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Repairing Kustom K100-1 [message #7380] Thu, 24 August 2006 17:29 Go to next message
kaiser_will is currently offline  kaiser_will
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Registered: August 2006
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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]

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Re: Repairing Kustom K100-1 [message #7381 is a reply to message #7380] Fri, 25 August 2006 07:51 Go to previous messageGo to next message
stevem is currently offline  stevem
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I am not clear on the events you are discribing.Are you saying that when you first turn the amp on you get a hum, or which ever channel you put a signal into you get a hum? The first thing I would do is check for DC voltage on the output of the amp.In that PA head and all kustoms any problem that is common to all channels must lie in the common sections to all the channels like the powersupply or the driver/ output stage, Plus the PA head has the reverb stage common to all 4 channels.Q118 and Q119 transistors hold the DC level on the output at zero.The output stage current limiting transistors are Q123 and Q124.Do you have the +8 volts and-8 volts into the preamps? the blue wires are the audio output of the preamps.

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Re: Repairing Kustom K100-1 [message #7383 is a reply to message #7381] Fri, 25 August 2006 10:38 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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There is a hum all the time. With or without an input.

I will check the VDC at the output and the +/-8VDC into the preamps and get back to you.

I was told by another friend to never power up the older Kustom heads without having a speaker cabinet connected as it will damage the output amplifier stage. Is this correct? If so, I am sure someone has come up with a modification for such a design flaw. Losing connection between the head and the cabinet is easy with a damaged cable. I sure do hope that a $10 damaged instrument cable does not allow for such a big equipment failure.

Thank you for your help.

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Re: Repairing Kustom K100-1 [message #7384 is a reply to message #7383] Fri, 25 August 2006 12:30 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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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 #7385 is a reply to message #7380] Fri, 25 August 2006 12:43 Go to previous messageGo to next message
stevem is currently offline  stevem
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Like bill says, the second and latter serise of kustoms,that do not have the the output transistors mounted on rear of the back, have a protection circuit to prevent a shorted cord or speaker from killing the output stage.Also as stated on kustom schematics, the amp voltages are stated with no output load, and all controls full up.Only the early kustom amps with the 2 fuses on the front lower right face of the amp have no protection circuit.

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Re: Repairing Kustom K100-1 [message #7390 is a reply to message #7380] Sun, 27 August 2006 09:44 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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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]

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Re: Repairing Kustom K100-1 [message #7391 is a reply to message #7380] Sun, 27 August 2006 15:56 Go to previous messageGo to next message
rodak
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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 #7392 is a reply to message #7380] Mon, 28 August 2006 07:30 Go to previous messageGo to next message
stevem is currently offline  stevem
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Time to start pulling and checking the semiconductors fed by the +30 volts that is getting loaded down from what should be +39 to 40 volts depending on you A/C wall voltage, starting in the driver and output stage.The item you are calling a bridge recto inbetween the 2 output transistors is the thermal circuit breaker, the output stage bias control diode is also clipped in on that aluminum strip.Since you are not poping the fuse(if its the right amperage) I do not think the positive rail 36892 output is bad.Check on my friend!

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Re: Repairing Kustom K100-1 [message #7397 is a reply to message #7390] Mon, 28 August 2006 12:08 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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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 #7399 is a reply to message #7380] Mon, 28 August 2006 13:12 Go to previous messageGo to next message
stevem is currently offline  stevem
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I would check Q107 the channel combining transisor.

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Re: Repairing Kustom K100-1 [message #7401 is a reply to message #7399] Mon, 28 August 2006 13:21 Go to previous messageGo to next message
C4ster
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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 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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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 #7404 is a reply to message #7380] Tue, 29 August 2006 07:28 Go to previous messageGo to next message
stevem is currently offline  stevem
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I`ve been at falt on this too.I though he has a k100 combo with the seperate preamp boards in which he would then have the blue preamp output wires of each channel going to the seperate driver board, his k100 like the 2 non-combo amps of mine have the preamp and driver board being the same item. His amps only blue wires are on the output transistors thats why he has DC voltage on them.
Sorry about that Kaiser will.
To bad the k100s do not have the preamp out rca jack on the back he could then atleast check if the preamps where free from hum.
I don`t about the open filter thing, I can`t` see how he would get good 8 volts positive and negitive to the preamps if either main power rail had a problem like that. I guess checking the 8 volt rails for a/c ripple would be a good thing anyway.

