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Sears by Fisher Receiver won't power up
Sears by Fisher Receiver won't power up
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Old 23rd March 2018, 12:40 AM   #51
Eric is offline Eric  United States
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Sears by Fisher Receiver won't power up
I made a composite image of the top and bottom of the board, maybe this helps a little...

The pair of blue wires on posts #3 and #4 are not what I thought there were. It looks like one wire comes from the transformer (?), is fused, and then goes off into the amp somewhere. It might be worthwhile to track down that set of wires and see where they go - things are not quite so clear from the image. If this comes from the transformer, the AC voltage needs to be rectified somewhere before it is actually useful.

The orange pair is clearly a secondary winding from the transformer that gets rectified with D5 and Q1 is clearly a voltage regulator (though, without a sink which is odd). Searching mouser reveals a "330D regulator" to be a 3v regulator. The ones they have are all surface mount, rather than through-hole, though. Not sure if this voltage reference is correct or not, seems a little low to me. It would be interesting to see what AC voltage is present on this orange set of wires. It would help to know where wires #7-11 go, this would provide some insight on whether 3v is a reasonable power supply for these circuits.

If the fuse pops when you connect outgoing wires to the main PSU cap, then you clearly have a downstream problem - my guess is another shorted cap somewhere on an amp board. This is not, however, evidence that your pain PSU cap is without problems, just that the downstream problem is larger.

Those three blue caps on the right side of the board separate the PSU voltage from ground. As they aged, they shorted and effectively grounded your positive voltage. This created strain on the rectifier and the associated resistors. This is the source of the heat and discoloration.

One thing at a time - this amp can be fixed! It will just take a while, so patience is key.
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File Type: jpg PSU Composite.jpg (345.5 KB, 47 views)
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Last edited by Eric; 23rd March 2018 at 12:51 AM.
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Old 23rd March 2018, 03:02 AM   #52
freeisforme is offline freeisforme
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The two blue, two red, and two orange wires all come from the transformer and are different voltages. The blue and red on the left go to the dual cap. The green, red, smaller orange, and pink wires on the right go to various boards. The two black wires are soldered to a ground tab just to the right of the board.
I found a used D330 transistor, and I used two new old caps I had for now in the 100uF positions and two 470uf 100V caps in place of the 330uF caps, and I found a good used 220uF cap. I took the parts from a few junk newer items I had laying around.
That allowed me to put the 4 and 1amp fuses back in. I powered it up without the two 6a fuses.

I added the voltages I have at the p/s board now. The transformer voltages were taken with the wires removed, with no load. The output voltages were with the fuses in place and the unit powered up minus the amp board or 6a fuses. The used rectifier seems to work fine, the numbers are the same as the old one. The transistor came from a small amplified record player, the numbers matched and it tested good so I went with it for now. The larger caps in the axial positions won't allow me to fit the lower cover so they can't stay but for now they got it working. They're 1/3 larger than the original 330uF/50V caps.

With it powered up, I was able to listen to it play through the headphone jack, and was able to use the tape out jacks to run though an old Niles amp with a level control that I've got here on the bench and it plays, the tuner works, the controls all work, but no amp, and no panel lights at all. It appears that every last bulb in this thing is blown. The panel bulbs look like fuses. It seems to me that the only issue keeping this from playing is something on the amp board?
I am getting a faint hum through the headphones though, I didn't hear it though the amp run off the tape out signal though. Not sure what that's coming from, but it seems to go away when I touch the chassis with my hand.
No doubt I'll need to get some proper new caps for the power board but the amp seems to be the main issue.
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File Type: jpg Fisher PS Board w-Values & voltages.jpg (274.9 KB, 43 views)
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Old 23rd March 2018, 03:53 AM   #53
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Also, the board that I stole the D330 transistor from had a small bolt on heatsink attached to it, what I did for now was to clip the leads off the shorted transistor and I soldered the new one to the old leads which were already bent forward. This lets the heatsink sit facing away from the board. Its not much but it may be an improvement. Its basically just a 3/8" x 3/4" aluminum finned heatsink attached to the back of the transistor. If it don't clear, i can cut the fins down a bit I suppose. Anything has to be an improvement if it helps get some heat away from the transistor.
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Old 23rd March 2018, 03:45 PM   #54
Eric is offline Eric  United States
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Sears by Fisher Receiver won't power up
Finding another D330 regulator was a nice trick! This makes things easier - I was not inspired by what I found on Mouser. Might be useful to carefully measure the output of the voltage regulator - in the event you need to source another one. Having a reading while it is working is very helpful. Retaining the heatsink is also a good idea! It is possible the old part just slowly cooked itself over time. New caps on the PSU board may or may not impact the voltage readings you made, though they will help insure that this board is functioning properly and provide confidence in their longevity.

