Fisher Model 40 power amp module troubleshooting

jberger

jberger

Super Member
I've been cleaning and repairing a Fisher Model 40 (pictures at the prvious link http://www.audiokarma.org/forums/showpost.php?p=8187928&postcount=51

I've been through several rounds of Faderlube, which fixed a lot, but I still have low output from the right rear channel. I've pinned it down to the Power Amp board (since it's a quad unit, there are two identical stereo boards in this unit).

Feeding a signal into the Input post on the board results in proper output from the left rear channel; feeding it into the right channel gives a much lower level signal, one that is symmetrical on a scope, and output varies with input level.

Because the output is symetrical, I'm thinking that it's not the driver or output transistors (only one half of the waveform would be affected, right?). I'm suspecting either the predriver Q805/806 or the input transistor (Q801/802). Unfortunately, Fisher built them so well that each leg of the transistors have insulating sleeves, so I can't easily get a scope on them to see how they look before and after, and I'll have to remove the board from the chassis to get to the back.

So, before I disassemble this further, am I on the right track?
 

Attachments

  • Fisher 40 power amp schematic.jpg
    Fisher 40 power amp schematic.jpg
    71 KB · Views: 38
  • Fisher 40 power amp component list.jpg
    Fisher 40 power amp component list.jpg
    113.3 KB · Views: 14
With the symptoms you are describing, the next component I would suspect would be C821, the large coupling cap feeding the speaker output.

In my experience, SS devices either work correctly, or they don't work at all. With the direct coupled design of this unit, a failure of the input or pre-driver transistors would upset the bias levels ultimately all the way to the output stage. But since you're still obtaining a symmetrical waveform at the output, that would imply that the bias system is still in tact.

On the other hand, a weak output coupling cap would not allow the normal current flow through the load via the top output transistor on the charging cycle, and also not allow normal current flow through the load via the bottom output transistor on the discharge cycle. This then would still produce a symmetrical -- albeit reduced output, consistent with the symptoms you are having.

Let us know, and good luck with it!

Dave
 
I had similar mismatched gain issues with an NAD solid state amp. It turned out to be a dried out electrolytic that was affecting the feedback circuit. Basically one channel was getting more feedback than it should have, so the gain tanked. Symmetrical output and all that, just the sensitivity was bad. It could be driven to full power but it needed about 3x the input signal to get there.

Anyway I think the equivalent on this one would be C805, or whatever that 220uf 35v electrolytic on the input transistor is. It might be C806, I can't quite read it.
 
If you find that those caps are bad, you might as well shotgun the whole board with new caps. If you don't do it now, you'll find you have it on the bench more and more for signal related problems as the rest die off.
 
Thanks, especially dcgillespie. I know that that particular cap is bad, as I can see leakage; now I know that it could be causing the main problem too. I plan on changing all four of them (quad, you know). I'll go ahead and do C805/806 as well.
 
I've been trying unsuccessfully to find cap #CE-22324-47, a 1600 uF 50v axial electrolytic. I tried Fry's, Parts Express, and was overloaded and unsuccessful at Digikey. I' m also trying to find the axial lead bulbs for this unit. Any pointers on current part numbers or sources? I'm a little unsure on what caps to order for this.
 
Use a 2200uf 50V axial. Mouser part # 647-TVX1H222MCD($5.04) it'll work fine, and the size is almost the same. You could use a 2200uf/63V also. 647-TVX1J222MDD ($5.72) plus shipping.

Any 1600uf or 1800uf caps @ mouser start @ $15. and go up to $44. I don't think you want to go that much for one cap. And they are radials to boot. The TVX Nichicons are fine working as coupling caps. I use the 50V version in my klh 20's.

Axial lead lamps you can find at www.wjoe.com/all_lamps.htm. measure the voltage going to the lamps. Either 8 or 12.6v
# 210270 8V .150a
# 206258 8V .200a
# 202213 12V .150a
# 204231 12v .250a
# 204232 12v .100a
$0.90 ea or $7.50 pack of 10.
 
Well, it's been a while, but the new lamps are in place, and the electrolytics on the power amp boards have been replaced. The turntable has been cleaned and lubed, and runs through a changer cycle successfully.

The next issue is with the tuner board. There is no output from the left channel of the tuner board at K1/K3. If I inject a signal there, it passes successfully through the rest of the amp, which tells me the problem is prior to this point and is on the tuner board.

What threw me was that there was signal from all four speakers (quad) when the mode switch was on mono; it wasn't until some time that I realized that it was just the right channel signal being summed into mono at the preamp, and sending it to all four channels.

Suggestions on where to start testing? I have a meter and a scope (that I'm an improving novice with), but no FM testing or alignment gear. I've been hesitant to remove the board to get to the bottom of it, as that means restringing the tuner.
 
We'll need a schematic of that section to be of any real help to you, but based on the description of your symptoms, you can assume that:

1. The problem is not in the RF, Mixer, IF, or Detector circuits, or there would be no FM signal to produce from any channel under any mode of operation. Therefore, the vast majority of the tuner circuits are not your problem.

2. Problems of no output from one channel in an FM Stereo MPX receiver invariably will send you to the MPX decoder section. Based on the age of the unit, and not having a schematic to reference, it's a tossup as to whether the design of that section is PLL or matrix based, but it too could be a simple cap that has gone south as well.

If you can, post a schematic of that section so we can understand what K1/K3 is, and look for probable causes.

