[Jamie]

   

I'm just starting work on a Philco 60B. Today I installed new E-Caps, and I put them in according to the photo of the schematic. The red marks indicate the negative sides of the caps. I need to know if this is correct.

Cap number 46 overheats to the point of exploding. I used to replace every cap and check all resistors before powering up a radio, but lately I've been doing one thing at a time. The only thing I've done to this radio are the electrolytics. Here is a link to the entire schematic...

http://www.nostalgiaair.org/PagesByModel...013793.pdf

If I've installed the caps correctly, what should I be on the lookout for that would cause cap 46 to overheat? The radio looks about 95% original, with the exception that, years ago, someone previously replaced the electrolytic caps. They did a hack job of it, using broken off bakelite block terminals as wire connectors.

[Bob Andersen]

Yes, you have the negative correct. The only thing I can think of is it's a bad cap or there's too much voltage across it. I'm working on a 60 right now and used a 450 volt cap.

[Mondial]

If it is installed correctly and operated within its voltage rating, then its probably a bad cap or the radio is drawing excessive B+ current.

You can check the current draw by measuring the DC voltage across the field coil. If it is more than 50 or 60 V then the drain might be excessive due to a leaky coupling cap between the 75 tube plate and the 42 grid. A leaky cap here will throw off the bias and cause the 42 to draw excessive plate current.

If the drain is normal them the cap is probably bad with a high ESR. This is the internal series resistance within the cap, and causes heating with a high ripple current as found in the circuit as connected right after the rectifier.

[Jamie]

Thanks guys,
It doesn't even give me enough time to check current draw. I turn on the variac, and as soon as I roll the volts up to about 90, the 46 electrolytic starts smoking. I was concerned that I might have it in there backwards. I've tried two different E-Caps in there, from two different batches... and both smoked. So, hopefully, you guys are right and there is a leaky cap elsewhere that's leading to high voltage. The other possibility is that whoever replaced the electrolytics screwed up the wiring somewhere... and I sure hope that's not the case. Everything looks right, but I guess I'll find out..

I've installed a fresh electrolytic in there, and I'll recap the entire chassis and check all the resistors before I apply power again.

[Mondial]

Before you go to the trouble of recapping the whole set, check to see if you have a short to ground on the B+ line with an ohmmeter. If the cap gets that hot, either you have a short across the power supply output or the cap has reverse polarity applied.

Its also possible you have a gassy or shorted rectifier tube which is putting AC into the first filter cap. Do you get a loud hum when the cap is overheating?

[Jamie]

No hum, no noise, nothing... to be honest, it doesn't even have time to warm up the tubes to put out any noise. It happens almost instantly. I get maybe 5 seconds after I hit 90 volts on the variac, and there is smoke. The rectifier tube tests good, which doesn't mean much. The only weak tube is the 42, which I think is the output tube. It's not a bad tube, but I'd typically replace it once the set is up and running. Nothing else gets hot, just that one electrolytic.

I'll look for a short in the B+.

If you look at the photo I posted, No 47 is a big wire wound resistor. Very easy to find, very easy to trace the wiring. I've got both negatives running to 47, and I've double checked it 20 times, so I hope polarity is correct.

[Mondial]

Does resistor 47 get hot at all? if you had a B+ short, the current would also be passing through it and it would also get at least warm.

You can confirm proper polarity by connecting your DVM across the cap in question, with probe polarity matching the cap marking. Then slowly turn up the variac and watch if the voltage goes positive or negative.

[BrendaAnnD]

A short in the B+ line shouldn't cause that cap to overheat. A short would drag the voltage down and so put less stress on the cap. What COULD cause it to heat immediately like that would be if one side of the transformer HT winding were shorted to ground. As said before, whether your error or an error in manufacturing, the cap polarity may be reversed (I've done this trick before while swearing I installed it correctly. More recently, I pulled a pair of 10 mF/160 V caps and installed them into a radio with a 265V B+ line... )

At any rate, I would pull the rectifier tube and check your transformer HT voltages to the chassis. Also check the resistor (47), as if there is a transformer short, it would likely be open.

[Jamie]

Ok, I just tested polarity. Voltage goes positive. I only turned the variac up to about 30 volts. I was nervous about ruining my last cap.

When it overheated before, It was too hot to touch for at least two minutes. During that time, I touched resistor 47, and it was not hot. I probably waited 30 seconds before I touched it, so it may have cooled off... but if so, it never got real hot.

[Jamie]

Brenda, resistor 47 checks out at 32.3 on the short side and 239 on the big side.

All of my electrolytics are rated at least 450v. The first one that burned up was a radial, the second was axial, otherwise I might suspect some bad caps. I get all my caps from justradios.com and I've never had a bad one, but I suppose it's possible.

I'll pull the rectifier and check voltages.

[Jamie]

Brenda, if I pull the rectifier to check voltages, is there a chance this cap will overheat again while I am checking? I only ask because I am out of caps, until more arrive. I did a continuity test from every pin on the rectifier to the chassis and got nothing.

[Mondial]

What are the resistance readings from each contact of the rectifier socket to chassis?

The resistance from each rectifier plate contact to chassis should be equal. From the schematic, across the two plate contacts you should have about 275 ohms.

[Jamie]

Pull the tube before checking or leave it in?

[Jamie]

I pulled the tube...
Ok, across the plate contacts I get 314.5 ohms.

From one plate contact to chassis I get .418k from the other .437k

[Mondial]

OK, so far so good.

Now measure resistance from the rectifier filament contacts to chassis.

Then measure from each rectifier plate contact to the negative leads of the filter caps.