Zenith 7s488 assistance request - Printable Version
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+--- Thread: Zenith 7s488 assistance request (/showthread.php?tid=23678)
Zenith 7s488 assistance request - Rob Quay - 07-24-2022
Hi Guys,
I’m in need of some help with a Zenith 7s488. I’ve recapped the radio and have the audio section working. I’m getting some bad voltage readings from some of the tubes. These voltages were recorded at 80 volts on a variac.
6K7G (RF)- P 205 v., G2 0 v., G3 0 v., G1 0 v.
6A8G (Converter) - P 0 v., G2 235 v.
6K7G (IF) - P 213 v., G2 0 v., G3 0 v.
6J5G (Det) - G 0 v.
I didn’t include readings from the 6X5G, 6F5G and 6F6G because they are apparently functioning. Transformers and coils show continuity so I assume they are good. This is a tough radio to work on because everything is packed tightly. Any direction from the Phorum Pfolks is appreciated. Thanks. - Rob Q.
RE: Zenith 7s488 assistance request - morzh - 07-25-2022
Your primary of the RF transformer (that goes to the plate where it reads 0V) is likely open. Start there.
And for good measure, if you have no smoke, do the readings at 110VAC.
RE: Zenith 7s488 assistance request - Rob Quay - 07-26-2022
Thanks morzh. I’ll get to work and report back. - Rob Q.
RE: Zenith 7s488 assistance request - Rob Quay - 07-26-2022
The rf transformer checked out fine but I suspect that the problem isn’t far away. The leads of the primary go to two solder connect points with multiple wire leads so I’ll go there next unless someone has another idea. Thanks! - Rob Q.
RE: Zenith 7s488 assistance request - Eliot Ness - 07-26-2022
Just to be clear, if you have 0 volts on the plate of the 6A8 (converter) did you ohm out the primary of IF coil #10? It looks like it should be about 22 ohms.
RE: Zenith 7s488 assistance request - Rob Quay - 07-26-2022
Yes I did. It is 22 ohms on the dot.
RE: Zenith 7s488 assistance request - MrFixr55 - 07-26-2022
Hi Rob,
This is an interesting issue, as the 1st IF Primary connects directly between the 6A8 plate and the B+ Bus (that supplies B+ to Audio and through resistors, to the plates and screens of the other tubes) and the B+ Bus is working. I would look for a cold solder joint. One technique:
Confirm the resistance of the 1st IF Primary again. At the exact same points that you are measuring the resistance, without moving your leads (Test clips help here), switch the DVM to DC Volts and fire up the radio. If you see a voltage in the 200V Range, then a bad connection exists between the 2 points that you are measuring. This can be a cold or a poorly soldered joint inside or outside the 1st IF transformer. The resistance reading is good due to the very low voltage and low current put out by the DVM.
However, if the voltage across this point is close to 0V, then look elsewhere, but you are close. I do not suspect that the plate voltage is 0V due to a short in the tube or at the socket, as this would put the full 250V B+ across the 1st IF Primary winding and burn it out!
After resolving this, I would worry about the 6A8. With no plate voltage (and no plate current), then the various grids connected to B+ will handle all the current and possibly burn up, shorting against adjacent grids. This is very common in the output stage of radios if operated without the output transformer connected. This is why most radios of this vintage that have the transformer mounted on the speaker and connected to the Output tube via a plug also interrupt B+ to the rest of the chassis when the speaker plug is disconnected.
Hope this helps. Good Luck.
Best Regards,
John, MrFixr55
RE: Zenith 7s488 assistance request - Rob Quay - 07-26-2022
Thanks for your input John. I’ll follow through with your suggestions.
After recapping, cleaning tube pins, variable capacitor, switches and contacts the bc band was working, along with the preset push buttons. When I went back to work on it the next day the radio had reverted to nonfunctional mode.
I’m sending a picture of the chassis. It’s definitely crowded inside! Thanks again! - Rob Q.
RE: Zenith 7s488 assistance request - Rob Quay - 07-27-2022
Hi Guys- Following John’s suggestion I found a reading of 0 v. DC. All tubes were tested earlier in the process but a second check of the 6A8G found the tube dead. I’ll order another one today. My question is whether another issue took out the tube, or was it just ready to go after 70+ years? Thanks again for the help guys! - Rob Q.
RE: Zenith 7s488 assistance request - morzh - 07-27-2022
Rob
This is not a higher math.
Take a meter, set it on volts, and strart going along the line of feeding the power to the 6A8 plate. 0V? OK, how about the other side of that same RF transformer primary? Again 0V? Go tro the bus it is connected to. Itr is literally 5 minutes to trace.
RE: Zenith 7s488 assistance request - Rob Quay - 07-27-2022
I’ll do that. Sounds logical.
RE: Zenith 7s488 assistance request - Eliot Ness - 07-28-2022
If you have voltage on the plate of 6K7 (RF) and the plate of 6K7 (IF) then something must be wired incorrectly. Check both sides of the IF (#10) for voltage. Then check at R23 and then the junction of R8 & R7. If you have voltage at the resistors but none on either side of the IF (#10) then it isn't connected to where it should be.
In-other-words, if you have voltage at point A and B but not C, then there must be a disconnect between point C and D.
RE: Zenith 7s488 assistance request - Rob Quay - 07-28-2022
I had wired the IF incorrectly. 6A8G plate voltage is back. Still no radio reception. I’m waiting for a NOS 6A8G to arrive. When the one I have is in the tube tester, readings are all over the place. It would be nice if that’s all that is wrong with the radio, but I’m not getting my hopes up yet. I’ll let you know what happens. Thanks guys! - Rob
RE: Zenith 7s488 assistance request - MrFixr55 - 07-29-2022
Good Deal Rob! I was wondering if here was a miswire, but was going with a cold solder joint first. I gotta get around to learning how to easily cut, markup and paste sections of a schematic like Elliot Ness did in his last post.
I have a sneaking opinion that the 6A8 is toast. Good luck!
RE: Zenith 7s488 assistance request - Rob Quay - 07-31-2022
Hi Guys-
A new 6A8 didn’t get the radio functioning. I’ll go back through and recheck tube voltages at 110 v. Since the AF section works maybe I should focus on the IF portion next. Any and all suggestions are appreciated. Thanks. - Rob