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K200B-4 [message #14456]

Mon, 27 September 2010 23:42

ampcabinets
Messages: 10
Registered: August 2010

Junior Member

Let's start by saying I usually work on and build tube amps, but have woked wit hdigital circuitry in the past, designing ang building automation equipment, which is totally different from tube or solid state amps.
I have penty of equipment, meters, oscilloscopes, etc.

Now....
I have recently been given a kustom 200b-4, by a friend, to keep, where the first channel (reverb/tremelo channel) is not putting out much sound.
what I can hear is clean, but nothing affects it (volume, bass, etc)
Reverb works, tremelo works, as far as I can tell.
I didn't know a bass amp (200B) had all of this (harmonic clipper, volume boost, reverb, tremelo, etc) on it. But that is the label on the chassis. I can plainly hear the reverb tank ,when I tap on it, and it is not dim in volume.

The reverb tank was backwards (tons of noise when in the chassis...very loud hum, almost a motorboat sound), so I experimented, and turned it around and now it is dead quiet. Cables were marked but backwards. (something about those power filters makes noise as far as I can tell)

Channel 2 is loud as all getout, but overall the entire amp has a nasty hiss, and it is reasonably loud, and is on both channels (I suspect something in the power section for this. (I shorted the resistor next to Q700 and it went quiet, so I replaced Q20 0and it still hisses, loudly. I had one of these, new, in 1970 and if I remember, they all had some kind of slight hiss.

what I've done:
I swapped channels going into the power section and the reverb channel stays dead, or nearly dead. It makes no difference on the clipper channel, it still is loud and clean.

I replaced the 2200uF filters with the suggested 4800Uf computer grade mallory's. It didn't change anything. I kept the originals.

The board number is pc-303, and I do have the schematic for both it and the 703 main amp board.

I have all of the transistors needed to gut the board(s) and completly rebuild them. That is a tedious process and is not the way to trouble shoot an amp, in my opinion.

any hints on where to start?

You may email me if you like...

thanks

p.s. any editing is only to correct my spelling........

[Updated on: Mon, 27 September 2010 23:44]

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Re: K200B-4 [message #14461 is a reply to message #14456]

Tue, 28 September 2010 09:43

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Hi.
what you have is a guitar amp, the model K200-1 is the Bass/non-effects amp. The B letter after the 200 stands for the second major serise of amps, 200As where the first.

The hum you had due to the reverb pan being turned the wrong way is due to the normal 60 HZ magnetic flux from the power transformer being picked up by that wrong side facing end of the pan, not the filter caps!

One side of the pan is just as likly to pick up 60 noise and hum as placing your guitars pickup(s) next to the amps power transformer.

All Kustom amps have a certain level of hiss, but should have little hum but for the following factors, you have the reverb up, the two main filters are bad or going bad, or the power supply rail regulator circuit that controls the preamps +8 and -8 volt supply is out of balance due to semicondutor problems or componet drift.

Basically the smaller the size of amp, the more the reverb will apply hum into the audio signal stream due to the the pans closer mounting point to the power transfomer.

As far as hiss goes in all Kustoms it comes about due to many factors.
First and for most are early noisy transistors.
replacing the first one or two transitors on each preamp board can help drop the hiss a fair amount as any excess noise at this point in the signal chain gets amplifide hundreds of times by the time you hear it at the speaker.
also the driver transistors on the output driver board (two 38737 and two 38736)can add to the hiss level heard.
Next on the list would be the Tantalum type caps that these amps use a bunch of.
The orignal power supply filters are 4500 uf caps,not 2500s. I guess if you do not want to let those 2500s just sit you can parrallel them with the current 4800s.

As far as trouble shooting your 303 board goes, since you have a O-scope and a schematic just follow a injected signal thru the circuit and see where it goes dead and stops passing thru.
99 times out of 10 its a transistor, and many times its the first gain stage transistor that got blown out from someone pounding the crap out of the that first stage with a very high drive signal level from a stomp box.
Many times a shorted Tantalum cap can be the signal killer also.

Once you get both channels working again if you find you still have a un-normal level of hiss regardless of channel used, that would be a sign of hiss from the output stage driver board.

Let us know how you make out and if you need more help.

[Updated on: Tue, 28 September 2010 09:47]

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Re: K200B-4 [message #14463 is a reply to message #14461]

Tue, 28 September 2010 12:22

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

There is one thing that I will add here that relates to the reverb control.

The reverb control on the K200 series amps is a balance control. Full counterclockwise is no reverb and all straight signal. Full clockwise is all reverb and no straight signal. So while you are troubleshooting, be sure to turn the reverb control down.

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Re: K200B-4 [message #14471 is a reply to message #14463]

Wed, 29 September 2010 04:58

ampcabinets
Messages: 10
Registered: August 2010

Junior Member

I'll get started today ,tracing the signal.

The original caps I pulled out were, indeed 2500uF, with the original rings.
I have no use for them, if I now, have the correct parts inside.

I have ordered all of the transistors I will need to complete a 'complete'overhaul. I had all but about 5. (those you mentioned. I figured that out, last night, when going through the schematic.
I had all of the rest, in my old stock 'drawers of things never used'. I can't even remember what I got them for, originally.

I have plenty of tantalum caps, also.

thanks,
I am sure I'll need a bit more help in finding the culprit.

all I have worked in, in the p ast is those tube amps and digital circuitry.

I have determined that all voltages and negative voltages are present and in compliment to the other (plus and minus numbers are the same)[+/- 8vdc and +/- 40vdc]

Thanks for the info as to 'why' the hum in the reverb went away.
I had figured the 60 cycle hum was from the caps.

[Updated on: Wed, 29 September 2010 04:59]

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Re: K200B-4 [message #14472 is a reply to message #14456]

Wed, 29 September 2010 09:16

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

When you get it back up and running, just note like I posted that if you find you want to tame the amps hiss level more that Tanalum caps used in certain locations add to the noise.

Changing out small value ones (1uf or less) with orange drop types and larger ones with even cheap electrolytics can drop the noise floor a good amount.

I too cut my teeth on repairing tube TVs and stereo gear and to this day restore, modife and repair tube gear and tons of guitar amps for local players.
When it comes to replacing parts give me a point to point terminal strip or turret type amp any day comapred to removing componets from a dubble sided circuit board in SS gear and even some mass produced guitar amps!!

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