[RodB]

Roy, when I have to replace the candohm resistor I usually strip out the wirewound guts and use the case and terminals to mount the new resistors.

[morzh]

Yes, I was able to do this with a Candohm in Zenith T/O 7G605. Sometimes it will work, sometimes it is inconvenient (in case of the flat Canohms; the T/O one is quite boxy)

[glyphmon]

Mike,

This one: https://www.tubedepot.com/products/termi...-terminals
does have fastening points in the middle, I presume exactly for that purpose.  If they could simply post the physical dimensions of the part on the product listing, I would have either ordered it by now, or moved on, but alas...

Rod,

I may end up needing to do that, so will keep that option in mind.

Best wishes,
Roy

[RodB]

Try this for terminal strips

https://www.amplifiedparts.com/search/no...20strip%20

[morzh]

What you will have to do if using the long one is to have standoffs (spacers), 4 of them (or at least 3).
Which makes the effort comparable to using shorter ones.

[RodB]

Mike, have you ever tried using silicone adhesive to fasten large wattage resistors to the chassis for better heat dissipation?

[morzh]

No, I have not.
A few times when I had to replace Candohms (the very first was a Zenith 9-S-262) I simply used the wattage that would be appropriate to not worry about the dissipation. My personal preference is 3x the dissipation. I mounted them on a barrier strip which then was bolted to the chassis.

You obviously could try to glue the resistors, but then you 1) have to make sure that wherever it is mounted, it will not fall under its own weight, should the cement soften. 2) Nothing is mounted on that chassis nearby so local heating (which could be pretty intense) while warping the sheet metal a little bit (some chassis are thick, some like Zenith's are thin) could not crack anything. A resistor in the air is still OK if sized correctly, but will not warm the actual metal as much.

This said, that red RTV they used as a gasket between the exhaust fan and the combustion chamber in my furnace probably would hold just fine.

[radiorich]

Hello guys,
Yes, I have a Zenith model 908 with one of those flat styles, but it seems to be working fine, so I’ve left it in for now.
Sincerely Richard

[glyphmon]

So, I lined up all five (I think) of these 10W resistors pretty much end to end and they came to about 9 inches.  On the Amplified Parts website (thanks Rod) they had both the 13 and 26-lug strips that Tube Depot listed for about the same price, though on Amplified Parts they were listed as a backorder.  However, Amplified Parts they showed the dimensions and the overall length of the 26-lug strip is 9.5".  Unfortunately, that's a little longer than the candohm, but I think I may be able to finagle something if I bring it down a bit and use, on at least one end, one of the screws that goes into the "legs" on the bottom of the chassis.

I'll have to drill at least one hole for a center mount and standoff, but I feel like this will ultimately be a cleaner/more elegant solution than going in multiple pieces.  While it's possible that it's an entirely different product between the two websites, I took a chance and we'll see if the one from Tube Depot matches up and if they actually have it in stock to ship.

Best wishes,
Roy

[glyphmon]

Just starting to look at the electrolytics on this - going to try to replace components in phases, then test, with this one.  The crumbling insulation on many of the wires may make that more challenging.

I wanted to verify how we generally identify the polarity of electrolytics on schematics.  While this is an RCA, I have to admit, even the Philco schematic for the 37-620 wasn't super clear on polarity with electrolytics, unless there's a trick that my eyes missed or a rule of thumb that makes it obvious.

On the other hand, it would be simple to replace the original with the modern part as long as the original (or at least currently-installed) part indicated which side was negative.  The Aerovox Hi-Farad Dry GL-type I have graciously says "Red Positive, Black Negative".  Alas, the wires coming out of them are white and black.  While I suppose I could assume that red or white, meh, who cares? - black will be negative, I know what happens when one assumes.

I do know how to identify the polarity on the modern components.