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Re: Repairing Kustom K100-1 [message #7407 is a reply to message #7404] Tue, 29 August 2006 08:45 Go to previous messageGo to next message
C4ster
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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 #7409 is a reply to message #7380] Wed, 30 August 2006 07:26 Go to previous messageGo to next message
stevem is currently offline  stevem
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On an interesting note, I opened up my 1968 K100 last night to to fix its intermitant RCA cable on the in of the reverb pan and I noticed that it has not got a thermal circiut breaker on the outputs. But I opened up my 69 k100 with the metal warning and output tag on the upper right back of the amp, an it has a breaker.It might be interesting to try and pin down in what production number range this change was made. Maybe this happened when the k200`s went from the A models to the B models. Does anyone know if the A models have the thermal breaker?Hey! Les, whats your take on this?

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Re: Repairing Kustom K100-1 [message #7410 is a reply to message #7409] Wed, 30 August 2006 07:55 Go to previous messageGo to next message
LesS is currently offline  LesS
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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]

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Re: Repairing Kustom K100-1 [message #7413 is a reply to message #7380] Thu, 31 August 2006 02:16 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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Thanks to all for the help with this troubleshooting project. I would love the resurrect this old amplifier. I was sent out of town for work, but expect to jump back in this weekend using the tips provided by this board.

On the other topic, does anyone have specs on the power on indicator lamp? I expect to nail down the component failure this weekend and get the parts on order, then give the amp a good servicing (lubricate the pots and give it a good cleaning).

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Re: Repairing Kustom K100-1 [message #7414 is a reply to message #7380] Thu, 31 August 2006 07:26 Go to previous messageGo to next message
stevem is currently offline  stevem
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Hi, 1829 is common industry number for that bulb, mouser electronics carrys them you can call them and ask them to cross a GE 1829.One thing you might want to mod is that 5 watt resistor that is either above or below the lamp housing depending on your k100 model.These 5 watters always near cook the board. I extend the resistors leads with some heavy gauge copper wire or the clippings off of other components and get it to stand up 1 inch off the board.
One other thing you may want to do as a cover your butt move would be to replace that bridge rectifier if its the original round one. They have been known to fail for no reason and the problem with that amp even though it has not blown the fuse could be stressing it.Some of the early models all use what seems to be a very light gauge wire to bring the hot of the output to the output jack, I have never seen any of them melt and its a short run, but the speaker cabinets are made with a larger gauge wire or atleast wire with more insulation.Also make sure the biasing diode mounted inbetween the outputs is clipped in tight and with some heat sink compound filmed on it.

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Re: Repairing Kustom K100-1 [message #7447 is a reply to message #7380] Wed, 06 September 2006 14:17 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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(1) I could not determine the 120VAC line fuse rating as it is soldered in. I have a chassis-mount fuse holder that I plan to mod the amp with soon.
(2) I disconnected the green and red leads from the output filtering capacitors to the main circuit board, powered up, set both channel's volume to full and get ~ 0VDC at the speaker jack. So the filtering caps (C2 and C3) are pulling the output high? In receivers, I usually replace the filtering caps when rebuilding as the large electrolytic caps are prone to leaking after 40 years of life. I am sure I can find a direct Mallory replacement.
(3) I understand now that the transistors mounted to the short piece of heat sink is the thermal cutout switch (Q1 and Q2 on the K100-1 schematics?).
(4) I will re-check the voltages at the pre-amps with no load and volumes set to full.
(5) Does anyone have specs on the round bridge rectifier? If I should replace this, I might as well start building the shopping list.

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Re: Repairing Kustom K100-1 [message #7449 is a reply to message #7380] Thu, 07 September 2006 07:08 Go to previous messageGo to next message
stevem is currently offline  stevem
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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 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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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]

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Re: Repairing Kustom K100-1 [message #7457 is a reply to message #7453] Thu, 07 September 2006 11:43 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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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 #7458 is a reply to message #7457] Thu, 07 September 2006 15:27 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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Chicagobill, per the suggestion from C4STER, I removed the red and green leads from the filtering caps to the PC103 board and took measurements. However, the K100-1 schematics are not 100% correct in that removal of this lead sends no power PC103 with those leads disconnecting. Later, STEVEM notes that the filtering caps are probably not the fault.

The confusion I probably created is when instructed to remove the blue leads at the preamps, I then assumed that the output transistors (they are external to the PC103 board and have blue wires) were the preamps. Not so. I have correctly identified the preamps and have verified that they have the proper +/- 8VDC rail power.

