BF Pro Reverb Problems

[ricach]

I've volunteered to troubleshoot my friends BF Pro Verb. It's been in the closet for probably 10 years, but worked fine at that time.
Problems:
-Clean channel. Nothing but loud hum. No tone. Hum volume not effected by volume control.
-Vibrato channel. Loud hum, crackling, broken up tone but good volume. The crackling iincreases whenever I drive the amp by hitting the strings.

I don't know how loud this should get, but right now I'm guessing it sounds more in the '15watt cranked' ballpark. Also, the ground wire on the 3-prong power cord is not attached. I'm guessing it should be, but with the ground toggle switch option, where does it ground to?

One more thing. After I powered down, I took a v-meter to all the caps and had no residual voltage register whatsoever. This can't be right - what am I doing wrong? Or is this a good indication of a problem.

Thanks

[Jerry]

Go here:

http://www.theguitarfiles.com/guitarfile644.html

This will show you how to wire up the grounded cord.

[Paleo Pete]

Black Face means 40 years old. At least 10 years in storage and no voltage on the caps my guess-and it is a guess-would be cap job. Electrolytics are 40 years old and probably dried out, and cheaper "chocolate drop" caps were used in the black and silver face amps. Those are probably weak, I replaced them with new orange drops. My Super Reverb is a silver face '73, I had to replace every cap in it except filter caps, which had already been changed.

Definitely check the tubes, that's a good chance of being the source of the popping/crackling, along with the 1 watt resistors (resistors were my problem) retension the sockets, check all solder joints and I had to replace all the 1 watt resistors in my Super, they were causing it to make a sizzling sound like frying bacon at idle. I went ahead and replaced allthe resistors just to be on the safe side. Overkill, yeah but it works and sounds great now and I don't have to worry about it.

Schematic and layout diagram can be found at the Fender Amp Field Guide.

Some of the good folks around here who actually know what they're doing might have some troubleshooting ideas, but what you're describing is very close to what my Super Reverb was doing, except that it would play, it just dropped to about half volume after 10-15 minutes. I would avoid powering it up too much until the caps are replaced, if what I've read is correct you can fry a transformer if a cap shorts...

And be careful, you're dealing with deadly voltage, if you don't know that already.

[ricach]

Thanks for the suggestion - if it were my amp I'd probably go ahead a do all that but I think my friend would just like it up and running as cheaply as possible so that he can sell it.

I've taken all the voltage readings and compared it to the schematics. Now - when I figure out how to convey intelligently where I found the descrepancies, I'll post them here. This is new territory for me. My (limited) expertise is in the opposite ends of the spectrum in digital circuitry and high voltage. So how might I better convey that pin 6 on the 12AT7 tube that proceeds the 6L6GC tubes is only reading 160V instead of 230V as marked?

[Wild Bill]

Thanks for the suggestion - if it were my amp I'd probably go ahead a do all that but I think my friend would just like it up and running as cheaply as possible so that he can sell it.

I've taken all the voltage readings and compared it to the schematics. Now - when I figure out how to convey intelligently where I found the descrepancies, I'll post them here. This is new territory for me. My (limited) expertise is in the opposite ends of the spectrum in digital circuitry and high voltage. So how might I better convey that pin 6 on the 12AT7 tube that proceeds the 6L6GC tubes is only reading 160V instead of 230V as marked?

I think Pete already gave you the best advice. The filter caps are most likely shot! You can dink around all you want but nothing else will help. The amp doesn't care if you want a cheap solution. Parts work or they don't.

Dead filter caps can also account for that 160 volt reading. They're supposed to charge up and raise that voltage. Dead ones can't do that.

Once you've got the power supply working with new caps THEN you can go ahead with any other little problems!

Not trying to be negative, but if you want to sell your car bad but it won't start don't waste your time hoping changing one cheap tire will get it working well enough to sell.

The good news is that once the caps are changed most if not all of the other problems may disappear. You likely won't need to change out the caps for another 30 or more years!

[ricach]

My gut feeling has always been to, at the very least, replace the caps first. Then see what problem remains. But being new to this, my gut feeling is not based on experience - just from reading.