I don't know what to make of the blue pair of wires that come from the transformer... Seems strange to have a lead out from transformer just for purposes of a fuse, and then have it return to the transformer. One possibility might be that the blue winding is in series with the orange winding to boost the voltages - this would allow Fisher to use a single transformer with multiple receiver models. Might be interesting to see if the orange secondary voltages change with or without that 4A slow blow fuse in place. Do any other wires pop out of the transformer elsewhere?

With a functional power supply, it's just a hide-and-seek game now. Replacing the blue caps should be easy - it looks like each is marked with values. Some of them are probably still "functional" but given the age of the unit, they are all suspect. The orange ones even though they are not marked, all look to be the same size (fairly common in amps). I would pull a few of them to get a measurement, then replace all of them. Be careful to note proper orientation with replacements. The green caps are likely polypropylene/film caps and I would tend to trust them to still be good. Electrolytics first.

If this doesn't restore functionality, I would then go after the small signal transistors on the amp boards.

Another thing to look at might just be a careful inspection/cleaning with an air compressor. Make sure there is nothing stuck or "hiding" under one of the PCBs that might be shorting things out. No telling what might have been dropped in through the cooling vents on the top and slid around a bit. Paperclips, staples, etc.

Thinking about the fuse that blows when you reconnect the wires leading from the main PSU cap to the amp boards, is there more than one wire leading out to other PCBs? Can you disconnect/reconnect them individually to potentially narrow down which board is causing the problem? Anything that provides greater granularity here is helpful...
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Old 23rd March 2018, 03:59 PM   #55
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I updated the p/s board diagram to include voltages, and wire destinations.
The rectified voltage is with the 4700uF cap connected but the amp board disconnected.
The amp board is seeing 75.4 volts DC.

The fuses are between the rectifier and the 4700uF cap, not the transformer.

Doing a basic test for resistance to ground, I don't think its a short to ground but a ground short to hot.
If I understand this right, the blue wire from the rectifier, to the 4700uF cap is negative volts DC, this connects to the cap, then splits in two and goes to two pins on the amp board.
As does the red wire. The two blue wires and two red wires connect to the amp board via four pins, if I leave off the first pin, the closest to the cap on the amp board, the dim bulb tester doesn't light and the fuse survives. The fuse that would blow with this connected is on the negative side of the rectifier. The other blue wire connects to a pin on the opposite side of the board. I checked both blue wire pins for a short to chassis ground, and I get 0Ω to ground, but when I check both pins for short to the positive red wire, I get 9877Ω on the right side and 0Ω on the left side. This makes me think that the blue wire is shorted to power on the board somehow. There is no resistance between the blue and red wire's pins on the left side while there is huge resistance on the right side.

