Philco Model 38-7: what caps & resistors do fail typically? - Printable Version
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RE: Philco Model 38-7: what caps & resistors do fail typically? - RodB - 06-22-2025
Take a look at the Service Bulletin information in the Library for 1938 Wiring Diagrams, on the right menu is Philco Service Bulletins. Look at the several bulletins, there's one for January 1938 and May 1938. There's several bulletins concerning parts changes to stabilize oscillator frequency.
RE: Philco Model 38-7: what caps & resistors do fail typically? - Musaeum - 06-24-2025
Thanks for the tip. I see... asbestos, too. But only in certain "run". How can I tell which "run" my unit is?
RE: Philco Model 38-7: what caps & resistors do fail typically? - Musaeum - 06-24-2025
I swapped half a dozen of caps. Electrolytics first, that improved things a bit. Then I swapped those paper caps that connect stages (2x 15nF) and 2 or 3 others that are in a low-impedance high-voltage path. They all tested fine, worst has been 5nF instead of 15nF between audio stages. The swapping did not result in any tangible difference in performance. In particular the audio stage still distorts a lot when its getting a bit louder.
All other paper caps measure fine, most display more capacity as they should, like 30nF instead of 10nF. And not a single resistor was out of spec! Even the 1M's all show 1,1M at most. I tested in-situ, cross-checking with the circuit diagram to see if the reading should be exact or could be influenced by other components. Unsoldering components from the lug for checking is a nightmare. I bought a solder pump, but that doesn't work. I hate cutting component-leads just for checking, but for some components there seems to be no other way.
Next I'll empty the bakelite thing in the mains and put new caps inside. Perhaps that will relieve the on-station-hum somewhat. According to a thread in "Readiomuseum.org" this is due to RF on top of the mains 60Hz coming into the set through the power cord. Some suggest clamping 10-100nF directly across the rectifiers.
cheers
Martin
RE: Philco Model 38-7: what caps & resistors do fail typically? - MrFixr55 - 06-24-2025
Hi Martin,
First off, welcome to the Phorum.
If you are still using a UPS, it may, in fact not isolate you from the AC line. Only true "On Line" UPS units where the power is rectified, batteries are in line, and then inverted back to AC will isolate.
Your set has a power transformer and full wave rectifier. I tried running an "AA5" radio (AC - DC Set with 12SA7, 12SK7, 12SQ7, 50L6 and 35W4 half wave rectifier) on a portable 120V AC supply with "modified sine wave" output. It was not listenable due to the "hum" which was very raw. A set with a full wave rectifier fared much better, but not like the power line.
Unfortunately, there is so much noise pollution in a house nowadays. Between the Internet of Things (IoT), WiFi, Cellular, CFLs, LEDs, Computers, etc., AM is almost unlistenable. This is worse in areas where the house is wired with Type NM (Romex) cable as opposed to jurisdictions requiring Type AC (incorrectly referred to BX), actual BX (God help you) or all wiring to be in metallic conduit.
AM reception in my house is horrific, even with an outdoor long wire and even with battery power. My house is wired with NM cable, no conduit, recent service equipment upgrade, performed by me, fully in line with the 2014 NEC and inspected both by the power company and an Underwriters inspection service. All water pipe is coper and system ground is at water service inlet to house. 38ft of copper pipe from house to water main, buried 6ft in sandy loam, bonded around the water meter. Second ground as required by NEC. No change in noise between new service and previous service. However all lighting is CFL or LED, office computer, LED display, WiFi, etc. in house. I need to get outside the house to have good reception on line powered or battery powered radios. Even the car radio reception is sketchy in my neighborhood (typical suburban development from the mid 1950son Long Island NY.
Most of my radios do not have EMI filter caps, and those that had them, I removed them, but that's me. Those that do have EMI caps do not seem to work better in my house than those that don't. Seems that the best performing radios in my house are those with loopstick antennas. The worst "hum" in my house occurs "on station" as opposed to off station. On sets with Aux inputs, audio from audio sources, like MP3, CD, etc., are very clean. I am a big fan of adding "Aux inputs as they allow "Divide and Conquer" troubleshooting. If audio is good on an aux input but bad on radio, you have eliminated the audio amps, speaker and power supply as the source of hum, distortion , noise, etc.
The best thing I can suggest is that if you have a set from a similar period, compare performance between that set and the set that you are restoring.
Hope this helps.
RE: Philco Model 38-7: what caps & resistors do fail typically? - morzh - 06-24-2025
>>All other paper caps measure fine, most display more capacity as they should, like 30nF instead of 10nF.
That does not make sense.
To measure fine, a cap should be within tolerance; it does not work as "the more the better".
The best thing you could do is replacing all paper caps and not trying to separate good from bad: there are no good ones there. Good paper caps from that era simply do not exist. Or, if they do, it is considered a miracle and such a cap would be displayed in a local place of worship for people to marvel at.
RE: Philco Model 38-7: what caps & resistors do fail typically? - Musaeum - 06-24-2025
The mains filter is back, but that did not change the on-station-hum. Some people call it tuning-hum or modulation-hum.
With a scope I could see the audio signal was actually overlayed by a 120Hz rectangle. Probing various stages of the set suggests its already present at the output (anode) of the IF amplifier 6K7G. Channel 1 in the first pic is connected to the junction of components 20 (51k), 21 (490k) and 24 (10nF), prior to the volume control. Channel 2 is connected to the anode of 6K7G. That weakened the signal somewhat, but it shows how the rectangle is AM-modulated on the signal.
The two other pics show probing further upstream with channel 2 on the anode of 6A8G, showing a steady signal of the oscillator. With FFT I could also see the IF, looking rather steady. Same on the connection to the grid of the IF stage.
Could the AM-modulation happen somewhere in the 6K7G? I often read about shorts between heater and cathode. Adding capacitance to the AGC (3) and supply (14) did not change anything.
BTW, I have no reception at all on short-wave. The oscillator is running, but with much less amplitude than on "broadcast". Any ideas? When I first applied power I remember having reception of many stations, although they seemed to broadcast only noise or maybe digital stuff. Since then I twiggled this and that, cleaned the upper side...
cheers
Martin
RE: Philco Model 38-7: what caps & resistors do fail typically? - Musaeum - 06-24-2025
Quote:If you are still using a UPS, it may, in fact not isolate you from the AC line. Only true "On Line" UPS units where the power is rectified, batteries are in line, and then inverted back to AC will isolate.
Actually I use (or used) the UPS from battery only, no connection to mains. So I could be quite sure there was no galvanic connection to anywhere.
But since I had rather strong hum all over the scale I ran it from mains which improved things a lot. Leaving the dreaded tuning-hum I'm currently fighting with.
Perhaps I'll leave it like that. My friend will certainly be happy using it as a couple of stations come in nicely and the "dangerous" parts have been changed. With todays line voltage I get 326 volts on B+ instead of 315. That is less than 5% above nominal, should be OK.
cheers
Martin
RE: Philco Model 38-7: what caps & resistors do fail typically? - morzh - 03-10-2026
OK. Let's not hog other threads. Let's stick to the one you are riunning on the subject.