| another k200 problem [message #15847] |
Mon, 13 June 2011 21:44  |
ampcabinets
Messages: 10
Registered: August 2010
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Junior Member |
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I recently acquired a k200A-1 early ross kustom amp.
Problem #1 is hum, and the board PC-702. It also does not have the "Kustom 200" on the front, as later models do. Serial number is 20223, and was marked 1972 inside the chassis by the previous owner in the dust and dirt, telling me it had been sitting around for some time before 1972 to gather all of that dirt. I did clean up the inside, nice and sparkly.
I disconnected the preamp section to work on the power section and replaced the 5 1 ohm 5 watt resistors as they were all either open or sitting at some rediculous resistance such as 250k, etc.
After that I replaced all of the power transistors with new ones, and 2 diodes on the board, 1 near the input and one near the output, as one checked shorted, and one checked open, even after removing them from the circuit.
The 1 ohm resistors still heat up, so I am suspecting the driver transistors, now.
question, has anyone run acrss this before and do they have an ultimate solution?
I also changed the power filter capacitors with new and it made no change and the bridge checks good, also.
This amp is a bit different than the one I fixed before, in that the power transistors are mounted to the bottom of the chassis, and not to an aluminum strip used as a heatsink. Also the PC-702 board has another board just before it, with the same circuitry, but only about 2 inches by 4 inches.
I guess if nothing else I can just shotgun the parts.
anyone have any help to offer?
thanks
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| Re: another k200 problem [message #15857 is a reply to message #15847] |
Tue, 14 June 2011 12:12  |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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The emitter resistors should only get hot after the amp has been pushed really hard. They normally stay cold when the amp is at idle.
Rather than taking the chance of doing damage to the parts that you have just installed, I'd suggest that you test all of the transistors and diodes in the power amp. If you turn on the amp while there are still some bad parts in it, it could cause the new outputs to blow.
I've seen a lot of these power amps and you can never tell if it's just one bad part or twenty, each one is different. I see that you have started a number of threads for this amp. I'll suggest that you use just one for all of your questions, so that we can all follow along and not get lost.
I will check to see if I have the schematic for this amp. If I do I'll send it to you. It will help to be able to discuss the parts that you need to check or replace.
As for your question about the driver transistors, I've recently had to replace 8 of them on a couple of heads that I was working on. My stock of NOS ones has finally run out, so I used some modern replacements. I was able to remove the dead originals from their heatsinks and solder on the new replacement transistors. Some of the old ones just popped off of the heatsink and some of them needed to be drilled off. If you place them face down on your bench and take a screwdriver and a small hammer, you can see if yours will pop off or not.
One thing that I found is that the new replacements have a better high frequency response and may cause the power amp to oscillate. Adding a small value snubber cap to each side of the driver stage will tame this problem.
I can't remember the board layout of the A-series power amp, so I don't know if you need to connect the heatsink terminals to the collector or not. It should be fairly easy to figure this out though.
I'll look for the schematics.
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| Re: another k200 problem [message #15872 is a reply to message #15857] |
Sat, 18 June 2011 13:37 |
ampcabinets
Messages: 10
Registered: August 2010
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Junior Member |
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yes, it is the A series with no >200< on the front and it does say Ross.
I did receive the schematic and do have the amp working, although it does have a hiss (as do most old kustoms).
However I did decide to make a tube amp out of the chassis, 6G6b bassman style and have pulled all of the components out, with the exception of the input and output jacks,knobs and lamps.
I have always wanted to do this and this is the only amp that lends itself to the job (has the correct pot locations...vol, bass, treble, bright/deep sw). The only thing it doesn't have is the presence control, which I will fix in place, with a resistor.
I did save as much wire on the original components as I could, and without cutting any terminals off, if I could help it.
I am offering up the transformer and other parts to anyone who wants them for the shipping cost. This includes the preamp boards, tranny, bridge rectifier (this thing is huge, and made by westinghouse), and power caps ( which are all still good), cap mounting rings, etc. Be aware that shipping will be upwards of 20 bucks, because of the weight of the transformer, but I can estimate that after I get it boxed up.
