Left channel BAD in stereo! HELP please! It's my first tube set - diyAudio
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Old 3rd July 2009, 07:40 PM   #1
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Default Left channel BAD in stereo! HELP please! It's my first tube set

Hi, I am a newbie in the wonderful world of tube technology.
I picked up a beautiful RCA Victor Stereo console at a thrift store for a mere $40. I am excited about it because it is my first tube stereo, it has FM, and to my surprise it can pick up shortwave with a apropriate antenna. I like the glowing blue magic eye too.
It was dead at the store for some reason, no lights no nothing.
So they chopped it down from $80 to $39. When I got it home, I took the back off and looked in to see all kinds of neat architecture of probably the 40's or 50's. I crossed my fingers and plugged it in, after a few secinds; the tubes glowed and the magic eye came on, and the dial lit up via two lightbulbs behind it.

Well I tuned in to an FM station, and followed the magic eye.
Rich deep sound came out of the left channel, but not the left.
I played with the ballance and if I turned it to the extreme left I could get weak sound on the left speaker.

I went into the unit and dissconected the unamplified stereo signal from the tuner/radio chasis, and fed it dorectly into some seperate speakers. I cranked the volume to the MAX and plugged the radio in. I got weak sound on BOTH channels, so there is left and right un amplified sound going INTO the Amplifier chassis.

There are two output transformers for each channel, I pray that the one for the left is not fried. There is a seperate tube for each channel so I switched them to see if it was a bad tube and no there wasn't a difference.

I only know basic electronics, and only basic understanding of the use of a multimeter. The machine is a big beautiful
RCA Victor with a turntable, FM/AM stereo speakers with bas and tweeters. Oh and I tested both speakers by touching a double "A" battery to the leads, they are both fine.

I am dying to get the old girl sounding good again. Its my first radio with tube goodnes. Thanks
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Old 3rd July 2009, 08:39 PM   #2
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Correction:

"Well I tuned in to an FM station, and followed the magic eye.
Rich deep sound came out of the left channel, but not the left."

SHOULD BE:

"Well I tuned in to an FM station, and followed the magic eye.
Rich deep sound came out of the RIGHT channel, but not the LEFT."
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Old 3rd July 2009, 09:49 PM   #3
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...."I went into the unit and dissconected the unamplified stereo signal from the tuner/radio chasis, and fed it dorectly into some seperate speakers. I cranked the volume to the MAX and plugged the radio in. I got weak sound on BOTH channels, so there is left and right un amplified sound going INTO the Amplifier chassis.'....

Maybe the problem is in FM stereo decoder....?
Try to change left and right output of stereo decoder.
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Old 3rd July 2009, 10:21 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by azazello
...."I went into the unit and dissconected the unamplified stereo signal from the tuner/radio chasis, and fed it dorectly into some seperate speakers. I cranked the volume to the MAX and plugged the radio in. I got weak sound on BOTH channels, so there is left and right un amplified sound going INTO the Amplifier chassis.'....

Maybe the problem is in FM stereo decoder....?
Try to change left and right output of stereo decoder.
From the description it does not sound like the tuner is the problem, but you need to proceed cautiously. First off you should never plug old tube gear into the wall without a thorough check out first, and the power supply caps should be replaced or reformed gradually - not brutally as when you plugged this thing in. (You are lucky no electrolytic exploded which is not an unusual outcome for this sort of mistake.)

That said you have identified that the tuner/radio chassis is working correctly for the moment and the problem resides in the amplifier section. It could be a bad tube, socket, open/badly drifted resistor or even a coupling capacitor. The OPT is least likely unless it got disconnected at some point in the past and was playing loudly at the time in which case internal arcing could result in a short in the primary.

You will need to find someone local who can perhaps guide you as you learn to rebuild the electronics in this console. Get some books on basic trouble shooting technique, tube era books so much the better. Morgan Jones has written an excellent book on hifi tube amplifier design aimed at people of all knowledge levels - "Valve Amplifiers" and I would get a copy right away if you do not already have it. You can get it on line at Old Colony Books (AudioXpress) or Amazon.

FM stereo arrived in 1961 so this unit is certainly no older than that.

You are very lucky you did not let out a lot of magic smoke when you plugged the thing in the first time.
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Old 3rd July 2009, 10:31 PM   #5
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---Civic..
Did you read the section in tubes titled "General safety....high voltages"??? Be very careful poking around on the inside of your pride & joy................she can & will bite you if your not careful.
We don't want to hear any electrocution stories from you on down the line!
________________________________________Rick...... ...
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Old 3rd July 2009, 11:42 PM   #6
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Okay, I just tried that, and no there is no difference.

Could a bad capacitor cause a channel to be dead?
I notice that there is a seperate capacitor atached to the socket of each channel. I am narrowing it down to an issue in the sound amplifier section, because I have found that the tuner section is putiong out proper L/R unamplified sound on both channels, so I think the issue is in the amp. I pray that it is not a bad transformer, there is a big transformer per channel one for left and right. But I think those are very expensive to replace.

I hope it is just a matter of a bad cap. Can a bad cap cause a channel to be bad?
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Old 3rd July 2009, 11:46 PM   #7
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Quote:
Originally posted by CivicProtection
Okay, I just tried that, and no there is no difference.

Could a bad capacitor cause a channel to be dead?
I notice that there is a seperate capacitor atached to the socket of each channel. I am narrowing it down to an issue in the sound amplifier section, because I have found that the tuner section is putiong out proper L/R unamplified sound on both channels, so I think the issue is in the amp. I pray that it is not a bad transformer, there is a big transformer per channel one for left and right. But I think those are very expensive to replace.

I hope it is just a matter of a bad cap. Can a bad cap cause a channel to be bad?

YES

But before you go indiscriminately replacing parts you need to learn some basic diagnostic skills and borrow an oscilloscope.
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Old 4th July 2009, 12:56 AM   #8
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Thanks for the warning about safety! I am learning more and more as I creep along. I'm enjoying playing with this stuff.
Actully, I am glad the thing wasn't perfect to begin with, it is fun to figure it out and have the fun of making it work again and learn along the way. I never touch anything unless I unplug the cord. I read about the possibility of something letting go with a bang on power up. For all I know, this old girl has been sitting in a must damp basement for ten years. Everything looked ok, but I understand the risk.
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Old 4th July 2009, 12:56 AM   #9
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Quote:
Originally posted by kevinkr



YES

But before you go indiscriminately replacing parts you need to learn some basic diagnostic skills and borrow an oscilloscope.

I see only two (electrolydic?) capacitors on the amplifier stage.
One is atached to the sperate output tubes, "left" and "right".

I did an ohm test on each of the capacitors. They are the exact same types with the same values. But the reading on the meter was different on both capacitors. I don't understand.

At anyrate I hear it is standard practice to replace all the caps in one shot even if they are good, they will probably fail soon.
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Old 4th July 2009, 03:43 AM   #10
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You could also do a few other basic things, like unplugging the tubes and re-installing them to get better contact on the pins if the unit has been idle for decades (with it powered down, of course), and checking to see if all the tubes are glowing with the unit powered up, that will eliminate heater/filament issues. You can also swap tubes from one channel to the other one at a time to see if the problem is tube related.

As mentioned, it'll probably be a good idea to change the caps at some point.

Another good beginner book is Bruce Rosenblit's Beginner's guide to tube audio design.

What tubes are in the console?
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