[ntsc525]

Hi, everyone:

I have an early Model 70 (non-AVC, brass escutcheon) I'm working on, and I just spotted a new-ish 5.6 Megohm resistor hiding behind and shunting the 500pF capacitor (#28 in Bulletin 57) from the 2nd detector (24A) plate to ground. It looks like it was added some time in the 1950s or '60s, because it's the brown smooth cylinder shaped carbon resistor common in those days, though it's a 5%er so it could have been later when those were cheaper.

Basically, it goes from the plate of the 24A to ground, in parallel with the .0005 uF capacitor (28).

Wanting to keep the set as original as possible, I'm inclined to remove it, but I'd like to know why it was likely installed there. What was the purpose of adding this resistor to the circuit? Is there any reason not to remove it?

I just recapped the radio and it sounds good other than needing an alignment, and I wanted to figure out this resistor before I do that. If I know what it was supposed to fix, maybe I can anticipate what I'll have to expect on removing it, or I'll know I already fixed it with my recap.

I don't mind telling you this is my third, favorite, oldest and most pristine 70 I own, and I can't wait for it to go on display! (It was a deal because it doesn't have the coveted Mershon capacitors, but it does have the original Sprague ones, and I want whichever ones it wore out the factory door!)

Thanks in advance. I tried Googling this, thinking this was a common practice, but any searches containing the words "Philco" and "resistor" come up with volume control discussions.

[Ron Ramirez]

Moved to proper section.

[morzh]

I do not see any reason for it to be there. The circuit is the plate detector, the resistor would barely change any characteristics of that filter, and as the DC voltage goes, it would barely affect it being more than 10 times R32 and R33 put together.

[ntsc525]

Well, I removed the 5.6 Meg resistor and the radio still works when it's not dragging down B+ intermittently. <sigh> So, now I go wiggling wires and tapping on the chassis, and EVERYTHING I touch causes a change. Just gotta love intermittents!

Still curious if anyone has heard of a "fix" involving adding such a resistor.

[WallaceRoger]

So this issue didn't exist until you removed the resistor? I'm too much of a n00b to understand how that works, but logic tells us if the issue didn't exist before you removed the resistor, then someone back in time knew more than we did.

I say put it back and leave it. Maybe throw a little dab of paint or a sticker label by it to note it's not original, if you feel like it.

[ntsc525]

I didn't really run it long enough to know if the intermittent issue was present before I removed the resistor, and reinstalling it didn't make any difference during the failure mode.

If there's an intermittent problem with the radio, I have to figure out what it is and fix that anyway. It's not related to the resistor that was installed many years ago.

If I don't make progress on the B+ problem today, I'll start a new thread on that topic.

[morzh]

Resistor of this magnitude installed this way cannot possibly account for B+ dragged down.

[ntsc525]

The resistor that is the subject of this post was, indeed, NOT responsible for dragging B+ down. (Especially since I'd already removed it and confirmed it was still about 5.6 Megs.) 

It was not one, not two, but THREE intermittent 24A's, all National Union (so now I'm prejudiced against NU), such that my single tube substitution didn't really yield much, and I suspected something under the chassis anyway after my recap and volume pot swapout.  I knew I was in trouble when setting down my screwdriver caused audio to drop out! (Wait: Did that really happen?  Picked it up and set it down again.  Yes, it did!)

With its original tubes in, I could tap on anything in or even near the radio to bring on the problem. Flex the cabinet, poke at the dropping resistor, move a wire (any wire)!  It turns out, one tube mostly worked, one sometimes worked, and one worked every once in a while, but all made noise if you tapped anything in the room.

It looks like someone just re-tubed the entire radio (as if that's ever a good practice) with all NOS looking NU tubes, and they just developed or already had internal intermittent shorts.  I just re-tubed it again from another 70 chassis I had, and started swapping original tubes in one at a time until it got sensitive to taps again.  I chose the second detector position to test all of my 24A's one at a time.  Yep, THREE of them were bad!  I suspect a heater to cathode short, and now I don't even trust the one that seems to be working fine.

I don't know if the 5.6 Meg resistor was intended to help the situation, but it certainly did not!

I'm still puzzled on the theory behind adding a 5.6 Meg resistor to the 2nd detector plate going to ground.

Now that I've gotten my intermittent 24A's out of the way, I'll go finish the restoration and see if anyone remembers any kind of "fix" involving adding a resistor like this.

