| Repairing Kustom K100-1 [message #7380] |
Thu, 24 August 2006 17:29  |
kaiser_will
Messages: 10
Registered: August 2006
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Junior Member |
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I do electronics repair and play guitar, so a friend paid me for some repair with a non-working Kustom K100-1. I purchased the schematics for this model from Ebay and have gone through the basics.
I have plugged the head into a 4x12 cabinet that is 16 ohms, turned on the power and an input to all 4 channels produces the same output...low-freq hum. Not sure where to start with the troubleshooting?
[Updated on: Thu, 24 August 2006 17:37] Report message to a moderator |
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| Re: Repairing Kustom K100-1 [message #7384 is a reply to message #7383] |
Fri, 25 August 2006 12:30  |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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KW:
You don't need to worry about powering up a Kustom solid state amp without a speaker load. If fact this holds true for nearly all solid state amps in general. A lot of Kustom schematics will note that all voltage readings marked on the schematic were taken with no speaker load.
You need to be careful about shorting the output of a solid state amp. The Kustom power amps have a current limiter circuit to protect against shorted speakers and cables, but in general never run the amp into too low of a speaker load or into a short circuit.
Just the opposite is true for a tube amp. Never run one without a speaker load hooked up, as it will damage the power amp.
Some more questions about the amp you're working on, does it pass any signal? How loud is the hum? Is it low freq hum or more buzzy?
Bill
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| Re: Repairing Kustom K100-1 [message #7390 is a reply to message #7380] |
Sun, 27 August 2006 09:44 |
kaiser_will
Messages: 10
Registered: August 2006
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Junior Member |
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Guys, thanks much for the tips.
I gathered some field data before I was made aware of doing the testing with all functions 100% and no load or source connected.
Data with all functions set to 0% and speaker cabinet connected...
1. Power on and hum with or without a load connected
2. The hum does not vary in magnitude as the channel outputs, or their bass/treble, are varied
3. Power on and get 25VDC at the output
4. 58VAC out of XFMR T1 into bridge rectifier CR1
5. 73VDC out of bridge rectifier CR1 (+32VDC to ground, -40VDC to ground)
6. 8.2VDC and -8.4VDC into preamps to ground
7. +30VDC and -40VDC at blue wires of preamps (audio output of preamps)
8. At best, I could measure 4.7VDC to ground into the preamps (tough to get the drain leg of the transistor, so I measured at the leg of the sourcing resistors.
One thing I could not figure out...there is a small piece of extruded aluminum on the bottom of the chassis with 2 transistors on it and what is probably a bridge rectifier. I could not find these on the schematics. I looks like they are powered off of the on/off switch (S1) and transformer, with heavy-gauge wires off of the transistors into main circuit board (PC103). These guys have blue and orange wires coming from them (with the +30VDC and -40VDC readings as stated above).
Based on that, at power on, it appears that full load (25VDC) is sent to the output. The fact that it affects both input channels, and both high and low-gain inputs, leads me to hone in on the output amp stage.
On a side note, the power on indicator lamp is burnt. My schematic set has no detail on lamp DS3 (part #1829, 40V rating?). But that is not a biggie at this point.
[Updated on: Sun, 27 August 2006 09:48] Report message to a moderator |
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| Re: Repairing Kustom K100-1 [message #7391 is a reply to message #7380] |
Sun, 27 August 2006 15:56 |
rodak
Messages: 522
Registered: October 2001
Location: Georgia
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Senior Member |
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Just a minor clarification and observation:
"Power on and hum with or without a load connected" - A "load" is (usually) speakers. If you have no load, i.e., no speakers connected, then you're probably not getting hum. I think what you meant to say was no input connected.
If by "output", you mean at the speaker jack, and you're getting 25VDC there, that's definitely bad news. That's about as far as I can take it, though. I'm sure some of the other, more knowledgable tech=types here will help you track it down.
www.combo-organ.com
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| Re: Repairing Kustom K100-1 [message #7397 is a reply to message #7390] |
Mon, 28 August 2006 12:08 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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KW:
First off unplug the speakers before you do anything else! If you're seeing 25 volts on the output, the speaker voice coils will overheat and either burn out or change shape enough to start to rub on the frames.
Check the fuse for the proper rating. I'm surprised that it hasn't blown yet.
As Steve noted, there is a thermal cutoff switch that is attached to the heat sink that you are talking about. This is the thing with two twisted wires going to the ac primary wiring. This is supposed to shut off power to the amp if it overheats. So be careful around this!
I can't understand the high voltages you have on the pre-amp outs. Are you sure you're checking the right wires?
The power supply should be at approx. + and - 40 volts. The voltages to the pre-amps is correct at + and - 8 volts.
You're correct in suspecting the power amp. The first thing I would check, are the output transistors for shorts. Then check the driver transistors. Also don't forget to check the bias diode string and the protection transistors as well.
Keep us informed as you progress.
