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Old 2nd Feb 2019, 8:49 pm   #1
sprint750
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Default '71 Blaupunkt E

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
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Old 2nd Feb 2019, 9:53 pm   #2
MotorBikeLes
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Default Re: '71 Blaupunkt E

In pic 3, are you sure the brown wire and grey wire SHOULD go to that one point?
Remove ALL solder and confirm it is not TWO separate junctions.
It just looks a bit wrong with so many track from one junction.
Les.
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Old 2nd Feb 2019, 10:22 pm   #3
sprint750
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Default Re: '71 Blaupunkt E

Hi Les,

Thank you for your interest in this tthread

I checked and everything is OK.
These are 2 separate +12VDC circuits, running very close one from each other.

The long brown wire come from 12VDC input to the power switch/potentiometer.

The 2 other one comes from the switch to power feed all the circuits, incl.the push-pull transistors.

If the wiring was wrong, the unit would be powered permanently

Best regards
Sebastien
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Old 2nd Feb 2019, 11:33 pm   #4
'LIVEWIRE?'
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Default Re: '71 Blaupunkt E

I think the grey wire should simply bridge the burnt track, near where the track is 'L' shaped. This is not common with the track to which the brown wire is soldered (as far as I can see from the picture) Les is, as far as I can see, correct. Does the diagram you have not include a picture of the PCB Layout? Most Blaupunkt manuals & circuit diagrams did have this. Incidentally the AD156/157 transistors can be replaced with AD161/162. but, in a DC coupled circuit the other transistors should also be checked. If one or more of these are faulty, replacement output transistors could be destroyed as soon as power is applied.

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Old 3rd Feb 2019, 4:16 am   #5
sprint750
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Default Re: '71 Blaupunkt E

Thank you for the input,

Be assured that I checked and double checked everything before soldering the white strap to bypass the burned section.

In fact, this specific area is a common point for +12VDC. From here (cathode of C279) , power coming from the main switch goes: (see pic attached)

- to driver transistors V207, and rest of the circuit
- to power antenna remote
- to AD157 (by-passed by the white strap)

I appologize, the pic were not that good, and the crappy soldering didn't help.

Unfortunately I don't have the PCB layout seems for that specific model, i don't even know if it was included in the technician booklet. At least it is missing in the documentation available through the site you all know (you can scroll the pages before downloading):

https://www.radiomuseum.org/r/blaupu...n_7631340.html

On the diagram (attached) I try to make this common point clearer (thick line)

About output transistors, I was clearly optimistic.
To be honest I didn't even checked they were working correctly before installing. They seemed brand new, out from a Blaupunkt labelled box. I put them in place and no more short circuit. That was enough for me. Now i'm starting to ask questions.

What is sure:

- DC Power is distributed 'as it should' to all stages of amp circuitry. I checked and double checked. This is DC, after all...

- Fresh output transistor didn't get hot at any moment. I kept them under survey, as it was the root problem of this unit. If they died, it's not from overheating...

However, it seems that trouble starts on driver stage, around BC 238B (V206). See the voltages when I tested the whole circuit. (1st post)

BTW, and for the sake of my curiosity (I'm here to learn after all), can you tell what is the purpose of BC 328. Most of the units I came across had only 2 transistors on driver stage... and 1 negative feedback circuit (2 here)

Thank you again guys for the inputs and sharing your experience
Sebastien
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Old 3rd Feb 2019, 12:23 pm   #6
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Default Re: '71 Blaupunkt E

AFAIK the BC328 is simply an extra pre-amplifier/driver stage. The circuit(with minor variations) was used in a wide range of Blaupunkt Car radios in the early/mid 1970s ( I used to service & repair them back in those days) such as the Hamburg (MW/LW), Frankfurt mono & Frankfurt Stereo, and others. The signal at the collector of V207 will be out of phase with that at it's base, which will be in phase with that at the base of V205. I admit to have partly forgotten how these DC coupled amplifiers work, but the phase of the signal presented to the bases of the output transistors may be important. (Someone refresh my memory, please!) All such amplifiers are, so far as I know, based on an early Mullard design, which did only use 4 Transistors. I built such an amp. maybe 40 or more years ago for use in a SW Radio. This only had 4 transistors (AC127 & AC128),with a max. output of about 500mW, but most car radio amps. of that type I've seen, used 5.
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Old 3rd Feb 2019, 6:00 pm   #7
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Default Re: '71 Blaupunkt E

Hi Sebastien,

Nice period radio! (I imagine, the cars are too.)

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.

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.
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. Th 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.

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.

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.

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

Regards, Peter

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Old 3rd Feb 2019, 6:18 pm   #8
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Default Re: '71 Blaupunkt E

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.
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Old 3rd Feb 2019, 7:07 pm   #9
Leon Crampin
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Default Re: '71 Blaupunkt E

If the bias stabilising diode D207 is selenium, and especially if it is labelled AEG, change it on sight. By now, these diodes become intermittent, turning the output stage hard on with consequent damage, made worse by the lack of emitter resistors.

