[vecher]

All, Just completed the Recap and out of Tolerance Resistors.. ( All of them) ..

Radio brought up on Varaic and at 115 Volts draw is 400 Ma.. Radio does work.. Receives many stations.. However there is a LOAD Hum.. Doesn't change volume.. I even added caps to C22a and C22b..  Original values are C22a  50 UF and C22b 30 uf  ,, I added 22uf to each with NO change in Hum..

Here is Radio and schematic..

[Image: http://i292.photobucket.com/albums/mm38/...4axjk5.jpg]

[Image: http://i292.photobucket.com/albums/mm38/...aysmrf.jpg]


Any clue on how to get rid of the LOAD Hum..

Skip...

[morzh]

if you rotate the power plug, does it change anything?
Does changing 25Z6 change anything?

[vecher]

I did try reversing plug No change I will look and see i have another 25Z6

Skip...

[Radioroslyn]

Remove C-24, the schematic is kinda goofy. I would think that C-24 should be connected from the plate back to the heater not the cathode. The way it's shown it's coupling the ac line to the dc output of the p/s. Not good hum like crazy!

[vecher]

Ok removed Cap Still same problem Load HUM ..

Maybe I should try adding more Capacitance again ??

Skip...

[Radioroslyn]

Ok Do you have the + side of the filter caps connect to R-13 and the - side connected to the off/on switch? As Mike suggested try a new 25L6 and also a25L6 (may have a heater to cathode short).

[vecher]

Terry, OK correct R13 is across the caps. Neg is isolated from chassis and goes to switch..

I will see if I have some tubes to sub...

thanks

Skip..

[Radioroslyn]

The thought is that the tubes that have the most cathode dissipation are a bit more likely to have cathode to filament  or grid leakage causing hum. Those tube being the 25Z6 and the 25L6 @7.5w all the other are abt 2w.

[morzh]

Here's something I don't like the way the sch is arranged: the output stage is powered from the very first cap C22, before the R13, which will have lots of ripple. In a different radio they had a 2-pole filter with an RC after the what would be in place of C22, with a smaller resistor, and then another RC to power the rest.
I wonder if that C23 is there to counter that ripple a little.
The max C22 can be is 40uF, so you cannot increase it much.
Adding another RC between it and R13, and then powering the output from that point would be interesting.

[morzh]

Kinda like it is here:

http://www.nostalgiaair.org/pagesbymodel...007442.pdf

[vecher]

I think that C22a is 50uf and C22b is 30 uf at 150 volts.. I will have to go out and look its in garage ready for alignment when I get rid of the HUM...

Skip...

[vecher]

Well Of course I cannot find any 25Z6 's I found 3 25L6's

So will have to make a trip up to Joe's friends shed to see if any are up there..

So for now its on the back burner util I can get some replacement tubes..

Thanks to all

Skip...

[morzh]

Try 25L6. 25Z6, now that I looked at the hookup again, won't affect anything much.
And if you are up to it, try that idea with 100 Ohm and extra electrolytic cap.
You could possibly leave the existing filter in place, disconnect only the wire that goes to the output xfmr primary and connect R=100 Ohm (1-2W for the experiment) and C=30uF to 100uF (with proper WV rating) between them, the cap being on the xfmr's side and the resistor going between that and where the wire used to go.

[vecher]

Ok subbed a different 25L6.. Hum almost totally gone.Can hear some but as soon as you turn up volume a little you cannot hear it anymore.. So I tried the IF alignment.. Worked good. Then the RF alignment on AM band Also good...

NOW is were it starts to go wrong As soon as I switch to SW band I get a VERY load Motor boating sound Doesn't change with volume..

So I assume its somewhere in the SW Osc circuit??? But how do I trouble shoot this I am afraid it will blow speaker..

Skip...

[Radioroslyn]

Try replace the 6SA7 and or detune the primary of the 1st IF transformer.