He who speaks first can have it. I will pack it in the smallest box possible, that I have in my attic (bubble-wrapped, etc).
Email me your shipping address and I will post back when it is spoken for. In case of multiple emails, the one with the earliest "real" date/time stamp will get them. I do take paypal.
I'll wait a few days before I can them, to decide if no one wants them.
I DO (100%) appreciate all of the help in fixing this amp. It only took 3 days, after I got it to complete the task, thanks to you all.
Chicago Bill...THANKS!!!!!
You are 'the man'......
as a note, The collector leg of the driver transistors was cut off (originally by kustom), but the heatsink was welded to the body of the transistor, and soldered to the board ,to take it's place, so I beat the heatsink off (not very hard), and re-drilled the heatsink to fit the new replacements and put them back in the board, as they were, originally. I had a very tight fit to the transistors, and the solder connections held them down tightly, also. I did use a drop of solder (3 on each transistor)to make sure the heatsink would not pull off, but they were a really tight fit, anyway. This board actually made less white noise than another "A" series K200 I have and so I used this board in it, and sure enough it was quieter than the old one.
[Updated on: Sat, 18 June 2011 13:54] Report message to a moderator |
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| Re: another k200 problem [message #15876 is a reply to message #15847] |
Sat, 18 June 2011 22:38 |
ampcabinets
Messages: 10
Registered: August 2010
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Junior Member |
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believe it or not, with the stand-offs of the model A, there is enough room for tube sockets to be mounted inside the chassis, and the wiring will go underneath. I drilled holes and used rubber grommets to make sure the wiring did not short or get cut by rubbing, through the chassis. The cleareance is about 3/4 inch, underneath the chassis. (By the way, I have some older NOS omron 8 pin sockets that fill the shallow socket bill and some ceramic 9 pins that are also, reasonably shallow, plus I can lay down the lugs on the 9 & 8 pin sockets, for more cleareance, but the 8 pins will fit without laying them down. The 9 pins are very close, but they clear, when placing a flat board across the chassis to check fit, by about 1/8 inch, before trying to lay down the solder lugs.
The transformers fit nicely inside the chassis as well.
I cut an opening large enough for the tubes to get air circulation, in the back and I have some z-channel and a nice piece of expanded metal, from the back of an old marshall (flat with holes in it)that will be very nice for the back. The z-channel will get rivited in, as well as the sheet metal vent.
I will have to scrunch the board, a bit from the original size, but that amp's board was loosely fit, anyway, and spread out. The capacitor board will fit sideways, and the bias board will be on the same board as the rest of the components.
Since the inside of the case is wood, I shouldn't have to worry about shorts, underneath the amp, but I will be using tie-downs for all of the wiring, anyway, inside and out, with tie-wraps ,also.
I'll be more than happy to post pics as I build this, if anyone is interested.
I also decided to mount a variable 25k linear pot, inside the chassis, in order to set the presence to a desireable level. If I remember, I used to set mine at about 6 or 7 on my Fender 6G6b.
I was also thinking I should vent the top, with the same z-channel and expanded metal, but I am not too keen on messing up that tuck and roll, so I may throw that idea out the window.
If I DO use that idea, I will not just cut out the naugahyde in a square, I'll cut a slit, pull it back, cut the hole and then wrap it under the opening and glue/staple it to the wood inside the cabinet. That way it won't tend to pull out from underneath the vent, from just cutting a square hole. I'll also make the entire vent first and then fit the opening to it.
I have collected 90% of the parts and will start with the chassis, this next week.
So far everything looks good for the conversion, but you know about murphy's law.
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| Re: another k200 problem [message #15877 is a reply to message #15876] |
Sun, 19 June 2011 19:51 |
stevem
Messages: 4754
Registered: June 2004
Location: NY
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Senior Member |
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You could always install a small fan you vent it out and not cut the T&C.
What I have done on a bunch of my tube amps that have a presence control is get one with a on/off switch so I can have no gain cutting feedback circuit at all when I want it.
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