Replacing link cable in 1950s Blaupunkt Car radio

[Mark Hennessy]

Pictures?

[Old Sparky]

Pictures?

I'll take some, and post them in the next day or two, Mark. In the meantime, having made up a new link cable with no outer screen, I have proved that the radio is working, but with a continuous whisle of some 0.5 to 1khz , which isn't affected by the volume control. I haven't been able to pinpoint the source, but I think I'll have to remake the cable with an outer screen. The audio cable is screened, but modern cable, or at least that which I have doesn't have high density screening like the original.

[Mark Hennessy]

That's a shame. I suppose the question is this:

Is it getting past the screen on the audio cable and in turn being picked up by the audio core, from one of the other wires in the bundle?

If so, will an outer overall screen help? Probably not.

On the other hand, if interference is leaking from the power cores to something else outside of the bundle, then yes, perhaps an overall screen is the answer.

A difficult question to answer without doing the experiment first.

Other thoughts that spring to mind (sorry, you've probably already thought about them):

Where do you think the whistle is coming from? Is that too high for it to be the vibrator? I ask because I have no experience of them, but somehow expected the mechanical ones to run at a lower frequency. I don't know...

I suppose it might be possible to pull the audio cable from the bundle, then you could try spacing it away to see of the interference changes?

Was the noise there before you changed the cable, or is this the first time you've had the set working? If so, it's entirely possible that the cable isn't the problem.

Either way, I'm sure you'll get there

Mark

[Refugee]

Have the decoupling capacitors been checked or changed?
Vibrators usually run at 50 hertz.

[Old Sparky]

This is a radio in which I had already replaced all the waxed paper capacitors, the reservoir & smoothing capacitor, and a number of other faulty components. The original mechanical vibrator was missing, so I fitted a solid state one from the 'Antique Auto Co.' ( obtained from AC James of Paulton, Bristol ) All of this was done some time ago, and the radio tested. At the time it was working OK, but left in store. When I retrieved it a short while ago, I discovered that the internal insulatiion of the wires in the link cable were perished, so I conclude that the oscillations are due to my new, unscreened cable. This could be because I've routed a lead carrying HT+ down the same length of sleeving as the screened signal lead, perhaps due to capacitive/inductive coupling. THe LT +, -, & switched + leads are in a seperate piece of sleeving running alongside the signal & HT leads, with the two fastened together in several places with cable ties. Not a good idea, maybe, but all the leads are, of neccesity, in close proximity to each other at both the head unit end, and at the plug/psu end! Because all the two-unit valved car radios I've ever come across used a multi-core cable with an overall outer screen to connect the two parts together, I was probably wrong to use unscreened wires for the job. After all, those designers back in thee 1950s knew what they were about, and the setmakers surely wouldn't have used expensive screened cable unless it was necessary! The Radio is a Frankfurt 'R' series dating from about 1956.

[Old Sparky]

[attachment=10975][attachment=10976]         [attachment=10978][attachment=10979]

[quote='Mark Hennessy' pid='46051' dateline='1403948104']
Pictures?

I'll take some, and post them in the next day or two, Mark. In the meantime, having made up a new link cable with no outer screen, I have proved that the radio is working, but with a continuous whisle of some 0.5 to 1khz , which isn't affected by the volume control. I haven't been able to pinpoint the source, but I think I'll have to remake the cable with an outer screen. The audio cable is screened, but modern cable, or at least that which I have doesn't have high density screening like the original Pictures attached ( for some reason only two of my four attachments have uploaded/appeared. Why is this?)

[Old Sparky]

Attached (I hope!!)

                   

[Mark Hennessy]

Wow - that's a complicated beast! Well done for tackling that - I'm not sure I'd be brave enough

[Old Sparky]

It certainly wasn't the easiest radio I've ever worked on, Mark! Took quite a few hours and considerable dismantling to get at, and change, some of the faulty components. I hope I can resolve the remaining problems and get the set fully operational again. Apparently Radios of this model were fitted in Mercedes cars in the 1950s, so it must have been quite an expensive thing in it's day!
(P.S. why some of my photos have appeared more than once, I don't know. Tried to delete one earlier, but the system wouldn't let me!)

[Mark Hennessy]

I find it incredible to think that they managed to cram FM reception in there as well. I know that FM was much more popular in Germany at that time compared to the UK, so it would have been an expected feature, but even so

Was the demodulation done using germanium diodes by this time?

Mark

Edit: Hopefully the pictures look better now