[Guest]

An early model Philco 90 (45 P-P output) is on the bench.
1) The voltage readings I'm getting, even allowing for a modern
meter, don't make sense. Is the chassis supposed to be the
common point to take the readings?
2) The radio is sensitive enough, but there's a peculiar distortion
that shows up on some music, especially that with a strong bass
line. It's really evident on something with a Hammond organ background.
I'd describe it as a hum modulation but it's lower than 60 Hz, more
like a gargling effect. Straight spoken voice sounds great. Is this
intermodulation distortion, and am I expecting too much of 1931
technology?
Cheers,
Nelson

[Ron Ramirez]

Hi Nelson

You will find that a set's voltages, as read with any modern DMM, will read higher than what is published in old service literature. This is because in 1931 when the 90 was new, the voltage measurements were taken with meters that tended to load down the circuit under test. Now when we test those same circuits today using modern multimeters, we get higher readings than they did back then.

That having been said, are your voltage readings significantly off? Yes, you should measure your voltages using the chassis as common.

Now let's discuss the sound quality. The early 90 with push-pull 45 tubes used what is called a power detector instead of the diode second detector which came into general use later, along with automatic volume control (AVC). In your radio, that is the 24 tube just before the 27 first audio amp. One of the characteristics of the power detector is distortion. So you can't expect pure high fidelity sound from any radio with a power detector.

I don't think you should hear gargling noises, though. Maybe your voltage measurements may indicate a problem?

[Guest]

Thanks, Ron.
Do you know offhand what the resistance of the early test sets might be? I have a voltmeter here that's about 20k per volt, but
the readings I got with it were quite close to those of the DMM. Line voltage here is about 117VAC. Most of the plate voltages I'm getting are, for example, 284V rather than the specified 250V for the 1st RF. However, the info I have specifies 48V on the 2nd detector plate; I'm getting 247V! Maybe a "2" is missing from the literature? And all the cathode voltages listed at 15V are measuring at anywhere from 2V to 7V.
The only thing I can think of is to lift connections and measure the power resistor. It appears to be OEM, but could have been a swap during some repair effort in the past.
I think you may have put your finger on the distortion issue with regard to the power detector, and I might be too fussy. My wife says the radio sounds okay to her.
Nelson

[Ron Ramirez]

Yes - 1000 ohms/volt was the standard for many years. 20K ohms/volt is much better, and the modern DMMs are in the 1 meg/volt or better range.

If your line voltage is 117, you're lucky - and your radios will like that. Most of us in the States have line voltages in the neighborhood of 125 volts. I once measured close to 130 when I was living in Evansville. Higher line voltages will also translate into higher voltages in your set.

284V on the RF tube plate is not surprising. When I was working on my Model 370 last year, I measured 280 volts on its RF, 1st detector and IF tube plates. Philco specs call for 250V on the plates of those tubes. As for the cathodes, I measured 3V on the RF, oscillator and IF cathodes; 7V on the 1st detector (mixer) tube. The front end of the 70 is the same as the early 90 front end. I was using chassis ground as common.

But 247V on the detector plate? That does sound quite high! Have you checked resistors (23) and (34)? Both 250K, 1/2 watt. And what about the 2nd detector cathode resistor (21)? 50K, 1/2 watt. If these check OK, then check your B-C resistor (38).

Try measuring again, using B- as your common point, and see what the difference is. You will probably read close to published specs on the cathodes by doing this.

[Guest]

Thanks for your detailed responses, Ron.
I took some time to do some further measurements, and in the process found a couple of inconsequential wiring discrepancies from the diagram... taking cap #17 to the top end of resistor #21 instead of ground, for instance. Correcting them made no difference.
I double-checked resistors #23 and #34, along with the cathode resistor #21. They're all new, and within 1k of spec.
The B-C resistor, #38 is within a handful of the readings you gave for the model 90.
Using B- as the common, all the cathode voltages are very close, as you suggested they'd be.
The mystery is the detector plate voltage, and the only idea I have is that the voltage at point D might be too high. I get 290 volts there, but it seems the only way the detector plate voltage could be so high, is if it's not drawing enough current across #23 and #24.
Still scratchin' my head...
Nelson

[exray]

That's one of those critical locations when using a modern voltmeter. There is only a tiny current being drawn and the plate actually DOES operate at higher voltage than 48 volts. The discrepancy is in the old measurement method and documentation.