With the left side blue wire off, fuses in place, the right channel will play through speakers. So the issue is to find what may be shorting out the left channel side of the rail voltage.
I'm thinking of swapping the modules from side to side to see if the problem switches channels? Is there a simple pin to pin resistance check for one of these STK 082 modules that can be done with the module out of circuit?
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File Type: jpg Fisher PS Board w-Values, Voltages, & ID.jpg (311.2 KB, 34 views)

Last edited by freeisforme; 23rd March 2018 at 04:16 PM.
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Old 24th March 2018, 04:01 PM   #56
Eric is offline Eric  United States
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Sears by Fisher Receiver won't power up
Your voltage measures look pretty reasonable - I was expecting to see somewhere from 50-70v on the power supply to the amp boards. You are correct about the polarity of wires here: red is +75v and blue is -75v.

The resistance measures you have of where the red and blue wires hit the PCB provide some interesting insight. Looking at the amp board again, they were nice enough to indicate the traces on the bottom with a silk screen on the top - this makes following the circuit much easier. Looking at where the blue wire connects to the amp board, you have two Rubycon caps right where it hits. The same is true for the red wire - the PCB trace goes directly to a cap. These are my primary suspects for now. Having one functional channel will clearly be an aid in troubleshooting. Most amps run on a biploar power supply like the one you have here. You need a +v, a 0v, and a -v. These are typically joined together in the circuit a +v through a cap to 0v reference (ground) then through another cap to -v. If either of these caps shorts to ground, it places double the voltage across the remaining cap, thus making it more likely to short. For example, each Rubycon cap on the amp board should be rated at 100v with a 75v power supply. But if one cap shorts, then you have ~150 across a cap that is only rated at 100v- so it fails. Now you have both caps shorted, effectively shorting the power supply for the entire channel. If you are lucky, both caps shorted and blew the fuse before the extra voltage on the ground plane blew any of the small-signal transistors on the board. An easy test might be to remove these larger Rubycon caps and see if the short between power supply lines remains on the bad channel.

Swapping the amp ICs is a good experiment to try if they are socketed and easily movable. This will eliminate another variable, though you said they played in another amp, so these aren't a prime suspect right now. With the amp ICs out of the circuit, you have the ability to make another host of resistance measures for comparison that might help you trace things down a little. Check resistance on the pins of the small signal transistors to see how they measure.

Without a data sheet on the amp IC, there is no references for any resistance measurements you might make.
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Old 24th March 2018, 07:28 PM   #57
freeisforme is offline freeisforme
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All I've found online is this spec sheet on the STK082 amp chips
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Old 25th March 2018, 09:00 PM   #58
Eric is offline Eric  United States
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Sears by Fisher Receiver won't power up
My to do list for this would be:

1) swap amp ICs, if this is relatively easy, to make sure both work and your problem stays put. You might want to compare resistance measures across chips - might reveal something. Swapping is likely to be faster, comparing measurements is likely to be safer...
2) Check/remove caps to start tracking down the source of your short in the non-functional channel.
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Old 27th March 2018, 11:55 PM   #59
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I'm leaning toward just replacing them if I pull them again. To swap them, I just unscrew the modules, then the board and tilt the board over towards the front of the unit. I have a good desoldering gun so removal is easy. Its soldering them back in without accidentally bridging two pins is the hard part. The pins are super close.
I ordered a list of parts so I'm hoping to have most everything this week, it ended up having to come from three different suppliers to get what I needed. The axial caps came from a ham radio repair shop, the caps came from Jameco and the rest from a shop about two hours from me here.

Meanwhile I dug into the display panel a bit to see why the lights were out, it needs all the bulbs, but I found that the indicator lights are in stamped sheetmetal sockets that are badly rusted, the bulb holder and surrounding tin is rusted pretty bad, it looks like acid damage, but only to that immediate area around those indicators?
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Old 28th March 2018, 02:37 AM   #60
Eric is offline Eric  United States
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Sears by Fisher Receiver won't power up
Given that the ICs have to be desoldered, I would start with the caps. If you've already removed the ICs, I'd be reluctant to recommend doing so again.

If you have rust, check out a product called "naval jelly" from your local hardware store. Apply a nice coating with a cheap childs paintbrush or Q-tip and let it sit for an hour or two. Be careful not to drip - it removes everything it touches.
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