[morzh]

My first radio to restore was a 20, and the first tubes to swap were 24A.
I got some success but very soon realized that without a good tester it is still shot in the dark, so I bought me a Hickok 700A. One does not necessarily has to have a full mutual conductance meter but some sort of tube tester is a necessity, unless this is intended to be your last restoration.

[ntsc525]

Yeah, a tube tester is great, but only if you use it! ;-) I'm embarrassed to admit I have a few Hickoks around, and in fact, a 600A was right there, under not too much stuff (OK, another 70 chassis was on top of it that I was using for reference...)

Since the radio seemed to work at first, I didn't bother to test the tubes, but you can bet I drug the tester out and sure enough, one tube was good but if I tapped it, it would register a short, one was mostly shorted but I could tap it and clear it, and the third one was pretty much always shorted but the lamp would flicker if I tapped it.

Anyway, it was only my self doubt that had me looking under the chassis, and who knew there'd be three tubes with the same short doing the same thing! No wonder my B+ was dropping at different levels! I didn't listen to it much, and in fact when I bought it, I don't think I even let the tubes completely warm up when the guy was trying to demo it for me. He said it was "scratchy" and I knew I was going to recap and clean it.

I'm sure, however, that the B+ issue was not the reason for the addition of the 5.6 Meg resistor.

Maybe it was a workaround for something caused by a leaky capacitor block, which I fixed when I restuffed them all. Also, the two "250K" (240K) resistors at 32 and 25, as well as the "50K" (51K) resistors at 11 and 27 had drifted pretty high, so I replaced them also.


Dan

[morzh]

I no longer power up a radio without having checked the tubes prior to it. Granted even that is not a guarantee, but gives you some degree of assurance.

[ntsc525]

I'm of two schools of thought on that:

If a radio is AC/DC and not too expensive, I'll just boot it up to see where I'm at. (I'll usually use a dim bulb test at least, or else there's the thrill of wondering if any magic smoke will come out.) A transformer set that's been playing, I'll listen to it before I start, because it'll tell me right away if the transformers and coils are all good if I can get some kind of signal all the way to the speaker, and I'll know how bad the filters are.

In this case, the guy had it plugged in and on display in the living room, so he just switched it on.

If it's a garage find, or clearly one that hasn't had attention in a while, I'll ohm out all the transformers and coils, test all the tubes, and fire it up at 12 volts to see if the transformer outputs are a 10th of what they should be. Still, I'll always try to at least do a listen before I start work. It's risky sometimes, but at least you have an idea what symptoms were present before you started working on it. Had I noticed the B+ issue before I started, that might have saved me some time. But of course it was intermittent, so what can you do?

If I already know the radio's working (or at least trying very hard to work), then I may take some shortcuts, but clearly it burned me this time because of the 3 tubes.

I got into tube radios because intermittent computer problems are not enough aggravation in my daily life, so I need more things to fix in my spare time! ;-)

[morzh]

Ever fixed the CPU unit built out of discrete 74 series logic with microcode burnt into antifuse ROM with an intermittent problem?
In the heyday of PDP-11 mini series I did.
Honestly, any radio is easy compared to that.

[ntsc525]

I did work for Digital Equipment, but I started after the heyday of PDPs, and in the 90s, it was all board swapping until the problem was fixed. (So now you may understand why testing tubes wasn't my first instinct, and why I found the problem by substitution. Sometimes you have to swap all the DIMM's and move the originals back one at a time until the problem comes back. A 24A is just a single bit DIMM to me.)

I'd rather work on radios because (a) I can get a schematic, (b) I can easily see and replace individual components (and a slip of the screwdriver won't dislodge a dozen microscopic capacitors), and © I don't have to use an anti-static strap.

The great thing about 80+ year old radios is you get to have imaginary conversations with the long dead people who built them. ("Boy, you really wrapped that wire in there, didn't you? Don't you realize that a baby will be born in a few years, who will have a son, and that child, after aging well into his fifties, will judge your work?")

Sorry about the rambling post...

Anyway, about that mystery resistor. Theories?

[Radioroslyn]

<Anyway, about that mystery resistor. Theories?

I think someone had a spare resistor to get rid of. Like Mike said at 5.6M it's not going to drawn hardly any current, maybe a micro amp of so I lost count of the zeros. Essentially it's invisible.