Even a 20k VOM is not enough to load it down to match the published voltages.

[Ron Ramirez]

Hi Bill

Thanks very much for the explanation! That's what we needed to know.

I had no idea a 1K ohm/volt voltmeter would load down the plate of the 2nd detector that much.

Another Phorum member measured the voltage between the second detector plate and ground in his 370 - he came up with 230V.

[Radioroslyn]

I'm the other member. As Ron said I did check the plate voltage on the detector and found 230v. Latter I was just sitting around and perusing the model 70 info. It sez plate voltage 105v. So I thought I'd give it another check. This time I used my old faithful Triplet 630 ( just a vom 20K per volt). Nice old Bakelite job that will measure up to 6KV handy for working on old 7" electrostatic deflection TV sets. Any away I digress. It sez 215vdc. I'm pretty sure that I did go through the resistors and check them when I serviced it. Something else to have a look at. Set play fine as a matter of fact it's on now playing Tales of the Texas Rangers. I'm using an old signal generator as a low power AM station.
Terry

[Radioroslyn]

You may want to approach the problem this way. Use a good sounding amp and speaker and poke around the output of the detector and audio stages of the '90 and hear if it is coming from the detector or other parts of the amp. If you have a signal tracer take a listen before the detector and see if you can hear it. To be honest with I've got a National NC-100X with the same problem. Low audio frequency sound garbled. I did a lot of work on the set to eletriclly restore it but so far I haven't been able to pin this one down. It too uses a plate type of detector. It's too bad it's a very kewl looking 1936 deco set.
Terry

[Radioroslyn]

Yes - 1000 ohms/volt was the standard for many years. 20K ohms/volt is much better, and the modern DMMs are in the 1 meg/volt or better range.

Just to clarify Very old volt ohm meters were 1K per volt. I would suspect these where used for testing B batteries. The meter would load the battery and give better more accurate reading under a load and 1000 ohms would do it. Most radio circuits you don't want to load. It will give you an inaccurate reading. Later I'm guessing late 1920's better vom's 10k and 20k per volt. Vtvm ( vacuum tube voltmeter) and Dmm (digital multi meter) all are 1meg per volt or better (higher). Personal preference is use dmm for resistor. Very accurate. Use vtvm for voltage. Two reasons: 1 Problaby what was use when set was made to measure the voltages that are on the documentation. 2 Dmm's sometimes can give an incorrect reading. They are sensitive to any sort of signal that maybe be mixed in with the voltage you are measuring. This happens a lot with measuring voltages on TV's and things like ham transmitters. This can be very frustrating!!!
I'm jumping off my soap box.
Terry

[codefox1]

So think back 40 years ago. If you have a 1000-2000 ohm per volt meter, and the plate on the detector has a 2.5 meg resistor, sure you are going to have a low value when you attempt to measure it.

[Guest]

Very interesting discussion. Seems the sound I'm hearing is typical of the "power detectors".
It is subtle enough that my wife doesn't notice it... but my "daily driver" here is a big Sparton
console with variable IF bandwidth, so most of the other radios suffer in the comparison.
As far as voltage readings go, I seem to recall reading somewhere that you can put a resistor
of 1K or so in parallel with the probes on a DMM or VTVM and get very close to the readings of
the early VOMs. Have any of you tried that?
Nelson

[Radioroslyn]

Never tried that. Do know that most vtvm have a 1meg in the + probe.
Terry

[codefox1]

Think of it as a voltage divider. If your old fashioned voltmeter had a sensitivity of 1000 or 2000 ohms per volt, and you had a really large value resistor between the plate and the B+, then you would be almost shorting out the circuit when you measured the plate voltage with the meter. E=IR, and the current is so small, ere the measurement error of yesteryear.

[Ron Ramirez]

Did a little checking...found out that Philco's 027 multimeter was rated at...1000 ohms/volt. I thought I had read that somewhere before.

So there you go...those voltage measurements made "way back when" were all inaccurate! Yet the engineers and service men did the best they could with the technology available at the time.