Help repairing crate xt65r

[shinychrome0]

well the bridge rectifier checks out ok.  It might take a few days to get a current limiter rigged up.  I'm in a dorm room so i need to scrounge some tools and materials. anything else i can try in the meantime?

[J M Fahey]

Hi shinychrome.
I guess you are quite close to the end of the trail.
You can make a bulb limiter with a desktop (incandescent) lamp and an extension cord, some black insulating tape and not much more.
I guess that you have some tools available or you wouldn't be working on your amp.
Of course maybe you can't get that if you can't get out of school or whatever.
To work safely you need that limiter.
The option, a Variac, is more expensive and harder to get your hands on.

[shinychrome0]

still working on getting a limiter together.  Haven't really had a whole lot of time on my hands lately.  What else are you going to have me test when i get it though?

BTW, from what i can figure out, i just need the light bulb in series with the hot wire in the extension cord correct?  Which color is this going to be?

[J M Fahey]

Don't know the standard in your country, but you should find it in 5minutes.
Yes, you need the lamp in series with the "hot" or "live" wire.

[shinychrome0]

Alright, so i've got my current limiting desk lamp extension cord gadget.  Now what?

[shinychrome0]

bump.  Still there JM?

[shinychrome0]

I'm going to pull the board and inspect the filter caps.  There is a significant amount (about 82v) of ac on both transistors.  There obviously shouldn't be any.

[J M Fahey]

Hi shinychrom, back here.
No speaker attached, all controls on "0", plug your amp and with black probe grounded to chassis, measure DC voltage on both power rails (you should have around + and - 40V respectively) and DC voltage on the speaker out hot pin.
I don't understand your previous posting of 82VAC on the transistors, there's nowhere a point in your amp that can show more than around 30VAC relative to ground.
Please tell me what scale was the multimeter on, and what were the probes touching.

[shinychrome0]

I'll check that again when i put the board back in a little later.  I've got it out right now to check some of the other parts of the power amp and filter section.  The measurement i took earlier was from the case of each transistor to the earth ground connection.  My multimeter was either on the 500v ac or 200v setting.  Probably the 200v.  The decimal point can be testy on this meter, so now that i think about it, its possible that it was 8.2 volts.  But it was exactly the same on both transistors, and completely steady.  I know there should be less than a volt of ac.  The filter caps seem to be fine though.  using the highest resistance setting on me meter they both show charging and discharging normally.  There is some heat discoloration around the diodes in the filter circuit though, mainly around D28.  It measures okay though as far as i can tell.  I tested it the same as the transistors earlier.  D28 read around 830 one way and infinity the other, D29 tested about 830 one way and 750 the other.  There are a few other spots that look like they've been overheated.  I'll figure out whats on the other side and test whatever i can.

[shinychrome0]

Ok so i found a definite problem.  the tracks connecting to Q19 are damaged, and one is completely broken.  attempting repairs now.

[J M Fahey]

Hi shinychrome
I don't like your D29 testing low both ways, just replace it.
Cheap available anywhere multimeters with only a couple AC voltage scales, typically 200 and 500 or 750 VAC do not really measure AC but use a diode in series with the DC measuring circuit, and approximately double its sensitivity because they expect to see one demicycle on, one off.
When you touch DC with that probe, the meter shows about double the DC voltage, so on a point that has , say, 40VDC you read over 80V "AC" which was never there to begin with. :duh
What is that scale there for?: to measure 110/220 V power lines, transformer secondaries, etc. ; *maybe* power output across a speaker, *never* to follow signal voltages across a preamp nor to measure ripple.
I see this error everywhere on this Forum, not really your fault but those greedy manufacturers (or merchants)  :trouble

[shinychrome0]

I will replace that diode.  Q19 was the problem.  I had to run short jumpers because the tracks were too badly damaged to repair.  It looked like a bad solder job from the factory.  But the amp appears to be working fine now.  No DC at the speaker output jacks, so i connected a speaker and everything seems to be back to normal.  I'll hook it up to its original speaker in a little while so i can get it a little louder.  My bench speaker is really low powered. 

So two questions:  one, what could have caused this to blow?  could it just have been a little heat from playing at full volume combined with the bad solder joint?

and two, is there a better way to repair the traces on the board?  Its lifted off the board for a good half inch on all three legs.

[J M Fahey]

1) GOOOOOOOODDD , it works  :tu:
2) maybe there was a short, the thin underdesigned traces worked as fuses.
The "bad solder job" probably comes from overcurrent melting the solder while it was busy burning tracks.
3) follow the tracks backwards, cut and discard semi-molten, unglued parts and scratch the board to get rid of carbon deposits.
Get o a healthy piece of track, scratch carefully the solder mask paint from a 1/2" section and solder there a piece of wire.
When you are happy with the repair, fix the added wire with a drop of contact cement or similar to avoid it wiggling and tearing the track section
4) Play at will.

[shinychrome0]

So what could have caused this meltdown in the first place?  Just overheating on this transistor?  It has a really dinky heat sink.

[J M Fahey]

Rather than only the transistor overheating (which also happened), eventually it shorted and that track passed 10 times what it was designed for.
*That* burnt it.
There's something else, although they will *never* recognize it to you, many try to "repair" the amp with a higher value fuse, or even some cigarette paper or bypassing it.
It never blows, so some part of the amp must, to stop that overcurrent.
Either that, or your house burns.
I've made and sold over 10000 amplifiers (in 40 years) and obviously I backup them, both under guarantee and years afterwards.
When the original fuse blows, and there's no foul play, that's to say, the original type and value is used (as all amps state clearly written on user manuals and back panels), 90% of them only have output transistors blown, sometimes only one, the board looks virginal.
I use regular (fast) blow fuses and always supply an extra one or two taped to the power cable, so it's always there if needed.
Sometimes amps come with no fuse at all (they pull the incriminating evidence) but with burnt tracks, wire resistors no simply "cut" but when open show sizable balls of molten wire, grey cracked solder, resistors burnt beyond recognition, cracked transistors, the full horror show.
All those used way oversized fuses.
I'm starting to include hidden in-transformer picofuses just because of that.
So now you can imagine what might have happened there.