Blaupunkt Frankfurt ca 1970 polarity stick issue

[MGBGT1966]

Hi! I just got an old Blaupunkt Frankfurt to go with my MGB GT 1966. The GT is positive ground and the Blaupunkt has a switch for polarity. However, the top of the switch that indicates the polarity is broken. But, there is a serial number on one side: XY 4158 /2Xx (see image). So the question is, does anybody know which side is which based on this. My guess is that the side with numbers are for negative ground (the front side being reserved for the most common setting), but thought i'd check if anyone had a similar one.

Thanks in advance.

Gunnar.

['LIVEWIRE?']

I ought to know which way round is which, having been an I.C.E. (Car radio) Service Engineer, but I can't remember. I may have one of those polarity c/o slides somewhere. If so, I can check, Gunnar, but in the meantime it should be possible to check the polarity setting with the cover off the radio. If you can identify the main smoothing capacitor - a1000 or 2200uf Electrolytic, then, using a multimeter(Analogue or DMM) on a low ohms range, check which terminal of the Capacitor is grounded. With the slide one way up the Negative terminal will be earthed, whilst with the slide the other way up, the positive terminal will be earthed. Be careful not to push the slide in too far - easily done with the end broken off. I may be stating the obvious (apoogies if I am), but this test should be done with the radio switched off and disconnected from any power supply. Incidentally that radio is probably over 50 years old - by 1970 they were more compact in size and almost certainly Negative earth only.

[roffe]

Some pictures of a Frankfurt X:-

https://www.thesamba.com/vw/forum/vi....php?p=2725369

['LIVEWIRE?']

Neither of the photos on 'The samba.com' shows a complete polarity c/o slide, though! Maybe Radiomuseum.org has the answer?

[MGBGT1966]

Hey guys! Thanks for the quick feedback and Livewire for the suggested test. I will try it tonight unless someone comes up with the solution before. I did google this extensively before and the only thing I could find that comes close is the same samba link posted here, so I hope it can be solved now. Thanks. Gunnar

[roffe]

Picked out of my Frankfurt X.

[MGBGT1966]

Thanks alot roffe, this was just what I was hoping for! I was just about to go with the numbered side as positive ground (like in your picture). However, now I see that on your other stick, the voltage one, the numbered side is 6V. This is different from the samba link from earlier. It seems there is no consistency in terms of how the numbered side is used. I think I am going to have to try the test.

[MGBGT1966]

Hi again, so I found the capacitor and tried the test. No matter how I positioned the stick (tried all possible positions), I couldn't get differences between the two sides of the capacitor, they were either both grounded, or not grounded at all. Always same reading on both sides. I don't think this is a good sign? I did turn off the radio prior to testing, no power connected etc. Maybe it has already been blown? Before I got it, it had been laying in a garage for several years, and not tested in a long time. Any thoughts on what could have possibly been wrong in the testing?

[MGBGT1966]

The polarity stick issue is now solved! Radiomuseum did turn out to be the solution. http://www.radiomuseum.org/r/blaupunkt_frankfurt.html There are several pictures of the radio with the polarity stick inserted in negative position. In one of the pictures, you can see from the inside how the stick is attaching to the circuit board, and from this it is possible to derive the rotation of the stick. From this, it is now confirmed that the side with writing is indeed the negative side (not the positive as mistakenly written in the above post). I now only hope the radio works, at least I can try it. Thanks alot for helping out!

[MGBGT1966]

Short update; the radio now starts up! On startup, there is a low poff-kind of sound from the connected speaker and a red light comes on the screen. This seems correct, yet there is no sound whatsoever from the speaker after this. I just started troubleshooting, but it doesnt look good, so I might have to come back and start up a new thread. Meanwhile, any quick thoughts or fixes would be appreciated.

['LIVEWIRE?']

Radiomuseum has details of the Frankfurt 'A' one-piece radio, which may be a later model than yours, but, as with yours, there is a blanking plug inserted, which connects pins 1 &2 of the socket together. If the plug is missing, or not inserted, there will be no radio audible,, although, from your picture, the plug is shown to be in place, so an internal fault in the RF/IF section is likely to be the problem, assuming that the aerial and it's coaxial cable are in good order. I used to check for faults in the car aerial by temporarily plugging in a known good aerial.

