Kustom K200A.4

[bsco]

Anybody here have any experience with this head?? It is original....of course it needs a good cleanup.....pots and jacks...etc...tremolo and reverb work...but there is a Selective Boost circuit and a clipper circuit.....I guess the boost selection will increase overdrive which then can be controlled by the Boost volume........this does not do anything so I have a problem there......but what about this clipper circuit......what exactly is it's function? Is it used to manipulate high freq content in some way?? any info greatly appreciated......
Cheers,
Bernie

[52 Bill]

Anybody here have any experience with this head?? It is original....of course it needs a good cleanup.....pots and jacks...etc...tremolo and reverb work...but there is a Selective Boost circuit and a clipper circuit.....I guess the boost selection will increase overdrive which then can be controlled by the Boost volume........this does not do anything so I have a problem there......but what about this clipper circuit......what exactly is it's function? Is it used to manipulate high freq content in some way?? any info greatly appreciated......
Cheers,
Bernie

Happy B-day!

The Selective Boost is sort of like a switchable tone boost. It emphasizes a fairly narrow band of frequencies, sort of like a parametric eq. Later versions of the amp used a wah type pedal to control the frequency sweep. The Clipper circuit is a fuzz or distortion unit.

Remember that all of the FX work on the one channel only, even though the controls would make you think differently. They also must be turned on with the footswitch.

That is the type of amp that Creedance Clearwater used on all of those hits back in the day.

[bsco]

Happy B-day!

The Selective Boost is sort of like a switchable tone boost. It emphasizes a fairly narrow band of frequencies, sort of like a parametric eq. Later versions of the amp used a wah type pedal to control the frequency sweep. The Clipper circuit is a fuzz or distortion unit.

Remember that all of the FX work on the one channel only, even though the controls would make you think differently. They also must be turned on with the footswitch.

That is the type of amp that Creedance Clearwater used on all of those hits back in the day.

Thank you for the birthday wishes.......I am not familiar with this particular amp.....so that info will come in handy.....The footswitch in this unit is a 4 pin female xlr cale and the male connector is mounted on the rear of the amp.....That works ok so I have problems with the associated circuitry for these functions......The owner was kind enough to supply the schematics.....hopefully I can get it sorted out....
Cheers,
Bernie

[52 Bill]

The worst part about the A-series amps is the stacked pc boards in the preamp and fx circuits. It can make following the signal path interesting.

[bsco]

The worst part about the A-series amps is the stacked pc boards in the preamp and fx circuits. It can make following the signal path interesting.

I just checked it out...The channel located to the left when looking at the front of the amp is the channel that all these functions control......I was using the wrong channel and didn't realize it....anyway the clipper section works fine....reverb and termolo work fine as well....The boost circuit is badly distorted and only produces this distorted sound when the boost volume control is maxed out......so I am willing to bet I have a bad transistor in the boost circuit and yes....I noticed that the boards are stacked......I am lucky to have the schematics which I will size up later tonight.....outside of this problem, the amp sounds really good...that tremolo sounds awesome.....

[bsco]

It has been awhile as the owner is away working....I had to put this to one side for a few months......just getting back at this again....I have been playing around with this for the last two or three days and I noticed that when scoping the unit....I have a signal generator hooked up, the signal comes in from Q401 and goes through a coupling cap and then through a white wire and down to the boost selector switch.....then it goes off to Q402 base....the transistors and all associated components test good....but there is no signal on the other side of this dual boost selector switch...I guess you could say something there but it is very low....especially when compared to the other side.....the board in question is PC 402A and PC 402B..... Lets see if I can add a schematic of the circuit in question.....
Cheers,
Bernie
P.S. I had to make two scans as it is on 11x17 paper....

[52 Bill]

....but there is no signal on the other side of this dual boost selector switch

The output of the circuit is on the yellow wire that connects to C407, coming from the emitter of Q406.

The second side of the switch is actually part of what I think is an electronic inductor circuit. It's not directly in the signal path.

Have you cleaned the switch? Scope the output from the emitter of Q406 and see what you get there.

