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Old 8th May 2012, 5:01 pm   #1
Martin L
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Default Blaupunkt Frankfurt repair

Hello all,

Just signed up here as I am after some advice

I have recently been given a Blaupunkt Frankfurt (circa 1971) for my BMW, however it is non functional. I believe it to be a 7-631-640 model, although this is not absolutely guaranteed as the model number label is long gone.

A test and strip down showed evidence of overheating in the amp/power module (burnt PCB track), and a DMM test showed the 12V line shorted to ground which would explain that.

I traced the short back to an issue with the two amp transistors - AD156 and 157 - however this is where my limited knowledge of 1970s electronics (and limited test equipment) probably lets me down.

I have tested the two transistors according to this, however the transistors appear to conduct in both directions so I assume they are u/s.

There is, however, a single component I cannot identify without a wiring diagram (see attached photo); can someone help with what it is likely to be and how to confirm it is OK?

For clarity it is a light grey plastic pot, 10mm in diameter, 5mm high and with two leads exiting from a black potted centre. There is a red spot midway between the two leads.

Any help would be appreciated to get this working correctly.

Thanks very much!

Martin
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Old 9th May 2012, 10:28 am   #2
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Default Re: Blaupunkt Frankfurt repair

The item in your photo is a Thermistor, Martin. It's resistance should fall with increasing temperature. These were fitted to compensate for temperature changes in other components, including those germanium output transistors, which, incidentally, are similar to the more common AD161/AD162 devices, one being NPN, the other PNP. You are correct about the part no of the radio, it will be 7631640xxx(I forget the other 3 figures.
As some forum members will know, I used to work as a service engineer for a Blaupunkt dealer, and still have some spare parts and service manuals from those days. The audio amp in those radios is what's known as a DC coupled type with complementary symmetrical output stage. Failure of one or both o/p transistors in such a circuit can be caused by external factors such as incorrect polarity or over voltage, but may also be due to failure of other transistors, etc., earlier in the circuit.
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Old 9th May 2012, 12:50 pm   #3
Martin L
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Default Re: Blaupunkt Frankfurt repair

Hi Livewire,

Thanks very much indeed for your reply, very helpful indeed

Thanks for the ID on the thermistor, it was mounted to the heatsink between the two output transistors so that makes complete sense.

A quick test shows it's reading zero ohms at room temperature, so I need to get it colder and warmer and see what it's up to.

I don't suppose you have any service information relating to this radio do you?

I'm loathe to replace the output transistors (I have replacement transistors - AD161/162 - on order) to find there is another "upstream" component I also need to check/replace.

Thanks very much indeed again, it really is appreciated.

Martin
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Old 9th May 2012, 7:45 pm   #4
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Default Re: Blaupunkt Frankfurt repair

Martin, I'm 99% certain that thermistor is faulty. Whether by 'zero ohms' you mean short or open circuit it should be neither. I may have a manual for your Frankfurt, since I have maybe 2-300 of them, many in binders. Will have a look and let you know if I have one
as soon as I can.
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Old 9th May 2012, 9:16 pm   #5
Martin L
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Default Re: Blaupunkt Frankfurt repair

Sorry, yes it is measuring as a short circuit. Now to find a replacement!

It'd be fantastic if you could find a manual, thank you
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Old 9th May 2012, 10:50 pm   #6
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Default Re: Blaupunkt Frankfurt repair

It is worth a quick check to see if it is a overheat cut out.
Heat it to about 100 deg C and check it again while it is hot.
It should go open circuit and close again once it has cooled down.
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Old 10th May 2012, 10:29 am   #7
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Default Re: Blaupunkt Frankfurt repair

The device is actually an NTC Thermistor, which, IIRC, should be a low resistance(maybe about 30 Ohms)cold, rising to at least several hundred ohms when heated.
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Old 10th May 2012, 10:51 am   #8
Martin L
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Default Re: Blaupunkt Frankfurt repair

