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So I have this Windsor on the bench..... It had this problem of unnatural note decay... and what sounds like a very bad ground hum... I started with checking all the solder joints, possible effects loop troubles. I also put a cable in the effects send and return... nada.... so I changed out the first preamp tube....fixed most of the note decay thing.... So I'm playing it and adjusting the controls and it starts oscillating when I adjust the presence and Texture controls... Adjusting the Resonance control doesn't affect the oscillation at all.... I looked with a magnifier to see if maybe I missed a cold joint in those controls.... no joy.... I noticed in the schematic that both the presence and Texture controls go to V3.... one to each side of the triode... So I changed that tube... no joy... I'm suspecting C23 in the V3 circuit... The presence control hits the negative side of that cap in the circuit...Also, between the 2 sides of the triode lies C17... I'm gonna change C23 in about 10 minutes as I have that value laying around... Has anybody had a similar problem with one of these? I read a thread that somebody posted about oscillation but they solved theirs in the effects loop... And they said nothing about the presence and Texture controls not affecting their oscillation problem....Any help would be appreciated.... thanks for reading...
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Oh....I also tried replacing all power tubes with new EL-34s as I had a set lying around.... -
Not too familiar with the internals of the Winsor but : is there a 3 pin connector for the output transformer primary wiring? (Br,Rd,Bl)
If so, it may have been reversed at reassembly. (causing positive feedback)
Note: found a chassis pic.
It appears that there is a locking tab on the molex connector to prevent inserting it wrong.Comment
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I suspected that actually! Thanks man....But the board is silk screened Blue Red Brown...I pulled that stupid connector off and just hard soldered those to the board in their proper place... it was on correctly, but the connector pulled off way too easily...Comment
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Also... This thing has 4 legged pots... what the hell is up with that? This thing is cheap as hell on the inside.... I was able to read 1 meg at the texture pot... finally after some poking around... But... the presence pot says it is supposed to read 10k....it reads around 1k... but... measuring the Resonance control the same way got me 32.5 k and it's supposed to be 1meg... so... I guess I will have to take it out and read it...Comment
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Look at the circuit rather than the parts. I assume the presence control is VR8? It is a 10k pot. One side is grounded. the other side is wired through 820 ohm R58 and 470 ohm R51, which in turn is grounded, so we have a 470 plus 820 in series, which is across that 10k pot. There is another parallel path to ground through 1 meg VR6, 33k R47 and the OT secondary. Due to the large 1 meg, I will ignore that branch for now. So, 1290 ohms parallel 10k. I get about 1140 ohms 1.14k. That is close to what you got. If the 10k pot were open you'd see the 1290 ohms.
Same with the 1 meg pot, it has this other stuff in parallel with it. We can ignore the OT secondary as it is an ohm or two at most. If you measure across the pot itself, you have the parallel stuff we just discussed plus the 33k R47 in series with that. SO 33k plus 1290 ohms or 34.3k. Put that in parallel the 1 meg pot and you get about 32k. The pots seem intact. Possible wiper issue, that you would check right at the pot. No reason to suspect them really.
The texture control is not really related to the NFB circuit of the presence. Look at the split load phase inverter. the plate coupling cap goes to the upper power tube grids through C7. On the cathode side, coupling cap C16 would normally connect the lower tube grids to that cathode, but they added the texture control. Note also the cap is twice the value of C7. That is because they added C15, which is effectively in series with C16. That makes the aggregate total cap value at about the same as C7. The texture pot itself is really just like a volume control for those lower power tube grids. The texture control takes a push pull stage and turns down the push, so to speak. Or was it the pull?
So oscillation. Oscillation means feedback. The NFB is by design feedback, though out of phase so it is negative. Most of this is on PC boards, so lead dress is less an issue, but consider the three wire cable from the presence control board to the main board, does moving it help?Education is what you're left with after you have forgotten what you have learned.Comment
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