[roffe]

I've been wondering about which Frankfurt you have (RM lists over 40!)?
A 7.639.600 perhaps?
The printed connections on yours and mine don't match and that's why they have different numbers, I suppose.
What voltage and polarity do you test it with?

[MGBGT1966]

Thanks again livewire, good idea about checking the coaxial cable. I did not consider it essential, as I asumed the radio would make at least some noise once the tuner is turned as is usually the case even with a very short antenna. The coaxial seems fine enough though, but this could be a good thing to go into. The speaker has been tested, I am also thinking about testing the speaker wires for electricity tomorrow as I think they should give a reading even with a "silent" radio broadcast. This might at least allow for connecting an external unit, as is the main idea anyhow (even though I dont have the proper connector yet so I cant test it). The external din/aux outlet seems to be a different standard din then the usual 5 or 3 pins i have at home.

[MGBGT1966]

Roffe, that is indeed the correct number, even though I cant find an A. There is another number as well, which i believe could be the serial number. This number has a B before it: B 3155576. I am testing it with 12volts and positive ground.

['LIVEWIRE?']

The serial No. is B3155576, indicating that it's a later model than the one on the Radiomuseum site, though it should be similar. The 7639000 quoted by Roffe is a Blaupunkt Part Number This would be the same on all examples of the Frankfurt, but will differ on other models. As to the fault in yours, you're correct in expecting at least some background noise, plus clicks when you switch wavebands. It seems to me that there is a fault in the RF/IF stages, possibly in the Oscillator circuits, assuming the AF Stages are working correctly. This can be checked by removing the blanking plug and injecting a signal, either from an AF Generator, or from a device such as an MP3 Player into pin 2 of the DIN Socket.

[MGBGT1966]

Hm, this sounds like it could be a complicated process. If it gets to this, i will probably have to find some local expert to go through it.. thanks again for all the help though! Highly appreciated

[MGBGT1966]

Here is an update; I got a different, but same model of Blaupunkt Frankfurt today and connected it. It was working perfect for some time, but then came same issue as the first one. It makes a sound as I turn the switch, but then nothing. I am now thinking two things; either there could be something wrong in my connections, or my speaker is going in and out. The latter seems unlikely though, as it just started working with the new radio. The ohm reading seems stable enough on the speaker when I test it, but it might be damaged as I now recall it did get flooded at one point. Another explanation could be that I was connecting a phone through a din-adapter around the time it happened (this was not the case on the first radio though which I never had a sound from). I am going to try a different speaker and see what happens later, but I assume the radios are in need of repair. Meanwhile, any thoughts would be appreciated.

['LIVEWIRE?']

It's likely the speaker has been affected if water has found it's way into it, but that's unlikely to damage the radio. Connecting another speaker will hopefull prove that. I wouldn't have thought that connecting a phone would damage the radio, either. As to your connections, provided the polarity is correct, the in-line fuse is alright, and of the correct amperage, an d the radio is properly earthed to the car's metalwork. there should be no problem with that either.

[MGBGT1966]

Thanks! The other speaker I tested did not work either. Both speakers give resistance reading, but now when I measure the original speaker, the value goes down each time, from 9 ohms down to 4 now (yet in each measure, the value is constant). Perhaps this could be what was causing the radio/s to break, ie. too high resistance? Also, it seems the fuse holder was equipped with a fuse of 800mA/250V, instead of a 5, which I think would be the correct one? Not good, but at least it wasnt too slow. Yet, it seems strange that this one didnt blow, because the radio still starts up. Everything else should be correctly connected, and positive ground is connected through the assigned spade.

['LIVEWIRE?']

Generally speaking, transistor radios don't ming higher impdance loads than that for which they were designed, but I guess it's possible that the output stages of the Frankfurt could have been damged by a faulty speaker. It would need to be bench tested and some readings taken to find out what has happened. I'm not sure what fuse should be fitted, but it certainly would be more than 800mA. Perhaps it could be tried with (say) a 2 Amp fuse in case the 800mA one has become high resistance. That may be all that's wrong with the radio.