March 20, 2020 at 3:13 pm #13466Les DicksonCVRS Member
Working on my Deforest Crosely 851 York/Mayfair using my signal generator to isolate the problem(s) and I’m at the outer edge of my understanding so I’m looking for advice.
Working from the output tubes toward the antenna I have success here:
- 400 cycle AF signal into plate and grid of both output tubes (247)
- 400 cycle AF signal into plate and grid of audio tube (224)
- 175 IF signal into the plate and grid of detector tube (227)
Then when I inject the 175 AF signal into the plate of the IF amp tube (58) I get nothing so my question is; does this mean the problem is in the IF transformer, or could it be in other circuitry related to the IF transformer?
I’m attaching a schematic and a photo of the IF transformer (yes, it’s dirty). I’m not getting capacitance values from either of the caps in the transformer (my multimeter may not be equipped for accurate readings for low capacitance). The caps look more robust than what I’ve seen in AA5 type radios. The mica in both caps looks good and I’ve tried adjusting the cap to see if I can get any readings. I don’t have the ability to test the coils in the transformer but i get a reading of 150 & 157 ohms measuring across each coil which could be high from what I’ve read.
Attachments:March 20, 2020 at 5:07 pm #13487Alistair ThomsonCVRS Member
You probably won’t be able to read the very low values of the trimmer caps unless you have a very good meter. Even then, stray capacitance in the meter leads will make accurate measurement difficult.
You might be able to check that the secondary tuned circuit is working in the transformer, i.e. the coil attached to the grid of the 227 detector. When you inject the IF frequency into the grid, tuning the trimmer on that side should affect the signal output level as it swings in and out of tune.
If that happens, the secondary is probably fine and, since the primary shows a similar DC resistance, it’s probably fine too. However, I agree that a value of around 150 ohms for an IF transformer winding seems high. Certainly at this low IF the inductance needs to be higher than for an IF of 455KHz when being tuned by a small mica cap, so that implies more windings, with a greater DC resistance.
Anyway, it’s difficult to see how these windings could go wrong.
I assume you’ve verified the correct supply voltage on the plate of the IF amp. Have you tried removing the IF amp tube (in case it’s faulty) while injecting the IF into the plate connection? That ought to get through the transformer to the grid of the detector.
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