Fisher 400 - cleaning up after someone-else's rebuild - help!

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Poor Impulse Control
The previous owner did all the work you see below with a few exceptions: the output coupling caps were a mix of brands and values, 2 .039's, a small silver polyester .047 (I think) and a random old .047. I swapped in the 4 orange ones seen now in .047.

One of the output transformers was an original early 6K item, the other was a later 10K item. I bought another early transformer last week on the bay, and installed it last night. The replaced Transformer is the one closer to the MPX unit.

I brought it up on my variac and when I got to the point of playing, I got a loud hum from the side I swapped the transformer on. I turned the speakers off and found there to be good even sound going into the volume pot. While testing, the load resistors started to smoke a bit and the output tubes on the loud hum side were just starting to red-plate. I checked the voltage to pin 9 on the output tubes for both sides of that output transformer and found roughly the 395v shown in the schematic.

What gives? I'm thinking there is something wrong in the amplifier section on that side... anyone see anything obvious I missed?













 
Matt -- You've got the Blue and Blue w/tracer primary leads of your new output transformer reversed. This in turn changes the negative feedback into positive feedback, making that amplifier section act like an oscillator. Reverse the leads and all should be good.

Good luck with it!

Dave
 
Inclined to agree here. The striped vs solid wires are opposite on the two channels, not sure which is correct but one ain't. When it oscillates it will draw heavy current and do all sorts of dumb things.
 
HA! Thanks guys. Hard to see when you make really dumb mistakes! I was babysitting a 5 year old while trying to work and probably got distracted between figuring out which wire goes where, and actually soldering them in.
 
Duct Tape works well on UNRULY 2 to 6 year olds. Add a small piece to their mouth and you can work/sleep in peace.

Watching Annie!!!.jpg
 
Matt;

It is a good thing that you used load resistors instead of your speakers! I have done that in the past too. I had to use a generic replacement output transformer on an antique radio I was restoring and before I found the right polarity I had a really strong audio oscillator on my hands. On top of that someone in the past had defeated the negative feedback in the radio...:no:

Glad you have it working again!

Joe
 
With no NFB, the transformer polarity doesn't really matter. Once you connect it though, look out.
 
Alright - as anticipated the 400 is now kicking out the jams, so on to the next problem.

Next Problem: Testing with my phone going into the AUX one channel sounds very flat - I believe it's the left. If I put the balance all the way to that side it just sounds kind of bland, like highs and lows have been rolled off by an EQ - when I turn the balance knob about 10 degrees I've got a decent balance, but the other side sounds great - very dynamic. Controls have been cleaned with deoxit. I tried swapping input RCAs and speaker leads, no problems there. I rolled a known good 12ax7 into all the positions that would matter - no change. I still need to swap output tubes around - they were too hot to touch last night when I was shutting my work area down.

I am inclined to think it's the pec at the tone control. Anyone have any other ideas?

Is this the point at which I buy a known good oscilloscope and break out my physics lab books to remind me how to use it and then follow the signal path until I find the offending component?
 
Matt; Make an inexpensive signal probe. Take a standard RCA Cable and a .01uf 600v cap. Solder the center pin on the rca and one end of the cap together. The unmolested end of the RCA goes into the AUX input of another receiver/amp. The cap now becomes a probe. Adjust the volume on the testing receiver so it's fairly low but audible, as you'll have pops when initially touching spots on the 400 to listen to. You may have to ground the 400 and the testing amp together. The cap is there to keep unwanted AC out of the testing unit. Work backwards from the output's until you get equal sound from both sides. The problem will be between there and the previous test point.
 
As a simple test to help isolate the problem, short together the two "hot" or high side terminals of the volume control. The sound will go to mono of course, but it should sound the same from both channels. If it does, then the power amplifier section is eliminated. If it still sounds flat, you're still looking for issues in the power amplifier section.

If the test indicates the line/tone stage section, then next, try the source through the Tape Monitor inputs. If it is still flat, then the problem is in fact somewhere in the single stage of the 12AX7 acting as the tone control amplifier (i.e.,right after the tone controls in the signal path) for the affected channel. On the other hand, if the sound is good through the Tape Monitor inputs, then the problem in in the first stage right after the selector switch, before the tone controls.

Also make sure that there is a good connection at the SpaceXpander jacks. A poor connection there will cause the issue you're having.

Dave
 
Guys, thanks for the quick input. I do have a signal probe made as you describe Larry. I'll follow your elimination strategy Dave, and check my work with the signal probe.

I don't think the tuner is currently working, so there will be a third problem once this one is solved.
 
Is there a Fisher Switch interconnect deciphering thread somewhere? Based on Dave's comments above, I started tracing signal paths in the 400 and realize I don't really understand how to interpret the arrows and circles of the switch diagrams. Seems like a good chance to learn the fundamentals of something so I can reduce my dumb question load in the future.
 
Not a Dumb Question. Remember dumb questions are the one's not asked.

On the schematic is a legend for the switches. All switches should be set for the most anticlockwise position, and each source position is listed vertically. The front and back's are read differently. The front's are read clockwise, and the back are read anticlockwise. Say phono 1 is the top source and FM AUTOMATIC is the 4th source. The switch is set to phono1 on the drawing. Rotate the center part to position 4. You trace up to the switch, then rotate the center part to the 4th source and follow the path (cut out some shapes to correspond with the centers). Rotate as needed, then follow the path thru the center to the other end and continue on. Eventually you'll be able to do it in your head.
 
I saw the switch position table, and assumed the 12 circles around the circumference of the switch diagram were the steps as counted in the table, but it hadn't occurred to me the front and rear distinctions meant different directions since you were facing the control from opposite ends of the shaft. Front CW, rear CCW makes perfect sense now.

What about the connection made by the arrows and concentric semi-circles in the center of the diagrams? I assume an arrow with the point touching the circle indicates a connection, everything else indicates no connection?
 
That's right. The tab that sticks out is the connection for the input selection. The tab rotates as you turn the knob. As it sits now it's in position 1. As you turn it to each stop it contacts a different source. The output is always connected (the longer arrows).

The numbers in the circles correspond to the individual connections of the switch. This is what gets me everytime. Is 1 and 12 on the top (with the unit upside down) or is 5 and 6 at the top (with the unit upside down). I have to trace out a couple lines every day I do them. So much for short term memory. LOL!
 
reading through rotary switches gives me a damn headache. I've mostly given in to just using an ohm meter to verify where I am.
 
yeah, especially fun when the schematic doesn't bother to tell you what the positions are, or which direction the switch is drawn in. I spent a couple of hours tracing the switches out in a Knight preamp just to figure out how to convert the tape head input to a phono input. That one has a multi-section rotary switch mounted to a circuit board with a bunch of other parts on the board. If you want to talk about headaches, try one of those sometime.
 
My goal in this exercise, in addition to getting better at diagnosing where problems may lie in the circuit based on eliminating sections of it based on the kind of logic Dave outlined above, is to be able to review circuits on paper while at work during downtime to be more efficient with the little repair time I get at home.
 
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