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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 10:15 pm
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Monkeyleg wrote:
I have no idea where to measure screen voltage, bias supply, plate, screen grid, and cathode voltages. I'd hoped there was a simpler solution.


The schematic has been posted several times in this thread. Here it is again:

http://support.fender.com/schematics/gu ... ematic.pdf

If you don't know how to read it and measure the voltages that are clearly marked on the schematic, then you have no business working on this amp or any amp and should stop now before you electrocute yourself.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 10:29 pm
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I just did a search for a few of the terms, and found them all.

People have always been telling me I have no business doing this or that because I don't know what I'm doing. I figure out what I need to, though, and get the job done. Biggest challenge was completely disassembling a 1969 Jaguar XKE down to the last nut and bolt, rebuilding it (including welding and replacing body panels and painting it), and turning it into a trophy car. Scared the hell out of my friends when I'd drive 125 mph and say "wow. Just last year this car was in a zillion pieces. I wonder if I forgot any."

I'll figure it out, but I'm sure I'll have more questions. ;)


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 10:55 pm
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Monkeyleg wrote:
I'll figure it out, but I'm sure I'll have more questions. ;)


Then I suggest you learn how to read a schematic fast, before you do damage to your amp.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 11:14 pm
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Quote:
Then I suggest you learn how to read a schematic fast, before you do damage to your amp.


I knew how to read them decades ago, when I needed to. When I no longer needed to, I forgot. I've come to believe that the human brain has maybe several terabytes of storage.

I don't touch anything unless I'm certain it will do no harm. If in doubt, I leave it alone. Except women.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 11:21 pm
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Good luck fixing your amp.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Sun Jan 19, 2014 11:53 pm
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Sorry, Bill. I didn't mean to come across as snarky. I appreciate your advice and suggestions.

I'm comfortable working on electronics, but I won't go further than my limitations. If I can't find the problem taking readings, then I'll hand it over to a tech.

I didn't get zapped replacing the largest capacitors, though. Lot less danger in there than working on a TV.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 2:08 am
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Monkeyleg wrote:
Yeah, I know the boards could be junk. This is just getting frustrating.

I have no idea where to measure screen voltage, bias supply, plate, screen grid, and cathode voltages. I'd hoped there was a simpler solution. (Edit to add: but I'll find out what those are and do them anyway).



The electronic device repair is a job that does not just happen like any other job if you want to be competent.
Amp repair as a hobby can be learned over time. But it does not just happen overnight.

For your amp there is no simpler solution if you do not have basic electronic knowledge .


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 7:40 am
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Monkeyleg, what I do is break the amp down into its component sections.

Basics. All amps need a healthy power supply unit (PSU). For most vintage Fender amps, the PSU section of the schematic is on the bottom middle-to-right area. Find where the power from the wall enters the amp. Plug, fuse, then primary winding of the power transformer (PT).

Next, find the rectifier and main filter section. The rectifier attaches to the secondary of the PT. The main filter caps and the resistors in the power rail attaches to the rectifier. See the voltage markings on the schematic, where the output and gain/effects stages get their power? You must have proper voltages (within a tolerance), off these taps (B+, X, Y, Z on your amp's schematic). More important, each tap must drop measured voltages in a relative sequence.

From the first tap (usually, the junction of the first filter stage and the standby switch) to each tap for the phase inverter (PI), gain stages, reverb and tremolo stages (EFX stages). The tap to the output tranny (OPT or OT) & output tubes, via the standby switch, being the highest. Then, the screen grid tap for the output tubes. Then, the PI (usually, directly after the choke or large wattage power rail resistor).

Next tap, down the line, is the tap for the gain stage and EFX stages. Finally, the input stage. This is where the amp first sees the guitar signal.

See how the topology works? AC Plug --> Fuse - -> PT - -> OPT & output tubes - -> screen grid of output tubes - -> PI - -> gain and EFX stages - -> Input stages. Think of the tap points as individual batteries to power each section. Each battery must have a specific voltage (within tolerances) for the section or stage to work. Moreover, the voltages of each section must be in a relative order for the electricity to pass correctly, from stage to stage.

Each stage or section has its own set of voltage values on the schematic and layout. Go step-by-step, from the PT to the input stages. Measure each noted value. Usually, the VDC, where the (+) or red lead of your DVM is the probe. And the (-) or black is clipped to the chassis.

Compare your measured results to the schematic or layout. This is the way to isolate the issue to one section or stage.

For the ’65 Super Reverb reissued amp, the TP points, in the rectangular boxes, on the circuit board layout corresponds to the appropriate points on the schematic. Try to go section-by-section (or stage-by-stage) to isolate the problem stage.

http://support.fender.com/schematics/gu ... ematic.pdf


It may be easier to study the schematic of the original AB763 Super Reverb. You can easily identify the stages or sections. The topology is the same between the AB763 and your re-issued SR

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


Good luck! Keep us informed.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 8:51 am
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Monkeyleg wrote:
I didn't get zapped replacing the largest capacitors, though. Lot less danger in there than working on a TV.


