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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Wed Oct 24, 2012 9:39 am
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bluesky636 wrote:
Now the OP needs to locate the two 100K ohm resistors associated with the two 220 mfd caps and see what is under the foam.


+1!

I think that's the critical issue here.

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Wed Oct 24, 2012 9:42 am
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Here's a better schemo. Free software, so not quite the best. But, better than my free-handwriting. :lol:

Those two 100K-ohm/1 watt resistors should be UNDER the first stage 220mfd/300VDC caps. Lift one leg and measure the resistance. Tap the resistors to see if the resistance changes --- indicating internal or solder problems.


HTH!



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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Wed Oct 24, 2012 10:05 am
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BMW2002Ti wrote:
Here's a better schemo. Free software, so not quite the best. But, better than my free-handwriting. :lol:

Those two 100K-ohm/1 watt resistors should be UNDER the first stage 220mfd/300VDC caps. Lift one leg and measure the resistance. Tap the resistors to see if the resistance changes --- indicating internal or solder problems.


HTH!


You're making this way too easy! :lol: :lol: :lol:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Oct 25, 2012 1:10 pm
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Bluesky, after all that "CADD-ing" around, I hope the OP didn't drop-off the thread.


:mrgreen:


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Oct 25, 2012 1:33 pm
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I hope he's been tracing out his amp to compare to the schematics!

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Oct 25, 2012 1:37 pm
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Ha. Not at all guys. I'm still on the case.

I wrote a massive rep,y with questions about method, but the forum keeps spitting an HTML error code back at me. I emailed the forum admin to see what was up.

On my iPad in a car right now, more info coming soon.

Thanks again


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Oct 25, 2012 1:38 pm
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Ironic to now be attempting to troubleshoot, my means of troubleshooting....


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Thu Oct 25, 2012 1:55 pm
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Parkside, ok cool! Few addendums. The "70/75" cap on the left of my last drawing of the BIAS POWER SUPPLY---should be 80/75 (as in 80mfd/75VDC).

And the 2700 ohm/10watt resistor in the B+ rail is a "sandbox type" wire-wound on the underside of the chassis (with all the wiring). Laying on the edge of the main breadboard, by the power transformer. If my mind recalls correctly.

Keep us informed. Good luck!

Latest schemo... (with corrections):

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 12:49 pm
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Hey guys

Currently uploading a video of my multimeter read of the capacitors after a safe discharge procedure. For your piece of mind (and mine) :)

The amp is fully discharged (reading was 1.5 v, on 200ma VDC setting, as the video will show), unplugged, and has a resistor attached so the caps can't regain charge. I think I'm safe to proceed, but I will wait for the go ahead. I realize that you guys have taken the time to help me, and you have all mentioned safety at one point or another, so I figured the video would be the best way to illustrate that the amp has no remaining current (please advise me if there is anything other than the power rail caps that store charge) I always work with one hand in pocket, and wearing rubber soled shoes.

My next move is to create a detailed look at whats in the amp, both in the doghouse and on the other side. I feel this is the best way to understand the schematic, and what you guys are telling me.

Thanks again.
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 1:38 pm
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ParksideDrive wrote:
Hey guys

Currently uploading a video of my multimeter read of the capacitors after a safe discharge procedure. For your piece of mind (and mine) :)

The amp is fully discharged (reading was 1.5 v, on 200ma VDC setting, as the video will show), unplugged, and has a resistor attached so the caps can't regain charge. I think I'm safe to proceed, but I will wait for the go ahead. I realize that you guys have taken the time to help me, and you have all mentioned safety at one point or another, so I figured the video would be the best way to illustrate that the amp has no remaining current (please advise me if there is anything other than the power rail caps that store charge) I always work with one hand in pocket, and wearing rubber soled shoes.

My next move is to create a detailed look at whats in the amp, both in the doghouse and on the other side. I feel this is the best way to understand the schematic, and what you guys are telling me.

