zenith: Ch= 5657; 6D315

I am trying to restore a Zenith 6D315, chassis 5657. The problem is that MY radio does NOT MATCH the schematics available on-line or in Riders Manuals! My chassis is definitely marked 5657 on the original Zenith paper serial number tag attached to the inside rear chassis. Some of the differences are:

1. The wavemagnet antanna on my radio is NOT SHIELDED as is the one shown in the schematic. The bottom socket where the wavemagnet shield plugs in is empty. The remaining plugs and wiring to the wavemagnet look original, including the lead to the external antenna screw. I just cannot imagine someone removing the wavemagnet shield!
2. My set uses a 6K7G IF amplifier tube, and not a 6U7G. And the TUBE SOCKET is marked 6K7G!
3. The 6Q7 grid resistor in my set is 15meg and not 1 meg. And bias resistors R5 and R6 are NOT INSTALLED in my set (and apparently never were - all resistors look original).
4. R4 in the AVC line is 2.2meg vs. 1 meg in the schematic.
5. The two wire wound resistors R9 and R10 differ from the schematic. R10 (the one that goes to ground) is marked 12 ohms and measures 12 ohms. R9 looks like it is marked 40 ohms (yellow/black/black) but measures 100 ohms - but sometimes colors fade or change with age and heat. The resistor is a Micamold type and is somewhat bulged. If it really should be a 40 ohms resistor then it needs to be replaced, but with WHAT? The schematic shows 60 and 50 ohms for R9 and R10. But R10 in my radio is original and is both marked and measures 12 ohms!

Has anyone else encountered this variation and can verify the value of resistor R9? Thanks!

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I have been looking at a 5646 chassis schematic for someone else and noted that these two are virtually identical. The 5646 uses a 6K7G for the IF and it turns out that the 6K7G and 6U7G are interchangeable, so maybe there was a shortage and a substitution was made. Things like that happened a lot from what I'm told. 

R4 for both chassis is listed as a 1meg and R4 is shown twice - once off to the 6A8G grid for AVC and the other to the grid of the 6Q7G. It may be that I'm reading the schematic wrong as it isn't very clear in places. In any event, the 2.2meg will probably be fine and is a typical value in other radios (2.2meg to 4.7meg) AVC circuits.

Both chassis show R9 as 60Ω and R10 as 50Ω. I can't explain the differences from your set and the schematic, but the change would certainly affect the bias on the 6Q7G. Maybe a modification. I think I would go with the schematic values - this shouldn't hurt anything and can be easily changed back. 

Welcome to the days of shortages and radio manufacturing.

Fred N.

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After some cleaning, the first dot on the Micamold wire wound resistor R9 is likely WHITE and not YELLOW. So R9 is 90 ohms.  So R9+R10 would be 102 ohms, which is close to the schematic values of 60 plus 50 ohms.  Thus the bias on the 25L6 output tube would still be correct.  By going to contact bias for the 6Q7G, R5 and R6 could be eliminated, thus the change in value of the 6Q7 grid resistor from 1 to 15 megohms. Eliminating two resistors is a major savings for a high volume manufacturer.  The change from the 6U7 to 6K7 may be related to either cost or availability - the tubes are basically interchangable. The only thing I cannot explain is the missing wavemagnet shield. That could either be a cost saving production change, or it could have been removed by a previous owner or service personnel, for some unknown reason.  My conclusion is that my chassis is simply an undocumented varient of the original schematic.

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Sorry, I completely glossed over your item 3 in the first post.Your installed values make a lot more sense now, but I'm still a little confused as to what you ended up with. So the 6Q7 grid resistor goes where ? Cathode to where ?

Yes, sounds like a factory mod was put into place and perhaps near the end of the production of this particular model, but that is just speculation on my part. Any time you could reduce the labor costs (don't think the resistor cost was significant) and time made the bottom line better.

It does sound like you have a radio that will probably work just fine once you get the other parts in the set that require replacement replaced.

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The cold end of the 6Q7 grid resistor, as well as the 6Q7 cathode and cold end of the volume control all go to the junction of R9 and R10.  I'm guessing that this junction would be at about -0.7 volts based on a B+ current of 60ma.

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