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Topic: PRRI, No Response From Bass Pot (long) |
Rich Hlaves
From:
Wildomar, California, USA
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Posted 21 Oct 2013 12:20 pm |
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I have a Princeton Reverb reissue on the bench that lost its bass control in the tone stack. The amp acts as if the bass control is turned completely off. The owner of the amp has attempted a repair and replaced the bass pot and bass cap. He did good work and didn't cause any additional issues however this did not cure the problem.
I opened it up and metered it all out and it seems fine as far as component value goes. I pulled the pot board and ran the amp with the pot board lifted from chassis ground. The bass circuit then rejoined the party and all controls on the pot board work correctly. If I jumper the frame of any pot to chassis ground It shorts out the bass. I've been all over the control (pot) board and can't find the short. Solder pads, between traces etc. Since the bass pot and bass cap have been replaced I think the short is between pin three of the treble pot and the frame/case of the treble pot. All controls except bass work correctly board installed, removed & floating above ground or jumpered to the chassis ground.
I have now found .27 ohms of resistance between all the common bass cap connections, treble pot, cap, bass pot and the frame of all pots on the board, darn near a dead short.
I also unplugged the ribbon cable that connects the volume control and tone stack to the main board. This does not open the short so I am assuming the short is not on the main board.
My next move is going to be pulling the treble pot and jumpering in and plain old pot to test. It may not be necessary once the pot is removed if I find the short from pin three to the case of the pot. What I don't understand is how the treble pot could still work correctly if it in fact is shorted.
What say you guys, This is a weird one. Did I mention I don't like RI amps with PCB construction?
I'm not yet out of ideas but I'm getting close.
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On man....let the smoke out of another one. |
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Mike Schwartzman
From:
Maryland, USA
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Posted 21 Oct 2013 3:01 pm
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Heck Rich, I guess (you may have already done this) you could try reflowing the solder patches in the control board neighborhood.
You know...it's a darn shame. I'll bet you have some serious time (hours) into this repair? My buddy Paul, brought that model amp to rehearsal a couple of weeks ago, and it was a real nice sounding amp with his Tele and his Les Paul too. And the price to purchase a PR reissue is far from cheap. Ah well...I'm not going to rant about these Fender reissues with cheap PCB's and reliability issues, but suffice to say, I feel your pain.
I think Moe (of the 3 Stooges) said: "Hey Porcupine, get me pick and shovel. If that doesn't work, we'll blast".
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Emmons Push Pull, BMI, Session 400, Home of the Slimcaster Tele. |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 21 Oct 2013 3:24 pm
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Mike, I consider this amp to be a challenge and the time is of little consequence as I accepted the job as a favor to a friend on another board.
Once the cause is found, the repair will be forever in my memory banks and if I ever come across another it will take no time at all. A friend of mine who has now past on would tell me, "If you think an education is expensive, consider the cost if ignorance." Words to live by for sure.
At least I know what it isn't at this point. I'm a measure twice, solder once kind of guy so stuff like drives me nuts. A Fender factory tech would order a new board for it and be done with a $250 repair bill. My buddy deserves better.
It will be correct before it leaves my bench however.
_________________
On man....let the smoke out of another one. |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 22 Oct 2013 12:08 am
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It was the treble pot. I pulled it as the way it mounts to the board you can't get a good spray of DeOxit in there. Sure enough, pin three to the case was reading 0.285 ohms when removed from the board.
A few blasts of DeOxit and a few wiggles and it cleared right up. I re-installed the pot and all is well.
Thanks for the moral support Mike!
_________________
On man....let the smoke out of another one. |
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Mike Schwartzman
From:
Maryland, USA
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Posted 22 Oct 2013 2:40 pm
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Hey now! Way to go Rich...high marks for perseverance and process of elimination. Your friend is fortunate to have you on the case.
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| A Fender factory tech would order a new board for it and be done with a $250 repair bill. |
That's probably an accurate outcome in most cases. That amp costs around 1000 bucks new. I think that model came out in 2006 or 2007 at the earliest. One small component (in this case a treble pot) is a problem and the owner gets a whole new control board with a beefy repair bill. And where does the entire old control board with only one bad component go? I don't know, but I could make a guess.
Anyhow, I'm sure your buddy will really appreciate your work.
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Emmons Push Pull, BMI, Session 400, Home of the Slimcaster Tele. |
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