Hello,
My name is Sebastien, from France
As a classic car enthusiast (several classic Alfa’s in the stable) I always try to have the period correct item in the dash.
Until now, I was successful with some Blaupunkt and Grundig units, overhauling them with my limited knowledge and a lot of patience. Most of the problem were simple and trouble shooting was straightforward. Mechanic (missing or broken switches, broken needle wire) or electronic: visibly burned or damaged component and some repairs to the welding.
Recently I came across a new patient that is reluctant to come back to life. I must admit that I got the point where the next step is beyond my competences. I definitely need the help of more knowledgeable people.
BTW: as a living, I’m a mechanical engineer, now technical trainer specialized in automotive diagnostic. So I practice on a regular basis troubleshooting methodology. About the electronics, learned the basics in University (PNP/NPN transistors, impedance etc..), but consider me as a dummy.
The technical level of this forum always leave me speechless.
Historical
- Purchased this ’71 vintage ‘Essen ’ UKML in 'unknown' condition, for the faceplate and knobs (salvage for re-commissioning a more modest '71 Mannheim that works perfectly). No info, no tags anymore. Apart that, the unit was cosmetically excellent.
- The item looked too good, so I plugged it 'just to see' ... no sound, no smoke, no light, no life at all .
- Removed the upper and lower cover: found 2 straps (previous repairs or attempts to) and one burned track. Seems that someone gave up at this point. I soldered a strap to bypass the burned section and I put a 1 A fuse on the power supply.
- Test: smell hot, then the fuse blown. 1 of the push-pull (AD156 / 157) was boiling hot....no good ("usual suspects" some will say...)
- Checked with the multimeter +12VDC is shorted to ground, I unsoldered the + 12VDC power supply to the NPN power transistor. The good news was that the short circuit was gone. The bad new: I had to find another pair of Germanium transistors...
- So prospected, and finally bought somewhere else on the planet a pair of matched transistors, AD156 and AD157 (N.O.S , in Blaupunkt box: 8 913 605 266 )
- Swapped the Push-pull transistors, according to schematics. BTW, Previous fitting was correct...
- Trial and ... still no joy.
A barely audible sound, with a lot of distortion, either through radio or AUX-IN (DIN socket)
Measurements
I checked the whole LF circuit with - what I believe to be - the right schematic (Remember: no label).
I assumed (front fascia, rear connexions etc..) that it is an ESSEN 7.631.340 or 341. Further experience showed that 99% of the diagram matched with what I found in the device.
- Power supply (battery) 13.5 VDC instead of 14V blueprinted.
- The measurements were taken without input but with output loaded (8Ω loudspeaker)
4 points, in 1st approach (see schematic with personal notes):
1 - Resistor R246 (820KΩ on the diagram), 71 KΩ measured at terminals. The color coding of the component says 820 K. I have not sticked on that because measurement was made plugged, so interacting with the other voltage divider bridge (W11 / W12): I certainly measure equivalent impedance. And why bothering as there is 0.65V at the base of BC238 for 0.7V expected on the diagram, so OK.
2 - Same for W11, 36KΩ measured (still connected) for 270K blueprinted (?!) checked and double checked. Impossible to open the circuit: resistor is 'thin film' style, specific to Blaupunkt. Consequence: BC238B, almost lacks 2.5 VDC compared to schematics!
3 - On R248, measured 4.6KΩ, for 6.8K on the schematic. Checking the color coding, it is actually a 4.7KΩ unit ‘in situ’. Do I have the right schematic? Previous repair? Everything seems original yet (welds etc ..) ... mystery.
However on BC 238B, I definitely do not have the expected voltage values: -2.5V of deficit at the Base and -7V at the collector ...
4 - As far as BC328 is concerned, it's not good either. With such a polarization it should be saturated (??).To be honest, the ignorant that I am do not see what it is the role of this transistor(??). On my ’71 Mannheim (same year), or a contemporary ‘Wolfsburg’ not such 3rd transistor to complete the drivers...
5 - Track that can be interesting: The PCB is badly damaged by repeated overheating (I’m responsible just for 1 of them).
The copper has started to come apart the mainboard in some places. During one of the manipulations, the loose damaged 12VDC track touched C275 capacitor, R249 side ... and the amp STARTED TO WORK !!
Clear sound, low power (10 to 15% of what is expected), but perfectly audible.
Certainly the 12V has nothing to do there ... but it is a clue and when nothing works, it was the only improvement in months! As I you start to see the light at the end of the tunnel.
I stopped after 4 sec. Before it smokes (or not, I’ll never know), but it's perfectly repeatable. Just as when I turn off the power, a decent sound comes out in the 10ths of a second before shutting down.
So I guess I’m not far from the solution, and there's lifr in this item, but I definitely need a helping hand, as I reached le limits of my own incompetence.
(very uickly, I must admit...
)
Now you knbow all the story.
Any clue or idea is welcome, and thank you in advance for your helping hand
Best regards
Sebastien