Strange problem with Grundig CS200 tube receiver

[Rodeodave]

So I have this Grundig CS200 tube receiver/amp combo out of an old Konzertschrank for quite some time now. It was gutted from a Grundig Mandello e/ST Konzertschrank and is identical to the CS200 tuner that came as a stand-alone device. I also have the matching stereo decoder, so it's a nice piece of gear.
It is completely unmolested, so all the wax seals of the tuning caps and coils are still intact.
The tuner part uses ECC85, ECH81, EAF801 and an EM87 tube, the latter one is the magic eye tuning indicator.
The output stage uses an ECC83 and an ELL80.

I slowly brought it up with a variac and everything seemed to work nicely. I then hooked up speakers and the stereo decoder. It works and it really sounds nice. I'm planing to use the tape out connection to hook it up to my main amp, thus using it simply as a tuner. Once I'm done fixing the thing I will include a switch so the output stage can be switched off to safe power, tube life and heat.

Now for the problem: The EM84 tuning indicator suddenly goes out after maybe a minute from a cold start, but the the tuner keeps the station and it's also good enough for stereo tuning. Then over the next couple of minutes the received signal grows weaker, and finally faints completely.
What is causing this behavior? Where would i start looking for errors? I have some experience with tubes and I have the necessary equipment to take measurements, but I have never dealt with tuners before.

I suspect the problem with the EM84 might just be due to it's age, but with the the other stuff I have no clue. If I exchange the ECC85 and ECH81 with known good tubes the problem is still there, for the other tubes in the tuner section I unfortunately have no spares.

There are 5 electrolytic caps in the CS200, one of them a multisection cap. I'll replace those for sure, but for now they seem to work good enough.
The other caps are listed as:
KT caps, which are Polyester caps with aluminum foil
KS caps, which are Polystrol caps
Ceramics caps
MKT caps, which are also Polyester with aluminum coating

I don't think there are any paper caps.


What would be causing the fainting signal? Some charge building up somewhere due to leaking coupling caps between stages?

One more question: I have a bad feeling about those Selenium rectifiers. I hear they stink up the place when they go bad, and they're toxic. What to replace it with? Simply 1N4007 diodes and a dropping resistor to compensate for the lower voltage drop of Silicone diodes? How big should I choose the resistor? The CS200 uses the B250C100, does anyone have experience with that thing?

[DF96]

When the volume fades does the main HT voltage change? Also, check grid and cathode voltages on ECC83 and ELL80. In this circuit they seem to have quite high values for the grid leak resistors, so any grid current due to gas would have a large effect.

[the_manta]

Hi neighbour,
How does the EM84 faint ? does only the bar go to its lowest position or is the complete tube "dead" without a single luminescence ? The first error would point to some problem with the detector voltage ("Ratiospannung") which the intermediate frequency amplifier uses for its variable-µ-valves (mostly EF89/6DA6) to change the µ.

Capacitor Issue: In most my radios I keep the big PSU capacitor when it doesn't get hot or lost its capacity. But when you're nor looking for authenticity you should change it.
What you really should change is the cathode capacitor at the power tubes cathode and most important, the ratio-capacitor. It's rated about 5µF to 10µF and is connected on the antiparallel diodes of the ratio-discriminator. It is C29 in the circuit
A good thing to check is also the coupling caps from the ECC83 to the ELL80.
Most other caps will be fine simply because they don't have to handle too high voltages. Replacing caps in the high frequency unit can lead to some problems like new finetuning of the intermediate frequency amp and tuner. So it's best to locate the error than replacing parts in bruteforce mode.

The selenium rectifier can be defective but in most cases it isn't. Such devices get warm maybe slightly hot, you should be able to touch it without burning your hand. There is nothing toxic on them as far as I know. And yes, you can replace it with a silicon rectifier and some resistors.

[nigelwright7557]

Sounds like a valve/tube at the end of its life.

[kevinkr]

Does this set have a selenium rectifier? In any event all of the electrolytics should be replaced before proceeding too far, then identify the film/paper caps in the audio section that may need to be replaced.

Edit: Helps to look at the schematic.. Anyway this set has a Siemens Selenium bridge, not only do these often let out the toxic magic smoke (no I'm not kidding) their usual failure mode is to become very resistive. The supply charges up as the tubes warm up but aren't conducting, once they start to conduct the supply collapses. Replace it! It is connected to the pink colored secondary wires. Probably to keep RFI from the diodes reasonable I would replace the bridge with discrete fast rectifiers like the UF4007 or similar - of course a bridge could be used - get something with a 800PIV/1A rating at minimum.

[Rodeodave]

Thanks for the input everyone!
I will check a few voltages next time I fire it up.
The EM84 turns off suddenly, no faintig, it just drops out and stays dead.
The station stays tuned for a few minutes and the signal then slowly faints. If the signal is strong enough the stereo decoder's indicator light lights up. When the signal gets weaker first this light goes out due to the weaker signal, but one can still listen to the tuned station. Therefore I suspect that there must be problem with the tuner section somehow. The cold unit works, so it should be related to heat our built up charge.
And the selenium rectifier must go, that stuff is toxic.

[kevinkr]

Originally Posted by Rodeodave

<snip>
And the selenium rectifier must go, that stuff is toxic.

Most of your symptoms really seem to point at a collapsing power supply, please do nothing else until you replace the rectifier! You might be pleasantly surprised or not...

[Rodeodave]

Okay, I replaced that B250C100 selenium rectifier bridge with a Si rectifier bridge. Between the plus terminal of the bridge and the circuit I fitted a 200R 5W wirewound resistor (only value I had in my stash) to drop the voltage to a capacitor-safe level.
Again I slowly started it up with the variac. The sound came on, the EM87 lit up but indicated weaker tuning than before, also the volume was lower plus quite some hum (guessing twice the line frequency) that wasn't there before. Then the EM87 dropped out and the tuning faded away, both faster than before the modification, but exactly the same symptoms.

Next up will be replacing all the electrolytic caps and measuring some voltages, that should give more clues.

Hm...now that I'm thinking about it I'm not exactly sure that that rectifier bridge is rated for those voltages...have to check that asap. I'll take pictures while I'm in there.

[DF96]

If possible measure voltages before changing caps. You now need to be in fault-finding mode, not change-everything-on-sight mode. You need to consider symptoms.

You have just found that with higher(?) HT the fault occurs more quickly. Or is the HT lower? Measure it. When the sound fades does the HT increase or reduce? We need some facts to work on.

[Rodeodave]

I'll pull some numbers from the circuit as soon as I get new batteries for my DMMs, they're all dead...and today's a national holiday here, so no store is open for business, hence no fresh batteries and therefore no measurements. Lame.

Like I said, I'll be back tomorrow with some numbers. Thanks for time!