A fair replacement is one silicon and one shottky (or germanium) diode in series. This will give about the right bias voltage. You may have to adjust the two fixed resistors in the bias circuit to get the right quiescent current for the output stage - about 7mA is reasonable for a cold radio if there is no other service information.

The npn germanium transistors are generally more fragile than the pnp devices, and usually fail first with this type of fault. Note that the base and pinout dimensions and hence the mica washers are not the same as those for TO-66 devices. (Euro-defiance...).

Leon.
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Old 3rd Feb 2019, 10:42 pm   #10
orbanp1
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Default Re: '71 Blaupunkt E

Quote:
Originally Posted by Leon Crampin View Post
If the bias stabilising diode D207 is selenium, and especially if it is labelled AEG, change it on sight.
In the simulation I used an 1N914 for D207.
You can leave in the 33 Ohm thermistor, remove W15 180 Ohm resistor, and adjust W16 for the idle current.
The simulation showed 25 Ohm value for W16 with 12.1mA idle current (and 0.5 Ohm emitter resistors).
You could thermally couple both the thermistor and the diode to the final transistors, a Si-diode also has a negative thermal coefficient.

At the emitter of V206, the driver transistor to the finals, the voltage was 8.7V with 13.5V supply voltage.

Regards, Peter
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Old 3rd Feb 2019, 11:33 pm   #11
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Default Re: '71 Blaupunkt E

IIRC the bias diode-a ZG1- is silicon, although, come to think of it, the 'G' in the type no. could indicate that it is Germanium. I may still have a few somewhere,and, if I can find them, I'll check. There may be a data sheet on the 'net for the XG1 aqnyway.
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Old 4th Feb 2019, 12:24 am   #12
sprint750
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Default Re: '71 Blaupunkt E

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

Quote:
Originally Posted by orbanp1 View Post
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.
Quote:
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 ?

Quote:
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
Quote:
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.
Quote:
Originally Posted by orbanp1 View Post
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
Quote:
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

Quote:
Originally Posted by 'LIVEWIRE?' View Post
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
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Old 4th Feb 2019, 1:16 pm   #13
orbanp1
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Default Re: '71 Blaupunkt E

Hi Sebastien,

That colored diagram with the voltages is rather confusing, there are several different voltages marked at one node in different colors, never explaining it.
But if you have your own measurements that is fine.

That 5.85V at the half point is not that bad, you can leave it like that, it only means that the output will start clipping on one side a bit earlier, but there is still enough output there.

You can "fine tune" the output voltage by adjusting W14, that 270 Ohm resistor, if you increase its value, the half voltage will increase.
You can change those thick film resistor values if you replace those boards with the printed resistors with individual resistors.

The gain of the final stage (the circuit consisting of the last 4 transistors) is essentially set by the ratio of the R250 and R249 resistors.
Other circuits, with no pre-amp stage probably have a different ratio there, making up the whole amplification in the final stage.

Regards, Peter
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Old 4th Feb 2019, 5:21 pm   #14
sprint750
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Quote:
Originally Posted by orbanp1 View Post
That colored diagram with the voltages is rather confusing, there are several different voltages marked at one node in different colors, never explaining it.
But if you have your own measurements that is fine.
Peter
You're quite right
Originally, this diagram was only for my own use, to trace all the sections of the circuit that I checked. Color (green) was just to say that this line was checked for continuity (no idea of voltage here) , then orange came to pinpoint a part of the circuit where I had to put attention...

Now I updated it to be more consistent toward voltage See updated attached pdf file:
- 1 color = 1 potential
- Voltage measurement on each link, black = OK or no idea (no spec available), red = out of manufacturer specs

Impedance measurements are still here, and - in case of mismatch between real life and documentation- color coding of the resistors. (there were some variations depending on mfg serial number, and remember that I got this unit without any ID tag)

Quote:
That 5.85V at the half point is not that bad, you can leave it like that, it only means that the output will start clipping on one side a bit earlier, but there is still enough output there.
I'm happy to learn that this value is acceptable
There is still a question pending. Should the reading be the same with or without load ?

Without load (loudspeaker disconnected) and unit ON, all the poles of the Ge transistors are on 13.5V potential, including common emitter with exception of AD156 collector, of course (0V). No voltage drop = no current....right ?
You told me about idle current. To be honnest I'm a bit confused here.
Should there be an idle current in any situation ?

Of course I should measure idle current as you suggested, but it requires to open the circuit, which is something I want to avoid for now, unless strictly necessary.
Quote:
You can "fine tune" the output voltage by adjusting W14, that 270 Ohm resistor, if you increase its value, the half voltage will increase.
You can change those thick film resistor values if you replace those boards with the printed resistors with individual resistors.
Your idea makes sense, when measuring, W14 was only 243 instead of 270 Ohm (diagram)

I'm afraid replacing film resistor with individual resistor is beyond my capacities. The pitch of film resistor is quite fine: even if this PCB is old school one, such operation requires precision, ability and specialised tooling I don't have. And this is why some people are surgeons and some other are lumberjacks.

So let's consider any operation on the "W" resistor as the "last chance attempt", when everything else failed.

