Grundig AM/FM portable "Transistor 305" troubleshooting

transistor base down to -9V, near enough.

I hope Lenny will get back to us with his findings.

:)

--Winston

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the transistor base down to -9V, near enough.

First of all let me say thank you to everyone who has become involved in my problem. It really is so nice to be a part of this community of kind people who are willing to help one another. I truly appreciate all your opinions and all the advice you've sent my way.

I have been working on this radio off and on since I posted this. This morning the alarm clock (bedroom radio) went off at 5:45 as it does every morning with soft classical music. As we gently awoke I looked over and saw that it was the Grundig on my bench! It was FIXED and it was WORKING! Imagine my big let down upon actually waking up a few minutes later and realizing that I really was in my bed, I wasn't in the shop, and it WASN'T the Grundig, GAH! This thing isn't driving me insane, I think it already has. Now it's haunting my nights as well as my days....

I would have agreed that the direct relationship between the time the radio is unpowered to the time it stays on the next time it is powered up would definitely point to an electrolytic, but so far I have not found it. There actually aren't that many in the signal circuit. I did replace C 44, not because it checked bad, but because it was suggested. However replacing it had no affect. I monitored the 1.4V zener during intermittent periods of go/no go and the voltage remained at 1,4V. Eventually the radio quit completely for a spell and I bridged the following resistors: R4, R5, R29, R27, R24, R26, R25, and then after measuring the resistance of R22, in circuit, (it was 13K), I bridged it with a 100K (not a conclusive test I know) but just to see if there were any affect There was none. I did resolder R22 but that was not it either. The voltage across R22 does fluctuate quite a bit during intermittent periods but it is a large resistor so I would expect that. However I'm not sure if 13K is whats in parallel with R22 and R22 is really open. I thought of pulling it but first I think that I have an idea.

Since the voltage never was exactly where it should have been, (1.18V) upon powering the radio on the next time I think I' ll see if I can set Vr124 to exactly 1.18V and wait and see if that might make a difference. For one thing it will prove if R22 is working at all, and also if the bias point is that critical to turning these transistors on when they shouldn't be on. I 'm also starting to wonder about the small capacitors among these resistors that have these voltage changes about them. I really hate to just start replacing things but I don't see any other way at this point. I wish there were a way to isolate the offending stage, but with the voltage changing as it is I don't see how I would do that. I think I've ruled out the above resistors but I'll have to start somewhere. It also frustrates me that I can't get on the collector of AF126II because it's inside a can, (FIV) and the can is soldered onto the board. There is another electrolytic, (C4) inside FIV but I don't think that is part of this.

Other than that there are the small caps in and around these two circuits and of course the ones in the last IF can (FIII) and the detector, (FIV). I tried small spot cold spraying again around the suspect area and even into the IF cans and detector assembly, and that yielded nothing. I'm pretty sure I've ruled out a mechanical intermittent also. This definitely appears to be something breaking down electrically. I'm also wondering about one of the IF cans, (coils) opening up intermittently. Does anyone remember how the old five tube radios would crackle when the IF cans failed in them. I never really knew how the cans were failing. We just replaced them and that fixed the crackling. It occurred to me that if either coil in the can in this radio opened up it would affect both AM and FM. I don't recall if I mentioned it too but all the transistor can connections have been lifted to rule out whiskers as well. .

I didn't get to work on the radio today and probably won't be able to tomorrow either as work, (the kind you get paid for) has to take precedence over my hobby. Although my "hobby" as of late has seemed to be more like my nemesis to be exact. Thanks once again for all your input. Lenny

(...)

Did you happen to re-solder F IV pin 10 and inspect the associated trace, Lenny?

I think Sylvia made a good point about it.

It would be hugely amusing to measure the voltage dropped across R21 and R28 during functional / non-functional circumstances. You could use them as current sense resistors to point you in the direction of the open or short.

--Winston

On Wed, 07 Mar 2012 20:35:40 -0800, Winston put finger to keyboard and composed:

I agree. I'm convinced she's pinpointed the culprit.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.

I know she was and it made perfect sense, but I was all over pin 10 and it's associated resistors. I had FIV out to replace AF126II in fact and reinstalled it. I can't see anything wrong with the trace or any traces for that matter. Tomorrow I'm going to take another look at this. I'm going to check those two voltage drops, go and no go as you suggested and I'll let you know. Lenny

Good.

I will Watch This Space. :)

--Winston

=A0IF

=A0 Voltage readingso

not 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 =A0-3.70V *

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

=A0 =A0 =A0 =A0 -0.80V

=A0 =A0 =A0 =A0 -2.95V

=A0 =A0 =A0 =A0 -0.62V

=A0 =A0 =A0 =A0 -3.10V

Sylvia must mean R28 in series; R26 goes from the emitter of AF 126 I to ground.

