Kustom 250-1 Hiss, Normal?

[misterc57]

Tihs amp has a low volume hiss, even with volume at zero, guitar plugged in or not plugged in.

Volume control has no effect on the hiss. As soon as you play you no longer notice the hiss, because the instrument is louder that the hiss.

Is this normal for this amp?

Thank you.

kustom-k250-schematic-diagram.pdf

[Chuck H]

I had one on the bench where that was one of the complaints. It turned out to be a noisy op amp. Unfortunately the exact part was hard to find and I had to risk purchasing an overseas "equivalent" shipped direct from China on the slow boat. Fortunately it worked just fine. But that was something like five years ago. I can't say about whether the part can still be found.

I'm not a chip guy, but I've read that you can blast a noisy chip with freeze spray it'll quiet down momentarily. So that may be a good trouble shooting method.

[nosaj]

I had one on the bench where that was one of the complaints. It turned out to be a noisy op amp. Unfortunately the exact part was hard to find and I had to risk purchasing an overseas "equivalent" shipped direct from China on the slow boat. Fortunately it worked just fine. But that was something like five years ago. I can't say about whether the part can still be found.

I'm not a chip guy, but I've read that you can blast a noisy chip with freeze spray it'll quiet down momentarily. So that may be a good trouble shooting method.

I've not used the freeze spray method, but have used the scope to find noisy opamps in a Kustom Casino, not fun or easy at that time since I was still learning but much easier now.
nosaj

[Chuck H]

Volume control has no effect on the hiss.

You'll need a schematic. Since the hiss isn't affected by the volume control, if it IS a noisy opamp it'll be one schematically after the volume control in the signal chain. I remember finding a really good site for Kustom board numbers related to models and years as well as schematics. I can't remember the site now. I'd go to work looking it up, but I suppose you could do that too.

[nosaj]

You'll need a schematic. Since the hiss isn't affected by the volume control, if it IS a noisy opamp it'll be one schematically after the volume control in the signal chain. I remember finding a really good site for Kustom board numbers related to models and years as well as schematics. I can't remember the site now. I'd go to work looking it up, but I suppose you could do that too.

Ba-Bamm......http://www.vintagekustom.com/literat...h/AmpList.html
Don't look for your amp model ,

Just find the boards pc5066 pc5067, pc5065
That will cover the entire amplifier
nosaj

[misterc57]

I do not have freeze spray but I am usinig a can of power duster. I believe it has a simialr effect of cooling components. Sprayed every op-amp I could find. No change.

The hiss is so slight I question if it is a real issue or not. More noticable when you are close to the speakers.

[52 Bill]

I just repaired a K250-4, it had a lot of hiss. Bad opamp caused it. Luckily it was a 4558 so easily replaceable.

These will normally have a certain amount of hiss, but the older technology opamps do seem to be noisier than newer ones. The problem is as Chuck noted that a lot of Kustoms use a 739 dual opamp, also RC4739 which is a later version just aren't made any more.

Rather than buying old or questionable sourced chips, I just install an 8 pin socket and plug in a new dual. There is some board modification needed, but it is minor.

The thing to note on the K250-1 is that the second preamp signal feeds into the first preamp and then to the power amp. So a noisy or dead channel may not be on that particular board.

[misterc57]

Unless it gets bad I am going to go with this is normal.

Thank you