Bill
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| Re: Repairing Kustom K100-1 [message #7401 is a reply to message #7399] |
Mon, 28 August 2006 13:21 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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Maybe there is a simpler answer. The voltage on the filter caps should be about 1.4X the RMS value of the secondary tranformer voltage. For 29 VAC (1/2 of the 58V you measured)that would be about 41 volts. If the filter capacitor were defective, open as it were, the voltage would be approximately the RMS voltage with high ripple. That would account for the low voltage and the hum. The output voltage could just be the mismatched voltages causing output. Disconnect the red and green wires from the caps to the output circuit board and read the voltage again. if the voltages are the same as before, the cap is bad. I would start there.
Conrad
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| Re: Repairing Kustom K100-1 [message #7402 is a reply to message #7401] |
Mon, 28 August 2006 16:44 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Conrad:
Really good catch! As you point out, one open filter cap would cause the same symptoms. Hum, drop in DC voltage, voltage on the output with no blown fuse!
This really points out one thing that is hard to deal with troubleshooting on line here; I use my ears a lot to troubleshoot these amps. If I could hear the hum I could better judge the problem.
So KW, I agree with Conrad. First thing check the positive supply filter cap.
Bill
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| Re: Repairing Kustom K100-1 [message #7407 is a reply to message #7404] |
Tue, 29 August 2006 08:45 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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The average DC voltage even without a filter cap is so much larger than the 8 volts, the regulator can output a full 8 volts without ripple. Also, the regulator has a 33uF cap on it's output. The negative and positive regulators are linked to track each other. I think the preamp voltages wouldn't be a problem.
Conrad
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| Re: Repairing Kustom K100-1 [message #7410 is a reply to message #7409] |
Wed, 30 August 2006 07:55 |
LesS
Messages: 479
Registered: December 2002
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Senior Member |
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I do not have any info on the switch from the "old" K100 to the "new" K100.
But, based on the K200 switch from K200A (no output transistor protection), to K200B (output transistor protection), I would guess that K100 started having output transistor protection starting about June 1, 1968 (approx serial number 23900).
Your older K100 might have the large power switch nut like the K200A’s; your 1969 K100 might have the smaller power switch nut. Also, I believe that K200A’s and early K200B’s had smaller diameter power cords than the later K200B’s.
[Updated on: Wed, 30 August 2006 08:06] Report message to a moderator |
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| Re: Repairing Kustom K100-1 [message #7449 is a reply to message #7380] |
Thu, 07 September 2006 07:08 |
stevem
Messages: 4785
Registered: June 2004
Location: NY
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Senior Member |
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Hi, I will check in one of my K100s tonight for the fuse value, though I think its a 3 amp slow blow.
The K100 models do not have a thrermal circuit breaker, those two TO3 case transistors mounted there are the output transistors. Sorry for the mis-information on that, in the plexi face line of amps, only the 200s have a thermal breaker!
Its unlikly that one of the main filters have gone bad in a kustom.
These are computor grade caps, and they all have a small outlined round circle on the top, if this is not bulged out then the cap has not dryed up and gone leaky, it may not still be 100 percent but the amp will still function.
Mouser and other parts houses carry the mollory brand and these filters can still be had, just confirm the can diameter size you need.A good upgrade if your hot on the idea is to replace the 2500 mfd cans with 5000 mfd values.
Any block type 25 or 30 amp bridge rectifier is a go, and I think radio shack stocks one or the other.
The mounting hole in the amps floor will have to be drilled out for clearance for a number 8 bolt and nut to place the new one down.
Keep use updated on your repair jurney.
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| Re: Repairing Kustom K100-1 [message #7453 is a reply to message #7380] |
Thu, 07 September 2006 09:26 |
kaiser_will
Messages: 10
Registered: August 2006
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Junior Member |
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Did some testing...here are the results...
(1) With the leads off of the capacitors to the PC103 board there is nearly 0VDC at the speaker terminal (57 mV)
(2) With the capacitors wired back in, no load at the speaker terminal, and all knobs turned to 100% I measure 41VDC and 8.2VDC and -8.5VDC at the preamps.
I started metering back from the output jack and have not found the culprit yet. It looks like all the values are very high compared to the schematic, such as 12.5VDC at the junction of Q118/Q119 (schematic shows 0.7VDC).
The saturated output voltage with no input acting for both input channels, with good pre-amp +/- 8VDC power and good +/- 40VDC system power indicates to me that the problem is most likely in the output stage (output transistors?).
STEVEM, you are correct that the transistors on the small heat sink are output transistors (Q1 and Q2).
Does anybody know the rating for the K100-1 2-channel solid state amplifier? The fact that it has separate bass/treble/volume for both channels and high/low gain inputs leads me to believe this was made for a guitar/mic amp. Did Kustom make a channel switching footswitch wired into both channels?
Thanks for the help!
[Updated on: Thu, 07 September 2006 09:27] Report message to a moderator |
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| Re: Repairing Kustom K100-1 [message #7457 is a reply to message #7453] |
Thu, 07 September 2006 11:43 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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kw:
I'm totally confused here. You removed the red and green wires going to the power amp board, from the filter caps and you get no voltage on the output? Do you mean that you are un-hooking the filter caps, and are sending unfiltered dc from the rectifier to the board? Or are you removing all power from the board and getting no voltage on the output?
