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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 8th March 2018, 05:03 AM   #1
freeisforme is offline freeisforme
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Join Date: Jan 2017
Default Sears by Fisher Receiver won't power up

I was given an old Sears/Fisher receiver the other day, its in great physical condition but it won't so much as light up. The Sears P/N is 143 92533800.
I removed the top and bottom covers and found two blown 6 amp fuses, these are on a small circuit board with the rectifier. The board is badly discolored and one transistor appears to have gotten very hot. The fuses blow instantly on power up.
I did an online search for this model and came up empty as far as a circuit diagram or manual. It uses twin STK082 amp chips so I'm guessing its around 32 wpc. This also leads me to believe this was likely built by Sanyo for Fisher.
Does any one have any pointers as to how to go about fixing this thing?
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Old 9th March 2018, 04:40 AM   #2
Ian Finch is offline Ian Finch  Australia
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Location: Coffs Harbour, NSW
Considering the blown fuses, its most likely that one or both STK082 hybrid power amp. modules has also fried. These are replaceable with refurbished (very dicey), copied versions (soewhat dicey), pulls (maybe good if the source properly tested them) or NOS parts (no, don't believe it).

A good, economical and DIY friendly alternative is to remove the hybrids and the rest of the damaged power amplifier section and replace with a current Chipamp design such as LM3886 based modules, kits of parts or just the PCBs. These are cheap, plentiful and work just fine as long as the scant information you get with them is sufficient for correct connections and you have an adequate power supply and heatsinks to suit the same power output as previous, which you'll need to control by the maximum amount of input signal fed to it. If you aren't careful with this, the amplifiers may deliver too much and the little power supply overloads and the fuses blow, before the inbuilt limiting of the chipamp can kick in.

Before buying, it might be wise to remove the power connections from the power supply rectifier and smoothing caps to the amplifier board and test that the power supply is functional when its fuses are replaced and no further problems are apparent. This should be the case, provided the rectifier is Ok, caps are fair and there are no shorts such as you get when someone isn't careful as they shove things that don't work, back together again.

That done, measure the main power supply voltages with respect to ground and check they there is something like +/- 25V or, since I can only see a single ballast resistor for each channel, it could have just a single voltage supply of about 50V. A single supply would also mean that the outputs to the speakers are via a large capacitor. Since all fuses, perhaps including output fuses, seem to be on a separate board, any output caps could be there.
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Old 9th March 2018, 09:40 AM   #3
freeisforme is offline freeisforme
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Could the STK chip cause the power supply board to fry like this?
I was thinking that maybe something failed in the power supply since the fuses are between the two wires from the transformer and the rectifier. the two other lower amperage, (1a & 4a) fuses seem to feed other boards The two fuses that are blown, are 6 amp slow blow AGC type. I tried replacing the fuses and new one's pop instantly. I powered it up with a variac slowly, the fuses popped at about 30v to the power cord. I unwound the wires to the amp board. Looking at the circuit board traces it looks like the two 6 A slow blow fuses are between the transformer wires and the rectifier. Testing the rectifier as if it were four diodes, I get a short between two legs, 0 ohms in both directions between the - marked leg and one 'AC' marked leg. in both directions.
Could the rectifier have crapped out and caused the transistor to overheat or vise versa? The transistor appears to have gotten so hot it looks burnt, but looking at the burn pattern I can't tell if the heat came from the transistor or the wire wound resistors ahead of it. That's a lot of heat there that apparently happened for a long time before the fuses blew.

What gets me is that this seems to have shut off all power, with the two blown 6a fuses, no power reaches any other part of the unit.

The two 330uF caps on the p/s board are the largest normal looking caps in the whole receiver. There is a 3 pin capacitor in one corner that sits recessed into the chassis with 6 wires on it. There are are few numbers on it and some Japanese writing but no values I can see other than one marking reading 85C.
Am I correct in thinking that this board is what supplies power to the rest of the receiver?
The two largest wires on the output side of the transformer appear to go directly to the pair of 6A fuses, then from the fuses to the rectifier.

The are a pair 1.5a speaker fuses on the amplifier board that appear to go out to the speaker terminals, both of those are good. There are two .5A fuses on the preamp board up front, those two are also okay.

