model 20 hum (detector circuit?) - Printable Version
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RE: model 20 hum (detector circuit?) - LASJayhawk - 10-29-2015
What is the screen grid voltage on the 24 detector with the volume all the way up and all the way down with no signal?
RE: model 20 hum (detector circuit?) - Tubesforme - 10-30-2015
(10-29-2015, 10:36 PM)LASJayhawk Wrote: What is the screen grid voltage on the 24 detector with the volume all the way up and all the way down with no signal?In LA, where I am there is always some signal even without an antenna attached. Source of the screen voltage is point F and it measures 19V using the PT CT as the common point, at all times. The screen of the 24 detector varies from a low of 9V to a high of 17V, using the PT Ct as the common.
RE: model 20 hum (detector circuit?) - LASJayhawk - 10-30-2015
Ok, blows that idea
You have checked the 1400 Ohm resistor from pin 5 of the power transformer to point B?
What is the voltage from point H to ground with the 27 in and out?
RE: model 20 hum (detector circuit?) - Tubesforme - 10-30-2015
(10-30-2015, 03:29 PM)LASJayhawk Wrote: Ok, blows that idea
You have checked the 1400 Ohm resistor from pin 5 of the power transformer to point B?
What is the voltage from point H to ground with the 27 in and out?
Yes, I have checked the 1400 ohm resistor and it is well withing spec. What is point H?
RE: model 20 hum (detector circuit?) - klondike98 - 10-30-2015
LasJ probably is referring to H on this schematic http://www.philcoradio.com/tech/images/20a.jpg
RE: model 20 hum (detector circuit?) - OZ4 - 10-30-2015
Maybe there is some leakage between the windings of the power transformer.
With the power off lift the #26 the 1400 ohm resistor then check the resistance from center tap winding #5, (that powers the filimants of the 71s) and the center tap #6 of the power transformer.
With #26 resistor lifted and even with all the tubes in place there should be no continuity between these two points. If there is that could cause AC hum.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-30-2015
(10-30-2015, 06:13 PM)OZ4 Wrote: Maybe there is some leakage between the windings of the power transformer.
With the power off lift the #26 the 1400 ohm resistor then check the resistance from center tap winding #5, (that powers the filimants of the 71s) and the center tap #6 of the power transformer.
With #26 resistor lifted and even with all the tubes in place there should be no continuity between these two points. If there is that could cause AC hum.
I tried this and there is no reading when 1400 ohm resistor is disconnected. My manual calls the resistor part #25. #26 is the main filter capacitor pack in my schematic, from Philco. By the way, I also tried replacing the 2.5V winding with another transformer and the hum remained so it isn't coming from the 2.5V winding either.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-30-2015
(10-29-2015, 07:57 PM)LASJayhawk Wrote: The interstage transformer center tap is connected to point B and not the chassis ground, correct?
Yes, it is connected to point B. If not there would not be any bias voltage on the 71's and there is bias on the grids.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-30-2015
(10-25-2015, 12:01 AM)Steve Davis Wrote: It looks the problem is after the 24 but before the 27. It could be lead dress. Try moving the 27 grid lead to see if that makes a difference.
Steve
That has been done several times, in fact the lead was replaced with a shielded one, grounded only at the one end. That didn't help so the original black wire was reattached. It was long enough to move around. The area of the problem that you suggest is exactly what I had come up with early on. I doubt that #16 is defective. The interstage transformer, while new, was manufactured overseas and I wonder if there could be leakage in that, but that seems unlikely. The hum sounds like 60Hz. I have ordered a 100uf @350V and once I get it to bypass point B to ground and see what happens.
RE: model 20 hum (detector circuit?) - OZ4 - 10-31-2015
To check the interestage transformer for any leakage. Just look for any continuity between the primary and the secondary windings and the case. Something else to try. Lift the center tap off the B line and use a separate bias supply here for the 71s. This will verify that it is or is not coming from this transformer. Just watch the bias on the 27.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-31-2015
(10-31-2015, 10:40 AM)OZ4 Wrote: To check the interestage transformer for any leakage. Just look for any continuity between the primary and the secondary windings and the case. Something else to try. Lift the center tap off the B line and use a separate bias supply here for the 71s. This will verify that it is or is not coming from this transformer. Just watch the bias on the 27.
I did test for continuity between primary and secondary and even tried it with a meg ohmmeter and there was none. I will try the bias idea.
Need correct voltages for det and af model 20 - Tubesforme - 11-07-2015
I need the correct voltages from a working Philco Model 20 or 21 for the 24 detector tube and the 27 audio tube. I have the readings from the literature, however these were taken in the days when meters were only 10K ohms per volt. I am using a digital meter.
I know the correct filament voltages as well as the cathode voltages, however because of a problem I am having I need the other voltages. Also I would like to know what readings you measure at point B, D, E, F, G and J. I have a good idea of the correct voltages, but I don't want to guess on this one.
Any help would be most appreciated. However I don't want to guess or use the voltage readings from a tube manual or the Riders or Philco literature.
Someone had suggested to load the DMM with a resistor, however that won't work because the impedance of the new Fluke is, according to Fluke, infinite..
RE: Need correct voltages for det and af model 20 - morzh - 11-08-2015
The voltages on det tube will be very different from the existing charts when read with a modern DMM. PLATE voltage will be on the order of 100V or so.
I restored two so far. Truthfully, the second time I did not bother checking voltages to be close to true values, all I bothered to do was (and often is with most radios) is making sure plate voltages look like.... plate voltages. As long as your radio is recapped/recarbed and your coils are OK, and tubes are good, the plate voltages will be good. I only start looking close if I have to troubleshoot.
RE: model 20 hum (detector circuit?) - Ron Ramirez - 11-08-2015
The voltages were originally measured at the factory with meters with a sensitivity of 1000 ohms per volt. That's all they had at the time. Those meters loaded the circuit under test, resulting in voltage readings that were actually incorrect!
Today, we can easily buy digital multimeters with input impedances in the megohms per volt, which do not load down the circuit under test as did the old meters from ye olden days.
You can acquire an old early 1930s voltmeter...or find a low-end analog multimeter made by Calrad and sold in the 1970s...or maybe someone will measure the voltages in their working model 20 and share the results here. I no longer own any Model 20 sets.
RE: model 20 hum (detector circuit?) - Tubesforme - 11-09-2015
(11-08-2015, 10:28 PM)Ron Ramirez Wrote: The voltages were originally measured at the factory with meters with a sensitivity of 1000 ohms per volt. That's all they had at the time. Those meters loaded the circuit under test, resulting in voltage readings that were actually incorrect!
Today, we can easily buy digital multimeters with input impedances in the megohms per volt, which do not load down the circuit under test as did the old meters from ye olden days.
You can acquire an old early 1930s voltmeter...or find a low-end analog multimeter made by Calrad and sold in the 1970s...or maybe someone will measure the voltages in their working model 20 and share the results here. I no longer own any Model 20 sets.
I have never taken the voltage reading very seriously in any radio I repaired, except for transistors and tube bias. Otherwise most problems are fairly obvious. It looks like I have found my problem. While I live in a building built new, four years ago everything should be fine electrically. I have two other radios, both transistor, from the early 70's and they work just fine. However today I took the radio to a friend who repairs radios for a living, in Santa Clarita and the hum wasn't objectionable at all. The radio worked as it should. His suggestion to me was to get an isolation transformer and said that might not even be enough, it might be the area I am living in. So thanks to everyone for all your suggestions.