A0 =A0 =A0-1.50V

=A0 =A0 =A0 =A0 =A0-1.20V

=A0 =A0 =A0 =A0 =A0-1.44V

=A0 =A0 =A0 =A0 =A0-0.90V

=A0 =A0 =A0 =A0 =A0-1.30V

=A0 =A0 =A0-3.70V *

=A0 =A0 =A0-4.00V *

=A0 =A0 =A0 =A0-3.70V *

=A0 =A0 =A0 =A0-3.60V *

=A0 =A0 =A0 -0.80V

=A0 =A0 =A0 -2.95V

=A0 =A0 =A0 -0.62V

=A0 =A0 =A0 -3.10V

The "whisker theory was also posed to me last time I took a crack at this repair. I disconnected all the cans at that time but the problem still persisted. I'm trying to understand though how C45 could affect this? It seems like a somewhat removed part of the circuit but perhaps I'm not seeing the whole picture. Could these two affected transistors be turning on, and if so wouldn't the collectors be very close to the same potential as the emitters? I never looked at that. Collector voltages are not listed on the schematic for those stages. Still if that isn't the case I have to try to determine how this voltage is so drastically rising. R22 seems to set a bias point. I subjected R22 to some stress. It's definitely not a cold solder joint on that pot. Unless it's breaking down internally but I would have thought that my poking and prodding would have helped that along. In fact I had hoped that once things became "thermal" they might also become "mechanically" intermittent as well, but that was not the case. Now hypothetically speaking in the case of one of the 820 ohm emitter resistors opening, would that affect both stages? So many possibilities here. I also thought of putting the soldering iron onto the leads of several components upon turn on to attempt to bring on the problem ahead of the time it usually takes but I am leaving that as a last resort. Here is another link to the schematic just in case the first one doesn't work or fails:

formatting link
Lenny

Reply to
klem kedidelhopper

When you re-soldered the connections to R22, did the intermittent go away?

--Winston

Reply to
Winston

This is a good example of the dilemma anyone fixing an electronic product faces -- do you want to find out exactly what's wrong, or do you just want to get it working again?

My own bias is towards the former, but eventually one has to stop experimenting and "fix the damn thing, already".

Given that it's battery-operated, a thermal problem seems unlikely. A bad solder joint, or a cracked trace or solder pad seem likely. It might also be a bad transistor.

I would unsolder all the components around the suspected-bad area, and troubleshoot for a cracked trace/pad. If you can't find any, replace all the unsoldered components with new ones.

Reply to
William Sommerwerck

=A0 Voltage readings

t working

=A0 =A0 =A0 =A0 =A0 =A0-1.20V

=A0 =A0 =A0 =A0 =A0 =A0-1.50V

=A0 =A0 =A0 =A0 =A0 =A0-1.20V

=A0 =A0 =A0 =A0 =A0 =A0-1.44V

=A0 =A0 =A0 =A0 =A0 =A0-0.90V

=A0 =A0 =A0 =A0 =A0 =A0-1.30V

=A0 =A0 =A0 =A0-3.70V *

=A0 =A0 =A0 =A0-4.00V *

=A0 =A0 =A0 =A0-3.70V *

=A0 =A0 =A0 =A0-3.60V *

=A0 =A0 =A0 -0.80V

=A0 =A0 =A0 -2.95V

=A0 =A0 =A0 -0.62V

=A0 =A0 =A0 -3.10V

I was able to open the pdf after downloading it.

While I haven't done the math, I would suspect C44. First, it is suspect merely by virtue of being a 40 year old electrolytic capacitor that is connected to both failing stages. Further: if, when you turned the radio on, C44's leakage current was initially low, but with operation it increased substantially, then a bad C44 would affect the bias point R22 was trying to set, as current began to flow through R21. Moreover, a short through C44 to positive should have an effect similar to the short through C45 to ground that Franc suspects. Thus, I would try replacing C44.

Reply to
spamtrap1888

ist,

g

What he says is what I would do also>>>>>

Reply to
hrhofmann

On Mon, 5 Mar 2012 07:02:27 -0800 (PST), klem kedidelhopper put finger to keyboard and composed:

Sorry, I'm not convinced that C45 is the culprit. I was confused by the negative voltages and the upside down circuit diagram. :-(

I still think that some component is reducing the bias on the base of AF126 (I). I was looking for a path to ground via a likely suspect, and C45 and C46 looked like possible candidates.

As for R22, I don't understand how an open circuit could reduce the voltage on the base of the transistor.

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

(...)

Emitter current through R24 is adjusted by the setting of R22, per the note to the right of R24. As R22 is adjusted higher in resistance, we can expect less current to flow from the base to emitter, causing AF126 to tend towards cutoff. (The base becomes less negative *in relation to the emitter*).

