[Bruce C]

This is my first post after searching other posts for answers and came to a wall in my 40-180 restore. A short history of events:

I salvaged it from the landfill in 1960 and it worked very good, yes I was 9 years old. Not used in the last 10 years but always in the house. Moved it to the family retreat and decided it was time to get it working on weekends. At this point lots of humming and one faint station received. Took the chassis to an Amp shop to test the tubes and he replaced the two e-caps and gave me spare tubes ($200 ouch). Still the same hum so I started studying what needed to be done and replaced all paper caps (and the e-caps with the correct value) and most resistors myself. Now no hum, no anything so I tested voltages and all tested good until I got to RF 1232/7G7 and they were very low so I stopped for that weekend thinking it would be easy to track down an error I may have made next trip. Came back this next weekend and the voltages are way off everywhere.

So what could have gone wrong while just sitting and where do I start now? I have linked a picture of the chassis upside down behind the console where it is plugged into the field coil and also a marked up diagram of the voltages from the last trip. Circled part numbers have been replaced or different values noted.

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[DeckApe]

Am I reading the diagram correctly that you are running a 7A7 instead of a 7B7?

[Bruce C]

That is correct. The 7A7 was in the radio when I got it 50 years ago. I read in some cases it can be used as a substitute so I didn't suspect it was a problem since it worked for so long but everything should be suspect now. I discovered one of the e-caps had been replaced before I got it and stuck under the deck so the tube was probably replaced back then.

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[rghines1]

In many cases the voltages can be 20% higher than shown on the schematic. If voltages are well past 20%, then would suspect the audio output stage is not drawing much current. Schematic shows 180V on the plates of the 41 output tubes. What do read for the plate voltage on the 41 tubes? Check voltage on the 41 control grids, would expect about -15 volts.

[Bruce C]

The plates of the 41s are recorded in red at 97V and 100V. I will have to put the control grid voltage on my to-do list for this weekend. Thanks

[rghines1]

Missed earlier your marked up schematic. High voltages on the RF tubes while low on the audio section. This leads to think the the electrolytic caps when replaced got wired wrong. Hard to say exactly but would reverify how the caps 58 and 61 are connected. Negative end of cap 61 should connect to the center tap of the power xformer. While on cap 58 the negative end connects to the chassis. Made my share of wiring errors.

[Bruce C]

I hope it is that easy but I am still baffled that I had mostly correct voltages and then with out me doing anything they changed so dramatically. I purchased one each extra of all the caps so I will be doing a lot of rechecking, thanks rghines1.

[Bruce C]

I can report the weekend evaluation went well. Resistors 34 and 60 were the wrong size. When I tested them out of the package I thought the values were right-on but in my hurry didn't notice the little "k" next to 150. The package was marked correctly but I was sent the wrong ones. After changing them out the voltages were correct. I now have reception but still have some humming and have since read my "Micamold" caps are really molded paper caps and should be replaced so they are on order for next weekend. I should have a signal generator by then also.

[rghines1]

Glad you got it worked out!

Refurbished this same radio over 10 years ago so my recollection will be fuzzy. Those micamold caps could be silver mica types instead of leaky wax paper types. Remember replacing only 1 of those in the push-button section to get the oscillator slugs to home in to the correct frequency ranges. May have been cap 24. Not aware of a way to tell the difference w/o destroying the caps but any caps in a tuning section less than 10,000 pF are likely silver mica which generally are good.

Mine had an intermittent hum problem that came days later that drove me crazy. Came down to a heater-cathode short on the 1st RF tube. The tube failed nicely in the radio and tube tester when it was tapped. The oscillator tube was really weak and would take minutes of warm up and suddenly blast sound.

Would soon get boring if it always worked the first time!

[Bruce C]

23 and 24 are listed as Silver Mica in the parts list and I planned on leaving them alone. They look very different from the rest. The remaining that I will now replace are all listed as Mica and branded Micamold, which is reported on anothrer site as formed paper and not reliable. They are easy to get to also. I am pressing my luck moving the push-button tuners out of the way. I had to repair one woven wire connection next to the coil already. Thanks again

[exray]

If they are listed as mica then that's what you should stick with. The company, Micamold, made all sorts of components, even resistors in a similar style package as the 'domino' capacitors not to mention tubular capacitors as well. Its just a brand name - not an indication of whats inside. As was said earlier, its typically the larger values that are paper. A lot of people are fooled by this brand name!

