Hi all!
I'm new to the forum, found it by searching for help on solving a problem with my amp. Occasionally while playing, the bass goes away and the volume drops a bit. It's okay when the amp has just been switched on, and only happens after I've been playing for a while, although "a while" varies from a few minutes to a few hours. Interesting thing is that I get a mild shock off the standby and power switches when this problem happens. Position of the ground switch doesn't seem to make any difference.
The amp is a completely original 1966 Fender Pro Reverb (built in February from the date stamp). I bought it about 2 years ago and the problem's only been happening in the last few weeks. I've got a set of AA165 schematics and chassis layout from the Fender Field Guide website.
Can anyone tell me where to start looking for the source of the problem? I can handle a soldering iron (built a tube radio and a number of solid state amps once upon a time) but haven't ever taken a Fender apart.
Best regards,
George
Man, you don't need to be playing guitars through that thing if touching it's shocking you! :shock:
Remember, the guitar is grounded to the chassis, so if there's voltage on the chassis, it's on your guitar.
Check for a voltage leak between the power transformer primary and the chassis.
I wouldn't keep an old amp in original configuration with the ground switch and "death cap." It needs to be converted to a three wire cord with permanent chassis ground.
"A cheerful heart is good medicine."
Thanks, Ricochet. If I do find voltage there, is grounding enough, or would I have to change the power transformer?
Best regards,
George
If there's a voltage leak to the chassis, it's got to be fixed. May not be in the PT, that's just what I'd check first. You'll have to go on a short hunt. (Hunting for shorts, that is.) Grounding adequately will keep you from getting shocked, but there's still a short to ground then, and it'll either blow fuses or burn something up eventually.
"A cheerful heart is good medicine."
I mentioned in passing the "death cap." That's the capacitor that the ground switch switches back and forth between the two lines of the power cord, or floating in between. It grounds AC signal voltages from the chassis, and works best when the switch connects it to the neutral line of the cord. When it's connected to the hot wire, a little AC power passes through it, which causes more hum and sometimes a little tingle you can feel. It's called the "death cap" because if it shorts out it connects the chassis to the power lines. It needs to go. Along with the two line cord and ground switch. There are instructions on lots of sites for how to do it.
"A cheerful heart is good medicine."
Hi Ricochet,
Thanks for all the advice. Found a site with a lot of technical stuff on how to do various modifications. So, first it's going to be changing the power cord to a proper grounded one, together with removal of the "death cap". Then it'll be checking for voltage across the PT and other likely places, followed by checking all the other ground connections and re-soldering any that are oxidized or look weak.
I've got a feeling that this will cure the problem.
Best regards,
George
Hope so! :D
"A cheerful heart is good medicine."
You might want to have a look at the Tube Amp Debugging Page. I have used this in tracking down problems on my 70's Traynor (and bouncing ideas off of Ricochet on this board) - thankfully nothing as shocking as yours. :shock:
If nothing has ever been done to your amp (or at least not for a while) you may find that you have multiple problems and each one you fix reveals the next. So have some patience and work it through. With an amp that old if it has the original electrolytic caps they are almost sure to need replacing and that can be a source of low volume.
Pop music is about stealing pocket money from children. - Ian Anderson
Hi Hyperborea,
Thanks for the link, it's a new page for me and very helpful. The method for draining the caps before touching them was particularly appreciated, before diving in blind (and maybe dead... :cry: )
Best regards,
George
Hi everyone,
Just to let you know that everything worked out well with the death cap removal. It took a while due to vacation, taking my son out of the country to start university, trips, and the simple fact that the amp's at the summer house and we're not. Further complications involved making a new 230V-115V stepdown transformer with IEC connectors (because the one that came with the amp was, to put it politely, not to UL standard). The amp now has an IEC male plug on it, there's no way to connect to any wall socket in the world. And there's an indicator to ensure that the right side of the plug is live, because wall sockets are wired at random in Turkey.
Some points of interest - all the preamp tubes were Mullards, made in England. The amp itself is completely original inside and in perfect condition. I checked all the solder joints for conductivity and they were all shiny and in good shape. It still has the blue tubular caps with "Made in USA" printed on them. Nothing had been changed since 1966. Even the QC stamps and a couple of faded notes in magic marker were still in place. The original power cord and one blue "death cap" are now in a little box for future reference: just in case anyone wants to buy the amp, restore it to original condition and kill themselves :lol:
The good news is it sounds just like it did before the modification, only quieter, and it doesn't lose volume. She's a keeper!
Best regards,
George
Way to go, George! :D
"A cheerful heart is good medicine."
Hello,
I have a 1969 Silver Face Pro Reverb (AA165) that I just ordered the large electrolytic capacitors for under the protective cover. While looking over the amp and the schematic (I'm a newbie so I believe its the layout) I noticed that the layout calls for seven 25mf / 25v capacitors and my amp has four 25mf / 25v and two 5mf / 25v that look to be original. Has anyone run across something like this? I saw on another post that some people change the value and quantity of these for a different tone. Please let me know you opinions and whether I should have the tech add the missing one and change the values on the other (I have all new ones).
Much appreciated,
Erick
