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38-116 died the 3rd time! Guidance Please? - Printable Version

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38-116 died the 3rd time! Guidance Please? - jerryhawthorne - 03-15-2013

Some of you may have watched previous problems with this chassis of a friend of mine. It works like a champ, at least it did for quite some time. Dead now from a station standpoint. I can shove an IF signal through it. Seems the problem is in the front end. On my scope, I can see the output of the RF amp and when on station, looks good. After that things go down hill. Did some voltage measurements of the oscillator area and it is no wonder why it went deaf.
RF tube 6K7G, good on all voltages.
Mixer tube 6Ag8G measures well on all pins
Oscillator tube, 6A8G Pin 3 should be 290V, measures 0 Pin 4 should be 90V measures 60
Pin 6 should be 225V measures 0

Osc Control 6Sn6G Pin 3 should be 225V measures 0
Pin 6 should be 225V measures 0

Would seem to me somewhere a voltage source just ain't hacking it. Any ideas?

Schematic link:

http://www.nostalgiaair.org/Resources/235/M0013235.htm

RE: 38-116 died! - Radioroslyn - 03-15-2013

Have a look at 40
Terry

RE: 38-116 died! - BrendaAnnD - 03-15-2013

Check for voltage on both sides of that resistor. If there is none on either side, follow it back to 117, which looks to be a candohm type, and those are notorious for opening up.

RE: 38-116 died! - jerryhawthorne - 03-15-2013

Thanks Terry and BAD, will look into those ideas in the morning and report back. Appreciate your ideas, the brain is going dead!
Jerry

RE: 38-116 died! - jerryhawthorne - 03-16-2013

Well, got back to the radio this evening and with the great guidance, did a little resistance checking. The #40 resistor looked good, went to the cadohm and INDEED, the 14K section was open! Kudos to MAD. Strange that the 1K section is quite long compared to the 14K (open) section. At any rate, the only 5watt resistor I had was a 10K and jumped across the open 14K and brought up the variac slowly and indeed I'm getting some plate voltage on the Oscillator.
I'm going to replace the cadohm and install a couple of dale type resistors, can anyone give me an idea of the wattage required? I'm dumb at that stuff.
Thanks for your help. Your great!
Jerry

RE: 38-116 died! - jerryhawthorne - 03-17-2013

Looking at Mouser and Digikey for Dale resistors it appears the 1K are available but the 14K, nope. It seems the closest is a 15K which I think would be fine. Just need to know do I need 5 watt or perhaps 10 watt ones? Would like to order but need a little help on the wattage. Perhaps the Philco P/n would help if anyone has a reference. P/N 33-3289

Jerry

RE: 38-116 died! - morzh - 03-17-2013

I think 5W is OK; 15K should also be OK.

RE: 38-116 died! - jerryhawthorne - 03-17-2013

Morzh, nice to have you around. I will place my orders tonight. Did bring it up on a 10K for the 14K and the radio sang! Love that sound. Now to order!
Thanks Morzh. Jerry

RE: 38-116 died! - morzh - 03-17-2013

Jerry

I saw similar wattage in similar places in Philcos, but here how I went about it:

You have about 300V going to that 14K resistor (I took the voltage of the output amp plates), and then the 14K after filtering forks into 10K and 40K resistors. If we took the worst case (which it is not) where both 10K and 40K resistorts go to GND then we would get 8K in series with 14K, hence 22K total.
Now we take 300V square = 90,000 and divide it by 22,000 which will get us about 4Watts total dissipation.
But this is split between the 14K and 8K, so the 14K will get (4W/22)*14=2.5W.

At this worst case dissipation 5W is plenty.


PS. Make sure when buying resistor the datasheet rates it at full dissipated power, no cooling.

RE: 38-116 died! - jerryhawthorne - 03-17-2013

Thanks Morzh, you know the math, I use to!
Searching now to find a couple of resistors.
Jerry

RE: 38-116 died! - morzh - 03-17-2013

Mouser's good, has pretty much everything.

Oh, and don't be afraid to combine values if the right one does not pop up or, as it often happens, is available but too expensive (non-typical). I do it all the time.

RE: 38-116 died! - jerryhawthorne - 03-17-2013

Morzh, I'm getting depressed. I went through both Mouser and Digikey and I must admit my computer skills for search these things suck. I could not find with either where I could get a 1K five watt or anything around a 14K 5watt without buying in bulk on a chassis mounted resistor. I think I will go and have a strong drink!
Jerry

RE: 38-116 died! - morzh - 03-17-2013

Wait up...let me look. I just have to go on a regular comp - the iPad does not work with Mouser search.

RE: 38-116 died! - morzh - 03-17-2013

Jerry

http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0wui7Z1yzekj6Z1z0z819

Pick one you like.

I like this (it is cheaper)

http://www.mouser.com/ProductDetail/IRC/W2151502JALF/?qs=sGAEpiMZZMtlubZbdhIBIMHG%252bmC7eAcWstpHW1OQeJ8%3d

This one is even better, 7W (yes 5% but it is fine)

And this is your 10W one,. 3%

http://www.mouser.com/ProductDetail/Vishay-Dale/CPW1015K00HE143/?qs=sGAEpiMZZMtlubZbdhIBIJKpT9GAIysMzeaY6IhWlvk%3d


And others 10W

http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1yzekj6Z1z0wt4w

Pick to your heart's content.


Most are STOCK!

PS. Considering the price for 7W and 10W is good, if you need a bigger size (I don't know what Candohm dictates) you could go with those too!

RE: 38-116 died! - jerryhawthorne - 03-17-2013

Mortz, you are the cosmic shopper. I was focusing more on the expensive aluminum fin chassis mount resistors. I like to attach to the chassis for better heat distribution. To heck with the price, this is for a friend!Icon_biggrin
I like to remove the old cadohm and JB weld the aluminum ones where the old came out. Makes for a rather neat instillation with no need for a terminal strip. Again, a 14K or about and a 1K both at 5watts or H**l, even 10 as I have some room length wise where the old one is. Just can't find that style for both resistors on Mouser. Again, my search skill suck.
Jerry