[Jazz P Bass]

I am confused about where you are losing the signal.

The first step is to verify the static voltages on Q2 & Q3.

K200A Pre_1.pdf

K200A Pre_2.pdf

[bsco]

The output of the circuit is on the yellow wire that connects to C407, coming from the emitter of Q406.

The second side of the switch is actually part of what I think is an electronic inductor circuit. It's not directly in the signal path.

Have you cleaned the switch? Scope the output from the emitter of Q406 and see what you get there.

The first thing I did was clean the switch and checked it with an ohm meter....It is late here...I'll check that out tomorrow and let you know.....Thanks for the reply....

[bsco]

I am confused about where you are losing the signal.

The first step is to verify the static voltages on Q2 & Q3.

[ATTACH]36131[/ATTACH]

[ATTACH]36132[/ATTACH]

Ok. I'll let you know tomorrow....as it is getting late here......Thanks for the reply....

[bsco]

The output of the circuit is on the yellow wire that connects to C407, coming from the emitter of Q406.

The second side of the switch is actually part of what I think is an electronic inductor circuit. It's not directly in the signal path.

Have you cleaned the switch? Scope the output from the emitter of Q406 and see what you get there.

Ok. Checked that out.....nothing there.......

[bsco]

I am confused about where you are losing the signal.

The first step is to verify the static voltages on Q2 & Q3.

[ATTACH]36131[/ATTACH]

[ATTACH]36132[/ATTACH]

Ok. Did that....For Q402....base is 5V, emitter is 4.55V and collector is 5.65V.......now for Q403 base is 5.20V, emitter is 4.55V and collector is 4.92V.....

[52 Bill]

Ok. Checked that out.....nothing there.......

Then keep tracing backwards through the circuit to find out where the signal stops. Q406, Q404, Q403?

As for the voltages, the collector voltage of Q403 is off by quite a bit. The schematic shows 12 vdc.

[bsco]

Then keep tracing backwards through the circuit to find out where the signal stops. Q406, Q404, Q403?

As for the voltages, the collector voltage of Q403 is off by quite a bit. The schematic shows 12 vdc.

The signal is at the base of Q402.......after that it is missing.....I removed Q402 and Q403and all related components....everything checks good....the gain of the transistors are over 300 each....which was checked on a transistor tester...you know...built into your DMM.....I was tempted to replace both of them with 2N3904's but when I checked out my stash of parts, I only had 2N3906's......as far as the voltages go, I have to ask.....this unit has a pair of 10W 330 ohm resistors attached to the rear of the chassis.....which have a diode from each resistor to ground....the voltage on one side of these resistors are plus and minus 39V....on the other side of the resistors the voltage reading is plus and minus 10V......that seems very low to me but what throws me off is the fact that the amp sounds very loud and sounds fantastic.....to me anyway......the owner had received the original schematics when he actually purchased the unit new way back.....and there are other schematics for a couple of other boards there as well.....which don't seem to belong to this amp......anyway, I am going to check out the power supply voltages first to see why there is such a big drop across these resistors and these diodes appear to be installed after the owner had it repaired once before many years ago.....I measured the resistor values and they check good......I shall check this out and get back to you........Thanks for the reply.......

[52 Bill]

......as far as the voltages go, I have to ask.....this unit has a pair of 10W 330 ohm resistors attached to the rear of the chassis.....which have a diode from each resistor to ground....the voltage on one side of these resistors are plus and minus 39V....on the other side of the resistors the voltage reading is plus and minus 10V......

Are the two diodes 10 volt Zeners? The K200A series of amps should have +&-40 dc supplies for the power amp and +&-24 vdc supplies for the preamps and FX switching. There should be two small pc boards on the back panel, one for each 24 volt supply.

The later K200B series amps used +&-8 vdc supplies for the preamp supply, but these were regulated supplies not Zeners. The much later Kustom 150 and 250 amps used resistors and 12 volt Zeners for the op amp supplies.

If this has been modded, what do the 10 volt supplies connect to? Are the boards all original?

If the signal dies at Q402, check to see if the circuit matches the schematic. Has there been any mods done to this board?