[sprint750]

Hi Peter
Thank you for your interest in this thread.
Very interesting tracks
My answers below

Originally Posted by orbanp1

Chances are that it was one of the final Ge-transistors that was bad in your circuit.
As mentioned, the AD161/AD162 is also a good good replacement.

Yep, if you read the full story, this was the root cause of PCB track burning. This was the conclusion of initial troubleshooting: I replaced both final stage GE transistors by NOS original parts (AD156/157) from a Blaupunkt box (so, I supposed they were paired). Did this with the schematic at hand and double checked everything, to be 100% sure. Original fitting was correct, BTW. So failure was not a mistake from P.O.

Two things you could check in the circuit once it is operating:
The voltage at the common emitter of the final transistors should be half of the supply voltage. A quick simulation shows that with the resistor values in the diagram that is assured.

Unfortunately, this is not the case (see my voltage measurements, attached pdf)
The schematic says as you suggested. Should be 7V here (14V supply)
I read 5.85 V (with 13.5V supply )
If I do my maths, should be 13.5/2=6.75V here.
All these measurements were done with a 8 Ohm load on the output. (by principle, I never power on these babies without a loudspeaker connected)
Without load on the output, all the teminals of the transistor show VDC (13.5V), as soon as I disconnect the loudspeaker.
Are we suppose to find 7V in the common emitter without a load on the output ?

The other thing you would want to check is the idle current of the final transistors.
It should be about 8mA to 12mA or so. You should open the collector circuit of V951 (or V952) and measure the current there. The idle current can be adjusted by selecting appropriate value for the W15 (180 Ohm) resistor. If the current is too much decrease the resistor value, if is too little, then increase it. You could use 5 Ohm increments.
Do not use pot there, those are not reliable in the long term. If you use a pot, adjust the current, measure the value of the pot, and select a replacement resistor. You could make a required value from two resistors, either in series, or in parallel.

Unfortunately W15 is like all the 'W' labelled resistor of "thick film" type.
I can't open the circuit here as it is a single unit of 10 resistors, used by all the BF section. (probably saves space on the PCB)
If I look carefully at the diagram, idle current seems also ruled mainly by R951 (thermistor) in // with W15. When measuring, I have a correct reading of 28 Ohms ( 33 Ohms // 180 Ohms =27.88 Ohms theoretical), so I have very little reasons to suspect these 2 components R951 and W15

The fact that you measured 12V at the bottom of R249 might suggest that the V206 transistor is shorted.
It was a sound advise to check all the transistors in such a DC feed-back circuit, so you might want to do that.
For V206, V207 replacement any small signal Si transistor is OK, e.g. 2N3904-3906 or similar.

Actually I DID NOT measured +12V at the bottom of R249.
A damaged track carrying 12V touched involuntarily this point when handling the unit (I just put a littel pressure on the PCB) and suddenly the unit started to sing. It stops as soon as the 12V is not applied. In normal situation, the actual Voltage here is 6.29V (see my measurements) for 8.7V blueprinted. This reflects the globally bad polarization of BC238B.
All the voltages measured are under the specs. The worst being at the Collector of BC238B, with 6.3V where 13.5 is expected.

Originally Posted by orbanp1

An interesting observation is that there are no emitter resistors at the final transistors!
Auto-radios with similar AD161/162 final stages (and other audio PAs) all use 0.5 Ohm emitter resistor (the actual value depending on the power rating) in the final stage!
The lack of those resistors was probably the reason for the destruction of the final transistor, it could have suffered a thermal runaway.
You could retrofit those, it would make your radio more "foolproof" in the future.
Simulation shows that the addition of such resistor barely changes the idle current of the final stage.

Maybe I'll try to do it , but first I have to have this unit working

As Livewire stated, the first stage adds about 10dB gain to the audio amp.

My question was more focused on the role of BC328.
I have several other unit from the same period, based on AD156/157 or AS161/162 and all of them features only 2 transistors in the pre-amp stage (see examples included)


Peter,

Thank you for your contribution
Hope my answers make sense.

Best regards
Sebastien

Originally Posted by 'LIVEWIRE?'

It may also be prudent to check the (zener) diode D2017 and the R951, which, IIRC is Thermistor. From memory, these seldom failed, but, given that the radio is over 47 years old, anything is possible! A couple of other points, one relevant, the other incidental. The relevant point is the speaker coupling capacitor C277. I've known these fail S/C in various makes of car radio, particularly if they are of the 'Frako' brand. The value isn't critical so the 800uf can be replaced with 1000uf, 10v or 16v. The other point is the pilot lamp. Should the fitted 7v type fail, it can be replaced with a 12/14v bulb. In this case R961 should be shorted out.

Thank for the input

After the Ge transistors, the first check was for the R951. I know that this is a sensitive element to prevent thermal runaway.
Reading was 28 Ohm (ambient temperature), which is correct considering it is in // with W15 (180 Ohm) The maths says I should get 27.88 Ohm.

As far as C277 is concerned, this is one of my potential suspect. The only problem, it is sunk under the power antenna outlet (see picture)

Something surprising. The capacitor casing is not insulated (raw aluminium), and very close to Power transistor heat sink on one sideand power supply unit on the other side. On the vert similar '71 Mannheim I have at hand, this capacitor is insulated with the usual plastic film.

Can't say about the brand and the rating, no marking, just bare metal. I can't say if it is original to the unit or already a repair.

About the zener diode, made some research, but it's not very clear to me about how to test it.
When measuring the voltage with unit powered , I found 0,7 V difference between the 2 poles of the diode. As this is the exact value stated on the Blaupunkt documentation, I guessed (maybe wrongly) that the trouble should be somewhere else

Thank you again for your helpful hand and expertise (experience talks, here)
Best regards
Sebastien

About the lamp, the unit was already fitted with a 12V bulb instead of the 7V OEM equipment, 80 OHM resistor is absent.

Best regards
Sebastien