I do have a few Sanyo built parts receivers, several of which use the same STK chips that work which I can borrow parts from for testing.
There is no sign of heat or burnt pins on either chips, the blurred lettering on the left chip is from me touching it, they both have a coating of something sticky, when I touched the chip, the paint wiped right off. I had pulled both chips back to add some fresh heat sink compound but the original grease was still soft. I wiped them off and replaced the grease anyhow.
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Old 9th March 2018, 03:48 PM   #4
Ian Finch is offline Ian Finch  Australia
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Join Date: Apr 2010
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I haven't worked on one these Fisher Sanyo models, just a few that were superficially alike and from the same era, which means I can only guess about the specific details from appearance. Anyway, the bridge rectifier would only have blown when there was a heavy load on the DC supply, specifically the main power supply which powers the audio modules and possibly other circuits too. That's why the power amplifier(s) would be suspect if the rectifier had blown. You'd then have to check the leads coming from the transformer and look for other rectifiers that might power the other circuits such as the tuner, preamp and front panel lighting (this is often just AC powered) to be sure how many windings and power supplies there were. The power for these minor circuits wont be much, so if there was a short on any of the small power supplies, the winding supplying it may just burn out without affecting the other supplies or blowing the mains fuse. However, this amount of burn mark, looks like there might have been a serious fault and a large flash that may have taken out the transformer primary winding and more. That would certainly leave you with no function at all. You can test the transformer windings for continuity by measuring resistance, winding by winding, after you have disconnected at least one end of a winding to ensure nothing else is messing with your measurement. Check for connections with other windings, in case there are actually tappings rather than separate windings. You should read only about 20 ohms for the 110V primary winding and less for the amplifier supply secondary winding(s).

Note, with any DC supply big enough to power 2 X 30W amplifiers , there must be at least 3,300 uF caps somewhere, or they've been removed.

Its better at this point to just check winding continuity unpowered, rather than power up again and risk more damage or your own safety. If your measurements lead you to think the transformer has indeed failed, you might want to consider a complete redesign and build because you won't be able to directly match multiple secondary voltages with any stock transformer.
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Last edited by Ian Finch; 9th March 2018 at 03:53 PM.
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Old 9th March 2018, 04:24 PM   #5
MAAC0 is offline MAAC0  Portugal
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Freisforme


Before blowing more things.. Can You identify the PA rail Supply wires ? Follow the datasheet corresponding pins to the board connectors.
Disconnect them from the Amp and try new fuses. If they blow agian, You know where the problem is. Pray the STKs are ok.


Regarding the caps.. Perhaps someone confused 330 with 3300. I'm sure it hummed like mad...if ... working...


Regarding the Sticky amps... CocaCola, Beer You name it perhaps.
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Old 9th March 2018, 04:37 PM   #6
radtech is offline radtech  United States
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What's all that black on the power supply board? Is it just from heat from the power resistors or is something blown up there? Looks almost like a disk ceramic cap blown up, hard to tell from the photo though. Looks a lot like lightning damage.
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Old 9th March 2018, 08:42 PM   #7
freeisforme is offline freeisforme
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Quote:
Originally Posted by radtech View Post
What's all that black on the power supply board? Is it just from heat from the power resistors or is something blown up there? Looks almost like a disk ceramic cap blown up, hard to tell from the photo though. Looks a lot like lightning damage.
All that black is in front of the D330 transistor, the transistor is badly discolored as is the wire wound resistor facing it. The burn and discoloration goes right through the board and it left a burnt mark on the metal chassis above it as well.
I don't think its lightning damage, it more looks like an area that's been exposed to a lot of heat over time. A quick flash of heat wouldn't 'cook' the circuit board so deeply. The transistor looks like it was in a fire for a hour. The aluminum tab on it was burnt and turned colors from heat, but not melted and there is no other physical damage to the transistor.
All three legs of the transistor are shorted together, in all directions at all times.

I don't have any manual or diagrams for this, all I can do is try and follow the traces on the circuit board and read the markings on components. I did an online search for this model but didn't find anything. A few were close but differed greatly in the power supply area.
Several of the caps on the burnt board have something leaking from them, it looks like brown tar. Those caps have no capacitance and only a few ohms of resistance. Only one of the 330uF caps tests ok.
The only larger cap is a single three terminal can in one corner of the unit, I have no clue as to what its specs are as its not marked in anything but Asian script.
If one or both of the STK chips were creating an extreme load like this, enough to burn through several components, wouldn't there be some sort of sign or heat on or around the chip itself?
The heat damage on the power supply board is very localized. I would think that if a load high enough to produce enough heat to make this kind of burn mark was present in the amp board, it would have melted wires or shown some signs of burning elsewhere?
The burn mark looks as if someone held a flame in that one spot for a long time. the surface of the surrounding board is burnt like firewood, but the trace on the other side is unhurt. It almost looks like the transistor got red hot.
The two ceramic caps both check out fine, I removed both and tested them off the board. The back side of the board shows no sign of damage, but the unprinted side is damaged to the point there's ash and a burnt layer present.
Usually when I see a lightning hit board there's burn through and missing pins, I don't have that here. The hottest point appears to have been the transistor and/or one of the larger resistors.
I'll see if I can check the transformer to see if I can detect any damage. I sort of doubt it because the two leads that area connected to this board still supply power to the fuses since it blew them right away once they were replaced. Every wire from the transformer goes to this board. A few go to unused pins, with no connection to any components on the board, just pins.

