EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

I am restoring my FM Tuner. I have discovered it is different than the factory diagram (not a huge surprise with Scott). Note the difference in the eye tube circuit - clearly original wiring, but they pick up the signal after the 6H6. Interesting, but that isn't the problem - that is the 2nd limiter. 

The plate and grid voltages on the 2nd limiter are much too low. If I simply lift the grid cap off the tube, the voltages return to normal. When the grid cap is connected, the 100k feeder resistor gets hot (obviously). Plate voltages are about 50 (normal) and about 15 with the grid connected. I have replaced both transformer assemblies on either side of the 2nd limiter - total replacement from another Scott set. 

Any ideas would be appreciated!

Kent

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Have you put a scope on the tube to see if it's unstable and possibly oscillating, I take it you have already swapped the tube,

have you checked the transformers etc, or even the wire positioning.

Had an audio amp once and the plates glowed when the volume was turned down, turned out to be the phase splitter tube taking off, 

Definitely try a couple different 6J7G tubes if you have not already done so.  FYI, the limiter tubes in the 800B often operate very different than identified in service data but as long as they are not overheating components, they work.

Norman

Does not appear to be oscillating. This evening, I tried almost every 6J7 tube I've got, including some GT and even metal tubes. Different tubes behaved differently in the circuit, but mostly they all behaved like the one I had originally. The screen voltage is always reasonable, but the plate voltage is dropped whenever the grid cap is in place. The 100k resistor is dropping almost all of the B+ (about 250V) down to about 14-15V on the plate, so I get why it is hot. 

I'll continue to scratch my head and hope someone has an idea.

Kent



Kent King said:

Does not appear to be oscillating. This evening, I tried almost every 6J7 tube I've got, including some GT and even metal tubes. Different tubes behaved differently in the circuit, but mostly they all behaved like the one I had originally. The screen voltage is always reasonable, but the plate voltage is dropped whenever the grid cap is in place. The 100k resistor is dropping almost all of the B+ (about 250V) down to about 14-15V on the plate, so I get why it is hot. 

I'll continue to scratch my head and hope someone has an idea.

Kent

Kent: have you checked for voltage on the grid lead? Since it is isolated by 3 capacitors, it would be odd for there to be any dc there.  I can only think that this would be the only reason that it would drop the plate voltage that much. If that isn't the problem, then I am out of ideas as well.  

Thom

A fellow on ARF asked the same question. With the grid lead off, there is 0 volts (what I'd expect if there is no cap leakage). When connected, I read -0.23v. The commenter on ARF said it should <not> be positive (and it isn't), so that may not be an answer either. I am really stumped - thanks for giving it thought!

Kent

Kent are we talking about the early “FM Tuner” stand-alone in the small cabinet ?

Yes. I think my next attack will be to do a full alignment, based on thoughts here and ARF. You have any other ideas?

Kent, You may not be as far off as you think. I agree that the plate voltages can be off quite a bit since the limiter stages work essentially saturated. What are the static DC voltages for the limiters with the mixer or IF tubes out. No signal and hopefully no oscillations? That 100k resistor I could see warm but not hot. Is it original? I'm traveling at the moment but would be happy to compare with my FM unit when I get back.

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