[morzh]

Jerry

You do not need the finned resistors or attaching it to the chassis. Keep in mind the Candohm relied on chassis for heat dissipation and were NOT rated for it without being attached to a piece of metal. The resistors shown above do not require any heatsink to be rated, so no derating occurs.

In my Zenith 9S262 I replaced Candohm with a terminal strip with resistors rated about 2-3 times actual worst case dissipation.

You do not need expensive, you need what works.

HOWEVER!!!

If you really want it, here it is!!!

http://www.mouser.com/ProductDetail/Ohmi...OagwhkI%3d

Mere $5.72. And hurry up, there are only 20 in stock.

[jerryhawthorne]

Morzh, thanks for your research. I missed that one as I was filtering for 5 watts, that one is 10. Have plenty of room.
Jerry

[jerryhawthorne]

As I promised in the revised title it's down again. I'm keeping this with the original thread. Worked for a while after the changing the cadohm out and slowly volume dropped to virtually nothing. Here is where I'm at. Great hum on the finger test, amp section seems fine. Ran a modulated RF signal into the antenna and chased it down the radio and all great until I hit the second detector 6K7G. Signal on the G1 strong, signal on G2/G3 nothing. Tube tests strong on my Precision 912.
Off for voltages:
Plate 35V
G1/G2 18V
Cathode 0 Grounded
Where is my plate voltage? Not certain what it should be but not that.
Resistance time:
Plate to ground 4K Seems right looking at the schematic
Measured R69 and R66, on the money, total of 16K

Any help would be appreciated, this radio has been a pest.
Schematic link"

Jerry
http://www.nostalgiaair.org/PagesByModel...013235.pdf

[TimC]

Thermal issue with tube or resistor?

Does it play again after cooling?

Tim

[jerryhawthorne]

No Tim, it is "stable" at all times. Extremely low volume on my local transmitter but blasting away (on my scope) to the grid of the second detector. Doesn't appear to be heat related. I should have been clearer. Each day I turn it on just for a check and for several days in a row it kept coming up lower in volume until such time (now) full volume can barely be heard.
Jerry

[morzh]

Jerry

have you checked AVC 6J5G tube and surroundings?

Like the cap #57, 110pF. See if it got silver mica disease.

[BrendaAnnD]

Those plate and screen grid voltages are pretty close to spec. It could be that one of the resistors feeding that circuit have gone high, but the factory spec is 20V on the screen and 40 on the plate.

I suspect the last IF can (52), and one of the mica caps therein that form the output filtering for the detector. There is a pi network in there consisting of two 100pf caps and a 51K resistor, any of which might cause your problem.

BTW, there should be no signal on G2 or G3, they are AC bypassed to ground. Your output from that tube is actually the cathode (yes, at ground), where G1 and the cathode form the actual detector. The other side of the signal path is the bottom of the IF secondary, as seen on the schematic. This, as I said above, is inside the can and runs through that pi network filter before going to the volume control.

[jerryhawthorne]

Thank you Brenda and Mortz , (ladies first). Mortz, would removal of the AVC tube upset anything in terms of testing that area? Obviously the set would lack AVC.
Brenda thanks for the explanation on how the detector is working. I see now what your talking about. Sorry but I didn't realize the detector plate was ran at such a low voltage. I was chasing ghosts but that is how one learns. Dang, those two caps and resistor are inside the can. Fun. Well, can off and I can measure the resistor but not the caps. Should I be able to put my scope on the input to the pi network and see an AF signal? I hope the secondary is not open but if it were I wouldn't get a solid modulated IF signal on the grid of the detector. Correct?
Appreciate both your assistance.
Jerry

[BrendaAnnD]

At the input of the pi network, you'll see a "dirty" audio signal. It will still likely have some RF (455 KHz) in it. But, you should see some audio there. Since those micas are an easy 100pf value, I would just go ahead and replace them and the resistor (50K 1/4 watt would be fine). If you need them, I can send you a couple nice little 100pf disc caps.

[jerryhawthorne]

Thanks Brenda, I need to check my stash but may take you up on it. Will let you know. Took off the IF can cover and checked the transformer and the resistor (all good) but alas, no caps. Philco documentation lied listing those three parts as a subset of the number for the transformer. They are about 4" away tucked under a bunch of wires and resistors! Fun exercise. As far as I can see about the only thing that's left is the two caps in the pi network or the one going to the AVC amp. May check the amp again. All three of the 100pf caps are of the domino type.
Still wondering if I just remove the AVC tube as test, would the radio still work? Might eliminate that as a source of the problem.
Regards, Jerry

[BrendaAnnD]

Pulling the tube won't do much, just remove the AVC action. I kind of doubt that tube would be your problem, though.

[morzh]

Actually if the cap in series with the AVC tube's grid is bad, removing the tube may have an effect.

[jerryhawthorne]

Thanks Mortz, I will give that a try today. Just as soon as I get the courage to go out to the shop.
Jerry

[jerryhawthorne]

Off to the shop to attack the pesky radio. One of my brain cells fired about 5AM when I couldn't sleep and I thought "what about the muting control"? Silly little devise in the tuning dial which I have had trouble with in the past. Naw, the finger test provided a very strong hum. Couldn't be. Thought a little more and remembered I was working on the top of the chassis at the time and instead of turning it over I applied the digit to the grid cap of the first AF tube not to the center tap of the volume. They are separated by a cap. This morning the first thing I did was measure the center tap to ground and indeed it was at ground. Works great now but need to do a little work on that muting devise to insulate it better.
Happy ending and I learned a lot about that type of detector and got to tear apart an IF can and not break anything. Can't wait to give this chassis back to the owner.
Thanks for all your help, Brenda and Mortz.
Jerry

[jerryhawthorne]

I know, I was an idiot on the last problem. That mute switch was giving me problems from the get go. I mentioned earlier in the post that after a while running the volume would drop. Let it cool down and great for a while, this was before the muting fiasco. Well it is now fixed at that lower volume and a little distorted. Voltage measurements are all over and low. The following were taken at 105V AC, I failed to check my variac setting. I did afterwards move it up to 110 but didn't change the values given below by more than a few volts.
Measurements were taken to ground with my trusty VOM and a few confirmed with a fluke.
Some highlights:
6L6s Plates running about 240V should be 290V
G2 running at 250V on both tubes should be 300V.

6Q7 First audio Plate running 100V should be 70V Double checked and with new 110V AC reads 105V. The only high voltage.

6J5 Driver audio Plate running 180V should be 260V

6K7s 1st and 2nd IF Plates running 170V should be 250V
G2 running at 73V should be 90V

6A8 Osc Plate running 130V should be 250V
Grids 3/5 running 75V should be 90V
Grid 2 running 125V should be 225V
Cathode to ground 2V no spec.

6A8 Mixer Plate running 200V should be 250V
Grids 3/5 running 75V should be 90V (corrected previous value)
Grid 2 running 75V should be 90V (corrected previous value)

6U7 RF amp Plate running 210V should be 250
G2 running 75V should be 90V

I should note that on powering up and measuring with the meter in place the first voltage to hit is well in excess of 300V, closer to 400V but then drops to the readings indicated. The rectifier tests excellent.
Any ideas gang? Schematic: http://www.nostalgiaair.org/PagesByModel...013235.pdf
Jerry