Balance woes with an old receiver

OK I'm trying to bridge the contacts where the balance knob used to be on a
Mitsubishi R11 receiver. I've completed this operation successfully on a Sanyo
and it was quick and simple, but the Mitsubishi receiver is doing nothing but
confusing me. The balance knobs on these old receivers always go bad and I
prefer removing them rather than trying to fix or replace them. With the Sanyo
I just removed the balance knob and bridged contacts on either side of the
wiper where the pot was - so simple! But when I try this on the Mitsubishi, I
get the channel I bridge coming over both Left and Right channels as mono, and
the level is somehow decoupled from the volume knob! I managed to poke around
and find a way to bridge the channels so that the volume knob works, and the
channels are 'weighted' in their proper directions, but the left still 'bleeds'
slightly into the right channel and the right channel bleeds slightly into the
left. This is not acceptable, and seems pretty weird to me. Could it be that
balance knob (which was two stacked 250k pots) used the 250k resistence of the
pot turned all the way in either direction to silence the bleed of one channel
into the next? Has anyone ever seen an amplifier wired up like that? Seems
like a really poor way to wire an amp, if that is indeed how its supposed to
work. The Sanyo had the balance pot grafted to another pot beside it in order
for the one knob to turn two channels, whereas the Mitsubishi has the second
channel pot stacked behind the first. Perhaps one was wired in series and the
other parallel? I even found a pdf of the service manual for the Mitsubishi
and it doesn't help with this at all.

Any comments whatsoever would be appreciated...

 

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:

http://www.hifiengine.com/download_centre/index.php?mitsubishi_da-r11_service.p
df

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.

 

Your link doesn't work

 

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

http://www.hifiengine.com/manuals_type.php?man=Mitsubishi&mod=DA-R11

 

Connect a short wire jumper between the "center" pin of each pot,and one
of the outer pins.
The other outer pin is ground,if you have a Multimeter,check which one
of the outer pins reads 0-ohms to ground.Connect the short jumper from
the center to the opposite (un-grounded) pin.

 

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:
<http://www.hifiengine.com/download_centre/index.php?mitsubishi_da-r11_service.pdf>
so they don't break.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

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.

 

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:
http://www.hifiengine.com/download_centre/index.php?mitsubishi_da-r11_service.pdf

 

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?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

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.