Balance woes with an old receiver

This probably doesn't help much, but can't you check the old balance pot with a meter and see how it works? That should solve any mysteries.

Mike

Thanks for the reply, Mike. But it's not so much how the pot works that is the problem, the problem is how the circuit works in conjunction with the pot. The pot is fairly simple, (though I will check it with a meter to be certain, that was good advice) either the back pot is wired in reverse to the front pot or the circuit is plotted in reverse on the pcb (so that turning the knob causes one pot to wipe one direction, while the other pot wipes the opposite direction , so that as you turn it to the left the left channel gets louder and the right channel gets quieter, and vice-versa). What I don't understand is, where the hell are the 'clean' or 'isolated' right and left channels? Like I said, if I bridge the pot contacts I either get the channel I bridge coming over both left and right headphones/speakers, or a louder left or right channel but with some of it in the corresponding channel at a much lower level.

If anyone really wants to waste some time helping me, take a look at the service manual to give yourself some idea of what I'm dealing with:

As you can see, if you look at the service manual, the balance knob, loudness knob, treble, bass, and tone defeat are all on the same little PCB. If I can't get the balance bridging sorted out, I'll probably try to bypass this board entirely, so any tips on doing that would be appreciated as well.

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You're right, that's odd. Well just go here and then click the link:

It looks as though there might be a discrepency in the manual between the schematics, and the layouts. The schematics quite clearly show that the balance arrangement is absolutely standard for a dual pot system. They are connected exactly as if they were two additional volume controls ahead of the main volume controls. The difference is that they are connected differentially, so as one turns up, the other turns down. To accomplish this in a 'smooth' way, one is almost certainly log taper, and the other reverse log. Both will have their wipers at the same resistance from one end, when they are the centre, but it will be the *opposite* end for each pot.

Again, if you look at the layout diagram, you can see quite clearly that the left and right inputs to the tone and volume circuits come into the board on points 24 and 26, and with the vol / balance assembly facing you and to your left, point 26 goes to the *left* end of VR401's track, whilst point 24 goes to the *right* end of VR301's track. Then it all goes pear shaped. According to the schematics, signal passes from the balance pot wipers, through the loudness arrangement and off the tone board at points 27 and 29, and next fetches up at the attenuator board to become the signals for the top ends of dual pot VR303 / 403. I'm assuming that is the main 'volume' control. The signals then apparently leave this board on points 30 and 32, to return to the tone board, where they continue on via R355 and R455. All seems good so far. The layout diagrams show an 'attenuator' board with the appropriate pot and wiring point numbers on it.

However, according to the layout diagram for the tone board, this is not the way it actually is. If you follow the print around, the wipers of the balance pots (correctly) connect to the loudness pots, but then, instead of the signals going off-board to the attenuator board, and then returning to the tone board, they actually stay on the tone board, and go to VR302 and VR402, which appear to be mounted behind the balance pots, making this a quad-coaxial control. These two pots must be the volume control, but do not appear on the schematic under those ref numbers. The wipers of these two controls then pass (correctly) to R355 and R455.

So, does the version you are working on have the balance and volume all on one pair of concentric knobs, or does it have the volume control separate ? Either way, you should be able to defeat the balance control by shorting the centre and left pins on the front-most pot, and the centre and right pins on the next pot back. This should not affect channel separation figures at all, as you are making no connections between channels. It might not be a bad idea to disconnect the ground-y ends of the balance pots.

Arfa

Put long URLs inside like this:

so they don't break.

Boy you really took a close look at things, didn't you! Thanks for doing so. I also studied the schematics a lot last night so I think I know what you're getting at. The attenuator is actually 'volume', and you're right, one drawing shows it being on the tone board, while the other shows it going off to its own board and coming back on. The latter is the actual case - the 'attenuator' (volume) knob loops out from right behind the balance pot. VR302 and VR402 are the volume knob, and behind the balance knobs are two sets of three wires to loop out to the volume knob and right back again. Now, since the signal eventually leaves the tone board at points 41/37 (approx, I'm not looking at the schematic right now), what I'm going to do is bypass the tone board altogether, by taking the input to the tone board, at 24, 25, and 26, and splice those wires to the wires going out to the volume knob, then I'll take the wires coming back from the volume knob and splice them to the wires that I'll lift off of points 41/37. That way the signal will go straight from the selector knob to the volume knob, then to the amp, bypassing the tone board altogether, which is definitely the best fix possible, as far as my needs go.

Anyway thanks to you and everyone else who looked into it for me, I managed to figure it out on my own but if I hadn't I'm sure I would have with your help.

Yeah that might just work, but like I said I've decided to bypass the tone board altogether, because even with the tone defeat switch, the loudness control remains in circuit and I would prefer as clean an amplifier as possible, and it's kind of lucky that this particular model is built in a way in which bypassing the entire tone board is very easy.

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Yes, but that wasn't the problem in this case. Even though the link is supposedly the correct location of the document, it won't go there directly.

Or use a newsreader that's smart enough not to break 'em:

On Thu, 04 Oct 2007 04:06:50 GMT, snipped-for-privacy@watspam.com (Tuner Watson) put finger to keyboard and composed:

Set the balance control to its midpoint, measure the resistances between the wiper and each end, and then replace the pot with discrete resistors.

- Franc Zabkar

Why change software when I like the way it works?

At the midpoint there is no resistence. Do you understand what a balance pot does?

OK, so given that it's a matter of which two of the three legs you are measuring, but the "output" side of each channel of the balance control would be near zero ohms to ground only when the control is turned over to the other channel. If for example the wiper of the balance pot is connected to the input coupling cap of the next stage, you would measure near the full value of the pot from that point to ground when the control is centered.

Mark Z.

The balance control consisted of two pots stacked together, which I mentioned in my original post. At the center position, both pots are supposed to be wide open.

Anyway I solved the problem weeks ago, and I appreciate the responders who responded back then who were actually quite helpful, unlike you johnny-come-latelies throwing in belated obfuscation due to not reading the first post of the thread...

That was unnecessary. My post was accurate and relates equally well to a dual pot, in fact that is exactly what I was talking about. Further, I was only responding to a recent post - not to your original. I have been on this group for years and have earned the respect of those whose shoes you probably aren't fit to shine.

'bye.

Mark Z.

I would hardly call Mark a johnny-come-lately. Anyone who reads this group very much knows his skill and knowledge are far beyond all but a few of the posters that you will see. He is straightforward and helpful. You have to be reading from a very strange perspective to see obfuscation in he attempt to help.

Leonard

AND MARK'S ALSO AN ASSHOLE TOO. GO SHOVE A CORTELCO FONE UP YOUR ASS.