Now that you mention it, removing just the leads from the filter caps to the PC103 board sends no +/- 40VDC to the board and is a useless test. The filtering caps do not appear to be failed. Even if they were bad, they would offer poor noise filtering of the output stage. I could go ahead and jumper the filtering caps out of service, but I do not see how they would be driving the output stage to full power.

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Re: Repairing Kustom K100-1 [message #7459 is a reply to message #7380] Fri, 08 September 2006 07:17 Go to previous messageGo to next message
stevem is currently offline  stevem
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At this point I will say you have a bad driver in the output stage, a shorted Q1 or Q2 will all most always blow the fuse unless its an incorrect value in their.
I am unsure of what you mean by the ratting question you asked.
These heads could be used with guitar, bass or mics.Their are no high and low gain inputs in the amp, if you check the schematic, one jack of each channel has a .005 cap that cuts some of the highs out of the tone, hence the low and high marking on the face plate.I guess they should have also put the word tone on each jack also.
The brite channel of the amp has a boost circuit that is on all the time, any difference in gain betwwen both channels is just from the added lows of the normal channel providing more punch to the sound.
The latter non-ross k100s have a metal tag on the back stating the output is 60 watts RMS in difference to the early heads 50 watts RMS.
My own late K100 that I used for my posted output test could only muster up 49.8 watts RMS not 60 on the 117 volts in my house.

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Re: Repairing Kustom K100-1 [message #7460 is a reply to message #7459] Fri, 08 September 2006 09:44 Go to previous messageGo to next message
C4ster
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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 Go to previous messageGo to next message
chicagobill
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Registered: April 2003
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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 #7462 is a reply to message #7380] Fri, 08 September 2006 12:25 Go to previous messageGo to next message
stevem is currently offline  stevem
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Maybe he should try swapping the leads on the two caps, if that same positive or negative voltage stays low, than its not eitheir cap.
No, early kustoms never had channel switching, infact in that amp, the brite channel may be out of phase with the normal channel so jack in into both preamps may not work, and all you can do is use an A/B box.

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Re: Repairing Kustom K100-1 [message #7464 is a reply to message #7462] Fri, 08 September 2006 13:06 Go to previous messageGo to next message
C4ster
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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 #7465 is a reply to message #7380] Fri, 08 September 2006 13:18 Go to previous messageGo to next message
stevem is currently offline  stevem
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Yeap, Thats what I was intending him to try out, I`am sure if some of the wires are long enough with out removing each cap from its hold down clamp.
I had miswired the negative rail cap in one of my amps I was cleaning out, turned it on and took out all the drive and output semi conductors on that rail.I can`t recall 100% but in was in a 250 head and I think it also took out one of the channel IC chips on one preamp board.
The old wire once and double check two or three times prevals now!

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Re: Repairing Kustom K100-1 [message #7466 is a reply to message #7465] Fri, 08 September 2006 15:21 Go to previous messageGo to next message
C4ster
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I hope KW doesn't get in over his head. You hear that KW??? I have to assume he knows better than I. I know I don't come to think of it. Maybe that's why nothing I have works, including me. Laughing 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.
Conrad

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Re: Repairing Kustom K100-1 [message #7467 is a reply to message #7466] Fri, 08 September 2006 17:40 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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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 #7473 is a reply to message #7380] Mon, 11 September 2006 09:08 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
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Data...

With filtering caps normally wired into circuit = 41.1V at speaker jack, 42.5V at cap C2 terminal, -42.6V at C3 terminal

With caps wired out of circuit = 28.4V at speaker jack, 29.9V at red wires (where cap C2 was), -31.3V at green wires (where cap C3 was)

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Re: Repairing Kustom K100-1 [message #7474 is a reply to message #7473] Mon, 11 September 2006 11:26 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
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 #7483 is a reply to message #7380] Tue, 12 September 2006 09:07 Go to previous messageGo to next message
kaiser_will is currently offline  kaiser_will
Messages: 10
Registered: August 2006
Junior Member
Output transistor test data (Q1, Q1 = RCA #36892)...

B-C = .45VDC, 0 Ohms
B-E = .45VDC, 0 Ohms
C-E = .08VDC, 110/116 Ohms
E-C = .08VDC, 110/116 Ohms

Based on the above, unless there is a 110 Ohm damper diode internally between the emitter and collector, I would say both output transistors are failed wide open. This makes more sense with how the unit is operating (full rail voltage at the output, 60 Hz hum in the speaker)

The RCA #36892 is probably a custom number for Kustom. Does anybody have a cross reference?

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Re: Repairing Kustom K100-1 [message #7484 is a reply to message #7483] Tue, 12 September 2006 11:26 Go to previous message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
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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