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This part needs a replacement but I don't know what it is - Printable Version

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What is this thing in my Philco 42-390, it is causing problems - IntrStelle - 08-20-2025

Hello Phorum, this is my first post here, I was fixing my first Philco (model 42-390) a day ago that I found a week ago, it was fine when I replaced the caps, when I switched the audio output tube, everything went wrong, the rectifier (Parallel 6X5GT of course) died in the most violently spectacular way (it even had cool flashing lights), the X-plate replacement almost did because it did the exact same thing (but it refused to die and it somehow works fine)

When I looked underneath the chassis and turned it on to find the source of the smoke and what I thought was smoke from the main components, this weird paper thing circled above was smoking when I plugged it in (it was shooting out smoke like a gas pipe from in between the paper thing), the only thing that connects to it is the negative end of the Electrolytics .

I need to know what it is, and why it's doing that, and if any component could be damaged.
I've looked at the schematics but the number is too low res for me to ID

Thanks

RE: What is this thing in my Philco 42-390, it is causing problems - morzh - 08-20-2025

I merged the two threads, as they have same names plus more or less the contents are close.

Please edit the content so it is not redundant.

RE: What is this thing in my Philco 42-390, it is causing problems - Arran - 08-20-2025

That is a old fashioned style of heat sinked wire wound resistor commonly referred to as a "Candohm". If it was smoking then something connected to it was drawing a lot of current. If not then the only way it would do that on it's own is if the insulation broke down and the resistor shorted to ground. Did you power the set up before you started working on it? I'm guessing that a mistake in the wiring may have caused this, such as something connected to the wrong point, or a filter cap connected in backward.
Regards
Arran

RE: This part needs a replacement but I don't know what it is - GarySP - 08-20-2025

I've moved your post to your other thread. Please keep all questions regarding your 42-390 within one thread. Take care, - Gary

sorry - IntrStelle - 08-20-2025

Quote:I've moved your post to your other thread. Please keep all questions regarding your 42-390 within one thread. Take care, - Gary
I'm so sorry, I didn't think they posted because I couldn't see the posts and I had slow internet since it's not very strong in the shop I work on them in

I deleted the ones in excess and edited the first post to add any excluded content

RE: This part needs a replacement but I don't know what it is - MrFixr55 - 08-21-2025

Hi IntraStelle, and welcome to the Philco Phorum, A Phamily Phriendly Phorum pfull of phine phriendly pholks phull of phun Philco pfacts. (Notice a pattern here?)

We are very happy to have you as a member and are happy to help you navigate this Phorum as well as help you with your radio. Icon_smile

Please take a moment or two to read through the Phorum rules and the "What not to post" thread.  Also take time to introduce yourself in the "Introduce Yourself" forum, located in the "Members Only" page and don't forget to visit Old Ron's Bar and Grill for a virtual brew Beerchug and to shoot the breeze.  Please also note that Under "Various and Sundry Categories", there are forums (oops Phorums) for non-Philco radios, HiFi, test equipment, etc.

Regarding your problem, the "Candohm" is a multiple tapped carbon resistor used in this set to provide various "grid bias" voltages.  Since this resistor is between chassis ground and the center tap (- return) of the HV power supply, all DC current flows through this resistor.  Since a short to ground would not be an actual dead short of the power supply, the failure of this resistor likely is from the same issue that caused your rectifier tube to exhibit the epic fail that you described.

If you are using a schematic from Nostalgia Air, the one from our library may be more clear. If you navigate to the Philco Library on the top of the Phorum Page, select Service Documents, then 1942 yearbook, you can find the schematic. For your convenience, I post the link here:
https://philcoradio.com/library/download/service%20info/yearbooks/1942/split/Part25-Philco%201942%20RMS%20Year%20Book.pdf

Please refer to this schematic and note that the 2 filter caps have 2 different connection points for the negative poles of these caps.

The negative lead of 18 uF cap, # 89 connects to the center tap of the B Power Supply (HV Secondary winding of the power transformer). The positive lead of this cap connects to the junction of the rectifier cathode and the input to the speaker field coil. The 8uF Cap, C49A connects between chassis ground (negative lead of cap) and the output of the speaker field coil, the center tap of the output transformer and the main B+ line (positive lead of cap).