What if we broke the connection to R22 by lifting it's rotor or fracturing either of it's solder connections? The base of AF126 will be biased more positive in relation to it's emitter (towards cutoff) because R22 isn't there to provide a parallel current path to ground.

See how the AGC rectifier (AA130) biases the base of AF126 more positive (via R21) as IF voltage increases? This also tends to push AF126 towards cutoff. It's a negative feedback loop that keeps IF voltage constant.

--Winston

Reply to
Winston

My $0.02 worth... Since two stages are being upset, look for what's common to both circuits. I don't know how the PCB traces are laid out, nut since the layout diagram indicates that both stages are enclosed by (metal?) shields, it would be possible that the PCB trace(s) that connect the shields together might be intermittently broken fron the rest of the circuit. That would cause the symptom of the measured B and E voltages being high negative, indicating that the collector circuits of both stages are open. Look for a broken trace or solder joint somewhere in the collector circuits.

I've seen broken shielding enclosure connections cause all sorts of weird symptoms.

--
Dave M
A woman has the last word in any argument. Anything a man says after that is 
the beginning of a new argument.
Reply to
Dave M

On Mon, 05 Mar 2012 15:39:48 -0800, Winston put finger to keyboard and composed:

AISI, the voltage readings on the circuit diagram are using the positive terminal of the 9V battery as the 0V reference. Therefore all the voltage measurements are negative numbers, which means that the transistor is being turned on harder during the fault condition. The voltage on circuit ground would be -9V.

Or am I having a brain fart? (Quite likely)

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

The bias of AF126 II is derived from the emitter of AF126 I (hereinafter 'the transistor'), so if the latter's bias arangements go awry, so will the former's, which is thus of no interest.

If pin 10 of F IV became open circuit, the base of the transistor would be pulled towards -9V through R5, R4, R26 and R21 in series, being a total of 47.2K. For that to put the base at -4V requires a base current of 0.1mA, there being no other source for the current.

For the emitter of the transistor to be at -3.7V requires a current of

4.5mA through R24 (the 820 ohm resistor). Given the calculated base current, this would be the case if the beta of the transistor were 42, which seems entirely plausible.

So if the transistor has a beta of around 40, the behaviour can be explained by an intermittent open circuit at, or in the vicinity of, pin

10 of F IV.

Sylvia.

Reply to
Sylvia Else

(...)

I had the fart. :)

You and Sylvia are much closer to the truth. I now see that R5, R4, R28 and R21 would tend to turn on the transistor (AF126 I) if the path from the positive terminal of the battery, (through the diode "1.4 St1", through pin 10 of F IV) were opened, because pin 10 of F IV is only one diode drop less than the positive terminal of the battery normally.

So, I retract my diagnosis and now think that pin 10 of F IV and both pins of diode "1.4 Stl" should be cleaned and re-soldered.

--Winston

Reply to
Winston

transistor'), so if the latter's bias

pulled towards -9V through R5, R4, R26 and

Yes, I now see that R5, R4, R28 and R21 would tend to turn on the transistor if the path to pin 10 of F IV were interrupted.

current of 0.1mA, there being no

through R24 (the 820 ohm resistor). Given

transistor were 42, which seems entirely plausible.

by an intermittent open circuit at, or in

I agree. Cleaning and re-soldering pin 10 of F IV as well as both pins of the diode "1.4 St1" are good next steps.

--Winston

Reply to
Winston

On Tue, 06 Mar 2012 06:40:59 -0800, Winston put finger to keyboard and composed:

AISI, if the diode were open, then that would disturb the bias on the preceding stages (AF121 and AF124/125).

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

OK, this HAS to be a bad electrolytic capacitor. That is the only component that has that sort of memory. Replace electrolytics in the area where the problem is suspected. They are often used in bias networks.

Jon

Reply to
Jon Elson

As Frank observes, if the 1.4 St1 connections are compromised, then the bias for the AF124 would be affected, and on the OP's readings, it is not.

Now, we don't know the nature of the link between that diode and pin ten of F IV, and I've seen Grundig do some rather questionable things, but if it's just a circuit board track, the fault pretty much has to lie either in the pin 10 soldering, or in the track itself.

Sylvia.

Reply to
Sylvia Else

(...)

for the AF124 would be affected, and on the

IV, and I've seen Grundig do some rather

much has to lie either in the pin 10

Good point.

I look forward to what Lenny eventually discovers.

Given the history of the radio, a cracked trace would not be out of the question.

It'll be amusing if it was C4 or C38 or R22 that was intermittently shorting, instead. :)

--Winston

Reply to
Winston

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