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[Raleigh]

Appearances of the flat plastic capacitor packages can be deceiving. There were paper capacitors and mica capacitors that were in similar flat packages. The way to tell them apart is that the unreliable paper capacitors had capacitance values in the microfarad range. They should be replaced with modern caps. Mica or silver mica capacitors might look similar, but the capacitance would be in the micro-micro farad (picofarad) range. These are usually pretty reliable, though I have found a very few bad ones.

A couple of years ago, I posted a question about how to tell these two types of capacitors apart, and one of the moderators replied that Philco never used the molded plastic paper capacitor packages, so all caps of that type are actually mica. The capacitance value is also definitive. If all else fails, refer to the parts list. If it says "mica" or "silver mica" it most likely doesn't need replacement.

Philco used mica capacitors in other package types, typically large squares or rectangles covered in wax. Sometimes there were two different caps in one of these packages. A 42-355 I recently worked on had two of these big wax-covered mica capacitors in the FM circuit. On both, the wax was cracked and had begun to separate, so I figured they were on their way out and replaced them. When I had them out of the radio, I measured the capacitance and found they were both very close to their nominal value in spite of the appearance of the wax. Just for fun I chipped all the wax off one to see what was inside, and the capacitance hardly changed at all.

I usually replace micas if I have to desolder them anyway to change another component or wire, but otherwise I leave them alone. It is hard to measure picofarad caps unless they are out of the radio because capacitance of the meter leads can vary quite a bit depending on how you run them. I have a meter with little push terminals for the capacitor leads right on the body of the meter, and I get consistent readings from that. Once I have a mica cap all the way out of the radio to make the measurement, sometimes it just seems wiser to replace it, even if capacitance is pretty close to nominal.

[exray]

One of the main problems is guys attempting to twist the mica caps over to see what might be written on the other side. Sometimes this is all it takes to break the seal between the casing and the lead. If you've ever looked inside to see how they are constructed you can see how the metal 'leaves' are so easily broken (or shorted) if the lead is free to twist. There ain't no going back once that happens. This is true of all of them whether paper or mica. Of course the paper ones were probably bad anyway

Best to leave them alone with the exception of those that are obviously paper because of their capacitance value or physical size.

War story...I bought a grab bag of about 100 NOS domino-style mica caps. All mica, no paper. About 25% of them were bad. Just goes to show that you can't trust them 100% but 75% is a relatively good ratio compared to paper caps.

Raleigh is right. If you're going thru all the trouble to remove them and test them then you might as well replace them. Otherwise just leave 'em be and troubleshoot later if you encounter problems.

-Bill

[codefox1]

I have a couple of dental inspection mirrors (You know, the ones the dentist uses to see around corners,) so if there is room, sometimes you can read the other side that way.

And it is good practice to try to have replacement components face "up" so you or the next person who works on the set can read them.

One more thing, a mica with the "dreaded mica disease" may check out ok on digital multimeters, but applying a little voltage, like my treasured Eico 950B RC Bridge does will reveal the truth. The eye will not remain open after a few volts. This test is help in the diagnosis of the set, (the crackeling and fading caused by these little monsters.)

Of course you have to remove one end from the circuit to perform the test, and at that point I usually toss it anyway and put in a new one. So most little sets can get a whole new population of caps and resistors for less than $10. The resistors and capacitors almost always read high, the electrolytics are always dangerously degraded, and all paper caps are leaky. About all I leave untouched are the controls (after cleaning) and wirewould resistors.

Good luck on your project!

[Bruce C]

Thanks for the great responses and information. Sounds like I am on the right track only because I still have humming and the dominos are next in line to test so when you desolider and have three and four components on one post just replace them. I really like the war advertisement but I know they are older than I am and I'm way past my prime, just glad I haven’t been replaced yet.