'71 Blaupunkt E

[sprint750]

After compiling all the precious informations I got from the previous threads, I did my homeworks on the weekend,

I decided to go deeper in the issue, and start to check components (usual suspects).

Step 1 : Check BC328

This is the worst area in comparison with diagram. Every voltage reading is wrong, around BC328. I unsoldered the transistor and my component tester said it is a 13 Ohms resistor between B and E...

RIP BC328....


Step 2 : Check C275

Since this is the only capacitor in // on the amp citruitry, I was wondering if it was working properly and not leaking. Well results were

C= 222 uF (220uF blueprinted) OK but....
4.5 % Vloss!

I'm afraid this is a bit too high, is it ??

Step 2 : Check C277 (last 3 pics)

This is the output cap. 800 uF blueprinted.
It was pointed me out as being a possible cause for the death of final stage transistors.
Getting access to the capacitor was a pain in the A It is sunk between the Ge final stage heatsink and the DC input filtration block, with the power antenna remote on the top ...
Finally I managed to get the heatsink out of the PCB. Not for the faint hearted

The results: Good cap, bad cap... ?
C=990 uF (800 uF blueprinted)
Vloss = 1%

Even if this component is now definitely out of spec, my guess is this is the NOT root cause for the no amplification issue (correct me if I'm wrong). I've seen fancy values on schematics based on AD156/157 pair, ranging from 470 to 1000 uF and it seems that the number itslef is not that important, assuming it stays in the same order of magnitude, say from 300 to 1000uF.
Vloss seems quite acceptable (lower reading was 0.9%, highest 1.2%), but once again, I miss some feedback to compare.

Any other checking I SHOULD do ?
Thank you in advance for your contribution

Best regards
Sebastien

['LIVEWIRE?']

Based on years of repairing Blaupunkt and other makes of car radios using Direct Coupled audio amps. of this type I would say that the speaker coupling capacitor is not the cause of the problem. In 99% of these circuits any value between 470uf and 1500uf will work perfectly well. Obviously the working voltage of the capacitor needs to be 10v or higher, and it must be otherwise in good order (low E.S.R. and low voltage loss), but a Vloss of 1% is nothing to worry about. The 4.5% Vloss of C275 is a tad high, but, before I had a component tester which could measure this parameter, all I used to do was check if the suspect cap. charged O.k. on the ohms range of an AVO 8 or other analog meter. That BC328 on the other hand is definitely faulty, and must be replaced. You may find this to be the only fault, since failure of one transistor can cause all the others in such circuits to misbehave. Those BP audio amp. blocks are a bit fiddly to work on, but manageable once you get used to them!

[sprint750]

Dear all,

On the past weekend, having everything needed in hand, I made surgery on the patient.

- Replacement of BC328, which proved to be toasted by my component tester (as guessed by forum members !!)
- Replacement of C275 (220 uF), which 'leaked' a little too much> 4% but still within the correct spec: 221uF measured
- I soldered back in place the output cap (800 uF on the schematic but measured at 1000 uF ). Impossible to find a spare and with less than 1% leak measured, it doesn't seem to be the culprit for final stage death.

After checking everything, I put the item under tension (without L.S). I measured the common transmitter, to see if something has moved there: 8V (!!) which is a tad high (before surgery it was less than 6V). A little surprised, because everyone knows that we must have about U / 2, as I am on a battery around 6.1V

Loudspeker hooked, (vintage Grundig bookshelf circa 1973, 4 Ohm) in place, I turn on the knob, and the guy emits the characteristic hissing of the FM band between 2 stations! Wink

MP3 on the phone via the DIN input.
It's just bearable first, then better and better as you go.
Having put back into service some old stations, I know that it takes a little time for the items to sound 'right', especially when we swapped some components.
After 15 minutes it's really good Shocked. Many (too much) bass, which is not surprising knowing that the output capa is on the high side, but frankly a beautiful, dynamic yet clear sound. At this time, I was the happiest man on earth....

I tested several tracks, it 's out really well. And OUCHH, after 30 min, on a classic track, with violin ... big skid, no more amplitude, big distortion. I immediately thought that the final floor just blowed (once more?) I immediately checked voltage at the common emitter: 10.5 V ....

I turned off, the time to try to understand, and to make continuity measurements in search of a possible short circuit between the TO-9 boxes and the heatsink. I turned on and measured almost Ubat on the common emitter (and of course no more sound this time). Fingertip test tool... Everything is at room temperature, nothing overheating.