Best regards
Sebastien
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Old 4th Feb 2019, 8:38 pm   #15
orbanp1
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Default Re: '71 Blaupunkt E

Hi Sebastien,

You do need the load, a speaker, or a dummy load, like 4 Ohm to 8 Ohm resistor at the output. The driver stage, W207 gets its collector supply through the speaker. If you look, W17, the collector resistor of that stage, it gets the ground through the speaker. Without a speaker the DC voltages will be totally off!
The reason for connecting W17 to the speaker and not to the ground is "bootstrapping", that resistor "sees" higher voltage than when connected simply to the ground, you can drive the stage to the fullest.
Not having a load nothing bad happens, nothing burns down, just the voltages will be off, you can not do any adjustment.
The idle current can also be checked only when there is a speaker attached.

Those thick-film resistors, if I see it correctly, are printed on a ceramic carrier, a number of resistors on single ceramic substrate. You can take a same size proto board, assemble all the discrete resistors on it, and solder it in place of the ceramic one.
I could be off here, the picture there is not that clear.

One more thing, those audio amps like the one in your radio, mostly used 4 Ohm or 5 Ohm speakers, you could get more output power that way, than using an 8 Ohm speaker.

Regards, Peter

Last edited by orbanp1; 4th Feb 2019 at 8:45 pm.
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Old 5th Feb 2019, 12:21 am   #16
sprint750
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Default Re: '71 Blaupunkt E

Thank you for the reply and the information
I keep all this preciously, and at the end, all the parts of the puzzle will come along. WHAT failed and HOW it failed...

Well I choosed 8 Ohm spekers for a very good reason... they were already in the room!
Note that listening music on Monitor Audio 2 way bookshelves is not really a punishment, if you see what I mean

This evening, I decided to remove the upper and lower cover of a '71 Mannheim unit. Choose this one because is is the same year as the injured Essen, and it' s working perfectly with a clear & powerfull sound. Sometimes I realy feel it sounds better than my Denon micro-system, specially on voices ... but this is another debate. Basically a Mannheim is a Essen without 'Kurtz' wavelength, when the amplification section is quite similar (see diagrams)

The only minor differences:
- There's 1 supplementary Negative Feedback circuit on the Essen
- The 'Thick film' resistors are replaced with discrete units of similar value (more space on the PCB , as the Mannheim is missing 'Kurtz' wavelength).
The rest of the stuff is quite similar, as far as the final stage is concerned.(GE transistors, polarisation & Bias stuff... copy & paste)

Now I performed the same test as I did with the Essen. I measured the voltage at the terminals of final stage transistors, with & without load. Surprise!... there was very few if no difference on the common emmiter with speaker connected or disconnected, (reading around V/2 in both cases) when on the Essen, both Emitter & Base set immediately to +VDC in the absence of load...

Any Idea about what is happening ??

I know this doesn't make a solution, but these are clues that we can add to the file. I will keep on studying the mannheim, it'll give precious informations to diagnose the Essen. The risk is to end up with 2 burned unit,...

Best regards
Sebastien
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Old 5th Feb 2019, 11:06 am   #17
sprint750
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Default Re: '71 Blaupunkt E

With schematics for AF Amp section

BR
Sebastien
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Old 5th Feb 2019, 2:58 pm   #18
Herald1360
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Default Re: '71 Blaupunkt E

Quote:
Originally Posted by orbanp1 View Post
In the simulation I used an 1N914 for D207.
Does that mean you have some decent Ge transistor models?

The easy to find ones are rubbish, with Vbe clearly for silicon types.
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Old 5th Feb 2019, 5:41 pm   #19
orbanp1
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Default Re: '71 Blaupunkt E

Sebastien,

I did not see any additional negative feed-back in the Essen circuit, though it is possible that I was not looking hard enough.

Some radios with similar PA stages do have an additional 100 Ohm - 200 Ohm resistor, inside the radio, parallel with the speaker, like R182 in that snippet that I enclosed.
(That output capacitor to the speaker, just before R182, it is probably 1000uF correctly and not 100uF!)
Such a resistor does not interfere with the working of the PA-stage when a speaker is attached, it barely dissipates any power, it sole purpose is to "keep the circuit happy" when there is no speaker attached. The DC voltages within the PA-stage are nearly the same in this case as with the speaker.
It is possible that the Mannheim radio has such a resistor installed, though the schematics does not show it.

Chris,

I collected a few GE-transistor models during the years.
Here is a thread about the "fake" AC128 models, and a pointer to an Excel program that helps modeling transistors:
https://www.diyaudio.com/forums/soli...odel-rare.html
(Someone mentioned that the program is no longer at the original site, though it still could be found with Google.)
Someone made a careful model of the AD161/162 transistors that I saved, I think it was on that site too.
There are a few models of old Ge-transistors in the LTspice example circuits too.
I also understand that Pspice has a utility to generate bjt models, though it is not enabled in the demo version.

Regards, Peter
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Old 5th Feb 2019, 6:38 pm   #20
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Default Re: '71 Blaupunkt E

D207 is definitely not selenium, Leon. It's a small diode similar in size to a 1N4148. As per my earlier post, AFAIK it's silicon, though I had asudden thought that the 'G' in the type no. might mean that it's Germanium.
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