VintageKustom.com: Repairing Kustom Amps

Home » VintageKustom.com » Repairing Kustom Amps » repair tips

Show: Today's Messages :: Polls :: Message Navigator

repair tips [message #2445]

Mon, 10 February 2003 07:42

stevem
Messages: 4863
Registered: June 2004
Location: NY

Senior Member

Hi. TO those of you who have amps that need electronic repairs, or are sending out amps for repairs I would like to put out a reminder. The 100 200 and 400s, in short, all plexi face amps are to have the circuit voltages read with all the controls full on, no test signal applyed, and no output load. The metal face amps 150s 250s 300PA 600PA, and probibly every Kustom and Kasisno from 1971 foward gets tested with all controls off, no signal and no load. Not following this will make a big difference in the voltages read in certain parts of these amps.I have never seen a kustom schematic for a product made after baldwin bought the company so their test procedure should be found and used.

Report message to a moderator

Re: repair tips [message #2475 is a reply to message #2445]

Sun, 23 February 2003 13:32

Anonymous

Hi, Oldee here. I have a tech question and from what I read here you may be the one to answer it. I am in the process of rebuilding a 250 Kustom. I have everything working and it sounds pretty good. The problem is that it seems to reach clipping sooner than I would expect. I am using a ocilloscope to look at the wave form. With no load and a test signal I get a nice symmetrical clip on the positive and negative wave form. When I hook up a 2-15 cabinet and hit high notes on the bass I get no clipping problem and a good symmetric wave form. When I hit a really low note I get clipping pretty easily. Also, and this is the gist of my question, should I still get a symmetric wave form. I seem to get a large deflection in the negative direction and early clipping. The deflection of the wave form on the up side is only about a third of the down side. I don't know if it is the complexity of the wave form or the indication of problem. Any thoughts would be appreciated. Thanks, Oldee

Report message to a moderator

Re: repair tips [message #2476 is a reply to message #2475]

Mon, 24 February 2003 08:11

stevem
Messages: 4863
Registered: June 2004
Location: NY

Senior Member

Hi. you really have to test with a 1ooo cycle test tone. What is the RMS speaker output with your high test note, and what is the voltage with your low test note? I would also measure the negitive and positive 40 VDC power supply rails and see what they drop down to with your two test notes.Some possible fixes.The orignal 4500 MFD filter caps could be getting tired, They last longer when they get used belive it or not. At this point I would replace them with 10,000 MFD or higher depending on what your pocket can handle. Some big value ones can cost over 30 dollars each. Or if yours check good on a cap meter add two more 4500s.you might also change the 4 38692 output transistors for new NTE 181MPs. MP for matched pair with in 10%, and also check the value of the 4 .47 ohm emiter resistors and any other high tolerance resistors on the driver board. The 4 driver transistors might not be a bad idea to change out etheir.They have been in their for what 30 plus years. New ones are much more equal for current gain.Let me know what you find.Chow

Report message to a moderator

Re: repair tips [message #2477 is a reply to message #2476]

Mon, 24 February 2003 10:24

Anonymous

Hello again, Oldee here. I tried to give you all the relavent information when I descried the problem. Seem I failed miseribly. The scope shows the clipping is somewhere in the power amp driver board. The + and - voltages measured on the filter caps does not seem to sag at all, also the output transistor Q point does not move. I did use a 1K hz signal when I checked the amp with no load. I was afraid to run the amp with the signal generator up to clipping with the speaker connected. You said something I found interesting. Concerning changing the transistors, I have always thought that transistors either worked of did not work. Can they weaken like a tube? If I did see what I thought I saw on the scope the clip seemed to occur in the differential pair of input transistors. I don't have a schematic for this amp so I don't know exactly what the voltages should be around these parts. It appears that the driver circuitry is similar to that of the 200 Kustom amp. I have a schematic for that model. Can I use it for ballpark comparison? Thanks for Help

Report message to a moderator

Re: repair tips [message #2478 is a reply to message #2475]

Mon, 24 February 2003 13:29

stevem
Messages: 4863
Registered: June 2004
Location: NY

Senior Member

Hi. I have schematics for all the boards in that amp. I can fax you,, or mail you a copy if you send me a letter sized stamped emvelope.Small signal transistors can become slightly leaky and pass a small amount of current when they should not, or a resistor or cap has changed value or the latter become leaky throwing the bias off for that side of the sign wave.If you have access to a dummie load resistor of enough wattage, I would drive the amp into that and recheck for clean RMS output. If the amp can put out 27 to 30 volts into a 8 ohm non inductive load with out the clipping its OK. S.Magnotti 548 yorkhill road, yorktown n.y 10598. Another thing you have to watch out for with bass guitars these days is high output active pickups. These old preamps can not handle that much drive signal.

Report message to a moderator

Re: repair tips [message #2479 is a reply to message #2478]

Mon, 24 February 2003 18:54

Anonymous

Hey Steve, I think the best thng I can do at this point is send you the envelope for the schematic. I will put it in the mail soon. Thanks again, Jim Talley (Oldee)

Report message to a moderator

Re: repair tips [message #2482 is a reply to message #2478]

Tue, 25 February 2003 07:33

stevem
Messages: 4863
Registered: June 2004
Location: NY

Senior Member

Hi. you misread my post I have schematics for that except the -4 effects section, I have a layout for the -2 effects section. Thanks for the schematic offer any way

Report message to a moderator

Re: repair tips [message #2483 is a reply to message #2479]

Tue, 25 February 2003 07:35

stevem
Messages: 4863
Registered: June 2004
Location: NY

Senior Member

Sorry, disreguard that last post, I though you where offering me a layout.My eyes are still half closed!

Report message to a moderator

Previous Topic:	24-L info needed
Next Topic:	Would Appreciate Some Repair Advice

Goto Forum:

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

]

Current Time: Sun Feb 15 06:20:12 EST 2026