zenith: Zenith (Ch=6A19); 6G501F - noise when operating on AC

 Please be easy on me, this is my first radio post!

   I have rebuilt a Zenith 6g501f. It functions perfectly on battery (actually quite outstanding), but on AC, it appears to have poor sensitivity to the point that the ACG runs the gain up and generates a lot of noise. There's no real hum and the power supply voltages look good - although the filament string voltage is noticeably lower on AC. The alignment also looks very good and the dial tracks remarkably well, far better than any of my transoceanics.

    My first thought was that my wall voltage was low, but no, it's right between 116.5-117. The B+ looks good at 85-86V, the filament supply is around 7.9V. Checking the individual tube filaments, they are all around 1.35V (and ~2.8 on the 3q5g) which is very close to specification. I have fiddled with the power resistors and when I raise the filament to ~9 (essentially the same as on battery) it seems to get slightly better, but the required power resistors are down around 850 ohms (vice 1006 ohms per the schematic). I know from Transoceanics that the filament voltage can dramatically effect the performance but I am surprised that the standard voltages seem to result in such poor performance. I have tried several different sets of tubes, some NOS, with essentially no difference in the results. Does anyone in this august body have any better ideas?

    Brett

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Dear Brett

You wrote that the set works well on Batterie. I don't have the set only the schematic.I'd say that you check the electrolyts C8, C9 and C10 in the heating way. The best way is a Scope, so you can see if the hum is too high. I think the 7,9 V filament will be a little higher when C8-10 is in a good condition.In this case you should use the original Resistor 1006 ohms.

Please check C11 also for less hum for the 90 Volts.   Regards  J. Leber

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 I will check, but I did replace all of those with slightly larger values than original. One quick test - I will stick 220 MFD temprorarily across the filament supply in parallel with the nominal 20 MFD. That ought to effectively crunch any residual AC on the filaments. I did check that all the caps were where they were supposed to be and that the polarity as installed was correct. I didn't see any consequential AC on the filaments. The noise I hear is much more like a hiss than a hum - much higher than 60 or 120 hz. 

    I am startled at how much extra space I have after replacing all the old paper/wax caps with modern components. I could easily fit 10x the capacitance in the same space. 

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 Well, I tried both doubling the power supply capacitance (80 mfd vice 40) on the main supply, and also 12x the capacitance (220 paralleled with 20 mfd) on the filament supply, and no luck. I checked the AC component before and after of each, and there was very minimal reduction and it was only about 3 volts on the B+ and much less on the filament. There was also negligible increase in the DC voltages. The improvement in the noise (not hum, very high frequency hiss) was essentially undetectable. 

I also hooked it up with variac and isolation transformer, and ran the input voltage up until it matched the voltage with a brand new battery, no help at all. So I think it has nothing to do with the absolute value of the filament voltage. playing with it, I get oscillation at about 7.7-7.8 volts - the oscillator seems to require the most voltage, everything aside from that works a lot lower (also just like a transoceanic). 

    I rechecked my wiring and it still looks good. At this time I am pretty well stumped. There's not much to this power supply, it seems simple enough that it would be hard to screw up very much. Any further thoughts would be greatly appreciated. 

    Brett

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Hi Brett,

I just want to summarize as follows:

1. It's not a hum generated by line because filament and +B is good enough.

2. Filament voltage is also high enough

3. The hum/noise is much higher, like a hiss.

4. This noise is only when the set is on line.

 

a.) my question: Is the noise only on a strong station and between the stations is no noise? Or is the noise permanently there?

 

b.) When using a scope the picture on G1 3Q5 and on a loudspeaker would be interesting.

 

regards, Johann

 

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 That's a good idea. Right now I am set up to work on something else but when I get done I will take some scope pictures. I didn't see anything strange at the frequencies I was looking at. 

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Thanks for the input and sorry for the delay in posting. Since the last post, I have restored 4 more zenith portables of the same vintage and acquired one that had been restored by someone else. Also an unrestored but working 5g401. 

   In every case they had a fair bit of the same sort of noise. Great on battery, noise and apparently low sensitivity on AC. I tried many other potential solutions - chokes in line with the B+, large caps (up to 1200 mfd! - which blew a rectifier tube on startup due to arcing as excess current was drawn) across the B+ and filaments, run with filaments on batterys and B+ on AC, vice versa, etc. The only thing that helped was adding tube shields to the RF amp (when there was one) and/or the detector. 

   My conclusion - they just don't perform as well on AC. I think noise from the rectifier tube radiates directly into the RF and detector tubes, and generates RF noise. That makes the noise, and also runs up the AGC that has the effect of greatly reducing the signal/noise on weak stations. On strong stations it works about the same either way. My problem was not knowing what to expect. I get similar effects on battery from my fluorescent work light, Shielding between the rectifier and the rest of the radio blocks a lot of it. 

    Bottom line is that I was such a newbie I had no idea what to expect. I also learned to hate the guy who invented rubber insulation, but that's another story. On my 6g601, I ended up replacing *every single rubber wire* in the God-forsaken thing. Ultimately it was stripped down to bare metal and built back up with essentially all new wires, all new passive components, and NOS tubes. It was probably worth it - this and the 6g501 work almost miraculously well on battery, with remarkable sensitivity and decent selectivity. First night I had it working, almost at the first fire-up, it happened to be tuned to 850 khz - KOA in Denver. Nice and clear - from *Sunnyvale California*, almost 1000 miles, by accident. With an external tuned loop, I had no problem with the 50,000 watt Lexington, Nebraska (1800-ish miles) and intermittently, Hartford CT (~3000 miles)

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