Fisher 400 Left/Right separation.

djef

djef

Active Member
I recently hooked my Fisher 400 back up and I've noticed it has developed a vexing channel separation problem. No matter which setting, stereo or mono, I have it on, it sums the signal to mono. Also, the phase is messed up somehow. Vocals and instruments disappear on recordings such as Beatles albums where they do a lot with phase and stereo.
Any suggestions? It does this with phono as well as FM.
 
djef -- This sounds like the common leads of your speakers are connected together, but not connected to ground. This would result in your speakers effectively being wired in series, and then connected across the hot output terminals of the two channels. Such a condition would cause the problem you are experiencing.

Dave
 
Dave- No such luck. The common end of the speakers are connected to ground. That does seem like the most probable answer though.
 
OK. Then how about such a basic test as this: Plug a high level input into only one channel's Aux input and see what happens. The channel without a signal applied should basically be silent. Let us know!

Dave
 
OK
I plugged my ipod into only one aux channel and sound is equally loud from both channels. Same result whether I plug into right input or left input.
 
OK. Next test: With the ipod into the Left Aux input only, pull the phase inverter tube of the Right channel out and try the test again to see if the sound still comes out of both speakers. Let us know!

Dave
 
I had a similar issue with my Fisher 400 when I had a small whisker from the left channel touching the right channel speaker.
 
Ditto, I had an issue when I had a solder blob short out a channel. Because of flux or chassis flex it took awhile to appear. Couldn't see it because the terminal strip was right behind the front of the chassis. So much for trying to be sure I had enough solder on the joint (I am now a minimalist).

Although Fisher is not the worst (as far as tight assembly) there is always the issue of generating more problems than you solve.
 
Maybe some pictures of the bottom side with cover removed can help reveal the tale tale signs of what happened to this 400,
some of these AK members have seen so many underbellies of the Fisher 400 and someone just might spot something out of the ordinary.

Sometimes little critters crawl inside and make a mess of something.
 
Dave, it looks like you're on to something. I plugged the ipod into the left aux jack, pulled out the right phase inverter tube, and the right channel turns into a loud 60hz buzz with that tube out.
 
IMG_7306.jpg
 
That is very, very strange if that happens. With no phase inverter in place, there is no signal to drive the output stage to produce that hum. Can you confirm which tube you pulled out? Also, with all tubes in place, does the balance control still function? I know you say it's a weird signal, but can the weird signal still be shifted from left to right and back again?

Dave
 
The tube I pulled is V13, the 12AX7 right in front of the output transformer.

As far as shifting from right to left-
If both inputs are plugged in, turning the left/right balance knob doesn't isolate the signal to only one channel, but it does attenuate it a little bit on one side.
However, if only one input is plugged in, say the left for example, then turning the balance all the way to the right completely cuts off the signal being evenly sent to both channels.
 
dj -- If an input into the left channel only causes no sound when the balance control is turned full right, AND, a right channel input only causes no sound when the balance control is turned full left, then it is an absolute fact that your channels are shorted together somewhere between the wipers of the volume control, through the phase inverter and output tubes of each channel, and ending at the output terminals.

The most logical place is at the output connections, or if internally, at the speaker on/off switch and its wiring.

Try connecting your speakers directly between the COM and proper impedance tap in each channel, and DISCONNECT the impedance tap lead coming out of the unit for both channels. If your Fisher operates correctly now, then you have a short connecting the two channels together in the speaker on/off switch, and/or the wiring associated with it.

Let us know!

Dave
 
Dave you´re right

It happened to me when i was connecting the wires from the balance pot to the volume control, so those are swapped!

Dj check the wires position in that area.
 
I'm still hunting down this short. I tried connecting the speaker directly to the impedence tap, like Dave described, and no change. So it's not with the speaker switch.
 
Comparing your picture with some references I have (I don't own a 400, only 500c's) it seems the wiring around the balance control has been moved. I suggest you check this area carefully and if you can do the chop stick test with the unit operating.

What's with the wire nuts? I would suggest doing a better job, if you have to do spices solder the wires and use doubled up heat shrink.
 
If the output connections are confirmed good then, the next most likely area to check is the volume control, balance control area.

Along with Drews' comments, I would check the volume control area equally as well, as it no longer appears to be rotated in the original position as built by Fisher.

Let us know!

Dave
 
I can't seem to find any shorts.
I wonder if the problem is in the balance pot. It's a 500k pot. If I measure from left tab to the wiper (ground) it goes from 0-160k. Measure from right tab to wiper goes from 0-106. If I measure from left to right tabs, it is 106 turned all the way left, 160 turned all the way right. This doesn't seem right to me.
I might have a 500k pot in my parts box. If not, I'll try sending both signals to ground via 250k resistors, since the pot is 500k. Or do I even need to bother sending them to ground, such a high impedance is basically open, right?
 
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