If you look at the pic of the burnt board, all of the heavier insulated wires on the right side go to the secondary side of the transformer, the two black wires are the primary side wires, also connected to one receptacle on the rear of the unit. The red and blue wires are the higher voltage, orange and blue gives half voltage. No power is making past this board with the fuses blown, with the fuses replaced, with low voltage applied, I can follow the voltage to the rectifier, where it stops, nothing beyond it. AC volts go in, nothing comes out, as the volts increase, it gets warm and pops the fuse pretty fast. I'm thinking I need to start with fixing the obvious damage first, a handful of new caps, a new transistor, a diode, and probably a new rectifier. Then see what happens as I bring up the voltage and try to get it to power up. If it needs amp chips, I guess I'll borrow a pair from another unit for now.
I wish I had a wiring diagram for this thing, at least then I'd have a better idea of the power flow. Does anyone have a pinout chart for the STK082 chips?
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Old 10th March 2018, 12:34 AM   #8
Ian Finch is offline Ian Finch  Australia
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Google is your friend:
Unknow - datasheet pdf
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Old 10th March 2018, 04:06 AM   #9
freeisforme is offline freeisforme
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I took a few better pics of the power supply board and did some testing.
I also drew up a diagram of the front of the board with the marked values. The values marked on this diagram are what is marked on each component along with the location number marked on the board itself.

I removed both STK chips and temporarily installed each one in another (LXI) receiver and both work perfectly.

While looking at the p/s board, I noticed that the red and blue wires next to the rectifier, which come out of the transformer secondary side, connect to the + and - terminals of the rectifier? These wires become live when the switch is turned on even without any fuses on installed on this board. All of the six heavier gauge wires on the rigt side of this board go into a hole in the transformer casing, the other six go from this board to various other boards on the unit and ground.

With the switch on, there is voltage at each of the six heavier wires, coming from the transformer, but nothing leaving the board with or without the fuses in place. With the 1, 4, and 6a fuses installed. I used a dim bulb tester to test for a short here first. This remains even if I remove the six smaller gauge wires to the other boards.
I tested each component, five of the resistors read less than half of their rated ohms, and only one capacitor functions at all. The one 330uF cap nearest the transistor is a dead short with .007Ω resistance and zero capacitance, I got the same from the single 220uF cap, and C8 showed twice the capacitance and a very low ESR reading.
R4 = 180Ω
R3 = 549Ω
R5 = 79Ω
R2 = 102Ω
R7 = 1000Ω
R1 = 320Ω
There is continuity between the + terminal and upper right AC terminal on the rectifier in both directions.
All three legs of the transistor, (Q1), are shorted together with zero resistance with it removed from the board.

I have several meters to test with, I double checked with both of my other meters after getting the way off resistor readings but all my meters agree.
There is one other cap on the preamp board that's leaking some sort of gooey oil also. I can't tell if its an adhesive they used or if its electrolyte from inside the cap leaking.

The extreme heat definitely came from the 330D transistor, its the most damaged by heat and the cap behind it is slightly melted too.
I'm just not sure at this point what the cause was? The damage seams to be limited strictly to this board. The STK's work fine in another receiver, and the short remains even with the other boards disconnected.
It does seem that all power flows through this board first, every wire coming from the transformer on the secondary side goes to this board first and no where else. Incoming power, 120vac comes from the power cord, through the fuse, to the power switch, then to the transformer and to one rear outlet, the second rear outlet is connected directly to the power cord.

I also included a wider view pic showing the single large cap in the corner, it has three posts on top and six wires on it. I can't read the value on it. Its about 4" tall and 2.1" in diameter
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Old 10th March 2018, 02:28 PM   #10
jaycee is offline jaycee  United Kingdom
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To be sure on that rectifier, you'll have to take it out of circuit... you may simply be reading a short due to the transformer winding.

That 2SD330 transistor is probably best replaced. A TIP31C will do the job.
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