Should I only change out all the electrolytic's or the ceramics as well? And while I've got the iron hot, are there any resistors prone to age problems as well?

[ricach]

Apparently somebody's been into the filter caps. The one of the two 70uF 350V caps off the standby switch have been replaced with one only being rated at 150V. The lead was broke (melted?) so it was open, and it took the two 1W resistors with it (meter shows open across resistors).

I'll obviously replace all that, my question is the remaining filter caps. There are 3 20MFD 500WVDC caps (525 in schematic) and are G.E. brand, and copper in color. They are called Alumalytic Dry Electrolytic. Was this stock, or were these replaced already?

Here's the link to the schematic:
http://www.ampwares.com/ffg/schem/pr...a165_schem.gif

[Bruce / Mission Amps]

Apparently somebody's been into the filter caps. The one of the two 70uF 350V caps off the standby switch have been replaced with one only being rated at 150V. The lead was broke (melted?) so it was open, and it took the two 1W resistors with it (meter shows open across resistors).

I'll obviously replace all that, my question is the remaining filter caps. There are 3 20MFD 500WVDC caps (525 in schematic) and are G.E. brand, and copper in color. They are called Alumalytic Dry Electrolytic. Was this stock, or were these replaced already?

Here's the link to the schematic:
http://www.ampwares.com/ffg/schem/pr...a165_schem.gif

Probably stock.. dump them all and start over but use two 100uF@350v in series to replace your 70uF caps and for the rest use any decent 20 to 22uF@500v caps.

[ricach]

Thanks Bruce. My local parts store carries Ruby Gold caps. Are these considered decent?

Also, has anyone done the "StevieRayVaughan" mod on a Pro Reverb? It's replacing the reverb/preamp 7025 tube with a 5751.
http://www.torresengineering.com/5751jan.html

[ricach]

Nevermind about the Ruby Gold. I did a search here and found your (Bruce's) comment on their acceptability. Thanks.

[Bruce / Mission Amps]

Thanks Bruce. My local parts store carries Ruby Gold caps. Are these considered decent?

Also, has anyone done the "StevieRayVaughan" mod on a Pro Reverb? It's replacing the reverb/preamp 7025 tube with a 5751.
http://www.torresengineering.com/5751jan.html

In my opinion, just about everything you read from Torres website is stated merely to sell you something.
The 5751 is OK but not the Holy Grail and not the reason SRV sounded like that!!!
The statements about chime, linearity and all that rot is grossly over stated as far as a simple guitar amp goes.

[ricach]

Thanks again Bruce. I most certainly didn't think it would give the SRV sound, but question is will it lower the breakup point enough to give more a bluesy sound for these amps at a lower volume since these amps don't have a master volume.

I discovered - after pulling the cans off the tubes - that there was no tube in the V1 (normal) socket!! I was thinking of robbing the 7025 tube from the pre/reverb and replacing that with the 5751, or whatever else might be suggested to help blues up the amp.

[ricach]

Amp is up and running. Only oddity is that the volume control on Vibrato channel does not turn volume down completely and it gets really loud by the second tic. This is not the case with the normal channel. With all other controls at zero, I would expect the volume control to behave pretty similar on both channels. I'm guessing the pot could be the culprit. The schematic lists it as 1M-A 2-35. I'm assuming that means its a 1M Audio taper, but what does the 2-35 mean?

[Alex R]

That would be an unusual fault - worth measuring the resistance wiper to ground with the pot turned right down to check. Could it have been replaced with a linear pot? Does it have a 'bright cap' or switch across it that is shorting?

Don't know what the 2-35 means but it doesn't matter, 1 meg log or audio will replace it.

[ricach]

Well once again, I can thank whoever was in this amp years ago. For some odd reason two wires on the volume pot were switched. I'm guessing the pot was replaced at one time and the wires were switched accidently. Because there's no obvious reason to switch them otherwise.

Ok, its mod time now. Even with my guitar volume pegged, I can't turn this loud enough to get anykind of breakup going in the tubes. Bruce, I'm thinking your Turbo Jax might be the answer. Is there any other minor mod I could try as well?

And any recommendations on a 12" speaker that will help this along as well?

As always, thanks for any and all help.