Home » VintageKustom.com » Repairing Kustom Amps » K-250 (output board)
K-250 [message #13599] Sat, 08 May 2010 20:44 Go to next message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
I'm working on a 5065 output board for a k250 I picked up to restore. A cap (C Cool shorted apparently and burned 3 of the resistors around it. I have a schematic but could realy use a clear ledgable one if anyone could sent me one. The resistors I believe to be all 470 ohm ( resistors 13, 15 and 16 )as best as I can tell. They burn the board pretty bad, but I replaced them and the cap (I didn't have a 27mf 35v and replaced it with the closest one I had, 33mf 35v). I pulled a licon power switch from another amp and powered her up. Everything works (Channels 1, 2,also reverb and trem/vibro),but she's heating up those resistors I replaced and the sound distorts a bit as you crank up the volume. Would the 33mf cap I installed cause this (too big or in backwards) or are we looking elsewhere? Steve C

Steve C
Re: K-250 [message #13624 is a reply to message #13599] Mon, 10 May 2010 07:02 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
It seems that you may have a bias problem.
With a meter set for ohms check across all those 3 diodes( cr1 thru 3)for shorts.
You only have to check one way as a shorted diode wil check shorted no matter on which end you have the black or red test lead.
Also note that CR3 id mounted in a clip inetween the 4 output transistors and many times its brittle leads break off, or peoples fat hands probing around break a lead off with out knowing it.
The higher value cap you uesed is fine, but if the replacement you used is maked for + and - then yes, it needs to go in the right way.

Also test to make sure that other 27mfd at 35 volt cap is not shorted.
Let us know how things progress.
Re: K-250 [message #13635 is a reply to message #13624] Mon, 10 May 2010 22:58 Go to previous messageGo to next message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
all 6 diodes check good I have another board and pulled a cap out of it. Caps are correct (27mf 35v) now. I pulled 2 of the power transisters out of the chassis cause they were giving me bad readings, but out of the chassis they were fine. I replaced the 38735 next to C-8 and thats where I'm quiting for the night. Now I have a hum and the .51 5W power resistor next to the plug from the pre-amps is getting hot. I would think I put a transistor in backwards, but I got the plugs exactly the way they came off and I don't believe you can put the transistors in wrong because of the offset pining. So now I'm scratching my head!!!! I got a Kasino 250 head that works fine and I'm guesing thats the same on the inside so I'll open it up and use it for a comparison and maybe figure this out that way. Steve C

Steve C
Re: K-250 [message #13639 is a reply to message #13599] Tue, 11 May 2010 12:06 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Read the speaker output for dc voltage, that's my guess for the hum and overheating resistor. If you do find anything over a volt, unplug the speaker, as it will eventually destroy the voice coil.

Check all of the transistors on the board.

To check the output transistors, just pull off the black plastic plug and test with your meter. No need to remove them or to unhook the red lead.

Be sure to test the 4 driver transistors as well. These may be harder to test in circuit, but test in circuit first and if you get really odd readings then either pull the plug on the related output transistor and test again, or unsolder the base and emitter leads and test.

Is this the amp from eBay that Kevin M was selling? If it was, I saw the photos of the pc board. How did you clean-up the burnt areas? If the charred areas are too bad, they will conduct current and cause circuit failures.
Re: K-250 [message #13640 is a reply to message #13639] Tue, 11 May 2010 20:27 Go to previous messageGo to next message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
Yes, its that one from Keven M. I scraped most of the chared area with a flat screw driver and brushed it with a stiff tooth brush to get any loose stuff off. All the solder lines were intact. I was thinking about the 2 power transistors I pulled out. Why do they apear to be apposing each other yet the offset pining only seems to alow you to install the one way? I'm thinking that I didn't put them back in the same hole they came out of. I don't know how that would change things, but other than that I really didn't change anything else and it worked ok before I pulled them out to check them. Steve C

Steve C
Re: K-250 [message #13642 is a reply to message #13599] Wed, 12 May 2010 08:04 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
At this point it`s beat to start from scratch.

Test all 4 of the 5 watt resisitors.

Pull all 4 of the T03 output transistors and test them.

Then test the 4 boxed cased driver transistors.

Unsoldering these factory boxed drivers can be a pain, so what I have taken to doing now is cutting open two of the traces on each for test out, and then if they test good I re-solder across the gap I cut if I used a Xacto knife to make the cut.
If I used a wide cut off disc or flat stone in my dremel tool I then use some de-soldering wire wick or a section of 18 GA wire to bridge the bigger gap that they make.
Re: K-250 [message #13643 is a reply to message #13642] Wed, 12 May 2010 09:07 Go to previous message
cassent5150 is currently offline  cassent5150
Messages: 341
Registered: August 2009
Senior Member
This is going to a bigger fix than I have time for right now. I have a couple good boards in a K-II SRS slave amp I bought a while back and I'm just going to replace the whole board, get it going, grab my Strat, take her for a spin on a few different cabinets and see how she drives. I'll deal with this later when I have more time to really go through it. Thanks for the testing tip, "man that is WAY KOOL". I've been pulling them out of the board to test them and you sure are right, its a friggen pain in the #@$%^ if ya know what I mean. You can bet I'll be making some use of that tip.

AGAIN: You guys are the BOMB!!!

Steve C


Steve C
Previous Topic: Kasino Reverb Tank info.
Next Topic: K200-B5 - One channel is dead. . .
Goto Forum:
  


Current Time: Fri Mar #d 07:25:35 EDT 2024
.:: Contact :: Home ::.

Powered by: FUDforum 3.0.9.