Kustom K100C-8

[ca7922303]

Was working, guy that gave it to me admits he may have bumped this transistor causing one(1) leg to come loose. Unable to find part or equal.

[ca7922303]

F2N4248 is the transistor

[vintagekiki]

2N4248 is the transistor
F is the manufacturer's label

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/51507-kustom-k100c-8-repair
1)

[vintagekiki]

Kustom Semiconductor Information 2

[Chuck H]

How can you tell where that transistor is in the circuit schematically by looking at those photos of the board? I've repaired one of these and I can't do that.

ca7922303, do you own one of these or do you have a photographic memory or something?

I remember having to source three different transistors from various sources. I also remember that two weren't the original part number but "equivalents" that could only be sourced from east Asia. Fortunately they worked just fine. I also remember there was one that I wanted to replace, but didn't absolutely have to. And I decided not to since I couldn't find any replacement for it. I would have had to do circuit modification for one option or glue the transistor to the board and wire the legs cattywhompus for another.

[ca7922303]

Replaced 2n4248 transistor and no change. Still a loud hum. Guy I got it from says it was working and he was spraying noisy pots and may have bumped something and then it developed the loud hum. I found the 2n4248 with one(1) leg broken from bottom of transistor and thought he had maybe bumped it somehow.

[Chuck H]

If this is like a REALLY LOUD hum then you probably shouldn't be plugging it into a speaker cabinet for the moment. These amps are all modular and I expect very similar overall. I failed to connect the flying leads from preamp board to the power amp board having not tested just what those leads do. Well, one was a ground lead. When I turned it on the power amp proceeded to bump my speaker cabinet full of DC voltage.

No guarantee this is your problem, but check contact on all flying leads and test grounds.

[Jazz P Bass]

You can view the schematics here: http://www.vintagekustom.com/literat...h/AmpList.html

[Chuck H]

You can view the schematics here: http://www.vintagekustom.com/literat...h/AmpList.html

Excellent. And... When I made my mistake I did have a schematic. I just hadn't checked it to see if the power amp board ground was via the flying leads from the preamp board because I never expected that. I know better than to take such things as obvious now.

[ca7922303]

Excellent. And... When I made my mistake I did have a schematic. I just hadn't checked it to see if the power amp board ground was via the flying leads from the preamp board because I never expected that. I know better than to take such things as obvious now.

If possible, due to my limited experience, could someone give me step by step details of how to test? I have auto ranging dmm, pc oscope, signal generator, and vatiac, but very little experience troubleshooting. Thanks.

[52 Bill]

That transistor is one of the two input transistors on the power amp, the differential pair. Breaking off the lead may have caused damage to the rest of the power amp circuit.

Do as Chuck suggested and don't power up the amp with the speaker connected until you check to see if there is dc voltage on the output. Unplug the speaker and attach your voltmeter to the terminals of the speaker jack. Set the meter to read dc volts. Turn on the amp and see if there is a voltage on the speaker jack. If there is, then you will need to test the rest of the transistors in the power amp.

Do the test and let us know what you find out.

[ca7922303]

That transistor is one of the two input transistors on the power amp, the differential pair. Breaking off the lead may have caused damage to the rest of the power amp circuit.

Do as Chuck suggested and don't power up the amp with the speaker connected until you check to see if there is dc voltage on the output. Unplug the speaker and attach your voltmeter to the terminals of the speaker jack. Set the meter to read dc volts. Turn on the amp and see if there is a voltage on the speaker jack. If there is, then you will need to test the rest of the transistors in the power amp.

Do the test and let us know what you find out.

Ouput jack(-9.5v).

[52 Bill]

With the dc on the output, do not reconnect the speaker until you fix the problem with the power amp circuit.

What did you replace the broken transistor with? Did you find a 2N4249 or did you use a substitute?

[ca7922303]

With the dc on the output, do not reconnect the speaker until you fix the problem with the power amp circuit.

What did you replace the broken transistor with? Did you find a 2N4249 or did you use a substitute?

Found an actual 2n4248 on eBay. I think i've got maybe four(4) more. The three schematics (900,1400, 1500) referenced on this forum don't seem to have any 2n4248s unless I'm not spotting them?

[Enzo]

Well, what board is your amp? It will say on it.