A reference I found says that the minimum hFE for the AF 126 is 50, so a beta of 42 here is kind of sad.

Now I'm wondering if the supply voltage is really 9V.

o

Today I took the time to thoroughly clean all the old caked on flux off the areas that I've been working on and others also in order to get a better look at the board. I removed R22 and it tested OK. I put it back and reset the bias on the emitter of AF126 I to exactly 1.18V With this bias all the other voltages seem to be very close to what's shown on the schematic as well. I also did reflow some other suspect solder connections using Kester mildly activated liquid flux. I measured the following voltages before it became very intermittent again. Group 1 is with the radio working.. Group 2 is after the radio became very intermittent. group1: Vr28 .078V vr21 .012V Vr29 .0067V Vr22 5.80V Vr26 5.41V Vr25 .057V

group 2 Vr28 2.00V Vr21 2.10V Vr29 .0011V Vr22 2.30V Vr26 2.60V Vr25 .105V

I don't know if this was a coincidence or what but I had taken the above sets of voltages and had then decided to try to measure the E - C voltage on AF126 I. As soon as I placed my probe on the collector I must have detuned the circuit as the radio completely quit. I removed the probe, it began working again briefly and then it started crackling and soon after became constantly intermittent. Did I "shock" something I wonder or was it just only a coincidence? Right now it's in limbo between working and not, so intermittent that I can't really look at anything.. Lenny

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Well this may be a bit premature but in looking at the circuit I was trying to determine what could possibly cause such a drastic change in all these voltages, among the most drastic the voltage across R 22. So on a whim I just replaced C38 and powered the radio up after a long failure period. Normally under this condition it would quit almost immediately. So far it has been working for about ten minutes. I hope this doesn't end like my dream did the other morning....Lenny

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C38 was a foil cap, right? What kind did you replace it with?

It'd be great if C38 turned out to be our leaker, current-wise.

I'm still curious why the increased base current through R21 (949 uA) is not completely apparent as an increased current in the resistor that is in series with it, R28 (only 130 uA more, not 949 uA more). Our increased current isn't going through R22 because during the failure, current through R22 *decreases*.

:)

Resistance Eworking Iworking Enotworking Inotworking Edifference Idifference R28 15000 0.07800000 0.00000520 2.00000000 0.00013333 1.92200000 0.00012813 R21 2200 0.01200000 0.00000545 2.10000000 0.00095455 2.08800000 0.00094909 R29 330 0.00670000 0.00002030 0.00110000 0.00000333 -0.00560000 -0.00001697 R22 15000 5.80000000 0.00038667 2.30000000 0.00015333 -3.50000000 -0.00023333 R26 180000 5.41000000 0.00003006 2.60000000 0.00001444 -2.81000000 -0.00001561 R25 10000 0.05700000 0.00000570 0.10500000 0.00001050 0.04800000 0.00000480 R24 820 1.00000000 0.00121951 3.70000000 0.00451220 2.70000000 0.00329268

--Winston

You're right about the supply voltage. I just looked at the battery, but in fact the radio takes its current from the battery through a resistor, and the markings on the circuit indicate that this reduces the supply to

7.5V. Applying the correction gives a beta of 50.4, which is admittedly still right at the low end of the spec you've found.

Offsetting that, the base is also being pulled down through R22, whose actual value we don't know. I assumed it would be closer to 1M, on the grounds that otherwise a smaller valued trimmer would have been used.

It's a bit moot, anyway, since the OP says that he's already resoldered pin 10 of IF IV.

My next step would be to look at the voltage on what I think is pin 8 of IF IV (i.e., the pin connected to R31 and R28.

Sylvia.

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=A0IF

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=A0 =A0 =A0Voltage readingso

adio not working

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

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

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

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

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

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

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

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

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

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

=A0 =A0 =A0 =A0 =A0 -0.80V

=A0 =A0 =A0 =A0 =A0 -2.95V

=A0 =A0 =A0 =A0 =A0 -0.62V

=A0 =A0 =A0 =A0 =A0 -3.10V

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I used an epoxe cap.. The original was tubular and assuming foil with a polarity stripe. It doesn't seem to matter though. I found R22 set to about center when I pulled it. It measured about even on both sides of the wiper. I probably turned it about 90 degrees after my first adjustment for Vr24.(before replacing C38). But now I'm not so sure that that all my readings are correct. If you look at the note on the schematic it reads: "voltages measured with Grundig VTVM at battery voltage 7.50V, "etc". This seems to suggest, and it was the impression that I got that for whatever reason I was supposed to lower the applied voltage to 7.50 V as measured at the battery terminals to make my voltage measurements. Sylvia pointed out that (with the positive ground arrangement) the applied voltage (negative) goes through the 100 ohm resistor R46 to what we would consider as "ground", the positive rail. This reverse polarity takes a bit of getting used to...