Thanks again.
Dave


I go to work once the caps drop below 20 VDC. :lol:

That's an excellent next move. :wink:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 2:33 pm
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Cap Drained Video (boring to watch, but proves Im not just gonna stick my hand in there)

http://www.youtube.com/watch?v=uCC6chPe ... e=youtu.be

I have written down every way the caps terminate in the power rail, and have traced the wires to the other side. Now I'm going to compare my findings against the schematic, and then bring my conclusions to the forum with some photos for reference.

Thanks,
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 4:01 pm
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OK... I usually turn the STANDBY SWITCH to the "on" position (or contacts closed) --- with outlet plug disconnected --- this helps drain all of the caps in the power rail.

Also, one of the two big 220mfd/285VDC (300VDC) caps SHOULD have reversed orienatation, compared to the others. It's because the two 220mfd are connected in series. The rest of the caps are connected in parallel. (Check my latest schemo, to understand this).

If you are going to do this work. Go slow and draw diagrams & take photos. Remove and replace each out-of-spec cap & resistor one-at-a-time. To insure proper orientation. Be sure to measure those critical 100k-ohm resistors, under the two 220mfd/285 (or 300VDC) caps.


It's important to have the correct resistor (resistance and wattage) in each position of the power rail or the PS supply. I use metal oxide or wire wound ones. If you have a tech do this work, you may want to pass this info on.

Remember to check or have the large white "sand-box" wire-wound resistor (on the underside of the chassis) also checked.

Good luck! Yeah, changing out amp parts SHOULD NEVER BE a no-brainer. You gotta know a little about how circuits work. And why they are aligned it this particular manner.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 4:57 pm
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Hey Bmw, guys

I definitely did the standby switch move first, I didn't want to bore everyone with the whole process, but just show that the amp is safe to be worked on.

I haven't done any work yet, I'm in the process of trying to label things and wrap my brain around the match from schematic to physical layout.

To that end, I have taken a few photos and labeled exactly where everything goes.

For today's photo's (and so there isn't information overload) I mainly concerned myself with the 2 caps you mentioned in the last post Cap 1 (300v/220uf) and Cap 2 (same rating, reversed polarity physically speaking)

Have a look, and thanks

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 5:41 pm
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I'm impressed. You've come a long way in a short time. :wink:

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sat Oct 27, 2012 5:46 pm
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Boy! Did you pick a difficult amp to re-do! :lol:

First off, you MUST pull one leg of the resistor (unsolder one lead) --- to accurately read the resistance.

You are going to have to do a step-by-step continuity check of each point of the power supply. To find the correct components. Then, pull a leg of each resistor to check for proper resistance. That 2700-ohm/10 watt resistor in Slide 8 *(better seen in Slide 5) looks a little toasted.

And as for the resistor in Slide 6, you need to pull a leg and measure its resistance. Then, try to figure out from the schematic where it may go. A lot of detective is going to be needed to really re-do this amp correctly.

As for those 100mfd/100VDC caps on the right side of the rail under the doghouse. Better check the polarity of them! If they are truly the BIAS SUPPLY filter caps --- the (+) side of the caps should go to ground. This is critical, as the BIAS SUPPLY MUST have negative VDC.

You know, there HAS to be a good tech in Toronto area. At least to consult with over the phone? This is a major project, that has the makings of disaster. You may want to step back and read a bit more about UL amps and tube amp theory and practice, before diving into this thing. At least, to see the "Big picture." :D

You have any friends up there that are experienced in any kind of tube amps? HAM Radio guys can be the very best, when it comes to knowledge and experience.

Just my opinion. Good luck. If you want to continue with this project --- let's go step-by-step. First step... Figure out where ALL the components connect in the MAIN and BIAS power supplies. Compare to schematic. Draw a layout. NO REPLACE ANYTHING UNTIL YOU UNDERSTAND WHAT PART IS PART AND WHY.

Like what is the Dale, sandbox resistor in Slide 6 (Made in Mexico)?

Little blurb on UL:
http://ampbooks.com/home/amp-technology/ultralinear/

Excellent photos!

:mrgreen:


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