The PHILCO Phorum

My philco model 84 I just recapted and replaced off value resistors has more hum than I think it should but almost gone with 2 detector removed and has some distortion in audio anyone know what could cause these problems?

Well a few things come to mind.
As for the hum try bridging a larger cap across your filter cap. Like 20 or 30 mfd @ 450v. If this helps then go with a bigger filter. Make sure that the negative side is NOT connected to ground but to the center-tap of the power transformer which goes though a resistor the ground. This provides the grid bias for the output tube.

Make sure your 2nd det tube is good and has the shield over it. Not only does it act as a detector but as a very high gain amplifier. It can pickup hum from the power transformer. Any leakage from the cathode to grid will give you hum.

Distortion can be from the wrong bias voltage on the output tube's control grid. Poor alignment, and having the regeneration control set too high.

Some food for thought
Terry

I tryed those things no change guess that is normal for a cheap radio.

Your 84 should play rather well for the kind of radio it is. It will have some audio distortion due to its cheap design, but it should have little if any hum.

Please double-check to make sure the negatives of the filter capacitors are connected to the center tap of the power transformer's high voltage secondary, as Terry has suggested. The 77 2nd detector tube or the 42 output tube could have a heater to cathode short, or leakage? This will cause problems.

Check all of your resistors. I've find that of the 80, 81 and 84 sets I have serviced over the years, that the resistors have almost always changed in value from their original ratings. If they deviate more than 20% from published specifications, they need to be changed.

Most of the hum is gone when the 77 tube in sheild is pulled out of its socket I have tryed 3 77 tubes with no change I changed all off value resisters which was all but one or two. Hum is not to noticeable when tuned to a station.

The service bulitation show a humbucker but I dont see one on the speaker looks orginal speaker.

Your hum might be just interferance . From light , almost anything can register a hum in theses old sets . My model 111 and 48-1253 pops hums to lights being turned on ect.
Try grounding it helped mine alot .
Kent. Oh plug it into a outlet separated from other devices .

I have changed the 4 meg resistor to 2 meg as on philco repair bench it play louder but still has the distortion and hum tried unhooking the winding used as the grid cap replaced it with 120 pf cap but still has the distortion and hum tried a 56 pf cap with no change. I read that a smaller grid cap would reduce distortion but it did not for me.

Any more ideas

Hi Vinzer,

Have you made any basic voltage checks in the audio section?

If the audio output is not negative biased enough, excessive current draw will result. This can overload the power supply and result in hum. And distortion will follow too. Would expect to see at least -15 volts on the control grid of the 42 tube for normal class A operation. Would also verify all the tube pin voltages including those on the 2nd detector tube. Hope this helps.

Richard

Yes I have -16.5 volts on grid of the 42 all voltages are close to what they should be. I found the cause of the hum today the ground for both 77 were hooked to a tube socket rivet and had 20 mv of ac on them. I ran a ground wire to a good ground might have help distortion hard to tell at night. yesterday I unhooked wire from the if to grid of second detector and put a 4 meg resistor to ground and a .047 cap and used a radio to drive the audio amp and there was no distortion.

I've run into a couple 84's that hum when everything checks good and the problem turned out to be a bad ground on both of them.

If memory serves me correctly, there is a ground lug attached to the 42 tube through one of the tube socket's mounting rivets. The mechanical connection corrodes and you have hum.

Hope this helps

Ed

Hi Vinzer,
I second the motion to check all the rivets used as tie points to ground.

In '06 I also recapped a Philco 84, and still had a hum. Not so much of distortion though. Turned out to be the ground tie point on the 77 tube. As Ron says, this set should perform pretty well for what it is.

When you think of it, there are two surfaces where the failure can be. Lug to rivet, rivet to chassis on the other side.

Bad rivet to ground connection.


IIRC, this connection was showing half an Ohm, which in the scheme of things is not a good connection. A simple jumper wire to ground as a test showed this was the bad rivet.

As a precaution, I soldered a small wire to ground on all the other tie points. You may have more than one bad connection causing distortion.

You could test each rivet to ground, just for fun, and see if you have one that shows even the slightest amount of resistance. I like to try and pinpoint where a problem is.
You can then do them all as a preventative measure
Here is my original thread on the 'alternative' site.
http://antiqueradios.com/forums/viewtopi...ight=rivet
A couple of years later, I recapped a Truetone that displayed the same hum. Checking the ground and tie points again solved the problem. I think that this may be a common issue, and one should check these points on every set we restore.

Hope this solves the problem,
Gary.

The 77 ground was the hum cause on mine to checked all other grounds.
Still have not found the cause of the distortion it seams be worse on some stations than others.

Thanks vinzers

Your distortion sounds like the I.F. regeneration control maybe is not set right.
Since the set does not have AVC, strong stations may "blow thru" the IF and DET stage and cause the audio distortion.
Be sure to follow the Philco alignment instructions.

Chuck

I changed the 4 meg resistor to 2 meg and followed the Philco alignment instructions then put 4 meg resistor back in and realigned still had distortion changed back to 2meg and unhooked the winding that is used as grid leak cap. and have tried different value caps no change in distortion.

Other things to consider.
Bad speaker voice coil rubbing?
Easy sub. Just disconnect the VC wires ONLY and hook to a PM speaker to check.

Partially shorted audio output transformer?
Harder to check without a full sub.
D.C. resistnace measurements may show significant difference from
service bulletin. Check both Pri and sec windings.

Changed the 42 audio output tube?
Antoher easy swap and check.

Improper grid bias on the 42 output tube. BE SURE the (-) end of the dual 8/4 uf electrolytic cap DOES NOT connect to ground or chassis.
A common mistake if you don't watch the schematic carefully. The negative end connects to the jct of the powers upply transformer secondary center-tap and R30 and C28. See schematic.

Have a scope? It would be easy to see where distortion is starting and trace.

Chuck