No grid voltage in model 20 - Printable Version
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No grid voltage in model 20 - Danny - 10-01-2007
1st and 2nd RF has no voltage on grid. Coils #2 and #7 are not open, primary or secondary. No issues with volume control and I've replaced all caps including filters and all resistors. A bit lost here. Can anyone give me a hint as to where to look next ? I'm done for the night but will look into forum tomorrow morning while at work. Thanks in advanced.
Here is link to schematic.http://www.philcoradio.com/tech/images/20a.jpg
- rickd - 10-02-2007
Danny, Assume you're measuring with a scope, VTVM or DM with a high input resistance. Anything else will load the circuit so much, you won't see anything. Also assume coils do show some resistance. Any chance there's a little leakage across the variable caps or their parallel caps? Sometimes those 24's take a long time to warm up. If you touch the grid of the second RF, do you get noise? Good luck.
- Danny - 10-02-2007
Using a Fluke 23 DMM for most all measurements. Resistance for coil #2 24
ohms for pri and 7 ohms for sec. Measured other coils but forgot to record in
my notes...They are not open or shorted I can tell you that much anyway.
No noise on the 2nd RF at all. Here is link to picture of variable cap and you
can see the compensating condenser, one on each stage. Leakage across
variable cond ? Not sure what you mean. Thanks for your time and assistance.
http://img182.imagevenue.com/img.php?loc=loc71&image=36963_Chassie_top_restored_122_71lo.JPG
- rickd - 10-02-2007
Danny, What I meant was any measurable resistance (less than infinity), across the variable cap and the associated padders (nos 4 or 9) could kill the grid voltage developed by the secondary of the coil. Might have to disconnect the coil to check this. Another possible problem might be poor pin/socket contact. If plate and cathode voltages are good, don't see why there is no grid voltage. Might have to wait on the experts to chime in.
- Danny - 10-14-2007
Rickd....I had to make a quick trip out of town and had not been able to check what you had suggested... your suggestions were spot on. The mica on two of the three padders had cracked because I over torqued them. I had them apart and cleaned everything and they were not falling apart when I reinstalled them. But they sure were over tightened because they fell apart when I backed off the adj nut. Replaced them and low and behold up she came. Just finished reinstalling tuning hardware and did a little tweaking with sig gen. Listening to reruns of last weeks Rushbo on my first FrankenPhilco. Thank you for your suggestions and time.
- rickd - 10-17-2007
Danny, That's great. I was out of town also. Glad you found the problem.