Dave
 
dcgillespie said:
We'll need a schematic of that section to be of any real help to you, but based on the description of your symptoms, you can assume that:

1. The problem is not in the RF, Mixer, IF, or Detector circuits, or there would be no FM signal to produce from any channel under any mode of operation. Therefore, the vast majority of the tuner circuits are not your problem.

2. Problems of no output from one channel in an FM Stereo MPX receiver invariably will send you to the MPX decoder section. Based on the age of the unit, and not having a schematic to reference, it's a tossup as to whether the design of that section is PLL or matrix based, but it too could be a simple cap that has gone south as well.

If you can, post a schematic of that section so we can understand what K1/K3 is, and look for probable causes.

Dave
Click to expand...

Thanks, Dave. I should also note that switching to "stereo" on the mode switch, and tuning to a stereo station lights the Stereo Beacon, and does appear to be decoding into stereo.

The full service manual is at HiFi Engine at http://www.hifiengine.com/hfe_downloads/index.php?fisher/fisher_40_service.pdf ; I'll attach a screen shot of the tuner section schematic.

K1/K2 are the left output/right output from the tuner board to the preamp. They feed from outputs 11 and 12 on the MPX decoder, through R51/R52 (15K), the 38 kHz filters, C41/C42 (2.2 uF/16v; maybe the prime suspect), R56/57 (82K), and R58R59 (100K, shunting to ground).
 

Attachments

  • Fisher 40 Tuner schematic detail.jpg
    Fisher 40 Tuner schematic detail.jpg
    93.8 KB · Views: 12
It would be easy enough to use a signal tracer to see if there is audio right at both the L and R output terminals of the MPX decoder IC. If so, then one of the components you mentioned must be at fault. If not, then that's a problem...... but I'm betting that the audio will appear at both outputs.

Dave
 
I went the other direction and injected a signal at pins 11 and 12, and the left channel was definitely lower and missing bass, so I'm suspecting the capacitor on general principals. I'll put them on the scope and see what's coming out from the mpx decoder too.

[edit] checked the signal at the mpx out, and they were both similar, so probably the caps. Looks like I can get the board out without too much disruption on the tuner stringing, too.
 
Last edited:
Got the caps in, and the sound is even on both sides. However, I have to really force the tuning dial to the left to get it to tune to 88.1, so Iight not have the dial string correctly, or it may need a tuner alignment. Wish I had the gear to do it.
 
jberger said:
Got the caps in, and the sound is even on both sides. However, I have to really force the tuning dial to the left to get it to tune to 88.1, so Iight not have the dial string correctly, or it may need a tuner alignment. Wish I had the gear to do it.
Click to expand...

No equipment needed for this. The radio stations are your signal generator.

First, mechanically align the dial either by setting the pointer at "0" on the logging scale or by equalizing the overlap at each end. If there are mechanical issues reaching either end of the dial, those issues must be fixed first.

Find stations at known frequencies around 90 and 104.

Adjust the oscillator trimmer capacitor on the high end and the oscillator coil on the low end to bring those stations to their proper place on the dial.

The adjustments are interactive, so repeat until both stations appear on the dial where they should be.

Tune to the low end station and adjust the RF coil for maximum reading on the tuning meter. Make sure that you are adjusting the RF coils and not the first IF transformer. Then tune to the high end station and adjust the RF trimmer capacitor for maximum.

Finally, adjust the antenna coil for maximum on a station at mid band. This adjustment usually has a broad range and is not critical.
 
jberger said:
Fantastic, I have to pick up some plastic hex wrenches for this, right?
Click to expand...

It helps, especially for the wrenches, but I have used a metal screwdriver for the trimmer capacitors. It requires adjusting, then removing the screwdriver and possibly overadjusting so that when the screwdriver is removed, the adjustment is ok. Sometimes plastic screwdrivers are simply not strong enough to get a good grip on the slot and turn.

Also, make sure that you are turning the slug and not the entire coil. Probably not an issue on the front end with the coils being exposed but I managed to break off all the connections inside a ratio detector transformer when the coil turned rather than the slug. Any other IF transformer and I could have possibly repaired it but the ratio detector has additional coils and too many wires to figure it out.
 
I can't believe I just did this...

I got the tuner aligned fairly well, took off the side panels to refinish them, and had it playing with the turntable off. I moved the turntable RCAs out of the way, and they fell onto the power amp module, shorting it out!!! Now, all it does is buzz and heat up the resistors on the back. Guess it going to be recycled after all of that.
 
Difficult to read voltages on the slightly fuzzy schematic in post #1, but it looks like the transistors may be all silicon. That opinion is reinforced by the fact that there are complimentary output transistors. The circuit is straightforward. You may be able to rebuild this for the cost of a few transistors. Even if it were a germanium to silicon conversion, the only major issue to address would be bias.
 
You must log in or register to reply here.

Similar threads

Rubicon383
More Fisher 500tx troubles... Balance pot
Replies
0
Views
122
Rubicon383
Rubicon383
A
Fisher CA-660 no left channel
Replies
0
Views
77
aurora01
A
I
Fisher console P294 B repairing one channel ?
Replies
12
Views
367
larryderouin
larryderouin
fushifushi
Help diagnose Fisher model 143 output problem
Replies
19
Views
3K
dneu201
dneu201
shelly_d
Fisher Model 180 Receiver
Replies
4
Views
10K
Blue Shadow
Blue Shadow
Back
Top Bottom