Mbira

Needing guitar amp guru help...
« on: September 19, 2005, 12:32:35 AM »
I'm still working on this dang master volume fender twin (circa 1974).
The vibrato section isn't working.  I'm curious at the part of the circuit where the "bug" lamp thing is.  If I am getting audio to that top right part of the bug and nothing out of the top left, does that mean the bug is bad?


The full layout is HERE
and the schematis is HERE

Thanks for the help. :?

Joel
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


dukasound

Re: Needing guitar amp guru help...
« Reply #1 on: September 19, 2005, 04:10:14 AM »
Quote from: "Mbira"
I'm still working on this dang master volume fender twin (circa 1974).
The vibrato section isn't working.  I'm curious at the part of the circuit where the "bug" lamp thing is.  If I am getting audio to that top right part of the bug and nothing out of the top left, does that mean the bug is bad?


The full layout is HERE
and the schematis is HERE

Thanks for the help. :?

Joel

Hi Joel
The "bug" is optoelement. In many case it would be reason why tremolo dont work.
Second reason can be tube.
Watch this!!!!!!
Do you have switch for reverb and tremolo. If not you must jump conector for tremolo. You can this with simple chinch and solder plus and minus.
Duka

VacuumVoodoo

Needing guitar amp guru help...
« Reply #2 on: September 19, 2005, 04:30:09 AM »
No audio on the upper left pin of the "bug" is correct, this pin is connected to ground.

The voltages you indicated are too high. 473V might be ok if your mains voltage is on the high side but 537V is definitely wrong. Check if the 100K and 10M resistors below the "bug" are ok. Open circuit  100K resistor would explain the 537V: the meter probe would be picking up anything "from the air"
Alex Niemand
_____________________________________
Life's a party but you get invited only once...
Tubewonder amps
"L

Scodiddly

Needing guitar amp guru help...
« Reply #3 on: September 19, 2005, 08:06:45 AM »
You should also be able to see the light leaking out of the optoisolator - basically a low pulsing light.

And as dukasound says, you need to either have the footswitch plugged in or a shorting plug.  The tremolo has to be switched on to operate.

dukasound

Needing guitar amp guru help...
« Reply #4 on: September 19, 2005, 08:18:18 AM »
Quote from: "Scodiddly"
You should also be able to see the light leaking out of the optoisolator - basically a low pulsing light.

And as dukasound says, you need to either have the footswitch plugged in or a shorting plug.  The tremolo has to be switched on to operate.


And Alex is right about voltages.
If you dont solved problem I have Dual Showman heda. It is same head like yours and I can measured voltages for you.
Duka

Test Point

Needing guitar amp guru help...
« Reply #5 on: September 19, 2005, 09:02:36 AM »
Hi There..I've spent more than my fair share of time in this series of  amps. 95% of the time it is either the "bug" is bad and/or one or all of the three oscillator caps to the left of the bug are bad (usually one .02 and two .01's).  I have also found that one or both of the cathode bypass caps for the trem tube have gone south (they are usually 25uf @ 25v 'lytic caps).
Good Luck
TP
"People of Earth, your attention please, a voice said, and it was wonderful.  Wonderful perfect quadrophonic sound with distortion levels so low as to make a brave man weep."

 "Prostetnic Vogon Jeltz

Mbira

Needing guitar amp guru help...
« Reply #6 on: September 19, 2005, 10:32:55 AM »
Thanks guys!
1) The footswitch is plugged in and shorted out.
2) I do not see any pulsing light in the bug working.
3) The 12AT7 is a working tube.

I'll check out those caps and resistors.

Thanks again!  But if I can't see the bug working, I should assume it isn't working right?
Maybe resistor went south and blew out light?

Joel
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

Scodiddly

Needing guitar amp guru help...
« Reply #7 on: September 19, 2005, 11:13:12 AM »
Quote from: "Mbira"

Thanks again!  But if I can't see the bug working, I should assume it isn't working right?
Maybe resistor went south and blew out light?


Yup, that's what happens.  I believe it's just a regular old neon bulb in there, so you could try to jumper another neon bulb into the circuit.

Hmmm... I wonder... would it be possible to remove the optoisolator from the circuit and connect 120vac with a series resistor to test the neon?

Anyway, replacement optoisolators can usually be found for under $20 at guitar amp parts places.
Ampwares has them.

OldHouseScott

Needing guitar amp guru help...
« Reply #8 on: September 19, 2005, 11:53:19 AM »
If you are going to order a replacement roach, may I recommend Hoffman Amps or Weber VST for best service. I've not personally used Ampwares, but have heard numerous complaints of slow service with them.

Just my .02,
OldHouseScott

"We put a little quality in everything we build..."

CJ

Needing guitar amp guru help...
« Reply #9 on: September 19, 2005, 12:54:33 PM »
Kendrick also.
Maybe Fender Custom Shop if they are still around.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html


Mbira

Needing guitar amp guru help...
« Reply #10 on: September 19, 2005, 05:01:05 PM »
Thanks guys.  I actually got them at Angela.  $11.00 each.
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

NewYorkDave

Needing guitar amp guru help...
« Reply #11 on: September 19, 2005, 08:09:47 PM »
For future reference: the roach is easy enough to rebuild. Get an NE-2 neon bulb and a suitable piece of shrink tubing and you're in business. I've never seen the photoresistor itself go bad.

Don't assume that the roach is bad because you don't see it firing. When trem fails to work, it's usually bad caps in the phase shift network or some other fault affecting the gain of the oscillator (such as the cathode bypass cap Test Point mentioned). Looking at the oscillator plate with a scope will tell you if it's oscillating or not. If you don't have a scope, turn the trem to its slowest speed and look at the plate with a DC voltmeter (preferably analog). The rate of change is slow enough that you should be able to see it on the voltmeter. If the plate voltage doesn't bob up and down, your oscillator isn't working. If that's the case, and it's a good tube, replacing the caps in the oscillator circuit will bring it back to life 90% of the time.

Mbira

Needing guitar amp guru help...
« Reply #12 on: September 20, 2005, 12:24:41 AM »
I was switching out the wrong tube (the 12AT7) thinking that was the driver tube for the trem.  The 12AX7 was bad.  switched it out, light works...

I have to do every mistake at least once I guess. :roll:

Joel
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


 

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