Help with Kustom 200

[Bigbirdga]

Hi, I'm new to this and need some help. I am trying to reapair a Kustom 200. The PC105 board input works fine, but the the PC203 board has not worked for some time. Long story short, I found a B/E short in Q200 and a C/E short in Q201. I replaced the SE4002 with an NTE123AP and I actually found a 2N3638 to replace Q201. However...I now only get a steady popping through this channel that varies in loudness with the VOl control. The new transistors test OK and show proper polarity as I have them installed. I'm using a scope and an in circuit transistor tester. Ideas, please?

[52 Bill]

Welcome to the place.

The two preamps are exactly the same with the exception of the final 2 transistor boost circuit on the PC203 board, so you can compare voltage readings etc. from channel to channel.

If you are sure that the new transistors are installed correctly, then check the voltage readings compared to the schematic or the other board.

Input a signal into the front panel jack and then trace the signal with your scope through each stage and see where the signal is and isn't.

One thing with the boost circuit switch, is that it is just like a bypass switch on an FX box. If the contacts are really dirty it can completely kill the signal coming out of the preamp.

[Bigbirdga]

Thanks. I traced the signal to begin with and lost it after Q200. Found Q200 and Q201 shorted. I'll check the voltages. t's gotta be another component around these 2 somewhere. Interesting, though, my amp does not have a Boost switch. It has a switch labled "Bright" on both channels. I do see the boost circuit on the 203 board, but not sure how it relates to Bright. I Thanks again for the help.

[Jazz P Bass]

Kustom 200

Can you post the schematics?

[Bigbirdga]

Kustom Preamps

Here are the scats I'm using. Got them a few yrs ago. My specific unit varies a bit from these in that both boards have a Bright switch, although the Bright switch on the 203 appears to also function as the Boost switch. The problem is in the first preamp stage, so that does not matter.
I'm going to double check the transistor lead designations next. The NTE123 came with a diagram. The 2N3638 did not, although both are TO-92 forms, there are in fact 3 versions of the To-92. My tester will let me determine E/B/C based on the waveform. I have to take them out of the circuit to be sure, though. If there are any other suggestions, that'd be great. Thanks

[Jazz P Bass]

SE4002

I did a datasheet search on the SE4002.
I cannot find the exact lead identification.
I did come up with (4) cross references :
BC183: C B E
BC237: C B E
BC238: C B E
BC548: C B E
If this is any indication of what came out, the pinout should be C B E.
The NTE 123AP, has a pinout of E B C.
You might want to check your installation.

Do you have a DVM with diode check on it?
That is a real quick way of checking transistor terminals.

[52 Bill]

The boost circuit on the PC203 is what they called the bright switch. As I noted earlier, the two circuits are basically the same except for the boost circuit. If you look at cap C206, it is the same as the bright switch cap on the PC105 board.

Yes, make sure of the pinouts of the new transistors.

The only other common problem on these boards is the tantalum electrolytic caps. They will show signs of failure after 40 some odd years.

[Bigbirdga]

Thanks to both. Looks like I have the E and B reversed on the transistors. Should have checked them before relying on the diagram from NTE. Live and learn.