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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 9:36 am
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Did somebody just spit into the wind?

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 11:08 am
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BMW2002Ti wrote:
...BTW, can anyone use the COLOR option for FONTS? Just to right of posting area?


Thanks!


Like this?

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 3:49 pm
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So, are we going to try and help the OP solve his problems or not? I'm still game.

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 5:41 pm
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Ok, just to be clear. The Tremolux is working fine. It is the Dual Showman I am struggling with, and being new at repairs.

I failed to realize that the tremolo pedal switch should be closed, big oops. All the voltages are close to those on the schematic. Right now the only variance I have found is the 2700 resistor in the tremolo circuit and the vibrato channel is about half the volume of the normal channel, all settings the same.

Also, I changed to a three prong ac plugs and removed the death cap. TR1 has a black and a white wires and I connected black to black and white to fuse to power switch to white TR1. This is correct?

Thanks for any and all help.

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 7:06 pm
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The tremolo circuit does in fact cause some insertion loss within the vibrato channel but certainly not to the extent of a 50% loss in gain and overall volume. There's another issue at play IMO.

HTH

Arjay

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 10:15 pm
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MattGuitarKy wrote:
Ok, just to be clear. The Tremolux is working fine. It is the Dual Showman I am struggling with, and being new at repairs.

I failed to realize that the tremolo pedal switch should be closed, big oops. All the voltages are close to those on the schematic. Right now the only variance I have found is the 2700 resistor in the tremolo circuit and the vibrato channel is about half the volume of the normal channel, all settings the same.

Also, I changed to a three prong ac plugs and removed the death cap. TR1 has a black and a white wires and I connected black to black and white to fuse to power switch to white TR1. This is correct?

Thanks for any and all help.


The cathode resistor for that half of the 12AX7 being out of spec will cause the bias for that part of the tube to be off which will change the operating characteristics of the tube. Since the guitar signal does not pass through the 12AX7, I wouldn't think that it would affect the volume level.

I think Arjay is right, although I wouldn't characterize the vibrato circuit's impact as "insertion loss" since the signal doesn't go through it. However, it does load down the output of that section of the preamp through the intensity pot and the photo-sensitive resistor in the opto-isolator. If the vibrato performance is not up to par, I would replace the 2700 ohm cathode resistor and the opto-isolator as well as check the intensity pot function. Maybe check the 220K ohm resistor also.

As long as you properly grounded the 3-wire power cable, you should be OK.

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Post subject: Re: Bias setting
Posted: Tue Jan 08, 2013 10:51 pm
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Also, I changed to a three prong ac plugs and removed the death cap. TR1 has a black and a white wires and I connected black to black and white to fuse to power switch to white TR1. This is correct?

Put the hot (black line) to the fuse and switch. And the neutral (white line) directly to the PT. As in schematic below:

Image

Matt, Maybe, replace the bypass caps off the 12AX7 halves of the Vibrato/Tremolo tube. I use 25mfd/50VDC Sprague ATOMS. In fact, I'd replace all the input tube's bypass caps (25/25 to 25/50). When the caps get old and dry out --- the bias setting of the tubes changes. Usually, goes too cold. Losing oommph, esp bass response.



http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Bias setting
Posted: Wed Jan 09, 2013 8:56 pm
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Again, thanks to all for your help. I am learning alot and appreciate all the time and efforts of this community. Thank you!

Here are marked up schematic and layout of the new parts and measurements. The blue highlighted elements are new, the green highlighted elements are good, and the blue numbers are measurements.

I will install a new 2700 resistor tomorrow.

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Post subject: Re: Bias setting
Posted: Wed Jan 09, 2013 9:31 pm
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Very nice editting of schemo. One thing, bypass (or remove) the ground switich. Your three-prong conversion basically negates this swotch's function. I put a new Fender type switch onto the chassis and SAVE the original ground switch. These old US made switches are hard to find these days. And can be used for other amps power or standby switches, with a bit of re-wiring.


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Post subject: Re: Bias setting
Posted: Wed Jan 09, 2013 10:03 pm
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All looks good to me. :D

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Post subject: Re: Bias setting
Posted: Thu Jan 10, 2013 7:02 pm
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The new resistor is in and buzz is still alive and well. Now when testing voltage at the 500pF & 1M point (53v preciously) it pops severely.

At this point I'm convinced the problem is in the vibrato channel. Turning up the treb, mid, bass and volume intensifies the buzz. Should I remove each element and test starting at the pots and work my way to the tube?

Again thanks!

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Post subject: Re: Bias setting
Posted: Thu Jan 10, 2013 7:38 pm
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Matt, check the tone stack coupling caps. Pull one leg of the 250 pico-farad treble cap. Listen to the result. Replace the 250 pico-farad cap. Then, pull first the 0.1 mfd and then 0.047 mfd and hear the results. If you find ONE open cap drops the excess volume and humming ---- try replacing it with a similar mfd value cap. I like to use 600VDC related caps, as they are more resistant to breakdown and passing DC voltage.

Check voltage before and after that 0.1mfd cap just past the second half of the 12AX7 of the vibrato channel input stage. Should be 200+ VDC on one side and no more than 1-2 VDC on the other. If the low voltage side is in the 10-20 VDC raange, you need a new coupling cap.

This is true for any coupling cap, that links the previous anode stage to the grid of the next stage.

Good luck! Keep us informed.


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Post subject: Re: Bias setting
Posted: Thu Jan 10, 2013 10:48 pm
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+1. I can't think of much else. Sorry.

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Post subject: Re: Bias setting
Posted: Tue Jan 15, 2013 9:23 pm
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The three caps had very little effect. My meter gives no value for the 250pF (not in the range) but the other two check within spec.

I checked all the input wiring again for continuity and all wires are ok. The vibrato channel bright switch moved slightly differently than the normal channel. I took it out and 99% of the buzz is gone! I feel sure it is the switch and not the 120pF cap. Could I be off track?

Should I replace both bright switches?

Thanks again for your help!

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Post subject: Re: Bias setting
Posted: Tue Jan 15, 2013 9:54 pm
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MattGuitarKy wrote:
The three caps had very little effect. My meter gives no value for the 250pF (not in the range) but the other two check within spec.

I checked all the input wiring again for continuity and all wires are ok. The vibrato channel bright switch moved slightly differently than the normal channel. I took it out and 99% of the buzz is gone! I feel sure it is the switch and not the 120pF cap. Could I be off track?

Should I replace both bright switches?

Thanks again for your help!


Did you check the switch for proper open/close operation with an ohm meter? When the switch is off it will measure open and it and the cap are out of the circuit.

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