Emud Model 911 Audio Issues

[vortalexfan]

Originally Posted by Electronic M

May want to check if you have B+ to the outputs.... Especially if it has an eye tube And that isn't glowing green.

The tuning eye tube is working (it glows a bright green, probably the brightest glowing tuning eye tube I've seen on any of the radios I've worked on that had tuning eye tubes in them), but it doesn't respond to signal changes when tuning across the dial in AM, FM or SW; so I don't think its a B+ issue, unless the B+ issue can only affect the output tubes and none of the other tubes?

[vortalexfan]

OK, So a little update on this radio, I was able to get this thing to put out some audio, but its only coming out of the left channel, and its only coming out of the midradge and tweeter the woofer isn't working at all and the volume had to be cranked up full blast to even hear the audio.

I checked the preamp tubes (the 12AX7s that go before the 6BQ5s as this is a push-pull system) and they were fine (I tested them out in my Zenith Console amplifier) and 12AX7s didn't seem to be the problem.

Next I checked the 6BQ5s and there was nothing wrong with those either (they checked excellent on the tube checker) and just to be on the safe side I subbed in some known good 6BQ5s in my tube stash and still no dice.

Then I thought maybe I should try checking the 6CA4 rectifier tube, and I tested it out on my tube checker (a knight kit unit) and it pegged out the meter on the good side but then I checked the tube for shorts and pin 3 was glowing on the shorts test (it glowed for about 3 seconds and then slowly dimmed out.)

My question is, bringing up the idea that was brought up earlier about the low B+, would a bad or failing rectifier tube cause a symptom like that (low B+)?

[Penthode]

In a situation like this, there are some basic tests I like to perform. First check the voltages around the 6BQ5s. Ensure the plate, screen and control grid voltages are correct. Plate pin 7 and Screen pin 9 should be roughly the same between 200 and 250 volts. Control grid pin 2 bias should be around -8 volts.

If that is okay, inject a signal to the control grid of each 6BQ5. I like to use myself by touching a screwdriver to the control grid and listen for a faint buzz. If you are not used to doing this, you make wish to use another signal source such as capacitively couple the filament supply or a separate signal generator. A two or three volt p-p input shuld provide a noise.

Have you checked the voice coil continuity of the loudspeakers? Or tried connecting a transistor radio output to the speakers?

If the 6BQ5's are okay, then you move back. The cheap coupling capacitors to the control grid of the 6BQ5's are always suspect.