The PHILCO Phorum
38-7xx Low Volume - Printable Version

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462ron - 462ron - 09-04-2025

According to the service info it says voltages are measured with a Philco 026 circuit tester. I’m not familiar with that tester so not sure if it’s a VTVM or a VOM. My understanding is a VTVM usually presents a certain fixed resistance or load to a circuit, usually 10 megs regardless of what range it’s set on whereas a VOM is rated at a certain ohms/volt like maybe 10K/volt so the load it presents to a circuit under test changes depending on what range it’s set to. A VTVM or DVM probably are 10M regardless of what range it’s set to.

 As for the finicky volume change, check audio shorting switch #29 on the schematic. It is used to short audio to ground while rotating the cone centric dial to eliminate noises and audio until you release the knob once you are on a station. Just a thought!

Ron

RE: 38-7xx Low Volume - RGK20m3 - 09-04-2025

I cleaned, lubed, and adjusted the shorting switch and it works as designed; I also removed its connection at the volume pot and still has issue. I replaced the 2nd detector/AVC tube, removed the grid reference and volume resistors from the AVC tube socket and terminal strip, and cleaned the phenolic between, then resoldered- now it seems to be generating an AVC voltage ( more negative at the AVC buss than the fixed bias), but still have issue of the volume dropping. Going to start using a signal generator to try to pinpoint what is actually going on. Stay tuned (pardon the pun)…..
What confused me on the Fluke was the “or less”. Do they mean less loading (higher resistance) or less resistance (higher loading)? At any rate, the DMM is showing some AVC voltage generation where it didn’t before (previously was less negative than the fixed bias). But still fades out, probing the B+ brings it back frequently.

462ron - 462ron - 09-04-2025

RGK, that statement of “10M or less” has me baffled too, I would think it would be a constant 10M. Not sure if there’s a difference between AC or DC settings. At this point I think your next step of using a signal generator or as MrFixr suggested using a CD player or MP3 would be a good choice. A signal tracer is another good method or an oscope if no tracer is available.

Ron

RE: 38-7xx Low Volume - DaleHCook - 09-04-2025

Ron Wrote:  According to the service info it says voltages are measured with a Philco 026 circuit tester. I’m not familiar with that tester so not sure if it’s a VTVM or a VOM.

The manual for the 026 is in the Philco Radio Test Equipment Manual (1944) which I posted here a few years ago.

https://philcoradio.com/library/index.php/documents/philco_docs/manuals/philco-radio-test-equipment-manual-1944/

The 026 is a 666 ohms per DC volt VOM. As Ron noted the input impedance varies with range. To determine it multiply 666 by the maximum voltage of the DC range you are using. Because of that readings with be higher when measured with a VTVM, an AMM, or a DMM.

RE: 38-7xx Low Volume - RGK20m3 - 09-04-2025

Well, here is a new wrinkle-
Still haven’t hooked up the signal generator. But I decided to try a stronger broadcast signal and hooked up my milliwatt transmitter to see if a really strong signal might confirm AVC operation. Hooked the DMM to the AVC buss and voltage went to -9 volts on carrier, and dropped to -2.7v off carrier; when program material transmitted, reception was clear and of adequate volume. Decided to probe the B+ and audio volume did not change, but AVC voltage went to -13v
This to me indicates that the AVC is working, the audio stage is operating normally, and that the volume/gain drop is in front of the detector (as the AVC is compensating for the gain change).
New wrinkle is that the radio has been playing fine for a hour, and AVC is a steady -12.8-13v. Remember before I probed the B+ it was -9……

RE: 38-7xx Low Volume - 462ron - 09-04-2025

I restored a Philco 620 many years ago and played fine. Now recently developed a symptom like yours where the volume would drop significantly. If I wiggle the band selector up and down the volume returns to normal. Apparently a touchy or dirty contact on the band selector switch which I haven’t got around to looking at yet. This set worked great for years ! Sometimes it’s the easily overlooked things.

