Admiral 20B1 Combo restoration

[SwizzyMan]

I have but I have not completely anchored down the tube and the grounding bar is not making contact with the crt. I will investigate more tomorrow as it is getting late.

[SwizzyMan]

Checked my HV again about 7.5 to 6.9 KV. Should be giving me a raster at least! Messed with the ion trap still nothing ugh is the CRT too weak?

[SwizzyMan]

OK finally traced that smoking smell. It was a completely shorted out canned ohm resistor in the power supply. I saw smoke rising out from below the 40/40 filter cap and went to investigate (pulling the plug of course). I lifted up the metal chassis and get an intense burn on my index finger dropping the chassis cussing out the pain I noticed the metal directly under the canned ohm resistor on the chassis was RED HOT No wonder my skin was blistered and red! I have a reasonable suspicion that the resistor may be what is dragging down my high voltage. Know the set was on for about 2 minutes. I hope that didnt hurt anything. Hopefully that may fix my no sound too?

[Electronic M]

Canned ohms like to short to chassis.

[SwizzyMan]

Replaced the canned ohm resistors in the power supply. Plugged it in and the 5u4 immediately started to red plate. I pulled the plug and the tube was extremely hot! what did I screw up?

[Gregb]

My first thought would be a shorted cap down stream of the candohm resistor.

Gregb

[Eric H]

I'm guessing a short also, possibly why the original resistor was overheating.

Backwards cap could do it but it would likely explode at some point so possibly something else is shorted or wired wrong.
Start disconnecting the B+ circuits one at a time and see where the short disappears.

I always take some high resolution digital pictures before I replace anything, that makes it easy to check for mistakes if things go wrong (and they always do!)

[SwizzyMan]

I traced everything out and all looks well... The plates are glowing and looking at the schematic the canned ohms and the filter cap have no connections with the plates at all.

[bandersen]

That's the rectifier tube with it's plates going to the transformer secondary transformer winding. It's red plating because the load on it's cathode/filament is too heavy.

[SwizzyMan]

Just going to go over it this week or weekend disconnect the filter cap from the 5u4 and see if that solves it.

[wa2ise]

Originally Posted by bandersen

... plates going to the transformer secondary transformer winding. It's red plating because the load on it's cathode/filament is too heavy.

Don't let it run like that for too long, minutes at most. It will ruin the transformer. Check for shorts on the B+. Replace the filter caps if you haven't already.

[SwizzyMan]

Went through it all. Everything read ok. So I switched the wiring on the filter cap lugs and the 5U4 is still red plating! What can I do to fix this!?

[Zenith26kc20]

I would disconnect the wire from the 5U4 to the capacitor connected to the 5U4 filament/cathode and turn the set on. Measure the voltage and watch the tube carefully. It should have some DC but without anything connected to the output of the 5U4 it will be low. Make sure the wire goes to the + terminal on the electrolytic. If voltage is there, turn the set off and check for a short on the + terminal of the capacitor.
If you have a variac it can simplify troubleshooting power supplies.
Also, check the capacitors that you disconnected to be sure they are the right voltage rating.
If all goes well, connect only that capacitor to the 5U4 to see what the voltage is. As stated earlier, if anything starts to overheat, turn it off and look for a short or a backwards capacitor.

[SwizzyMan]

Disconnecting the filter cap lead made the red plating go away. Now I need to check if I wired the filter cap correctly.

[old_coot88]

Troubleshooting by red-plating a tube is very bad practice and ought to be nipped in the bud. Or at least use a metered variac to avoid brutalizing the tube and unobtanium power xfmr.
http://videokarma.org/showthread.php?t=265257