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Post subject: Issue with BFPR
Posted: Thu Dec 22, 2011 4:06 pm
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I replaced the cap can of that BFPR (AA 764) with a new FP 20/20/20/20 at 525 VDC cap can. Ramped it up with a 5Y3GT in place of the GZ34, so the amp would "see" voltage under 80 VAC outlet, which is about the passing voltage of the IDH GZ34.

All voltage points read fine. Speaker connected. All tube inserted. No input.

At 100VAC, I replaced the 5Y3GT with a GZ34. Ramped to 120VAC. All ok. But, when I went to play the thing. No sound. And within 30 seconds a GIANT blue spark came out of the square point on the can (point that goes to OPT). Had to change my underwear.

Went to the hassle of replacing the can with another new can. This time a Sprague 20/20/20/20 at 525VDC. Same symptoms. Including a bigger blue spark/flame. Another underwear.

I've replaced quite a few cap cans in PR's, VC, & Champs. With FP, JJ, or Sprague cans. Never had this happen before. Not to mention replaced lot of cap cans in hi-fi and other guitar amps.

Is this an issue when the OPT is starting to go hinky? Anyone else ever have this issue? I can only find the schematic for the AB1164 which I believe is close to the AA764 Princeton Reverb.

Thanks, for any input!


http://www.webphix.com/schematic%20heav ... aa1164.pdf


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Post subject: Re: Issue with BFPR
Posted: Thu Dec 22, 2011 8:20 pm
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Here are the OPT static measurements, out of the chassis:


I pulled the suspect OPT. Here are the static cold measurements:


Primary:
Blue to Red (one side to CT): 161.5 ohms
Brown to Red: (other sire to CT): 158.7 ohms
Secondary: (Black to green): 0.6 ohms
___________________________
This is a new Fender PR OPT's measurements:


Primary:
Blue to Red: 162.2 ohms
Brown to Red: 167.8 ohms
Secondary: (black to green): 0.8 ohms
_____________________________
Brand new Mercury Magnetics OPT:

Primary:
Blue to Red: 129.5 ohms
Brown to Red: 104.8 ohms
Secondary: (black to green) 0.5 ohms.
_____________________________

Of course these are all static COLD resistances. And not impedances. What do you think? The suspect OPT shorts when it heats up?

Much thanks, for any input.


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Post subject: Re: Issue with BFPR
Posted: Fri Dec 23, 2011 2:32 am
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All is well, now... after about 6 hours of going over the amp.

Amp was DOA. Put in new Fender OPT. Still no music. But, all sparking and blue flames stopped.

I measured every voltage point, esp on the input/reverb/tremolo/PI and was getting zero voltage on Pin 1 of the input tube V1. Even though the branch off point measured 230VDC (two 100K-ohm branch point to V1 at pins 1 & 6. See attached layout). Pin 6 was getting 176 VDC and Pin 1 was getting 0.02 VDC.

http://www.webphix.com/schematic%20heav ... aa1164.pdf

So I changed the 100K-ohm resistor going to Pin 1. Still 0.02 volts. Then I pulled all attached resistors, 250 picofarad tone stack cap, and all coupling caps. . So that the D voltage from the cap can was going strictly to Pin 1 of V1. Still 0.02 volts. Finally pulled the jumper wire going from 100k-ohm resistor to Pin 1. And replaced it. Guess what? 165 VDC!!!!


The crazy knuckleheads at Torres, or whomever modded this amp actually poked the lead of the jumper line right through the bottom breadboard & into ground. So the D voltage to Pin 1 was going straight to ground.

Now, the amp really sounds good. That GZ34 based AA764 PR is a great amp (once you actually have music and not blue sparks and flames). I ran it against my AB1164 with 5U4GB and the tone is amazing. The Vibrato and Reverb are almost as good as a BFDR. The amp had a Weber P10Q (back when the Vintage Series had Jensen designations and Ted Weber was building the speakers). An amazing speaker.

The GI caps and the SBE 418P caps are really great combo. And the third cap can makes the amp near dead silent (with no input).

Guess what is the best thing. The owner gave me the amp as a hopeless cause. It had been to several techs and they could not fix the no sound problem. The sparking OPT was on my watch. I think someone may have run the amp without a load, trying to fix the no sound issue. Just a WAG. Pretty cool, eh? Now I have two AA764's.

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Post subject: Re: Issue with BFPR
Posted: Fri Dec 23, 2011 6:34 am
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Good thing you fixed it... you might have run out of underwear!


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Post subject: Re: Issue with BFPR
Posted: Thu Jan 19, 2012 5:12 pm
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Excellent job BMW. Perhaps you would like to sell it for a marginal profit? :wink:

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Post subject: Re: Issue with BFPR
Posted: Thu Jan 19, 2012 5:56 pm
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BMW2002Ti wrote:
The crazy knuckleheads at Torres, or whomever modded this amp actually poked the lead of the jumper line right through the bottom breadboard & into ground. So the D voltage to Pin 1 was going straight to ground.


Nice.

That's the reason I put 1/2" spacers between the circuit board and the insulating board in my 5F6A build. I was concerned with soldering one of the flying leads and having the wire poke its way to ground with the two boards resting right on top of each other.

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Post subject: Re: Issue with BFPR
Posted: Thu Jan 19, 2012 6:41 pm
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Way to go, Beemer! That's certainly one to keep in the memory banks. I've always been wary of those stalactites that can form under the eyelets. Cleaned up quite few of those, but never found one that punched through the bottom board. Good diagnosis! BTW, I put one of those FP cans in my old Valco on your recommendation. Of course it didn't fit, but now it does and is doing a great job. Bluesky, those spacers are a really good idea! Art

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Post subject: Re: Issue with BFPR
Posted: Thu Jan 19, 2012 11:55 pm
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Thanks, for the compliments! :D

Nope. I have to keep this one. The AA764 tone is like a hybrid of the "brown" VC tone with the reverb of a DR. Really, haven't heard quite the same tone in other PR's (with AA or AB1164 circuits). I'm thinking it's the 5AR4... so been running that 5AR4 labeled as 5V4GA rectifier in one SFPR. And it does tend to change the tone. Will report back, after break-in. Which can be awhile with these mil spec rectifiers.


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Post subject: Re: Issue with BFPR
Posted: Fri Jan 20, 2012 6:56 am
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Wow!
Good report BMW,I looked under the board for the "stalactites" as Art said,when I changed some caps on the Vibrolux Reverb...I'll look even closer next time.


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