help with a Kustom 200B

[Valvehead]

Ive got a Kustom 200B im trying to figure out. It has power but no output.

power board is the pc703

Here is what i have found so far :

No voltage to the 2 preamp boards. should be 8V there.

power from the 2 huge caps is 40V and is at the power board.

Found 1 burnt resistor, R746, replaced it. No change.

The 5 diodes all test ok in circuit.



The schematic has the voltages on it so i went down the line on all the transistors.

the 4 main output transistors all have correct voltages at the legs.
that would be Q1,2,3,4 .

i have a hard time tracing when there are this many transistors going on...

so i will list what i found on each one , maybe someone with the scem can help me trace it ?



Q701 0v , 0v , 0v (0 = meaning 10-100 mV )

702 17V , 0.6v , 0v

703 17v , 23v , 18v

704 ok

705 0v , 0.6v , 38v

706 1v , 0v , 0v

707 0.6 , 0v , 0v

708 OK

709 0v , 0.6v , 38v

710 0v , 0v , 0v

711 0v , 0v

712 0v , 0v , 38v

713 0v , 0v ,0v

714 0v , 0v , 38v , 38v

715 0v , 0v , 0v

716 0v , 0v , 38v

717 0v , 0v , 0v

718 0v , 0v , 38v , 38v

i hope that makes sense - if you look at each tran. on the schematic it might...

[Enzo]

Come on, be a pal, link the schematic in the thread for us.

If the preamp circuits have no power supply, then we can;t expect any sound to result.

See that MONITOR jack? Plug a nice strong signal into it. DO you get sound? If so, the power amp works.

Now get the +/-8v back. The burnt resistor is a clue. Resistors usually burn up because something else is bad. I see 9 transistors there. Just check them all. Note some are connected together in ways that may confuse your meter, so don;t let it fool you. And check the various resistors and caps around them.

And while it is good of you to collect that sea of numbers, I have to wonder if some of those 38v readings are not really negatives? As in -38v? Also would be useful to know in each case which terminal was which.

[Valvehead]

i was hoping i would get mr enzo......

[Enzo]

OK, now test the stuff I mentioned. Also, check the preamp boards. Make sure the +8 and -8 are not shorted together somewhere, and neither is shorted to ground.

[Valvehead]

Come on, be a pal, link the schematic in the thread for us.

If the preamp circuits have no power supply, then we can;t expect any sound to result.

See that MONITOR jack? Plug a nice strong signal into it. DO you get sound? If so, the power amp works.

Now get the +/-8v back. The burnt resistor is a clue. Resistors usually burn up because something else is bad. I see 9 transistors there. Just check them all. Note some are connected together in ways that may confuse your meter, so don;t let it fool you. And check the various resistors and caps around them.

And while it is good of you to collect that sea of numbers, I have to wonder if some of those 38v readings are not really negatives? As in -38v? Also would be useful to know in each case which terminal was which.

Ahh------ i thought the Monitor jack was an OUT put.

i will try that friday.....FIRST Thing.


i think all the resistors checked fine-some didnt and i pulled one lead up and of course they checked good.

and yes --some of those 38v are negatives. if i posted them they are there just to show 38v --- and what the scem wants as far as polarity--could be pos or neg . Just a 38 to show its there

[Valvehead]

will get back to it friday i hope,,,-----------

thanks Master Enzo for helping !

[Enzo]

It may well be intended as an output, but looking at the schematic, it is a cap attached to the signal path. Signal can travel either direction through that cap... Cram a signal in there and see what happens.

I understand what you meant with the 38v readings, but when two circuits are connected together like that, an open leg to the -38v rail COULD result in a +38 being where -38 should have been.

[Valvehead]

i ran a signal gen through that monitor input--it screamed nice and loud through the speaker .

The +8 / -8 wires (red/ green) , test shorted together on the board connections , and they both short to ground , about 0.2 ohms . They have 0 volts at them

[Enzo]

OK, the +/-8 is loaded down. Looks like a red and a green wire taking it to the other boards, right? Disconect those wires. Are the +8 and -8 terminals now still shorted together at the power supply? Probably not, and that would allow you to at least repair the two power supplies.

And that would leave wherever the 8v rails are going. I assume a couple preamp cards. Disconnect the wires to them, at least enough to find out which board has the problem.

Wait, I am getting ahead of myself. Check the red and green wires. Any chance they go through a mis-fitted connector? Are they pinched under some hardware? if they are OK, then back to which preamp card is beefed.

WHich preamp boards are in this amp? I am looking at a few preamps that run off +/-8, and I don;t see an easy way to get the two rails shorted together or shorted to ground.

[Valvehead]

enzo -

there are 4 wires from the main power board to the other 2 "pre" boards.

blue/blue/green/red



mine has the pc703 power board

the pc105 ( non effects pre board )

and the other pre board that has the verb / trem , etc... id have to pull it to see the # .




so to make it easy , i clipped the 4 wires from the power board to the other 2 .
so these measurements are with the 4 wires clipped.

heres what i got:

green and red = 0V at all boards clipped.

blue + blue , 0v


blues to ground = 100k both

red and green on the 703 board to ground (clipped) - measure shorted to ground.

red and green on the other 2 boards = 1k to ground

blue wires measure open on the 2 pre boards when clipped (not shorting to ground)
but measure 100k to ground on the 703 board


hope this makes sense.....

[52 Bill]

The two blue wires are the preamp outs.

The red and green are the + and - 8 volt supply lines.

The negative regulator tracks the positive one to keep both voltages equal.

If you have shorted readings to ground on the red and green wires on PC703 check for shorted components in the regulator circuits.

Remember that there are small caps on the outputs of the regulators, and they may influence the readings to ground.

[Enzo]

OK, so you get the +8 and -8 showing shorted to ground ON THE POWER SUPPLY BOARD, right? And that with the wires going elsewhere clipped off, right?

Look at the schematic. How many places in the 8v supply circuits is there just one part between an 8v rail and ground? I see only one each. I see C713 on the +8 and C717 on the -8. Granted, a couple transistors could short and get things two or three parts away, but everywhere I look, I see resistors. ANd resistors rarely short. SO for example if Q715 went short to ground, that would still leave the 8.25k R749 between -8 and ground. Very few places for anything to short those 8v rails to ground.

SO check those two caps. Are they shorted? Are they installed facing the right way. Nothing funny on the solder side of the board like untrimmed leads?


And as always, watch for solder bridges. All the schematics in the world won;t point those out.

[Valvehead]

i worked on it a bit tonight.
I hope this makes sense....

replaced c713 , c 717 , c718 .

I now have signal on one channel ( the non reverb channel) - but its very low. the other channel has no signal

tested a few of those transistors =

Q717 2 legs short to gnd , 1 is 10k

Q715 all legs short to gnd

Q713 all legs are 4.4k to gnd

Q711 1 leg to gnd , other is 9k to gnd


with the red/green wires disconnected , i get 3v on red , negative 9v on green


red measures 4.4k to gnd. green is 12k to gnd.


both blue wires are 0.5v

[Enzo]

No circuit works right if it does not have proper power supply. Fix the +/-8 on the power amp.

[Valvehead]

i hope to spend some time on it in the next few days.....

One thing-- i looked up a bunch of the transistor numbers at Mouser/ allied / etc...

I cant find ANY replacements...

for example
2n4249

2n3638

2n3567


the rest seem to be 38870 , 38737 , etc....