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37-93 hot resister - Printable Version

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37-93 hot resister - Zapped - 08-20-2009

Hi, new to the phrorum and relatively new to tube radio restoring. I'm currently working on a 37-93, http://www.nostalgiaair.org/Resources/228/M0013228.htm . I have replaced all capacitors, all out of tolerance resisters and cracked/brittle wiring. Here's my problem, the first time I turned it on, after a few seconds, the 200 ohm Bakelite (23) resister let out all its smoke Icon_sad . I replace it with a 2 watt, 200 ohm metal oxide, turned it on and it started smoking. I have swapped out all tubes except 6R7G (no spare) but I took it out, checked resistance readings, capacitor values, can't keep it on long enough to take voltage readings. I can't find anything that out of place but every time I turned the radio on, the resister starts getting hot. Any ideas out there?

Re: 37-93 hot resister - philco phan - 08-20-2009

hi,lift the end of the 200 ohm resistor connected to the electrlytic cap and retest,if o.k.,short is towards the left on schematic,if still getting hot short is towards output tube.
phil

Re: 37-93 hot resister - Zapped - 08-20-2009

OK, I lifted the resister and it stayed cool. I took some voltage readings and here what I got.

The plates of the 5Y4G = 368 VAC
The center tap has 134 VAC on it
The filament voltage was 279 VDC

Let me clarify where I lifted the circuit, I lifted the connection at the 8 mf capacitor, 6A.

All ideas welcome.

Re: 37-93 hot resister - philco phan - 08-21-2009

hi,you will need to check the circuit to the left,from lifted end of resistor. make sure the electrolytic polarity is correct. check for solder blobs shorting to chassis.measure resistance from lifted end to chassis and see what you have.
you can also pull the 6r7 tube and reconnect resistor and check for short.
phil

Re: 37-93 hot resister - Terry Burns - 08-21-2009

Hi Zapped,
Check to see if the 10 mfd cap (part #6) has the polartity correct. Looks like the positive end goes to ground and the negative connects to the 200 ohm resistor. Make sure the 70 ohm (part 13) connects to ground. This is going to give you negative bias for the 6f5 output tube. Maybe -15 volts. You can also check resistance between center tap and plates of your rectifier tube. They should be within maybe 10 ohms of being the same.

Terry

Re: 37-93 hot resister - Zapped - 08-21-2009

Thanks for the input.
I have checked the resistance from the resister to chassis and it reads a nominal 270 ohms.
The resistance readings for the transformer seem right, 228 ohm and 249 ohm to center tap, about 479 ohm across the plate leads (schematic says 230 ohm and 250 ohm to center tap).
I did catch the 10 mfd. Capacitor. Luckily I read it on this forum. (Not used to connecting + side of electrolytic to ground).
I'll check the other suggestions when I get home.
Does the 134 VAC of the center tap of the transformer sound right?

Re: 37-93 hot resister - philco phan - 08-21-2009

with 270 ohms to ground something way wrong,you are only measuring the 2 resistors in that string,check everything upstream of those 2 resistors.
phil

Re: 37-93 hot resister - Zapped - 08-22-2009

Everything checks ok, resistance, capacitance, wiring.
I lifted the 1.5 meg. (16), still got hot.
I lifted the wire going to G3 of the 6k7G where it connects to the 70 ohm (13) and 200 ohm (23) and the resister remained cool.
I had already swapped out the 6K7G and the resister still got hot.
This thing has me stumped Icon_mad

Re: 37-93 hot resister - philco phan - 08-22-2009

with everything hooked up,measure ohms from g3 terminal on the socket,tube removed,to ground.
what do you measure?then measure from the same terminal to the electrolytic,what do you have there?
phil

Re: 37-93 hot resister - Zapped - 08-22-2009

Phil

G3 pin to ground is 61.5 ohms
G3 pin to electrolytic is 150 ohms

Thanks for taking the time to help out.

Re: 37-93 hot resister - philco phan - 08-22-2009

lift c17, end towards b+ and retest with everything else connected,tube still removed,does 200 ohm still get hot?
phil

Re: 37-93 hot resister - Zapped - 08-22-2009

C17, is that the 110mmf. across the diodes of 6R7G?

Re: 37-93 hot resister - philco phan - 08-22-2009

yep.
the minimum resistance measured from g3 to ground is wrong,at least should have been 70 ohms,shortest path to ground is the 70 ohm resistor.also lift 70 ohm and 200 ohm resistors at thier junction so that all you are measuring is the 200 ohm and the electrolytic cap,if around 200 the cap is reversed or shorted.
phil

Re: 37-93 hot resister - philco phan - 08-22-2009

i've posted this topic on a.r.f. for some more help,hang in there,well get it fixed!
phil

Re: 37-93 hot resister - Zapped - 08-22-2009

I checked the electrolytic (6) and it checks good, correct polarity + to ground.
I lifted all the electrolytics and now I'm getting a nice blue arc from the power transformer to chassis. looks like i'll be changing out a transformer.