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Philco 71B Code 125 speaker thumping - Printable Version

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RE: Philco 71B Code 125 speaker thumping - morzh - 02-12-2026

See if switching tone control (BTW have you replaced the cap...39? is it?) changes the pattern.

Also, put a Voltmeter between the chassis and the plate of every tube starting at the 42 tube, and see if you see large voltage oscillation (an analog meter is better at this, more visual) at that plate. Move back tube by tube, see where it originates.


And. Remember this (I try to explain it to everyone): troubleshooting BEFORE the full recap is a waste of time.
So, pick one backelite, unscrew it, turn 90 degrees, see if it is re-stuffed. If not, well....see the above.

RE: Philco 71B Code 125 speaker thumping - dmblackburn - 02-13-2026

I removed all the vacuum tubes and tested. All were good. The 80 tube was ?. 
I got the thump gone when placing my  finger on the grid cap of the 44 (IF). I had to slightly move it around a bit.
Only thing remaining is a mild hum.

RE: Philco 71B Code 125 speaker thumping - morzh - 02-13-2026

what if you just wiggle the grid can clip (not with your fingers - touch it with some plastic rod) - does anything change?

RE: Philco 71B Code 125 speaker thumping - dmblackburn - 02-13-2026

Plastic rod or a wooden chopstick has no effect on the thump. Placing my finger on the grid cap of the IF (44) eliminates the thump but strong stations have a very low volume and I must hold my finger on the cap the whole time. If I remove my finger from the cap, the thumping resumes. When I turn the volume knob all the way up, I am getting a shrill squeal (without holding my finger on the cap). When I hold my finger on the cap (at increased volume), thumping is gone, and I have pretty clear reception but not the full volume that you would get with a normal set.

RE: Philco 71B Code 125 speaker thumping - morzh - 02-13-2026

I was trying to see if the grid cap has bad cold solder spot , so if you press it with a rod something changes pr it doesn't.
Finger is connected to you and presents all kinds of things, from capacitance to GND to an antenna signal.

Tracer would be nice. Or a scope.

Could you possibly swap the two 44 tubes places? (RF and IF)
I understand you tested them, but there are some strange things.

As for the voltmeter I mentioned, in case nothing else is indicative, it could tell us where the oscillation starts.

The oscillator is also a suspect.

PS. Have you confirmed the continuity of all RF coils?

RE: Philco 71B Code 125 speaker thumping - dmblackburn - 02-18-2026

Checked all the voltages and they are within specifications. Rebuilt the tone control with new capacitors and there was no change other than it works now at least for toggling tones.
Cleaned tube sockets with Deoxit and pipe cleaners. Again, no improvement. Tested all tubes and they all tested strong. Resistors were in spec as well.
I had planned next to restuff the Bakelites. I had already done the safety capacitors earlier on.
I started with the filter caps in the metal box. Changed all of these out and routed new wires. Eureka, this fixed the noise and brought in volume and reception.
Not that it would have made much difference, I put the new caps on the scope to find the outer foil and put those leads to ground

I want to thank everyone for their ideas and assistance with this radio. Truly it was a learning experience

RE: Philco 71B Code 125 speaker thumping - morzh - 02-18-2026

So...changing the caps DID get rid of the thumping? (I am not sure if you meant it by "fixed the noise").

RE: Philco 71B Code 125 speaker thumping - dmblackburn - 02-18-2026

Yes, thumping is gone. Stations are coming in clear. Volume is increased. There is a slight bit of constant static but barely noticeable. Also, there is a tiny bit of hum but again slightly noticeable. Do you think changing out the rest of the Bakelites would make an improvement in the sound quality ?

RE: Philco 71B Code 125 speaker thumping - morzh - 02-18-2026

Let me guess....you did not fully recap it?

See my post #16.

I have a rule: I do not discuss troubleshooting before full recap - waste of time.

RE: Philco 71B Code 125 speaker thumping - MrFixr55 - 02-19-2026

Hi DM Blackburn,

Yes, it is possible for that cap block to cause problems. The "Block" contains several "bypass" caps that is meant to prevent the undesired coupling and feedback between stages through the common power supply.

Morzh is correct. Unfortunately, these radios are very old, and the materials used to construct these capacitors deteriorate, sealed in tar or not. Newer caps using mylar or other plastics for dielectric are more stable and durable than paper and oil, etc.

It was very insightful of you to attempt to determine "outside foil" which is no longer marked on so many of these newer caps, even those from "reputable" manufacturers. However, some or many of these caps do not have an "outside foil" effect.

Do go about servicing the Bakelite blocks ASAP, as failure of these caps may actually cause damage. One specific example is the shorting to the bypass cap between plate and cathode r plate and ground of the output tube. This can burn out an output transformer. Shorting of the coupling cap between the plate of the 1st audio tube and the grid of the audio output can cause excessive current draw, "Red Plating" and destruction of the output tube, burnout of the output transformer, rectifier, power transformer, etc. Replacing these caps increases the safety of operating the radio.

When I was a kid, a filament to plate short of a 5Y3 took out a power transformer of an old Philco (I don't remember the model, it was a street find, but as likely late pre-war or early post-war) filled the house with smoke and there were flames when I discovered this. My Mom TOTALLY FREAKED OUT and demanded that I put the set to the curb immediately. Dad, who had the last word in our house (Yes Dear, No Dear, I'm sorry, Dear, etc.) would not overrule Mom's decree, therefore, I was not even able to scavenge tubes, speaker, etc or perform a "post-mortem examination". I don't know if the failure was a spontaneous failure of the transformer, a spontaneous failure of the tube (cathode to plate short), failure of a cap shorting the B+, etc.

2 Mods that I always try to do on transformer powered sets is to place silicon rectifiers in series with the rectifier, between the secondary and the rectifier plates, and to fuse the power input to the radio. This will minimize the effect of a direct short across the secondary of the power transformer if the tube shorts.

RE: Philco 71B Code 125 speaker thumping - morzh - 02-19-2026

I thought it was 6X5 that was notorious for the shorts (K to F).
5Y3 should not be any worse than 80, right? It is a direct heated tube. In essence, the two are not that different, the former having that extra plate resistance.

You got lucky!

RE: Philco 71B Code 125 speaker thumping - MrFixr55 - 02-19-2026

Or unlucky depending on your point of view. I believe that I had another 80 or 5Y3 short on me once. I would think that the older 80 type with the tent type heater may be more prone to failure. The problem with Cathode to heater shorts on 6X5s is that since the heater is powered by the same circuit as the other tubes, that allows for an almost dead short to ground of the cathode. Different illness, same baaaaad outcome.

The 5AR4 is an indirectly heated cathode full wave rectifier tube but is used in a lot of paces instead of the 5U4. The cathode is already purposely connected to one leg of the heater, and an isolated heater secondary is expected to be used, just like the 80, 5Y3, 5U4, etc.