400+ VDC in a guitar amp will kill you just as quickly as a TV will.

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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 9:01 am
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One other major section I forgot, in the above post...

The Bias Power Supply (BPS). In Fender amps, the BPS gets its power directly from an individual winding tap off the PT's secondary side. Or some amps get the power from the rectifier. If the tap is directly off the PT, this voltage needs rectification to give DC voltage. Usually, via a half-wave, single solid-state diode.

The idea is to make a source of negative VDC to supply the signal grids of the output tubes. This voltage helps set the idle bias of those tubes. Along with the grid-to-ground resistors. These are usually 220k-ohm or 100k-ohm resistors tying the output tube signal grid to ground.

Steady negative voltage is essential for the amp to properly operate. For your amp, it is -50.2 VDC (TP 5).

NB: Because the BPS supplies negative VDC, be very careful of the polarity of the diode and any filter cap(s) in the BPS.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 7:19 pm
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Thanks, BMW2002Ti.

On another amp forum, one poster gave some pretty detailed instructions on analyzing a problem with a non-Fender amp. A couple of things he suggested were measuring the resistors before powering up the amp (and making sure the capacitors are drained), just to look for any that are unusual. I know that some resistors can be read while in the circuit, while others than are in parallel with other components need to have one lead removed from the board to read. So far most resistors are checking out fine.

He also said he liked to check voltages at the various points that Bill mentioned (and others) with the tubes out, and then doing so again with the tubes inserted, just to see if there's any difference in voltage. I don't know if that's a good idea or not, but it sounds good.

I just wonder if I'll recognize a problem when I see it, or if this is just a circle jerk.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 9:22 pm
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There are many ways to approach checking & fixing an amp. As long as you use a logical, systematic approach to isolate the problem, you will be ok. It is important to follow a set procedure, so that you can not only isolate the issue(s), but also fix the problem with an idea how the "fix" is going to affect the rest of the circuitry.

That is why I take a "stage-by-stage" approach. If you understand how each section works and how it interacts with the rest of the amp, you'll have a better chance of solving the problem(s) correctly, the first time.

To understand each section or stage, you need a basic understanding of what the function of the stage is and how the manufacturer went about designing it. The strengths and weaknesses of each stage's design. Plus, the symptoms are often (but not always) related to a stage or section. So, knowing how section works and what happens when it goes bad can help narrow down problems.

Now, many folks, including me, are going to give you advice on this-or-that --- based solely on their personal experience. And what works for them. However, you are the one working on the amp. Therefore, you need to get a handle on simple, basic circuitry designs used in your amp. And figure out the best way to solve the amp's problem. I feel this is the only way you will understand why you are checking this-or-that value. The degrees of importance in each measurement.

Doing this will keep you from the dreaded "hit-or-miss" tactic, that can drive you crazy. When fixing one problem causes another. Not having a clue why this happened.

I say this, not to put you off --- but from years of fixing amps and helping others. There are many good books and online references you can read. To give you a basic feel for your amp's design and function.

You are welcome to ask any questions --- as long as the answers are not making you more confused. There is no ONE way to fix an amp. If anyone tells you this --- do not believe him. As you go through the process, you'll find out for yourself whose advice is good and whose advice should be taken with a grain of salt. This is the best way to fix anything, IMHO.

Sorry, for the yaddy-yaddy. I just don't want you to believe that there is any easy fix for something that may not be so simple. Just like learning to play the guitar. So many opinions. Some are relevant, some are pure BS. If you know what I mean.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 9:52 pm
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Thanks, BMW2002Ti. I'll follow your approach. Makes the most sense.

As I was going through the schematic today, I was surprised by how relatively simple it is compared to, say, a computer. There's not a whole lot of parts to go bad. It's just trying to figure out which ones, I guess. And your approach again sounds most logical.


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Mon Jan 20, 2014 10:11 pm
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monkeyleg, Yeah guitar amp circuits tend to be simple analog variety. With very little local feedback or voltage regulation -- -seen on hi-fi amps.

Here are a couple of sites that you may wish to peruse. They are by no means definitive. Much literature is out there on this subject. Try to choose one that makes the most sense to you.

http://www.valvewizard.co.uk/

http://www.geofex.com/ampdbug/ampdebug.htm

Good luck! :D


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Post subject: Re: Anyone have an idea on a problem with a 65 Super Reverb?
Posted: Tue Jan 21, 2014 11:24 am
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Monkeyleg, you have mail!

:D


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