| 250-4 [message #28618] |
Wed, 22 June 2022 16:29  |
kb0rex
Messages: 33
Registered: June 2022
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got brought a 250-4 from garage sale got it for 50 bucks... I can see why. It's smoked 9 ways to Sunday. Customer got this in black with the speakers wants the pa repaired. Found this site pulled the schematics. Someone had tried to repair and had no idea as the outputs were all the same wiring (2 backwards) the usual suspects 95.3 681s and 1ks burnt in half. driver also shorted. Relaced all the above and it's somewhat alive, has a nasty hum but does pass signal now that the effects board is also repaired the ic1 op amp 1/2 gone. reverb tremolo and vibrato good. It now limps along. It runs hot and thermal on the heat sink will shut it down. the 5065 board seems to still have issues, but I am apparently too dumb to figure it out, its pulling the power supply down to 37+- volts. I had no voltage on the outputs but after going back over the board and putting in the correct 95.3 and 681s I also tried using 4007s for cr1 and 2 as they tested better than the old white ones in there. I also thought this might clean up the Humm and heat but not I have a +1 volt or so on output . Steve or Bill I seem to be thrashing on this one. Work on lots of amps first time Kustom although it seems like a interesting amp built like a tank. Thanks Doug
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| Re: 250-4 [message #28619 is a reply to message #28618] |
Thu, 23 June 2022 15:49  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Welcome to the place.
I would first fix the power amp problems before working on the FX and the preamps. Almost all of the Kustom power amps use the same basic architecture, and are really basic. All the major manufacturers have used the same basic design for the past 40-50 years. You should not have any problem getting it figured out.
If the power amp is overheating, then there probably is a balance problem between the two halves of the output section. This can also be the source of the hum and of the voltage on the output. I don't know if you have a light bulb limiter available, but if you do, use it until you get the power amp working. Check all of the transistors in the power amp and take voltage readings to see if they come close to the listed schematic values.
The 1N3754 diode that clips to the heat sink is fragile, so be careful while you are working on the amp.
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| Re: 250-4 [message #28625 is a reply to message #28618] |
Fri, 24 June 2022 06:20 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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There held on with four 6/32" machine screws if I recall right , and you mean to tell me they rotted away, or did the metal reverb lock disc come off its handle and fall down?
With this lock on rear set I don't think that tank can fall that far!
I will open one of mine on the weekend and confirm the size of the machine screws for you.
Note that most times you can find the correct 1N3754 bias diode being sold on eBay, or 4 star electronics and other N.O.S suppliers have them.
I would get the correct one even if it's 15 bucks!
PS.
I too have been logged off the site somehow when typing a long reply and then going to post it , so don't feel bad, lol!
[Updated on: Fri, 24 June 2022 06:55] Report message to a moderator |
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| Re: 250-4 [message #28630 is a reply to message #28618] |
Fri, 24 June 2022 12:19 |
kb0rex
Messages: 33
Registered: June 2022
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The back on this 250-4 is different then the one on the reverb store pictures. It has on the back 6 1/4 inch jacks. Reverb,fuzz,boost,output,tape record, and way on the right corner module input. No place for reverb lock handle. Looks like a opening for a slide switch on the far left side by the slack wind up which is open, nothing in it. So I don't know if this is really old or came later without the reverb lock. Serial number 104867.
[Updated on: Fri, 24 June 2022 12:20] Report message to a moderator |
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| Re: 250-4 [message #28632 is a reply to message #28618] |
Fri, 24 June 2022 12:47 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Definitely check the differential pair transistors. They have a great effect on output offset voltage.
Bias is set by the three diodes that connect the bases of the driver transistors. The higher the voltage difference between the bases the higher the idle bias. The 1N4007 will work as replacements, but so will 1N4148s. There should be approx. 2 vdc different between the bases.
What transistors did you use as replacements for the outputs and for the drivers?
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| Re: 250-4 [message #28635 is a reply to message #28618] |
Fri, 24 June 2022 15:09 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Since it's dc and not ac ripple your reading on the output jack changing out the main rail filter's would not solve that issue.
Also do not power up the amp without that 3 pin molex connector being plugged in as that is where that whole 5065 board gets it's ground from, that's unless you want to blow up more parts, lol!
A long these same lines since the connection pins on the board are aluminum I have seen these solder connections go bad, so I now always reflow them with a really hot iron, and with the plug out of the connector.
[Updated on: Fri, 24 June 2022 15:11] Report message to a moderator |
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I'll go after the diff's... aye pull the board again. 
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