Replacing link cable in 1950s Blaupunkt Car radio

[Old Sparky]

An EABC80 was used for detection, Mark, two of the diodes being connected as a ratio detector, with the other being used for AM demodulation. The circui isessentially that of a conventional domestic superhet receiver with an added RF amp. using an EF89, the rest of the lineup being ECH81, EF89, EABC80 & EL84, the latter in the amplifier/PSU Box. An ECC85 was used for the VHF Tuner. Cramming all that lot into a 7" x 5" box with a push-button tuner means there is very little wasted space, to say the least! As to why VHF was more popular in Germany in the 19050s, I presume it was because in order to receive the 2nd & 3rd programmes in each Federal Land, you had to buy a VHF radio, as these were not on MW or LW.

[Old Sparky]

Having carried out some more work on this vintage receiver, I've come to the conclusion that I shall have to fully screen the interlink cable, which will hopefully minimise QRM (if I may use the 'Q' codes on this forum!) When run from my 10amp Bench PSU (actually an -ex Blaupunkt demonstration stand Power supply, BTW) a high level of noise is being picked up. IMHO the reason may be that the outer screen of the original is used as the common (earth) connection between the head unit and the amplifier/power supply box.

[AlanBeckett]

IMHO the reason may be that the outer screen of the original is used as the common (earth) connection between the head unit and the amplifier/power supply box.

You could always try joining them together with a nice thick piece of Braid.
Dumb question: Have you put all the covers back on the Radio? In my experience it's very important.

Alan

[Old Sparky]

Alan, no I've left the covers off the head unit, partly so I can take measurements at various points, but will try it again, maybe on Monday.

[Old Sparky]

3.5 years later, the Frankfurt is still in my workshop, as I never successfully solved the problem of QRM (Interference). Earlier this year, or maybe late last year there was a reference on UKVRRRDF to a German Firm (not 'askjanfirst,btw) who supplied modern cables made to suit radios of the 1950s & 60s, but, unfortunately I didn't bookmark thepage concerned, or make a note of the firm's contact details, and haven't been able to locate them. An ongoing request on UKVRRR's 'Sets & Parts wanted ' sectiom has drawn a blank. Does anyone on this forum have any idea as to the name of the company concerned?

[Murphyv310]

Hi.
Are you sure it's to do with cables? How are you powering the set, car battery or bench power supply?
The main issue with these sets is proper earth bonding, very hard to do unless it's fitted to the vehicle. I did one of these out of a 1962 BMW 1500 many years ago and had the same issue on the bench. It was perfect when 're fitted to the car. I've some aircraft quality screened lead if you need any.

[Murphyv310]

Hi.
The noise is definitely coming from the vibrator and combined output stage. I bet if you bond both head unit and power supply together either with thick braided wire or even put the covers on and tightly sit the power supply on the radio the problem will go.
This issue wasn't just confined to your set it was common on many sets that had a remote vibrator supply. Incidentally have you replaced all the caps and checked the chokes around the vibrator. I'm also sure from memory this is a higher frequency vibrator to enable smaller value decouplers that are also physically smaller.

[Old Sparky]

Reading both your posts, Trevor, and dealing with points as they arise. I was powering the radio from a variable voltage mains PSU (actually an ex-Blaupunkt demonstration stand supply, which, more than 20 years ago now, Ibuilt into a metal case, fitted a Voltmeter, and panel mounted voltage control, which allows any voltage fron virtually zero to 17vDC to be applied to whatever is being powered, The PSU can deliver up to 10 Amps, and is a conventional linear supply using 2N3055 transistors. As to the other points you make, first I have to replace the reservoir/smoothing cap. I removed the new one I'd fitted several years ago to use in a record oplayer, and have yet to fit a replacement. I replaced all thhe original caps, plus various other out of spec. components, and fitted a modern electronic 'vibrator' in place of the original mechanical one. What I haven't yet done, but will do when next i get the Frankfurt on the bench, is to bond the head unit and power pack together with braid, and run the radio with the covers on. I had also replaced the original selenium rectifier with 4 x 1N4007 diodes with a series resistor. IIRC the reservoir & smoothing caps were originally 32 + 16 uf, but the new one was 2 x 32uf. I won't take up your offer of Aircraft quality screened lead at the moment, but may well do so at a later date, maybe in the New Year. In the meantime thanks for your comments and suggestions, which should help me to solve the problem(s)
Re-reading some of the earlier posts, and in particular Mark's comments about it being a complex set, I'm sure there was an even more complex one, called the Koln, which had Motorised self-seek tuning.

[Murphyv310]

Ah ha.....
So is the modern vibrator running at the same frequency as the mechanical one? Have you added snubber caps across each 1N4007?

[Old Sparky]

Terry, I'll have to check as to whether I fitted snubber caps. or not. AS I mentioned when I revived this thread, the radio was put on one side for over 3 years. As to the electronic vibrator's frequency, unless it mentions on the company's website, I don't know what it is, and may have difficulty measuring it, but I'd imagine that over the years thousands of these solid state vibrators have been sold, and fitted into a wide variety of valved car radios, so there shouldn't be any pronlems caused by using one. I've no idea what the internal circuitry is, other than there being an oscillator of some sort, and they're sealed units, so I won't open one and reverse engineer it. Re-reading my old posts reminded me that the original mechanical vibrator was missing from the Frankfurt, which is why I bought the solid state one. Checking the Antique Automoblie Radio Company website, it seems that their solid state vibrators operate at 1Mhz. I've used them in other vintage car radios without problems.