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K150-1 Hum No Audio [message #28414] Tue, 18 January 2022 16:27 Go to next message
TelesnTubes is currently offline  TelesnTubes
Messages: 2
Registered: January 2022
Junior Member
Hello

I am new here. Thanks for sharing so much info on Kustom amps. I am a tube amp guy trying to navigate my fist solid state amp repair. It is a Kustom 150-1 head. Amp is not passing audio and has an audible hum thru my 8ohm test speaker. I have removed the signal input wiring harness plug to see if the problem is in the pre amp or power section. There was no change. So I suspect an issue in the PC5033 power amp board. I have 46 +/- VDC on the big filter caps and 12 +/- on the terminal block and wiring to the preamp boards. I also read 15 VDC on the speaker jack - my experience with tube amps tells me there should not be DC on the speaker output and maybe there is a coupling capacitor that is bad.

I have not started measuring/testing components or voltage at each component yet. I wanted to see if I am on the right track or should I start elsewhere? I also have a question about the correct value for C4. The schematic says 33uf. I wonder if the decimal point is missing and if the correct value is .33uf or 3.3uf. I can't see the code on the capacitor. I have found most of the transistors online except for the 40408/nte190. What is an equivalent for this transistor. I also see the IC and suggested nte725 replacement are obsolete. Any suggestions if these need to be replaced? I hope the issue is contained to the PC5033 board.

Thanks!
Re: K150-1 Hum No Audio [message #28415 is a reply to message #28414] Tue, 18 January 2022 17:13 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4546
Registered: June 2004
Location: NY
Senior Member
Hello and welcome!
Well the good news is that since your amp is not blowing its 3 amp then you likely do not have a blown output transistor , but one of them could be leaky.

The key here With the dc voltage you are measuring is if It's a negative or positive voltage as that will tell you what side of the power supply rail the issue is in.
The cap you question is a 33 uf .

Just as a matter of course and due to age I would start off by replacing all of the electro type caps on that 5033 board, C3, C4 , c7 and c8 .

Transistor 40408 is a NPN type and can be replaced by a NTE 128 or equivalent.

When you remove that board from the chassis be very ultra careful with the leads on the RCA brand bias temp diode mounted inbetween the two 3055 output transistors, they are fragile!

Also do not power up the amp without the 5033 boards connector plugged in other wise you will harm some semiconductors and blow the fuse.

You can use the model jack to pull signal off off the preamp section to see if that is working .
That jack is blocked from dc voltage by c2 , so you should not read any dc on it unless it's coming down from I bad Transistor or IC in the preamp section.
icon14.gif  Re: K150-1 Hum No Audio [message #28416 is a reply to message #28415] Tue, 18 January 2022 18:24 Go to previous messageGo to next message
TelesnTubes is currently offline  TelesnTubes
Messages: 2
Registered: January 2022
Junior Member
That's a great start - thanks! If it were a tube amp, I would be done already LOL
Re: K150-1 Hum No Audio [message #28417 is a reply to message #28414] Tue, 18 January 2022 19:19 Go to previous message
stevem is currently offline  stevem
Messages: 4546
Registered: June 2004
Location: NY
Senior Member
Yes, it's far easier to to stuff in a known good PI tube and set of 6550's once the bias is checked and see if it fly's again!

Note, you will have to make 3 more post for me to approve before your post will just go up to the site.

[Updated on: Tue, 18 January 2022 19:21]

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