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Old 11th Nov 2017, 12:19 pm   #1
JulienVW
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Default Blaupunkt KDB 951-804 problem

Hi,

I am new to this forum but seems like the place to be

I'm having trouble with one of my Blaupunkt radio's for my VW bug.
I already installed a similar Blaupunkt radio in the car with an aux 6pin connector and a new 4ohm speaker and it works perfectly but now i just tested the second radio (KDB 951-804) and i am running into an issue.

When i play a song at a very low volume all seems ok but as soon as i turn up the volume a little bit and there is some bass in the song (very little) it seems to get stuck at a certain bass frequency and won't stop untill i turn the volume down again. Even if i stop the song it is still stuck on that frequency.

I also tested the original speaker that came with the radio and it does the same thing.

Made a video of the issue which can be found here:
https://youtu.be/KX90spr1uZs

I am handy and would like to fix it myself but really don't know where to start looking.

Does anyone have any idea?

Thanks in advance!
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Old 11th Nov 2017, 1:42 pm   #2
ronbryan
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Default Re: Blaupunkt KDB 951-804 problem

I would first start by checking or changing any electrolytic capacitors on the supply rails as the audio amplifier is suffering from an oscillation caused by unwanted positive feedback.

Ron
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Old 13th Nov 2017, 11:45 pm   #3
JulienVW
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Default Re: Blaupunkt KDB 951-804 problem

Thx Ron for your answer. I will try to change these first. I disassembled the radio today. There is one capasitor that seems to leak a bit so probably this one will be the cause of my problems.
I tried to test them with my multimeter but they all charged up and released as they should.

Could you confirm the board on the picture is the supply rail you talked about?
Could the leaking seen in the picture be a sign of failure or is this normal with ageing components?

Julien
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Old 14th Nov 2017, 12:56 am   #4
ronbryan
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Default Re: Blaupunkt KDB 951-804 problem

A leaking electrolytic capacitor is not a good sign. It looks as if that capacitor is associated with the battery input filtering, but you could easily check with a meter to see if it connects to the yellow wire when the on/off switch is on. Try changing it as it is a likely suspect, or maybe tack or clip a similar one in parallel temporarily to see if it improves the situation.

I don't have a circuit for this radio, so there may be other faulty decoupling electrolytic capacitors nearer the audio output stage or the audio pre-amp transistor (I'm assuming that the output stage is elsewhere on the set.)

Ron
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Old 14th Nov 2017, 2:21 am   #5
Maarten
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Default Re: Blaupunkt KDB 951-804 problem

I'm not sure thats a valid model number. If you need a schematic, first see what's on the label as well or on other labels.
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Old 14th Nov 2017, 10:08 am   #6
'LIVEWIRE?'
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Default Re: Blaupunkt KDB 951-804 problem

Maarten, you are right. All 'open market' Blaupunkt Car radios were named after Towns & Cities, such as 'Bremen', 'Frankfurt', 'Hamburg','Windsor', etc. I don't remamber how the ones made for VW, Porsche, and other Car Makers were branded, but all BP radios had a part number which was in the style '76123456789', and a serial number prefixed by a letter, e.g. X9876543. Only paper labels were used on earlier models, either on the side or rear panels, plus, in some cases on the top cover. In the nature of things tese labels get torn, or peel off, so the full model no. may be missing. The serial number of later models, which always incorporated part of the '76' number, was also stamped in one side of the metal case. If the OP can post pictures of his radio, preferably front, top & side views, one of us may be able to identify it. That panel with the PCB shown in post # 3 was used in various models in the late 60s/early 70s. I have quite a collection of Blaupunkt service manuals dating from the late 60s to the 80s, so I may even have a manual for the OPs radio. In the meantime, the capacitor on that PCB is indeed the main smoothing /filter cap across the supply lines. I'd be inclined to change it in any case.
I've just listened to that YouTube video. The loud low frequency noise at high volume is probably caused by defective capacitors, though it could be an output stage fault. Does it get better or worse if the supply voltage changes? A 12v car radio should work between about 11 & 16v, whilst a 6v. one should be OK from about 5.5 to 7.5v.

Last edited by 'LIVEWIRE?'; 14th Nov 2017 at 10:14 am. Reason: To add a comment about the fault
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Old 14th Nov 2017, 10:38 am   #7
JulienVW
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Default Re: Blaupunkt KDB 951-804 problem

Thx. You are correct, there are other paper labels seen in the picrture with the number Y1474821 on it. If you could find some kind of shematic it would be great.
In the meantime i will take a trip to the electronic parts store to get some new capacitor to replace.

I will also try changing the voltage like you said and see what it does. I guess starting the engine and running 13.7v instead of 12v is a good test as i don't have a variable external power source at hand?

You can see the other 2 circuit boards in the pictures. I guess the main one will be for the for the tuner and the rear one the audio amplifier and last stage.
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Old 14th Nov 2017, 11:20 am   #8
ronbryan
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Default Re: Blaupunkt KDB 951-804 problem

It would appear to be a Blaupunkt Essen car radio. Maybe the 1967 model?

Ron
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Old 14th Nov 2017, 12:05 pm   #9
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Default Re: Blaupunkt KDB 951-804 problem

Ron, you may be correct. Each year the letter prefix ahead of the serial No. changed. I read the serial no. as being 'Y1474821', which, IIRC, would make it late 60s. If it was 'V1474821' it would be two years earlier. I'll check if I have the service manual, or at least a diagram, but please bear with me if it takes a day or two! Julien, the board in your 4th. picture is indeed the audio amplifier and output stage, with the two transistors (AD157 or similar maybe) mounted on the rear metal panel being the output devices, driven from one of the two transformers. The circuit will be typical of it's day, using Driver and output transformers, producing approx. 6 watts output into a 3 or 4 ohm load when run from a nominal 12v supply. A thought occurs that bad connections between the plugs & sockets linking the boards together could be responsible for the fault. Check/clean/tighten all, but particularly the cable(s) linking the input PCB (the one with the volume control) to the audio amplifier stage)

Last edited by 'LIVEWIRE?'; 14th Nov 2017 at 12:14 pm. Reason: to add further info.
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Old 14th Nov 2017, 5:10 pm   #10
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Default Re: Blaupunkt KDB 951-804 problem

Just wondering if the light grey colour patches on middle of the audio board and around the single capacitor on the other board could be leaked electrolyte from the capacitors. It may just be a camera illusion.
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Old 14th Nov 2017, 10:26 pm   #11
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Default Re: Blaupunkt KDB 951-804 problem

Hi Julien

It looks like this is the radio in question:
https://www.thesamba.com/vw/archives...unkt_essen.php

Peter
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Old 15th Nov 2017, 9:42 am   #12
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Default Re: Blaupunkt KDB 951-804 problem

Looking at those diagrams, Peter, that is almost certainly the correct service manual for Julien's radio. Hopefully they will help him find and rectify the fault.
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Old 15th Nov 2017, 10:08 am   #13
JulienVW
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Default Re: Blaupunkt KDB 951-804 problem

I ordered capacitors to replace them all.
Thanks Peter, this is the radio! I will take a look at it. Seems like a very complete manual
I also cleaned all the connectors and tested the radio again, problem remains. As soon as i have the parts i will replace them. I'll keep you guys updated. Thanks everyone for the great advice and help so far!
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Old 15th Nov 2017, 10:10 am   #14
JulienVW
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Default Re: Blaupunkt KDB 951-804 problem

The light grey color is just a reflection. The board is clean
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