Ron

RE: 38-7xx Low Volume - RGK20m3 - 09-04-2025

Could be something like the band switch; I did hit it with Deoxit and worked it a bunch of times- also have switched bands a few times, and resoldered a terminal or two. Doesn’t mean it isn’t intermittent or leaky.

RE: 38-7xx Low Volume - RGK20m3 - 09-05-2025

After listening to the radio for some time, the gain drops often; there is a subtle change in the audio, and the AVC voltage drops. Probing the B+ restores the audio quality and the AVC voltage increases. This cycle sometimes is a couple of minutes, other times hours. I suspect something is going on with one of the IF trimmers and am going to see if it will align normally.

RE: 38-7xx Low Volume - MrFixr55 - 09-05-2025

Reminds me of a Model 89 that I wish that I could forget, that would not playt at all, untill I measured anything, it would start.

One thing to check is the physical condition of any device such as a compensating capacitor that has an insulator such as mica. Look for cracks, dirt, grease, nicotine, 87 years of environmental pollution, etc.

Using the AM transmitter was genius on your part, glad that you were able to measure AVC voltages. If nothing else, you now know how AVC works.

Try the "Shock Test". Tap the chassis with the handle of a screwdriver. If volume changes, this confirms that there is a mechanical issue. tap more lightly and tap individual components, tapping as light as possible ill you tap the one that causes the issue.

RE: 38-7xx Low Volume - DaleHCook - 09-05-2025

MrFixr55 Wrote:  Look for cracks, dirt, grease, nicotine ...

Nicotine is not a problem. It is an odorless, colorless, and transparent chemical. Tobacco tar is the problem.

RE: 38-7xx Low Volume - RGK20m3 - 09-05-2025

Well this one certainly had 87 years of grime, I will clean and align, and keep you posted.
Tap test really didn’t reveal much, done before.

RE: 38-7xx Low Volume - MrFixr55 - 09-05-2025

Good luck with this and keep us posted. Don't forget that these sockets can be squirrelly, though the tap test should have detected this. Did you start by tapping "heavily"?

RE: 38-7xx Low Volume - MrFixr55 - 09-05-2025

You are right, Dale. The tar may be the stink. I have not tested the conductivity of nicotine, but as a militant non-smoker whose parents died at the age of 54 and 68 due to complications caused by smoking 5packs/day of Camel non-filtered cigarettes between them, I may easily misspeak. When Ms. Fixr and I bought our house from a family of smokers, we rented a rather powerful wallpaper steamer to remove the disgusting wallpaper, more disgusting cigarette residue mixed with the condensate from the steam (We were doing this in winter) rained down from the ceiling.

Early in my career, I was a clinical lab technologist in a NYC hospital. The histological examination of a smoker's lung vs a non-smoker's lung was absolutely an education.

RE: 38-7xx Low Volume - RGK20m3 - 09-06-2025

I performed the tap test again, with inconclusive results. I went ahead and connected my signal generator and was able to easily peak all 4 IF trimmers using the AVC voltage. At first screws were stuck and there was some crackling as they were turned but that went away as they were exercised. So far, no volume drop and selectivity has improved. I am thinking that there may have been a little oxidation on the trimmers.
I will start a new thread for antenna ideas….

RE: 38-7xx Low Volume - RGK20m3 - 09-11-2025

I am going to consider this radio to be working pretty close to as designed. Been listening off and on to it for some days with no volume drop after the alignment.
Yesterday I finally hooked it up to my milk crate tuned loop and after sunset picked up WHO in Des Moines, WBZ in Boston, and WNIS in NYC; as those are close on the dial I am happy with both the selectivity and sensitivity.
Couldn’t get a consistent signal on WSM Nashville, WLW Cincinnati, or the Montreal station at 690- so maybe a little weak at the lower end of the dial. Couldn’t get a just be atmospheric conditions….