What happens to the positive supply voltage when you remove the wires to the power amp? Originally you said it was down to 30 volts. Did it come back up to 40 volts?
Just wondering.
Bill
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| Re: Repairing Kustom K100-1 [message #7460 is a reply to message #7459] |
Fri, 08 September 2006 09:44 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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You did not understand my reason for disconnecting the leads to the amp board from the filter caps. If the voltage on the caps WITHOUT load was still 30 volts and 40 volts respectivly, the problem is with the caps with the lower voltage. If the voltage returned to 40 volts then the problem IS in the amp. If the voltage to the power stage is not balanced +/- then the biasing would be off and the output would have a voltage on it and not 0 volts as would be expected. Along with that, there would be a noticeable 60 Hz hum from the speaker. Just one more test to isolate the problem. Every component in the amp is subject to failure sometime in it's life. Some are just more susceptable than others.
Conrad
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| Re: Repairing Kustom K100-1 [message #7461 is a reply to message #7460] |
Fri, 08 September 2006 12:13 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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kw:
As Conrad suggests, remove the load from the power supply and check the voltages on the filter caps first, then move forward from there.
Just a note here, your output is not being driven to full power. The voltage on your output is caused by an imbalance in the power amp. When the amp is working correctly, the positive voltage side balances the negative voltage side at idle. The positive voltage on the output is either caused by a bias problem, or a shorted component that is un-balancing the output.
So the second thing to check if the power supply is ok, is the output and driver transistors.
Bill
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| Re: Repairing Kustom K100-1 [message #7464 is a reply to message #7462] |
Fri, 08 September 2006 13:06 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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NO!!!. Without rewiring both filter caps the polarity would be reversed. If he changes the polarity of the caps then that would be OK, but it has to be + to + and - to - or, poof. The smoke gets let out and I don't know of anyone who was sucessful at putting the smoke back in. Just remove the wires to the amp baord and check the voltage. If the voltage is 40 on both then the caps are fine, maybe. A low value cap will charge but cannot deliver the current when the amp calls for it and the voltage will drop even at idle. I am not discounting any other component issue just starting with the most obvious.
Conrad
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| Re: Repairing Kustom K100-1 [message #7467 is a reply to message #7466] |
Fri, 08 September 2006 17:40 |
kaiser_will
Messages: 10
Registered: August 2006
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Junior Member |
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Aye, Conrad, gotcha Captain! I am on the right path now. Float the filtering caps from the +/- 39.5VDC power supply (so the filtering caps are not buffering the 40V rails) then meter what is at the output jack.
I did this quick over my lunch break and measured...
1. Caps wired in to filter 40V rails, measuring ~ 40VDC at the output jack
2. Caps wired out of filtering the 40V rails, measuring ~ 25VDC at the output jack
From what others have stated, the +/- 8VDC rails not being matched may be the culprit (i.e., off-bias of the preamp power rails).
Have no fear, I do not plan to swap the caps around. I am already a tad dyslexic, so flipping around a pair of filtering caps could lead to trouble (heh, heh, heh).
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| Re: Repairing Kustom K100-1 [message #7474 is a reply to message #7473] |
Mon, 11 September 2006 11:26 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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kw:
From your latest voltage readings, it appears that the filter caps are doing their job and you do have a full plus and minus 42 volts.
So the next step is to check the output transistors. Pull the power cord out of the wall, and then use your meter's diode test position to check for shorts between the terminals of each power output transistor. These are the ones mounted to the chassis, and not the ones mounted on the PC board. Typically, there will be a short from the emitter to the collector.
Bill
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| Re: Repairing Kustom K100-1 [message #7484 is a reply to message #7483] |
Tue, 12 September 2006 11:26 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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kw:
Just the opposite, your output transistor is shorted not wide open. I assume that your readings are with the transistor out of circuit? If the transistor had a damper diode installed, there would be a low C-E reading in only one direction, not both.
The output transistor 36892 that all Kustom's used was a house numbered, selected (probably gain matched) 2N3055. Running at 40 volts, in a 50-60 watt amp, any 2N3055 you can find, should work fine.
I'd check the fuse value ASAP, because if it hasn't blown with a shorted output transistor, it won't protect your power transformer from damage.
Bill
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| Re: Repairing Kustom K100-1 [message #29339 is a reply to message #7380] |
Mon, 02 September 2024 06:21 |
stevem
Messages: 4785
Registered: June 2004
Location: NY
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Senior Member |
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You’re not answering the questions myself or others here on the board need answered to possibly help you.
1) was the amp working for you and then stopped?
2) if it stopped did this take place while playing, or the next time you went to power it up?
3) what hands on solid state electronics repair knowledge might you have?
[Updated on: Mon, 02 September 2024 06:23] Report message to a moderator |
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Not true, but KW be careful. One misstep and you may need more than a new component. I worked on a Yamaha power amp once, never again, and my components were not matched and I flamed the entire power amp board. Didn't take too long if I remember correctly. A fuse is never faster than the components it's protecting. The fuse just keeps the flames to a minimum.
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