Start out by measuring resistance ("Ohming") across the 8 uF capacitor labeled 49A in the schematic.  This "cap" is between chassis ground and the main B+ line that powers the output stage and the rest of the radio.  If you read a short or very low resistance here, troubleshoot this before powering up.  Verify that the capacitor is not installed backwards and that the working voltage is above 250 WV (Working Volts).  It should actually be at least 350WV.  Cap #80 should also be at least 350 WV, preferably 400 WV.  If you have the original "cans" check for capacity and voltage rating.  You can go a little above the rated capacity (10 uF instead of 8 uF or 20 uF instead of 16 uF) but not much higher.  BTW, don't just "bridge" the existing original cap, remove all connections from it, as the original cap may be shorted.  Another possibility is that if the output capacitor has an open winding, it will cause the output tube connected to that winding to draw all current through its Screen Grid, heating it white hot and likely shorting to the suppressor grid, which is at ground level.  This would amount to a short across the B+ supply taking out the Candohm resistor (and possibly the rectifier).  Measuring across Cap 49A, pull the two 41 Output tubes, one at a time.  If the short clears, you found the shorted tube.  It must be replaced.  Also, Ohm the transformer to confirm the open winding.  The transformer must be replaced before installing new tubes.  41s are easily available on eBay.

Once you locate and correct the short, you can replace the Candohm, #88 with 2 resistors, an 18 ohm from chassis ground to the center point and a 175 Ohm resistor between the center point and the center tap of the transformer (and the negative connection to Cap # 80). 5Watt resistors should work, 10 Watt resistors are even better.  As with the capacitors, do not just "bridge" the Candohm, remove all connections from it and use terminal strips to mount the resistors.  Consider the Candohm to be totally fried.

Hope this helps.  Like the old Home Depot ad, "You can do it.  We can help".

RE: This part needs a replacement but I don't know what it is - IntrStelle - 08-21-2025

Mr. Fixr

Thank you
I am going to buy 2 10W resistors to replace the candohm

And about the transformer, it is late as of now so I don't have a chance to check the transformer for an open winding, I'm worried it's open because the 6X5GT is the "transformer killer" but I happen to have a power transformer from a Truetone D-1046 (lovely radio btw, that's what got me into vacuum tubes, but the cabinet was beyond repair so I took out the chassis and took parts out of it), but I'm not sure if it's compatible as a substitute.

RE: This part needs a replacement but I don't know what it is - morzh - 08-21-2025

I hope you are intending to recap the radio.

RE: This part needs a replacement but I don't know what it is - IntrStelle - 08-22-2025

Morzh

I already did, I bought the caps from Tony's Capacitor Corner, recapped everything

All the caps are surprisingly healthy after this fiasco so I hope I won't need to recap

RE: This part needs a replacement but I don't know what it is - MrFixr55 - 08-22-2025

Hi IntrStelle,

I was worrying about the output transformer more than the power transformer. When an output transformer has an open primary winding or if a radio is powered up without the output transformer connected but B+ applied to the screen (which will happen with an open output transformer), then instead of the plate dissipating all the current, the screen will. The screen is designed to handle about 2.5 - 5 mA, not 40-50 mA. Therefore, it heats up, burns out and touches the suppressor grid, essentially shorting B+ to chassis ground. This may be why your Candohm is toast.

Ohm your output transformer for open windings. If you find one, this may be the issue.

To test the power transformer, pull all the tubes and power up. The transformer should be cool to only slightly warm, even after an hour. If good so far, measure AC Voltage between the center tap and each leg of the HV winding. They should be equal. Finally, install all tubes except the rectifier. The transformer should stay cool to warm, not hot. Since the rectifier is not installed, you can do this test prior to finding the short.

If you find a short in the B+ circuit, first, pull the 2 41s. If the short goes away, there is your smoking gun (Oh well, the gun did not smoke but everything else did)
One mod that I perform is to put a 1N4007 diode in series between the power transformer and each plate of the rectifier. This should protect the transformer in case of a plate to cathode short. However, with a 6X5 type tube where the rectifier heater is powered from the same winding as the heaters for the rest of the tubes, this mod will not protect from a cathode to heater short. This is the reason why so many folks fuse the power transformer primary. You could get away with putting a fuse between the rectifier cathode and the input cap and field coil, but most "AGC" type fuses are rated only for 250V. You would want only a 1/5A fuse max. 1/10 A is probably enough.

Hope this helps!!

RE: This part needs a replacement but I don't know what it is - radiorich - 09-16-2025

hello Instelle,
How is the restoration going?

Sincerely Richard