[morzh]

Curiouser and curiouser!

[dconant]

The radio played on the signal generator connected to the antenna terminals all day yesterday. Today I connected the outdoor antenna to the radio and it played normally for about a half hour when it did its thing. How can this be and why did the signal generator work so well at 100mv on 1600kc? Station I was using at the time of loss of reception was 740kc. Wonder if lower frequencies are causing the problem?

To be sure it was not my antenna I took the radio to a friend's house who has a long wire antenna and it did the same thing there. Did that about two week ago.

[morzh]

Have you tried it at the same frequency?

1. Try at 740kc.
2. Try it with the same type of antenna you usually use and the generator with a dummy antenna (load it with a loop or something) so it acts as a transmitter and not like wired signal source.

[dconant]

Turned radio back on tuned to 740 and within one minute is did it's thing. Made a loop for sig gen set to 740kc put loop near antenna wire and heard sig gen signal loud and clear but station is not heard even after taking away the loop.

[MrFixr55]

Try measuring AVC voltage under the conditions in your last posts. Remember, that unless you are about 5-10 miles from a very powerful AM station, the output of your signal generator or home AM transmitter connected to an antenna to transmit its signal is going to have a much stronger signal than a commercial radio station.

Where is Marion, NY? There are 2 stations in eastern North America on 740KHz; WNYH in Huntington, Long Island, NY, and CFZM in Toronto, Ontario, Canada

I am in Long Island and 740KHz is WNYH, a religious radio station within 10 miles of me that puts out about 25KW during the day and cuts back to 43W to protect the signal of CFZM, a 50KW adult contemporary / oldies rock and roll station in Toronto.

[dconant]

AVC voltage in disabled state is -7 volts.
AVC voltage with sig gen added is -12 volts. Sig gen is at 100mv.
Marion is about 20 miles east of Rochester. My 740 is Toronto. I have tuned to 1180 WHAM (strong Rochester station) with same results.

[morzh]

The AVC in disabled state has changed. It was around -2.5V when disabled and about -5.5V when working tuned at the station.

[morzh]

Oh....and another question.

Does it happen to all stations or just one local one at 740 kc?

[dconant]

I tested two stations, 1180 and 740. Yes the AVC has changed but I don't know why unless it was from Candohm replacement.

[tbone]

I am beginning to wonder if the signal generator level was set too high throughout all of the previous tests, which is why it never seemed to drop out. What leads me to believe this is the simple fact that it was stronger than the stations that drop out, the last test in particular. Usually the generator is set to produce a weaker, but audible signal. 100mv is really high, especially connected directly to the radio.

-7v AVC with no signal is really puzzling. What happened?

[dconant]

After playing with my sig gen I find I can go as low as 2mv. What would be best?
Yeah I don't get why the AVC is higher than it was earlier in my posts. I did replace the Candohm so I thought maybe that is why.

I will be out the rest of the day so I will lower sig gen and test tomorrow.

[morzh]

Is AVC at -7V with both weakd and strong sig gen levels?
And with the station playing through the antenna?

If he sudden change correlates with the Candohm change, that could be it.

[tbone]

Checking the AVC voltage to its source may help you identify where the -7V is coming from. That much of a negative voltage will not allow you to hear the normally weaker signals a good working radio does. This likely needs to be fixed before proceeding.

Once back to normal AVC, my suggestion is to turn down the signal generator until the audio disappears, the bring it up till the audio is clear but no more than that. Then wait and listen to what happens. Normally very strong stations could possibly give you a mv of signal, but most are in the uv range.

When you align the IF and RF stages, normally you only want enough signal to be heard and can be measured while tuning. This insures no AVC action is skewing your readings.

Swamping the radio with too much signal is not the answer in my opinion.

Food for thought.

[tbone]

Please see the voltage chart I took from the Zenith Service Manual. It’s easier to read than the Rider schematic. Looks like measurements are from chassis ground. Pin 9 would be the grid cap. Hope it’s useful.

   

[MrFixr55]

You should be able to get a rather negative AVC voltage. If you have a similar radio, make similar measurements of cathode current, plate and screen voltages and AVC voltage upon direct signal injection, then compare the AVC voltage under these conditions between the good radio and the radio with issues.