Hi!!
Someone saved a Fisher CP-7000 from being thrown out and brought it to me. One channel didn't work and I quickly noticed why. The output modules were replaced or at least unsoldered and put back really badly : one of them had the negative power rail disconnected as the trace broke, They're STK086 BTW. I repaired the connection and now both channels work but sound distorted. I checked with an oscilloscope and it looks a lot like terrible crossover distortion. It's spot on the same on both channels so I suspected a power supply problem but it's steady at +/-44V (the datasheet recommends 42V for these STK's so close enough) and there are not a lot of components around. A little input buffer transistor I think and two protection transistors connected to (from what I can see on the schematic in the datasheet of the STK because the schematic for the amp seems unobtanium) the bases of the drivers. On the negative side I have -750mV (pin 5) on the base of the driver but on the positive side only a few mV (pin 9) which would explain the crossover distortion. If only one of them had it I would suspect the stk to be faulty but to have both do exactly the same still points in a common fault somewhere, right ? The only question now is what ? If anyone has any ideas I would greatly appreciate it 🙂
Someone saved a Fisher CP-7000 from being thrown out and brought it to me. One channel didn't work and I quickly noticed why. The output modules were replaced or at least unsoldered and put back really badly : one of them had the negative power rail disconnected as the trace broke, They're STK086 BTW. I repaired the connection and now both channels work but sound distorted. I checked with an oscilloscope and it looks a lot like terrible crossover distortion. It's spot on the same on both channels so I suspected a power supply problem but it's steady at +/-44V (the datasheet recommends 42V for these STK's so close enough) and there are not a lot of components around. A little input buffer transistor I think and two protection transistors connected to (from what I can see on the schematic in the datasheet of the STK because the schematic for the amp seems unobtanium) the bases of the drivers. On the negative side I have -750mV (pin 5) on the base of the driver but on the positive side only a few mV (pin 9) which would explain the crossover distortion. If only one of them had it I would suspect the stk to be faulty but to have both do exactly the same still points in a common fault somewhere, right ? The only question now is what ? If anyone has any ideas I would greatly appreciate it 🙂
Here's a schematic. It has some kind of input muting/protection circuit that likely is bad.
Maybe you can just disconnect that circuit's connection to the inputs and see if the amp works.
Maybe you can just disconnect that circuit's connection to the inputs and see if the amp works.
Thank you, this is very helpful, I will check this tomorrow. It's now sure the STK's have been replaced because the schematic indicates SS1001. The person who did this might have bought these on ebay and gotten fakes or pulled this from an amp that had a fault which caused them both to fail in the same way. Still I keep my hopes up that it has something to do with the protection/muting circuit.
Try disconnecting the protection from one input and see if that channel works.
There could be (and perhaps likely are) several problems here.
There could be (and perhaps likely are) several problems here.
Well no luck removing the input mute/protection circuit. and even with the over current protection completely out of the equation there's no change whatsoever to the output. These are both channels at 10KHz one (top) with everything disconnected, bottom with both protection circuits still in place.
These are class b amplifiers really prone to 10 khz crossover distortions.Replace all electrolitic capacitors , especially ones decoupling the feedback path and the bootstrap ones c21, c22, c05, c06, c10, c11 a then check if any diode biasing the finsl trz is defective.
Severe crossover distortion. Check the parts in the output stage buffer. Resistors, diodes, transistors.
Had some time today to check on the Fisher. I replaced all the caps on one channel without any improvement. I tested all diodes and resistors on that same channel and they're all good. I can't really test anything in the output stage itself as it's an stk. They get a little warm which to me seems strange as on the bases of both drivers I have only a few mV indicating there is no bias at all. Could both internal bias stages have just failed in the same way ?
Hi , try to find a stk086 datasheet or schematic connections picture in another amplifier ,and compare with your schematic , maybe pinout is incorrect or some part missing, which were needed for stk ,but not needed for original ic . It would mean stk was not fully matching original .
I did, all components that should be there are there and the values are correct...😕 except the one going from the feedback loop to ground which is 2.2k in the amp and 2.7k in the datasheet for the stk.
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Damaged STK, has had heat history.
If oscillation and distortion cannot be controlled:
Modify it to 1875 or 3886, those STK chips are nearly impossible to find.
Mount the new ones higher on heat sink, and use the existing holes in the PCB.
STK were two amps or more in the same housing, so you will need to parallel the supply wires, as the new chips are mono amps.
7294 should work as well, and maybe a suitable discrete kit with transistors can be found..
If oscillation and distortion cannot be controlled:
Modify it to 1875 or 3886, those STK chips are nearly impossible to find.
Mount the new ones higher on heat sink, and use the existing holes in the PCB.
STK were two amps or more in the same housing, so you will need to parallel the supply wires, as the new chips are mono amps.
7294 should work as well, and maybe a suitable discrete kit with transistors can be found..
Did you located exactly that stk connection diagram ? If so ,then maybe both stk are faulty or fake ,and failure repairing was reason to recycle it .
You can also perform some checks .
1. How waveform changes ,if you adjust volume ?
2. Measure idle current of each stk , by disconnecting trace from pin8 or pin6 and passing it thru dc dmm in 10amp mode . If we see crossover distortions, then ammeter should show zero or almost zero with no signal.
You can also perform some checks .
1. How waveform changes ,if you adjust volume ?
2. Measure idle current of each stk , by disconnecting trace from pin8 or pin6 and passing it thru dc dmm in 10amp mode . If we see crossover distortions, then ammeter should show zero or almost zero with no signal.
According to schematic ,+-42v present ,too much for proper operation of 7294... Even 7293 can burn easily .
I found an old DIY post inquiring about the Sanyo SS1001 IC. ss1001-datasheet.39261
I'll be brutally frank. Assuming the schematic in post 2 is an accurate representation, ugly crossover distortion is inherent in this design, as there's no facility for reducing the class B distortion in the Q101/Q103 outboard power transistors. It best, the IC must carry the speaker load until the "helper" Q101/Q103 transistors begin to conduct. I see little hope of turning this sow's ear of a circuit into a silk purse.
I'll be brutally frank. Assuming the schematic in post 2 is an accurate representation, ugly crossover distortion is inherent in this design, as there's no facility for reducing the class B distortion in the Q101/Q103 outboard power transistors. It best, the IC must carry the speaker load until the "helper" Q101/Q103 transistors begin to conduct. I see little hope of turning this sow's ear of a circuit into a silk purse.
It is 0-42 or 42-0-42 in the supply?
If total 84 V, then only the Mosfet or 498xx series, which are good to 0-200, (and difficult to get) will work.
7293 is supposed to be good to 100V I think.
If total 84 V, then only the Mosfet or 498xx series, which are good to 0-200, (and difficult to get) will work.
7293 is supposed to be good to 100V I think.
If you mean q01 q03 in schematic from radiomuseum ,these two are only for limiting output current ,in short circuit or overload case . They have no relationship to crossover distortions.
I have burned 7293 3pcs , purchased from local shops , according to datasheet they supposed to withstand +-50 ,but with no signal .It was +-45v in my case. If you start playing ,they smoke . 7294 is even weaker in voltage terms ,same as 3886.