I'm working on a Wards Airline chassis 15W-2761A. I replaced all the was paper caps, and the main filter cap, so far so good. Switched on to AM I was able to get some reception, but it's distorted. The plate resistor from the 6BA6 1st IF amplifier was hotter than a two dollar pistol, faintly smoking. I know it's important to keep the grey smoke in your electronic circuits . That the resistor is getting so hot seems to indicate that I've got a short somewhere. But I don't know where to look or for what. Any suggestions ? Thanks in a advance.

Is R13 the 1K Ohm you are concerned about?
If so find which end of R13 connects to C42 and C65. Then check from that end to the chassis with an ohmeter. If you see a low value there is a possibility that one of the two capacitors are shorted. Also could be a wire touching the chassis.

Corby

It's actually R-37 that's getting hot. I tried to post a copy of the schematic but was unsuccessful. It's been tough learning how to trouble shoot these beasts on my own, but it's nice when I get it right.

Thanks

The Airline radio you posted has the schematic shown.

Is this the correct radio. I don't see R37.

Corby

http://www.nostalgiaair.org/resources/184/M0011184.htm

OK found R37 in the 6BA6 RF amplifier stage.
Measue each end to the chassis with Ohmmeter and see what you get.
Try in all 3 positions of the AM/FM/Phono switch.
If you find a low impedance try pulling the tube. I don't see a component likely to cause it to smoke. Suspect a wire touching the chassis.

Corby

Thanks...OK, with the tube in place it read "open circuit" no matter what position the AM/FM/PH switch was in. However, with the tube pull the ohmmeter read as follows. When measured from the switch side of the resistor PH "open circuit", AM 32.7 ohms, FM 32.6 ohms. When measured from the tube socket side pf the resistor I read PH "open circuit", AM 30.37, FM 30.35. Does this indicate a problem with the tube or tube socket ?

Thanks !

Seeing that you refitted the set; make sure that there is no solder, or protruding wire on the load side of the resistor, hitting an adjacent pin, a centre pole, or the chassis.

Marc

Marc,

Thanks for the advice, I double checked and there were no offending solder blobs or similar pieces of conductive detritus interfering with the tube pins.

Check tube for shorts, esp heater cathode, or substitute.

Marc

OK. Kindof odd that the readings looked different with the tube in!

The 30-32 Ohm readings indicate some sort of short somewhere!

Anyway with the tube out I would read across the C27 as it could be shorted. If youreplaced it make sure its leads are not shorting on anything.

Also check R4 resistance.

Corby

Hi

What voltage do you have on pin #5 of the RF 6BA6? Schematic shows 230 volts. If it's much lower R37 will overheat.

Does pin #6 have proper voltage? If too high plate will draw extra current and R37 will overheat.

Pin #1 should be zero or slightly negative.

C4 could be shorted but not common being 100pf.

True that a lot of SM's & ceramics do not often fail; but they can never be overlooked. The ones with HV B+ on them are the worst offenders. I have had one set where I had to replace the lot along with all of the resistors, and the rest of the caps?

Normally I use an insulation tester on them. Unless it is a dead short, or close to ohmeter is a waste of time. One end out of circuit for a test like that.

Marc

something shorted at 6BA6 from the plate to ground somewhere. either the socket or pin connections. Im thinking that if C4 is short then the next tube 12AT7 would cook

Some genius decided to put silver on the pins of some tubes. That has been known to track across.

It would be an idea indeed, to have a really god lookat the base of the socket. some of the wafer ones often break chunks out & release the pin/s. Is it actually a 6BA6 & not 6BE6, or something else that does not belong?

Marc

OK voltage at the plate seems high, on AM it registers 327 volts on FM about 320 volts, on the switch side of the R37 I get similar voltages. When I select phono the voltages drop to 20 on the plate and about 15 on the switch side. Guidance is welcome ! Thanks

Hi

Voltage will drop off in phono position as B+ is disconnected. B+ is high but your readings shows R37 should not be burning. You read high voltage on both sides so little is dropped across the resistor. Is the resistor still getting hot?

IMPORTANT : please disregard my earlier voltage readings, they were inadvertently taken with the the tube OUT of it's socket. I've got the radio upside down to work on it, and as I haven't touched it in a couple of days I'd forgotten that the tube was pulled.

NEW VOLTAGES

Im only getting 137 volts on the plate (pin 5) and 245 volts on the switch side, and the resistor is cooking itself.

The tube tests good with no shorts. I'm still checking other caps and resistors. This set is giving me a headache !

Thanks for all the help !

Since you have 3 6BA6's shouldn't be too hard to swap them around of take them all out and quickly take some readings. Leave the others in. I think you also should have a good look at the band switches, follow the volts. These damn things sometimes short out or arc. Tube sockets are sometimes bad too, but much less likely with a small signal tube that doesn't get hot.

The tube is either gassy or grid #1 is positive? What voltage do you have on pin #1. It must not be positive.

Norm,

Grid 1 is positive. When I switched the set on the meter showed negative voltage, this rapidly climbed. I stopped watching the meter and switched it off when I saw 75 volts (positive) on the meter. What does this mean ?