There is a note near the battery connector which indicates what appears to be a quiescent current between .020 and .022A. In fact when I started this project I measured .025A on the bench power supply with

7.50V applied to the battery terminals. Therefore, given my initial quiescent current reading, if all that current is indeed flowing through R46 then I should have dropped 2.50V across R46. However I never monitored that point so I didn't know. This morning the radio was still working and I noted that Vr46 was .840V. Quiescent current is still .025A. So now I'm really confused. Are you supposed to start off with 9.0 Volts at the battery terminals for voltage measurement purposes or is it 7.50 Volts? I can say with every certainty that with 7.50V applied the actual circuit voltages did almost mimic what the schematic shows when the radio was operational. Or has my supply been at least 1.50V - 2.50V too low throughout this experiment? Lenny

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=A0 =A0 =A0Voltage readingso

=A0Radio not working

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

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

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

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

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

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

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

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

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

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

=A0 =A0 =A0 =A0 =A0 -0.80V

=A0 =A0 =A0 =A0 =A0 -2.95V

=A0 =A0 =A0 =A0 =A0 -0.62V

=A0 =A0 =A0 =A0 =A0 -3.10V

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I used an epoxe cap.. The original was tubular and assuming foil with a polarity stripe. It doesn't seem to matter though. I found R22 set to about center when I pulled it. It measured about even on both sides of the wiper. I probably turned it about 90 degrees after my first adjustment for Vr24.(before replacing C38). But now I'm not so sure that that all my readings are correct. If you look at the note on the schematic it reads: "voltages measured with Grundig VTVM at battery voltage 7.50V, "etc". This seems to suggest, and it was the impression that I got that for whatever reason I was supposed to lower the applied voltage to 7.50 V as measured at the battery terminals to make my voltage measurements. Sylvia pointed out that (with the positive ground arrangement) the applied voltage (negative) goes through the 100 ohm resistor R46 to what we would consider as "ground", the positive rail. This reverse polarity takes a bit of getting used to...

There is a note near the battery connector which indicates what appears to be a quiescent current between .020 and .022A. In fact when I started this project I measured .025A on the bench power supply with

7.50V applied to the battery terminals. Therefore, given my initial quiescent current reading, if all that current is indeed flowing through R46 then I should have dropped 2.50V across R46. However I never monitored that point so I didn't know. This morning the radio was still working and I noted that Vr46 was .840V. Quiescent current is still .025A. So now I'm really confused. Are you supposed to start off with 9.0 Volts at the battery terminals for voltage measurement purposes or is it 7.50 Volts? I can say with every certainty that with 7.50V applied the actual circuit voltages did almost mimic what the schematic shows when the radio was operational. Or has my supply been at least 1.50V - 2.50V too low throughout this experiment? Lenny

(...)

'Sounds like your 'rail voltage' ended up at 6.66 V considering the 0.84 V drop in R46 or about 90 mV below the minimum indicated on the schematic.

Were it me, I'd adjust the power supply until the rail voltage measured, say 6.88 V.

It's an exercise anyway because you apparently found and fixed the bug.

That's the important thing. :)

--Winston

t

Well I still have my fingers crossed but it does look hopeful. Lenny

The situation mostly doesn't arise. With a few (possibly only one) exceptions, the annotated voltages are between two specified points, not a voltage relative to ground. Indeed, most are between two points neither of which is anywhere near ground.

The base and emitter voltages are relative to the positive supply rail. Given the way that the biases are obtained, they wouldn't have been sensitive to the supply voltage, which is no doubt intentional given that the supply voltage from a battery will change as the battery is consumed.

On another note, while replacing C38 has apparently cured the problem, I'm totally mystified as to why. It would imply that C38 had failed such that it intermittently behaved as if it had a resistance of many kilo-ohms in parallel with it. It seems an unlikely failure mode for a capacitor.

Sylvia.

(...)

totally mystified as to why. It would imply

resistance of many kilo-ohms in parallel with it.

C38 could be a total red herring.

Lenny said yesterday afternoon:

"I don't know if this was a coincidence or what but I had taken the above sets of voltages and had then decided to try to measure the E - C voltage on AF126 I. As soon as I placed my probe on the collector I must have detuned the circuit as the radio completely quit."

It may just be that the base of AF 126 I is shorting out against its grounded can, causing it to saturate intermittently. If C38 is close to AF126 I, Lenny might have 'bumped' the short open whilst replacing C38.

--Winston

If the base were shorted to the grounded can, then the base voltage would be at ground level, but it's not. That's the puzzle with this fault - finding a credible intermitten failure that will put the base at the voltage level seen.

Sylvia.