Kustom 250 Fuzz issue

[ReginaldBisquet]

Alas, no. I've tried a variety of speakers and the amp reacts the same way.

Anyone have ideas?

perhaps a speaker, ??

[lowell]

I'd check the filter cap solder joints.

[ReginaldBisquet]

I'd check the filter cap solder joints.

Here's a photo of the power amp section. I'm guessing the white rectangular things are filter caps...



No?

[Enzo]

No. The four rectangular things, one next to each finned transistor, are resistors. They are wire-wound power resistors. We call them cement resistors because the tan body is a sort of ceramic concrete substance.

[ReginaldBisquet]

No. The four rectangular things, one next to each finned transistor, are resistors. They are wire-wound power resistors. We call them cement resistors because the tan body is a sort of ceramic concrete substance.

Hmmmmm. Okay. Then there's this section:



Massive capacitors! I am still guessing the issue is in the power amp section as the loud buzzing occurs on all four inputs. Could one of these Mallory capacitors be going bad?

[lowell]

There ya go... yes they could be. First I'd unscrew those terminals on the caps, spray some contact cleaner on them and rescrew them in. Then follow all wires from them and make sure those connections are good... whether that be resoldering them or the contact cleaner treatment if they aren't soldered connections. Try that first and report back.

[ReginaldBisquet]

Okay - I cleaned off the capacitor terminals (they weren't dirty to begin with... but why not) and sprayed some cleaner at various joints and connections from the capacitors.

Plugged the amp back into the speaker cabinet and turned it on.

Both inputs for channel 2 work perfectly fine now. I still get a raspy breaking sound when the amp is cranked 100%, but I do not get the loud squealing buzz.

Channel 1 is a different story. Here's the board for channel 1:



The loud squealing occurs when the volume on the amp is turned up past 65% even with nothing plugged it. I think it could be one of two things: either the input jacks on channel 1 or the treble circuit.

I peaked under this circuit board with a mirror and all the joints look fine except for the solder point of the ground wire coming from the input jacks on channel 1 (red and black wires on far right). Looks like there's a bit of rust (?) building up on the ground solder joints under the board.

The two wires coming off the treble control (black) also look like they have a slight bit of rust at the join connecting to the control.

Am I on target here or am I overlooking something?

At this point, I'm pleased as punch that channel 2 works at all! So anything beyond this is gravy.

There ya go... yes they could be. First I'd unscrew those terminals on the caps, spray some contact cleaner on them and rescrew them in. Then follow all wires from them and make sure those connections are good... whether that be resoldering them or the contact cleaner treatment if they aren't soldered connections. Try that first and report back.

[Enzo]

Rust? lead doesn;t rust brown. The brown residue you find around some solder connections is resin flux from the solder itself.

[ReginaldBisquet]

Hmmmm. Okay. What should I be looking for with this issue? It's definitely channel 1 that's causing the loud buzz as the issue doesn't happen when using channel 2. What am I overlooking?

I checked on the Vintage Kustom board for ideas... someone mentioned the Molex connectors with the pre-amp boards can be an issue. I'll try that next.

Also found this nugget of info: The k250-4 model outputs about 108 watts RMS into a 4 ohm load, with 169 peak watts. Into a 8 ohm load it will output 85 watts RMS.

Rust? lead doesn;t rust brown. The brown residue you find around some solder connections is resin flux from the solder itself.

[52 Bill]

Spray a little cleaner on the input jacks and be sure to work those extra switching contacts as well. Make sure that they are clean and tight to the chassis.

And yes, those push on Molex connectors can cause all sorts of problems. Pull off the plug and spray the contacts there as well. Also check for broken solder connections where the pins connect to the pc board.

[ReginaldBisquet]

Okay... done. Still getting that loud buzzing noise when I turn up the amp. BUT... Channel 2 is still working just fine. I suspect it has something to do with the treble section of Channel 1's board. What else should I look for?

Spray a little cleaner on the input jacks and be sure to work those extra switching contacts as well. Make sure that they are clean and tight to the chassis.

And yes, those push on Molex connectors can cause all sorts of problems. Pull off the plug and spray the contacts there as well. Also check for broken solder connections where the pins connect to the pc board.

[lowell]

Do you have a phono plug that shorts the tip and sleeve? If so I'd try plugging it into channel 1 and see if the squeal goes away. If not, get a plug and solder the tip and ground connections together. This will short the input jack hot and ground and tell you if the jacks are bad. Aside from that make sure that you ohm the jack ground to be sure it's good. Also check resistance of the top of R2 to ground. Should be around 79k.

If none of this reveals anything you may need a signal probe to see where in the circuit it happens. It's gotta be before the volume control in the preamp, we know that much.

[keyboards2]

What to check for.....

Okay... done. Still getting that loud buzzing noise when I turn up the amp. BUT... Channel 2 is still working just fine. I suspect it has something to do with the treble section of Channel 1's board. What else should I look for?

I've had tons of amps that I've worked on that have bad preamp bypass caps. THese are the electrolytic caps that filter out the + and - voltages just before it hits the preamp transistors and/or op amps. On units this old, typically the smaller value caps dry out and don't filter out anything. As the power amp begins to superimpose the audio signal on the power rails (where the preamp sections also get their power), the audio signal is fed back unfiltered to the preamp and it creates a direct feedback loop.

I'll take a look at the prints and post back which caps these might be.

[matthias62]

Hi Enzo,
Please you help me to open this two files about the Kustom PC 5065 ?
Thanks anyway, matthias62

[g1]

Here's the schematic but I don't have the layout.