Put the radio in the freezer for 1 hour, no improvement, nearly Ubat on the common emitter.... RIP NOS transistors....

The next day as everything (power, loudspeker) was still in place, I decided to redo all the measurments to write a post ​​on the forum. As I powered on ... the radio started to sing. Clear & Loud. ! scratch
I put the red lead on the common transmitter and I read 8V, as the day before ... but not for long. 8.1 ... 8.2 .. 8.3 ... The voltage climbed slowly but surely, whatever you do. Power knob position, input or not.
At 10.5V I stopped the experience, and turn the button to OFF.
Still no overheating or evidence something is going wrong.

In short, the station works, but what can explain this inexorable drift of the output stage ...?

- I imagine that it would be necessary to measure the idle current on the DC power line to the Ge transistors, to have a better idea of what's happening ...?
- I also did not measure the voltage at the bases of AD156/AD157, in operation

Any idea of what's happening ?

Thanks in advance for your contribution

Cdlt
Sebastian

[orbanp1]

Hi Sebastien,

The last four transistors in that amp are DC-coupled, so a fault anywhere in that part of the circuit will result in that changed common emitter voltage.

Measuring the components when cold will not work as the radio works when cold.

Strategy is to replace components!
It is also a good idea to monitor/limit the total current uptake from the power supply so that you would not fry anything.
Freeze spray might work to find the faulty component.

Simulation has shown that a leaky C276 (or C212, depending which schematics you look at, the 1.8nF cap) would result in the increased common emitter voltage.
As Livewire mentioned, a leaky output capacitor (C277, or C213, 800uF) would not result in an increased common emitter voltage.

So do change C276, do change both Si driver transistors, V207, V207 (or V203, V204), you should also change that thick film resistor module, by making up a similar module from individual resistors.
I take the other components you have already changed in that part of the amp.
(My bet would be a flaky thick-film resistor module, though I am not a betting man ;-)

Also, here is a good article on repairing audio amplifiers:
http://sound.whsites.net/troubleshooting.htm

Good luck, Peter

[sprint750]

Hello Peter,

Thank you for the input. Your help is much appreciated
I made a hard copy of this message and started to work again on the unit.

Turned the unit in ON position, started to sing with the right tone.
I monitored again the common emitter, slowly moving high, as usual

But... I also noticed that the NOS Cap that I swapped for C275 was swollen on the top. "pffffff... Crappy 30 year old NOS part"... Picked up another one from my personal stock. Just for peace of mind I checked the polarity beforee installing.
SILLY ME I inverted anode & cathode when I soldered the NOS item.

5 min (and a new CAP) later the unit was singing beautifully (MP3 via DIN socket) with a steady 6.8V measured at the common emitter. ("like in the manuals")

The sound is really amazing. Clear, detailed, powerfull... (through a vintage Grundig 4Ohm 2-way bookshelf speaker) Happy !

So as a conclusion:

- Both Ge final stage transistors were toasted (replaced by NOS Ge AD156/157 from a Blaupunkt box)
- BC328 was dead (replaced by a "BC328-25" transistor)
- All the caps seems a bit out of specs.

As a personnal point of view, I learned a lot through your inputs.
And thanks to your post, I have some tracks to make this more reliable for the future. (thanks Peter-Orbanp1 and LIVEWIRE for your experience and amazing skills)

Now I'm only halfway. I coud get only 1 decent station on "U" (FM) band when I could get several when I swapped the Essen with another period unit ('71 Mannheim) with exactly the same arrangement. (antenna etc...)

So I have to work on that side. But this is a much more complicated job than fixing a AF amp !

To be continued...

Best regards
Sebastien

[orbanp1]

Originally Posted by sprint750

...
Now I'm only halfway. I could get only 1 decent station on "U" (FM) band when I could get several when I swapped the Essen with another period unit ('71 Mannheim) with exactly the same arrangement. (antenna etc...)
...

Hi Sebastien,

Good work on fixing the radio!

As the radio is old enough, it is possible that it is out of alignment a bit.
You should touch up the alignment of the radio!

Looks like you have the service manual, and if you have the test equipment you should tune up the FM section (and possibly the AM section as well) as per the manual.
That could make all the difference between receiving one station or more...


Regards, Peter