Safety questions about repairing guitar transistor amplifiers

[eh là bas ma]

2SD998 are at digikey
2SC1027 alternates:
2SC2383, 2SC3228, 2SD2184
BU 606...608, BUX 18(A...C), 2SC2769
2SA1023 alt:
2SA1013, 2SA1275, 2SB1456
BC 556, 2SA970, 2SA1049, 2SA1285(A)

Thank you very much !

I took some readings on these transistors.

Looks like only one of the power transistor is dead, i couldnt get any reading on it.
The others seem to be fine, here is what i read :


I checked the big white resistors without desoldering them, directly on the pcb.
On both of them I read 0.9 ohm on the DMM, and if i read correctly their values they're supposed to be 0.5 ohm, so i guess they're fine too.

Also checked the zener diodes near the big resistors, where the pcb is also slightly darker, and they appear to be fine.

So my next move is to find 2 or 3 fuses to replace the one that fried, a D998 power transistor and some thermal paste.

Any suggestion for a suitable thermal paste on digikey ?

Is this ok ?

https://www.digikey.fr/fr/products/detail/chip-quik-inc/TC1-10G/9681980

Edit : ah wait... Digikey is across the atlantic, with 25 euros delivery fee, it doesnt make sense if i can find what i need in EU.
i see my local electronic shop has some D998 transistors in their inventory, so i ll get some thermal paste there if i can.

 

[Gordo]

Any paste will do. Heads up it's a pain in the butt to deal with because it gets everywhere. At least with me, but I struggle with crazy glue as well


You can likely get away with existing insulators and make note if there's a plastic insulator for the screw. That keeps the chip from grounding out if need be. If grounding out isn't an issue you won't find one.

The worst is when the chip blows and you can't tell what it was or find a suitable schematic. Sounds like you're covered though.

 

[anevilspar7an]

Can anyone chime in if OP needs to replace both D998 transistors instead of just one? I know capacitors and potentiometers are a wear item but don't know if there would be uneven performance with one old and one new power transistor.

 

[eh là bas ma]

Can anyone chime in if OP needs to replace both D998 transistors instead of just one? I know capacitors and potentiometers are a wear item but don't know if there would be uneven performance with one old and one new power transistor.

I intend to replace both of them.

...if i can... because i just realized im being stopped by a bad screw, and i cant access the back of the pcb.

What do we do in that situation ? pierce around the screw with a drill ? Use a hammer to try to force the screw out of the chassis ?


Maybe try to use a thin drill and dig a little in the screw's thread to allow the screwdriver to get a better grip ?

Edit: I m a bit reluctant to start hammering on that screw like a mad man, so i will start by using a knife and try to carve the cross back into shape.
Patient work, let's hope it gets me somewhere.

 

[eh là bas ma]

Victory !

I tried a trick with an elastic rubber band between the screwdriver and the screw's thread to fill the gaps, and it worked !
Best trick ever.

 

[eh là bas ma]

I'm afraid the aeras under both D998 power transistors are seriously damaged, can i do something about it ?

Here are other pictures of the main pcb

...with other suspicious aeras, but i think i can clean that with some pcb cleaner, probably just some oxydation :

 

[jwin615]

Clean the PCB as well as you can. Hopefully that's just burnt flux and or mask and the copper is good.
As long as you have very low resistance between the transitor leg and another component on that trace, you should be okay.
If the trace is burnt, there's a couple of options.
1, run a bodge wire from the power transistor leg to another component on that trace
Use silicon or ptfe insulated wire.
It may be easier if you replace the component you are bodging too. You may be able to use the new lead as a new trace. Insulated as needed.
2, using a high fine grit sandpaper, remover the mask from the copper layer on the trace and solder to that. Add a short piece of wire if needed.

It may be beneficial to add addition heatsinking to the transistors. It doesn't appear that being bolted to the chasis has been sufficient, long term.