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 Post subject: Saalburg 5380
PostPosted: Jan Wed 24, 2018 6:29 pm 
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I've had this radio "shelved" for ages now. It was manufactured around 1964 in the German Democratic Republic. It's a very curious tube radio because much of it was built on printed circuit board. Also, the design is very different to what you'd find with a typical 1950s British set. Very curious is they used wires pretty close to telephone wire for circuits and very easy to solder.

Ages ago, I discovered what was wrong with the set. It basically worked on FM but nothing on AM and LW. After wading through the schematic and testing voltage on the tubes, I discovered there was no voltage on the L.O. triode plate of an ECH81 (used for the AM and LW circuits). The FM circuit has an ECC85 to oscillate on FM and that tube was working fine.

I'm pretty sure the fault is a break in the pushbutton, piano switches (the ones you get with Bush FM sets or Grundigs). Many times I've seen total loss of HT due to really dirty switch contacts. Nothing happens when you push the buttons and it stays on FM. Servisol helped last time I used it as the set had stopped working completely.

I shelved the set for some time as it was discovered the whole pushbutton mechanism on its board wasn't screwed onto the mother board but tab soldered. At the time I figured I had a real tough job ahead getting access. I even asked myself, is it worth all the effort and time when so many radios abound where access is plain simple.

Even so, if anyone is interested, here is what I find quite curious about this radio:

The FM circuit uses variometers. There is an ECC85 double triode tube. One triode is the amplifier and the other the L.O. for FM. To be honest, I'm not well informed about that kind of circuit where variometers are used. The whole thing is mounted on a platform and the tuning mechanism string fixed to it.

For some reason, they bypassed a lot of AVC diodes and just grounded them out. Why not just use the diode as intended I wondered?

If anyone can provide a link that shows a piano pushbutton key mechanism dismantled, please provide a link. I refer to the white knobs that run along the front of the set. I think it would help me just see the mechanism contacts in photos.

I recently dismantled a very early transistor radio where the switch contacts had been causing issues. I found them to be filthy after many decades.

One day I will just return to work on this radio as a challenge but felt at the time I needed a bit more experience.


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 Post subject: Re: Saalburg 5380
PostPosted: Jan Wed 31, 2018 5:34 pm 
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This particular German schematic gave me some headaches. I never really quite understood the switch diagrams. Finally the other night, after looking at it for some time, I finally understood how the waveband coils are switched in and off. On a British schematic this is all far clearer.
Something particularly puzzling is the AM detector stage. The first diode on the Saalburg fed by the 455Khz IF can is actually for AVC. By memory I recall this is the EABC80 tube.
The first time I saw this schematic it did come as a bit of a shock.


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 Post subject: Re: Saalburg 5380
PostPosted: Jan Wed 31, 2018 6:07 pm 
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Location: Near Brandon, Iowa
Usually on these sets one diode out of the three in the EABC80 tube is used for AM detection (and also AVC voltage generation) and the other two are used for FM detection.


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 Post subject: Re: Saalburg 5380
PostPosted: Jan Wed 31, 2018 6:43 pm 
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The FM part is pretty straightforward with a ratio detector. The audio frequency is easily traced from one of the ratio detector windings. The 455 KHz circuit I found a bit strange, given sometimes diodes are earthed out. Whereas in a typical British vintage set you'd be looking at one diode for the Am signal and the other for AVG.
I now have a lot more experience working on fiddly soldering circuits than when I first encountered The Saalburg. At the moment I'm working on a BEREC 1960 transistorised set where I had to really solder some fiddly bits. So, I guess I look forward to tackling the Saalburg at some point.


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 Post subject: Re: Saalburg 5380
PostPosted: Feb Mon 05, 2018 6:28 pm 
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According to my investigations of the schematic, the switching mechanism either opens or closes H.T. from the smoothing capacitor feed to the triode oscillator plate (for AM and LW). This is an ECH81 mixer/triode oscillator.
When AM is off, the HT will be directed to the permeability FM oscillator circuit. Given FM is stuck in the "on" position, no matter what waveband buttons you push, it's clear H.T. isn't being directed to the AM oscillator while breaking from FM I confirmed this ages ago by finding no voltage present on the 33 K dropper resistor (that feeds the triode plate). Plus nothing on the oscillator plate itself.
By the way, the first pushbutton for "on" does switch the set on but only on FM.
Before I do get around to pulling this set to pieces to service the waveband knob contacts, I plan to try an easier option of trying to get enough Servisol switch cleaner onto the switch contacts. I can't see these at all. I did consider the use of some sort of makeshift probe with a pinpoint light. Then lots of working of the switch manually to see if the contacts break down the grime.
Another very brave (but possibly nerve-jarring) experiment would be to simply feed HT to the AM oscillator circuit, via its dropper resistor, of course. The concern here though is the FM circuit would need to be switched out first. Also, another thing I do is check there is no leak to chassis where you plan to feed HT. Otherwise you get one big short circuit.
Otherwise, it's a real big job for the future. Removal of PCB, knobs and a complete overhaul.
Yesterday as I had an idle moment, I tidied up the output transformer. On this set the small boards on the transformer have hefty voltages. One lug is practically at full HT. All of the wiring is using very thin wire and quite rigid. Maybe that was due to cost as the set was produced in the GDR, around mid sixties.


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