Best wishes,
Roy

[RodB]

I just checked back to look at your photos to see what type electrolytic caps are in the chassis. If they were the aluminum container type then the outside can would be negative. For the dry paper/cardboard covered type like you have you'll have to rely on the schematic and the markings on the piece itself. I don't remember seeing one that had a negative wire that wasn't black. But you should always verify (ASSUME). So follow the black wire to where it is connected and find that connection in the schematic. Once you're there (schematic) you can see what the drafting person used to designate the negative lead. It was all up to what they wanted to use that week. In the early days, they used just the standard capacitor symbol. At some point they started using a different symbol for electrolytics and the Bracket ( [ ) laying flat became the commonly used negative. But not always; some egos get in the way of progress and flip the symbols. It wasn't a persistent discipline. Anyway, the best way is to verify with the schematic. I'll dig one up and check out the symbols used for your radio. I can come over and help, I've always wanted to see Maine.

Something I do when replacing crumbling insulated wire is not remove the wire but lift one end and run a piece of spaghetti insulation or heat shrink tube down the wire then reconnect it. It cuts down the error rate.

[RodB]

So, I checked the schematic from Nostalgia Air and there is no way the symbols reflect the difference between standard capacitors and electrolytics. So you use experience and capacitor values to find the electrolytics. I see 3 electrolytics in the schematic, C16, 30 and 31. C16 negative is grounded to chassis and C30 and 31 negatives are connected to the power transformer center tap, which is the most negative point of the power supply, also known as B-. In tube radios of this era, the B+ and C- power supply are generated from the same source, the high voltage secondary winding of the power transformer. You'll see resistors R17 and 18 form a voltage divider connected between the center tap and ground. These create the negative C- bias voltages for the audio tubes.

You can also check the parts list and this one lists the capacitor values in ascending order. At the end of the list it shows 3 electrolytics. Sometimes the list will tell you what kind of capacitor is being used; mica, paper or electrolytic. If any doubt at this point you have us to contend with. Just ask.

[morzh]

I would use C30/31 with 500V rating and C16...450V just not to think of it.

[glyphmon]

Rod and Mike,

Thanks.  That's why I asked about the schematic clues.  The diagram does specify which are electrolytic, but again no polarity.  I find the Philco part lists to be much more helpful in identifying types and other characteristics than the RCA ones, at least for 1937.

None of these cans actually had the paper insulating wrap on them - they were self contained and screwed tight through the chassis, metal to metal, so straightforward in a sense, with but two wires coming out of the hole in the center.   Two of the three did in fact have red and black wires, so I simply matched black to negative and red to positive on those (C16 and C30).  The "as-found" connection points for C31 were: black to the end of the candohm (R17); and white (red?) to a mid-air junction between the 2nd IF transformer and 6F6 tube, so I expect I'll do the same and route negative to the previous connection point for the black wire.

I had already gotten the 450V for all of them based on previous comments, but I did go with a presumably heavier-duty 10 uF for C30 since it was available and 34 cents more.  I could certainly set 500V as a standard for replacing electrolytics in the future unless a higher value is clearly called for.

And of course, I would welcome a visit from any experts that want to visit Maine.  I happen to be about a 45 minute drive from the primary section of Acadia National Park and I work in the park's smaller, mainland piece, so I'm conveniently located.

Best wishes,
Roy

[morzh]

You only need 500V for the firts cap and rarely for the second.
YOu do not need that for the rest: many will do just fine with 300V or so, but you have to know the voltages they are being kept at; if you do not, eveything being 450V takes that question off your mind.

The first cap charges first and his is worse with the 80 tube as it is direct-heated and starts rectifying fast while the rest of the tubes are still warming up; this way the 1st and 2nd caps are not loaded, and even with the choke in between the 2nd cap can get some signifcant voltage; however it is a filter and the voltage there is an average whereas at the 1st cap it is peak-to-peak ripple. So most times the 2nd cap is OK with 450V. But since I use the same capacitances, I buy them all with the same 500V rarting.

Now with the 84 and the likes of it, it warms up slower, almost in sync with the rest of the tubes, so the voltage even across the 1st cap might not reach 500V or even 450V.

But then again, since the capacitors should be of ths same type and quality to work well in the rectifier, rather than keeping several WV, it is easier to keep them all the same.

I think I bought bunch of Panasonic 450V ones, the reason being the earlier radios had 10uF (or sometimes 8uF) value and I get it by serializing the caps I have, so 22uF gets me 11uF which is fairly close. And the voltage is 2x450V (and even without equalizing resistors it is still much higher than 450V).