| K200 preamp extreme clipping [message #9163] |
Thu, 24 January 2008 03:41  |
alexr
Messages: 4
Registered: January 2008
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Junior Member |
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Hi from a new poster.
I have a K200 preamp to repair. I have it out of the amp, on an independent power supply. It is the one with the true boost section with two extra txstrs. I fed it about 100Mv. Up to around 4 on the volume knob it amplifies ok, then above that it goes into clipping. The weird bit is that as you get around to 6 or 7 the clipping gets quite extreme, and the output amplitude is a lot less than when the wave is clean. By the time you get to ten the signal has been clipped almost right down to zero.
About a half way through the circuit there are a pair of txstrs - Q203 and 204 - that seem to set the bias (correct me if I'm wrong). The voltages on these don't seem right and it's at this point in the circuit that the clipping sets in (once it's really set in the whole circuit is affected).
I've heard that these amps give full output at about 4 - but is the extreme clipping also normal?
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| Re: K200 preamp extreme clipping [message #9168 is a reply to message #9163] |
Thu, 24 January 2008 16:48  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Welcome to the board alexr. You'll find a great group of guys (and Deb) here to discuss the ins and outs of tuck 'n roll.
As to your problem, what version K200 do you have? I'm guessing a K200B-1, as I think it is the only one that has that boost pre-amp in it. If it is, the circuit board should be marked PC203.
It's been awhile since I've worked on one, but I do own a K200B-1, so I can always pull it out to refer to, if needed. I can't say that the symptoms that you describe sound familiar, and they certainly don't sound correct. Sometimes these amps will sort of "hit a wall" at mid rotation of the volume control, but I don't think I've ever seen one die out at higher settings before.
Could it be going into high frequency oscillation at the higher control settings? How do the tone control settings affect the distortion? How about the boost switch? Also, what happens to the plus and minus 8 volt supply voltages when it goes into distortion mode?
I don't have the schematic for that board, so I don't know about the two transistors that you mentioned. Maybe I should pull out my amp and see what I can find.
Again, welcome to the board.
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| Re: K200 preamp extreme clipping [message #9172 is a reply to message #9171] |
Sat, 26 January 2008 12:45 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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alexr:
Last night I pulled the chassis on my K200B-1 and ran some tests. My results are as follows:
Power supply voltage + and - 40vdc
Pre-amp supply voltage + and - 8.25vdc
Input to normal jack: 1K sine wave @ 100mV ac
Treble and Bass controls set at mid-point.
Bright boost OFF
Scope attached to the pre-amp output at the input to the power amp.
At onset of clipping, output voltage was 1.399vac
The volume control was approximately at 1:00 o'clock rotation.
With volume set to mid point, output voltage read 0.995vac
with volume set full clockwise, output voltage read 1.668vac (grossly distorted waveform)
I ran the same tests with the tone controls set to maximum and got the following readings:
Onset of clipping: 1.561vac (control at 11:00 o'clock rotation)
Volume at mid-point: 1.86vac
Volume at full clockwise: 1.81vac
I checked some of the circuit voltages, and found most were within 10% of the values shown on the schematic. But at the colletor/base connection of Q203 and Q204, the voltage was +4.25vdc. This did not change with any change to the volume control setting.
I hope this helps you find your problem. Let me know if there is anything else you need measured.
Bill
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| Re: K200 preamp extreme clipping [message #9183 is a reply to message #9182] |
Mon, 28 January 2008 11:55 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Steve: Good point about the control settings. I always forget that's what on the schematics.
Alexr: I don't know about C222 and R234 values, I will check it out and get back to you.
Regarding these parts, if you look at the schematic, there are small circled numbers by these parts. In my experience with Kustom paperwork, this indicates revisions from the original schematic values. The schematic that you posted was marked "Rev.3, 5-69". It is possible that your amp is older (or newer) than the schematic, hence the value differences.
If you search the board you'll find a serial number dating chart that Les Strickland posted. You can look up the serial number of the amp that you have and get an idea of is manufacturing date. Les' chart is really amazingly accurate, as he has spent many hours researching the handwritten date codes that he has found marked on the amp chassis top rails.
Bill
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| Re: K200 preamp extreme clipping [message #9185 is a reply to message #9183] |
Tue, 29 January 2008 12:04 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Alexr:
I checked my pre-amp board, and it matches the values shown in the schematic for C222 and R234. The serial number dates my amp to mid 1969, so it would make sense that it would.
Bill
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