You're quite right Livewire - I've found some info on generic AD161/2 amplifier circuits, and also schematics for similar Blaupunkt radios using AD156/7, and it should be an NTC thermistor of ~60 Ohms cold.
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Old 10th May 2012, 11:50 am   #9
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Default Re: Blaupunkt Frankfurt repair

Martin, I've just found some Blaupunkt service data, including an audio amplifier circuit dating from the early 70s(a 7631 series). In this the NTC is connected between the bases of the AD156(AD162)&AD157(AD161) and is a 60ohm type with a 270 ohm resistor in parallel. Unfortunately I have trouble scanning in legible copies of documents otherwise I'd post the circuit on the forum. It's a conventional DC coupled amplifier, using silicon transistors(BC108c and BC178A or similar in the early stages) Some versions include a protection lamp in series with the speaker connection, which detects any DC flowing due, e.g., to a leaky speaker coupling capacitor.

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Old 10th May 2012, 1:33 pm   #10
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Default Re: Blaupunkt Frankfurt repair

Apparently it varies from 33 to 60ohms.The value of the other two resistors and the diode type number should make it possible to determine correct value on the thermistor.
Examples:
http://www.thesamba.com/vw/archives/...111035101K.jpg
http://www.thesamba.com/vw/archives/...i/diagram2.jpg
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Old 10th May 2012, 2:09 pm   #11
Martin L
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Default Re: Blaupunkt Frankfurt repair

Livewire, thanks for looking - is there another way I can get a copy from you?

Roffe, they are similar to the diagrams I've seen (on that VW site). As you say the other components should help identify what I should have there.

Itching to get in front of the radio again now to have look!

Thanks for all the help
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Old 10th May 2012, 3:42 pm   #12
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Default Re: Blaupunkt Frankfurt repair

Quote:
Originally Posted by 'LIVEWIRE?' View Post
The device is actually an NTC Thermistor, which, IIRC, should be a low resistance(maybe about 30 Ohms)cold, rising to at least several hundred ohms when heated.
If the resistance increases with increasing temperature, surely that indicates a positive temperature coefficient?
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Old 10th May 2012, 9:51 pm   #13
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Exclamation Re: Blaupunkt Frankfurt repair

You're right, AJS-the memory is playing tricks, methinks. I don't have any data for the thermistors BP used, but somewhere I have some old Mullard data books, which include thermistor data.
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Old 10th May 2012, 9:53 pm   #14
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Default Re: Blaupunkt Frankfurt repair

Martin, if you send me a PM with your details, I can scan & print a copy of the diagrama I have and post it to you.
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Old 19th May 2013, 11:55 am   #15
Martin L
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Default Re: Blaupunkt Frankfurt repair

Hi all,

As a year has gone by since I last appeared on here I thought it was worth an update for you all.

With the new matched pair of eBay sourced amplifier transistors (AD161/162) and a replacement thermistor very kindly supplied by Livewire I got the amp repaired and working again (tested with the aux input).

However, there was some other problem as I had absolutely nothing from the radio stage, not even static.

A bit disheartened it was consigned to a box and I've only just dragged it out this week - a year on

A couple of hours of component testing found a duff (short circuited) tantalum cap in the voltage regulator circuit, replaced and I've got a fully functional radio

I do want to say a very belated thanks to everyone who helped last year, especially Livewire who kindly supplied some parts and schematics for similar sets which helped me on my way.

I'm really pleased to have resurrected what I thought was going to be a dead radio, just got to fit it to the car now
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Old 20th May 2013, 10:49 am   #16
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Default Re: Blaupunkt Frankfurt repair

Glad you were able to get your Frankfurt working, Martin, and to have been some help in the process. Tantalum capacitors, IIRC, were supposed to be the 'cat's whiskers' but do occasionally seem to fail short circuit. I usually replace them with miniature conventional electrolytics when they do.
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