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  1. Replies
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    That indicates the trim pot is working, and the...

    That indicates the trim pot is working, and the adjustment range seems reasonable.

    If the problem is intermittent, keep the tubes out, watch pin 5 while you poke the circuit board with a wooden...
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    No. TP30 is a voltage, but not the bias voltage....

    No. TP30 is a voltage, but not the bias voltage. TP30 is a voltage that equates to tube current. The bias voltage is what is adjustable, and is the negative voltage put on the control grids of the...
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    I can see the cap has no excess wires, what I...

    I can see the cap has no excess wires, what I meant was, get out the ohm meter and with power off, MEASURE resistance to ground from the supply line to ground. Across C38, does it measure shorted...
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    You bypassed my question. I do not doubt you...

    You bypassed my question. I do not doubt you connected the cap to the resistors correctly. But I asked if that connection was ALSO shorted to ground.
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    OK, so the socket circuitry has the issue. The...

    OK, so the socket circuitry has the issue. The most likely part there would be the drive transistor. How does it test out?
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    OK, well check the neg end for resistance to...

    OK, well check the neg end for resistance to ground. Is it shorted to ground somehow? I had a guy once bring me a unit he had recapped. He forgot to trim the excess lead wires off one of the...
  7. Thread: Korg Keybed felts

    by Enzo
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    129

    Google piano felts. I just did, and first thing...

    Google piano felts. I just did, and first thing I found was this:
    https://www.howardpianoindustries.com/felt-cloth-leather-parts/

    Just one of many...
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    Your new trace showed continuity for its original...

    Your new trace showed continuity for its original path, but perhaps it has gained continuity to some other adjacent trace where it ought not be?

    C38 is the filter cap for the bias supply. That is...
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    https://en.wikipedia.org/wiki/Stretched_tuning

    https://en.wikipedia.org/wiki/Stretched_tuning
  10. Seriously, look up those old Princeton...

    Seriously, look up those old Princeton schematics, that will show you how Fender drew the circuit. Yours will be very similar.
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    COuntersink the hole. If you look at commercial...

    COuntersink the hole. If you look at commercial amps, you often see the bolt hole has an indentation in the bottom panel to clear the bolt head. And they use a carriage bolt - the bolt with the...
  12. Agree, the trem was changed to the bias wigle...

    Agree, the trem was changed to the bias wigle type.

    First example I could think of was old Fender Princetons, like AA964 or 6G2.
  13. One of my favorites of those is the Peavey...

    One of my favorites of those is the Peavey Classic 30. And you can see it on the board layout drawing. Over by V1, the input tube, you can see in the traces a bunch of parallel lines in the traces....
  14. A short piece of wire is called a gimmick...

    A short piece of wire is called a gimmick capacitor.

    I remember a Mesa that had a short piece of bare wire soldered to a socket pin. I mean like one inch long. Just bare wire sticking straight...
  15. How is your bias pot wired in? Just making the...

    How is your bias pot wired in? Just making the 100k resistor variable won't do it.

    There is no grid leak bias in this amp. The power tubes are using fixed bias. Adjustable bias is still fixed...
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    You have a good circuit nearby to compare. SO...

    You have a good circuit nearby to compare. SO all I wanted to know was if the IC output was there or not. It seemed like before you were looking for output at the jacks. 5532 is specified, but if...
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    NAh, I was worried about 10 or 15v. A few...

    NAh, I was worried about 10 or 15v. A few millivolts won't matter.

    So if you can inject noise into the other AUX drive and hear it, but not here, then teh IC is suspect. First though, monitor...
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    FX1 send comes through a ribbon from the jack...

    FX1 send comes through a ribbon from the jack board. The signal comes from IC4. So is that working? Touch pin 6 and then pin 2 with a small screwdriver or something to inject hum. Does that come...
  19. Thread: Tascam Porta One

    by Enzo
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    No, I am just remembering some other unit. The...

    No, I am just remembering some other unit. The transistor was a 2SC916 or something like that. Ignore it, I am leading us down a false path it seems.
  20. Thread: Tascam Porta One

    by Enzo
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    I forget which model now, but Porta1 was popular...

    I forget which model now, but Porta1 was popular back when. But there was one transistor in the power supply regulator circuit, that screwed to the tape transport frame as a heat sink. There was a...
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    SP = solder post, or something like that. Look...

    SP = solder post, or something like that. Look at the schematic, they are simply places on the board to connect the wires from the choke, which is not on the board itself. Look at the parts layout...
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    My concern is not for the impedance. We all know...

    My concern is not for the impedance. We all know we can get away with being off one notch. My concern lies in the tube bias. The transistors biasing the cathodes might get confused. Think cathode...
  23. Yes indeed. and that is why military and similar...

    Yes indeed. and that is why military and similar use 400Hz instead of 60Hz. And I suppose ultimately that is why we use SMPS with frequencies as high as MHz.
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    V means vertical. OEMs can specify lead...

    V means vertical. OEMs can specify lead configuration. Like TDA7293 or TDA2050 cone in H and V versions. V versions have the two rows of legs bent to the thing stands up on the board. The H...
  25. Helmholtz. It was just a joke. I was implying...

    Helmholtz. It was just a joke. I was implying the line from the movie SPinal Tap, where all their amps "go to eleven... Well that's one louder, innit?" ("innit" being the written form of "isn't...
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    exchange places with the power tubes. Now does...

    exchange places with the power tubes. Now does teh same tube red plate in its new socket? Or does the other tube now red plate in this socket? Or put another way, does that excessive 1640mv move...
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    wire across it. Because the current...

    wire across it.



    Because the current through the 1 ohm resistor is causing a voltage across the resistor - Ohm's Law - so you measure in VOLTS. Since the current and voltage will be the same...
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    You measure the bias voltage at the tube grid. ...

    You measure the bias voltage at the tube grid. If the pot turns that voltage up and down, the pot works.

    Do I have this right? You put new power tubes in it, and found 119mv on the TP. Then you...
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