Kustom IV Bass Head Schematics?

[Corona Blue]

Greetings all!

A fellow musician in my local area brought me a Kustom IV Bass head and cabinet he recently bought.

The amp sounded very nice, quiet and clean, with a nice sounding tone stack.

But after the amp warms up, it gets the fizzies real bad. Makes a constant hissing.

I opened it up and it is a beautifully built amp! Date codes on the pots indicate a vintage of '77 or so. Visually, there is no evidence of any trouble. No hot discolored areas on the boards, no over-heated resistors, no swollen caps. I kind of wish there was some kind of clue, but nothing!

I have looked high and low for the schematics and/or service manual for one of these. They must be somewhat rare because I have found nothing on this particular Kustom model. This is not a "tuck and roll" era Kustom.

Anyone here have any experience with these? Or maybe some documentation???

Thanks in advance,
Allen

[Edit] I tried to I.D. the various circuit board numbers in this amp. All the numbers are printed on their under side and difficult to read with them mounted in the amp. The pre-amp boards look like they can easily enough come out for service. But the power section is mounted on the rear chassis wall and it does not appear like it will be easy to remove. Best I can see with a magnifier, here are what appear to be the board numbers:

Upper Pre: 009 5290 00
Lower Pre: 009 5277 00
Power: 003 5380 00 (or possibly 009 5388 00... difficult to see clearly)

[BasicBassVI]

If there are any socketed IC's, remove them, clean the pins, and re-seat them. Doing that helped quite a lot in my CV bass head of the same vintage.

[Corona Blue]

Thanks Basic. I will double check. I noted that the preamp boards use 4558 DIPs but they were soldered fast to the board.

Both preamp channels and the power board are full of carbon composite resistors. The sound it is making sounds a lot like when a tube amp has a noisy resistor. Carbon composite resisitors can pick up moisture from the air and cause this sort of thing.

The circuit DOES use socketed connectors to join up the various boards... hmmmm... that seems like a good place to start.

(I will try to post a couple of pics of this monster).

[Corona Blue]

Here are some pics of the unit. It weighs in at just about 50 lbs!!

Fully sealed and shielded chassis...




Preamp section... this is a "4-holer" amp. Two channels, each voiced a bit differently and each with high and low inputs...





Power board mounted to heat-sinks... this board looks like it could be difficult to remove for service...

[52 Bill]

Wow, biggest Kustom that I've ever seen. My files don't cover the later Tolex amps, but I have fixed a few of the III heads. I guess that there wasn't a schematic glued to the top plate like the older ones had? They seem to be all based upon the original basic power amp design, only more outputs and smaller ballast resistors. Is this thing bi-amped or single power output?

The Molex connectors are well known problems in all of these later amps, give them a good cleaning and make sure that the pins are tight.

What sort of noise is it making?

[Corona Blue]

Hi Bill!
It is a single output with a paralleled jack.
The cabinet that came with the amp is a Kustom 2 x 15 that is loaded with EV15B-SRO's. One of these had an open voice coil. The other sounds great.

Aside from some scratchy pot noise, the amp sounds fine until it warms up. Then I get the "pink noise" sound, like a noisy resistor in the signal path. The amp still plays, but the noise is up there pretty good.

I found a guy who had a schematic for the Kustom IV Bass, but the schematic is dated 1974. I think this amp is later. I went ahead and bought it ($20) just to have something. But until I can get the circuit cards clearly identified, I'm not sure how exact that schematic will be. "Close" may be enough for what I need though.

[52 Bill]

Aside from some scratchy pot noise, the amp sounds fine until it warms up. Then I get the "pink noise" sound, like a noisy resistor in the signal path. The amp still plays, but the noise is up there pretty good.

Try and isolate it to the preamps or the power amp and then get some freeze spray to find the bad component.

[Corona Blue]

OK, will try that.

On the Kustoms that you have worked on, did you ever have occasion to remove the power board and was it a chore to do?

It looks like to remove this one, I would have to de-solder all the power transistors from the board before the board could swing away.

[g1]

Looks to me like the OP devices are socketed?

[Corona Blue]

Looks to me like the OP devices are socketed?

So the device legs plug into those plastic receivers coming through the board?
I wondered about that.
That would be great if so!

I'll get back into the shop this weekend and will look at that closer.

Thanks g-one.

[g1]

Yes, you can see a pair of screws (no nuts) coming through for each of the OP transistors.
At that age you will probably want to change out the heatsink compound as it will likely be dried out. Careful with the mica spacers, swaps and alcohol should help clean off the old stuff.

[Corona Blue]

Yes, you can see a pair of screws (no nuts) coming through for each of the OP transistors.
At that age you will probably want to change out the heatsink compound as it will likely be dried out. Careful with the mica spacers, swaps and alcohol should help clean off the old stuff.

Yes, those are captive nuts there in the board. I guess that silver rivet in between the transistor screws is what holds the plastic socket to the board.

I think I remember seeing that there were sil pads under the transisitors rather than mica, but either way I should have a few spares on hand.

PS g-one, love your sig line!

[g1]

I was trying to figure out what was up with the paint spatters on the chassis floor?. I guess they painted the heatsink without covering up the holes .
As far as the sig line, I was talking to a friend about how tone will only take you so far, and that was his reply!

[Corona Blue]

UPDATE

Ok, I had a couple hours to mess with this Kustom yesterday.
I unplugged and cleaned ALL of the board connectors and cleaned all the pins and receivers and jacks with DeOxit.
Removed all the pre-amp boards and sprayed and blew out the pots and really worked them over.
Re-assembled everything, plugged in the speaker cab and powered up the amp.

I did not have a bass there in the shop, so I just let it run for an hour and worked the pots to see if any gremlins popped up...

No problems! No excessive heat. I could hear the amp and EQ stages working as I ran the pots. No appreciable hum, no hiss.

I checked the DC voltage at the big filter caps and read -58 to +58 and 0V at the PT center tap.

I will get a guitar and try it out this weekend, but so far so good.


Best,
Allen

[Corona Blue]

UPDATE: Schematics Found!

Long time since I have revisited this thread, but I wanted to complete it with the schematic of the amp so others can benefit.
Special thanks to Kustomoholic over at the VintageKustom Forum for these hard-to-find documents.

Kustom4_Bass

Thanks to everyone here who chimed in to help!

Allen