Zenith 7s488 assistance request - Printable Version
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+--- Thread: Zenith 7s488 assistance request (/showthread.php?tid=23678)
RE: Zenith 7s488 assistance request - morzh - 07-31-2022
Rob
I your audio works, take a generator and try to put a signal through, after you verify the voltages are at least in the general vicinty of what is required. Start from IF via antenna. See if the oscillator even works.
RE: Zenith 7s488 assistance request - radiorich - 07-31-2022
Hello Rob,
Do you have a Signal generator?
That could really help another thing is did you clean the selector switches contacts I had more issues with dirty switches.
double check all of the resistors and triple check all of your work and old work too!
Sincerely Richard
RE: Zenith 7s488 assistance request - Rob Quay - 07-31-2022
I have an old Heathkit signal generator that is marginal. After changing a couple more dogbone resistors I get an audible change in the sound of the speaker when sweeping the bc band. That seems to correspond with the frequencies of the two transmitters in the house. I’ll get out the signal generator tomorrow and try to dig a little deeper. Thanks. - Rob
RE: Zenith 7s488 assistance request - radiorich - 07-31-2022
Hello Rob,
It sounds like you are gaining ground and getting some were.
Sincerely Richard
RE: Zenith 7s488 assistance request - Rob Quay - 08-03-2022
I’ll admit that super het theory is largely a mystery to me so please bear with me. I tried to get a signal through the radio- both through the antenna and through the IF section with no luck. My signal generator is marginal so that may not mean much.
I’d read that you can check for an operating oscillator with another AM radio placed nearby. For what it’s worth the operating radio responded when tuning the non operating radio but not the opposite.
The next plan is to take out the band switch. That entails a LOT of part removal. I have a donor radio coming next week that might help things. Hopefully I can double check my work and compare voltages.
It would be easy to fake things by putting another radio in the console and running it through the TV plug. (Can you imagine TV audio on AM?) Then there’s the phonograph end of the console! Fortunately I like a challenge. As always your thoughts and encouragement are appreciated! - Rob
RE: Zenith 7s488 assistance request - Rob Quay - 08-11-2022
Hi Guys- The radio is now working. Thanks for the help. There wasn’t a single issue, but a number of problems. All rubber wire is gone, out of spec resistors replaced, etc. It’s not finished but a functional radio makes running down issues much easier.
The record player is next. I’ll update when the project is done. Thanks again. - Rob Q
RE: Zenith 7s488 assistance request - MrFixr55 - 08-16-2022
Congrats on yor progress Rob. As far as "TV Sound through AM", this radio's TV Input sends the TV Audio directly through the 2 Stage Audio Amp (6F5 and 6F6) A basic but nonetheless a good sounding amp. TV would sound fairly well through it. Philco modulated their early TVs that had no Audio Amp, to play through a specific frequency when the TV Button on a Philco Radio such as a 41-255 "TV Sound, the Wireless way" says the sticker inside my 41-255. Dunno what frequency is used, but since the TV plays all the time in our house, I have it piped through my AM Xmitter and through an amp and speakers in my office / shop. I can tell you that AM definitely degrades the sound, likely due to the relatively low RF / IF frequencies more than AM vs FM.
RE: Zenith 7s488 assistance request - Rob Quay - 08-17-2022
Thanks for the comments! I’m still trying to track down some distortion and hum. The Zenith 5678 that I restored previously has the same RF & IF circuitry so I’m using that as a benchmark. There’s still more work to do. - Rob Q.
RE: Zenith 7s488 assistance request - MrFixr55 - 08-17-2022
Hi Rob,
Divide and conquer. Think of the radio as an old school HiFi with a standalone Tuner (RF / Convertor / IF / Detector and standalone AMP (1st AF / Output / Power Supply). Inject a good Audio signal (MP3, CD, etc.) into the Phone or TV Input and evaluate the sound. The Audio section is no Fisher 800, but it is as good as any console radio that employs a 6F6. If sound is good and pleasant, then troubleshoot the "Tuner" section. However, if distorted, check wiring, etc. Check the electrolytics C13,14, 15. These do not share a common negative. Radios from the early to mid 30s sometimes have interesting ways of obtaining bias voltages and filtering them. Hook up the negative terminal of the 'lytic and you will get hum. Check the resistor Stack R13, 14, 15 and dropping resistor R16. I always wondered if the capacity of the 'lytics was so low (4 - 15uF because of the expense of making bigger ones or if they really engineered some wild and crazy Pi Filter. One can't go to crazy on the filter caps but maybe a little bigger? Don't discount a rubbing voice coil either.
Hope this helps!
RE: Zenith 7s488 assistance request - Rob Quay - 08-18-2022
Interesting! I tied the negative ends of the electrolytics and ran them to ground. That’s something to look into as well as your other suggestions. I’m going to recheck tube pin voltages also. Thanks! - Rob Q.
RE: Zenith 7s488 assistance request - Rob Quay - 08-20-2022
Hi Guys,
I took care of the hum in the radio but a new glitch cropped up. The radio motorboats unless I keep my hand on the antenna leads. Then it gets less noticeable or goes away entirely on some frequencies. If I can take care of that I am just about home. Any suggestions? As always- thanks for your help! - Rob
RE: Zenith 7s488 assistance request - MrFixr55 - 08-21-2022
Hi Rob,
Put the Unit into Phono and TV and raise the volume control with an open input. Does the motorboating occur? If so, issue is in Audio. See below discussion on shielding. Also "dress" the speaker cable so it is not near the Audio Ouutput or 1st Audio tubes.
If issue occurs only in radio mode, does the volume control affect the motorboating, or does it only motorboat on weak signals?
Check all shielding, B+ Bypass Caps for RF, Conv and IF. Check C3. This is the cap between the AVC Line (Returns of secondaries for IF, Convertor and RF transformers) and Chassis Ground.
Verify that all tube shields are in place. If no shields were used (No collar around socket), verify that the tubes are metal or are the Zenith "metal glass" tubes. These tubes have a grey or black coting on the inside walls of the tubes. This Shield for the "Metal Glass tube or the shell of the Metal Tube must be grounded. These are almost always Pin 1 and either are connected to chassis ground or jumpered to the Cathode.
Hope this helps.
RE: Zenith 7s488 assistance request - Rob Quay - 08-21-2022
Got it! The radio is working very well- no hum and good reception. The final step was checking the wiring at the volume control and cleaning up the tube shields. There’s still a thrill for me when these relics from the past come back to life! And that’s after sixty some restorations!
This particular radio was bought by my grandparents in 1940 and was their WWII information source. Thanks again for your excellent advice! - Rob Q.
RE: Zenith 7s488 assistance request - Paul Philco322 - 08-21-2022
Allright Rob! Yes it is always cool when they play again, think of what came through that speaker over the years.
Paul
RE: Zenith 7s488 assistance request - MrFixr55 - 08-21-2022
Big Thumbs up, Rob!! Some of my collection is from my childhood, given to me by neighbors. I learned electronics on these. Others Were from relatives. The interesting stuff (too many to list, some of it is under my profile) came from The Bay. In all cases, while working on these, you can picture what news, entertainment education, religious services , excitement or even distress the original owners may have heard. The fact that this was your parents WWII Set, back in the days when shortwave was that generation's internet makes heirloom even more precious.Again, Congrats on your success!