Musical Fidelity X-A200 mono power amps

[PinkFloyd]

Mar 11, 2020 20:40:56 GMT eightdigitword said:

hey, also got hold of a pair of XA200! soon i can open an X series museum.

i'm sure i seen on a forum way back that someone populated the missing transistors? was this you? or do you know anything of this?

what would this even do? more power? questions then are if the supplied PSU and output components would be up to it? im more curious than anything and will most likely never bother. 200w available to each LF channel will be more than enough. my existing XA-100's with 100w seem limitless.

Im gussing the A1001PA and had these populated? although i see from the manual it too was 200W.

mine were spares/repairs find so i have other jobs before this is looked at, thought id fling it out there early.

Also FYI/info for folk searching in the future that may need a replacement. The transistors in mine were either different from the SAP fitted at another time or have been replaced. either way they work. chips used are Toshiba A1943 (PNP) and C5200 (MPN)

Cheers.

Awesome, look forward to a bit of activity

[eightdigitword]

so, amps bought were listed as faulty. reported thump on turn on (on one), and distortion in one channel (didn't say if it was the same unit).

opening up the amps:
scorch mark on the plastic wrap of the transformer, windings tested fine no shorts. on close inspection i could see the X2 cap had been replaced meaning the old one likely catastrophically failed and burnt the toroid. old one was a .22uf new is a 0.1uf
some resistors looked a little scorched, values were out but not by miles. All other resistors tested with correct values.
some work could be seen done to other resistors than had previously failed. other than that no mods.
caps looked old but not terrible. PSU filter caps not in great shape, one was beginning to bulge, both measure near stated capacitance.
DC offset on 1 amp was 15mv, 60mv on the other. minor adjustment to the VRes increased but would not lower the DC output.

both power supplies give out +/-76v DC
scope on output signal showed no distortion to amplified sine wave input.
listening to audio with crap workshop speakers, heard no distortion. wont test on good speakers till thump/DC offset is gone.
both amps give out exactly 52.59v from the same 1.399v signal, making me think the all fet/transistors must be okay on both amps.
swapped burnt Res, same thump and DC on output.
swapped all small lytic caps, no change
measured all transistors, no shorts found

i will change the filter PS caps next
after that i ill remove the mosfets and test
then replace the on switch (thump source?)
closer look at the resistors?

Any suggestions welcomed.

Thanks.

[nando]

My xa200’s both thump over the speakers when turned on.

As far as I know the transistors are hard to find because they have MF coding on them.

I guess these amps have power enough. I read a test report stating they have 200W @ 8 ohm, 400W+ @4 ohm and 500W @ 2 ohm.

The pcb had A1001 printed on them. Identical maybe.

[eightdigitword]

I managed to reduce the turn on thump to acceptable levels (still thumps) and get the DC on the output down too. I have swapped out all of the small high voltage transistors on the board and replaced any resistors that showed the slightest sign of drifting. I also swapped the 0.1uf X2 cap it arrived with as it may be damaged and replaced with a 0.22uf one as originally fitted. attempting to set the bias tonight then see how they sound at the weekend.

have you managed to set the bias? looking for info on this. Im looking at R28 (0.22R), i believe this is the emitter resistor to measure across. 40ma?, across 0.22R, so 8.8mv?

fyi
All small sig transistors are mpsa42 & 92.
big ones are Toshiba A1943 (PNP) and C5200 (MPN)
mosfets are buz901 & 906.