How do fix a Amp when you Do Not Know Whats Wrong

[R.G.]

Sigged

I can't take credit for anything but retelling that. It was old when I heard it, decades ago.

It comes up in my head every time I see the serial number on an amp affixed to the easily removable wooden back panel.

[J M Fahey]

In a *fully* rebuilt to original spec, with new good parts, the bias cap will not pop.
I'm 100% sure about that.
A fully rebuilt to original spec amp, not only the "electronics" but jacks, pots, switches, *tube sockets*, in sum keeping only Chassis and iron, will be a new original amp in all aspects, and will live a long happy life ahead.
And, of course will sound original too.

[Chuck H]

Here, here!

But I really think the problem shouldn't be that hard to find. It just happens that anyone who's looked at it hasn't experienced the problem in an accute sense like Bob. Not to mention the possibility that those "techs" may not be able to handle anything that isn't easy. They just keep changing things and inventing things to fix because the amp was sent to them, they can't find or understand the problem and actually doing something to the amp is what gets them paid. I just thought of a nifty bench tool that shouldn't be too hard to make...

A device that could be set for a specific voltage and clipped into the circuit. If the voltage drifts beyond a given tolerance the device beeps. Loudly. Alerting the tech that the intermittent fault they are looking for is occuring. You could set up the amp for burn time and clip the device in place. You could even hook the amp to a dummy load and feed an input signal if needed. Then just go about your business until you hear the beep. Alerting you that NOW is a good time to poke around in there with the meter.

[J M Fahey]

Just guessing, but I think the *main* propblem with that amp is that it has been messed to death.
The same way that a misbehaving computer must be turned off and then on again, to start with "a clean sheet of paper", for the same reason that amp should be returned to normal, original state and *then* see what happens (which I think will be that it will behave properly).
I think this endless chasing of ghosts comes from the incessant messing itself.

[JoeM]

......
I think this endless chasing of ghosts comes from the incessant messing itself.

Exactly. (Ghosts is an appropiate word)

[Rick Erickson]

I cringe when I read threads like this. My shop sees amps from all over the country, as well as ebay purchases from customers who want their purchases evaluated & repaired. So often I see amps with components replaced that I know weren't necessary - the blue "hotdog" coupling caps in bf Fenders that have been replaced with Orange-Drops are a great example. I also see bad mods, (yes- some mods can be good) terrible soldering, etc., etc.. I would never replace original tube sockets, pots, transformers, jacks, or coupling caps unless I felt it was absolutely necessary - and it rarely is in a 64 Fender. Electrolytic caps and carbon comp resistors are another story. I say replace them all. They are so far beyond their life expectancy that they are no longer reliable. Also get rid of the ground reference cap or "death cap" and install a grounded AC cord. (a good mod)
A conductive component board can cause all kinds of problems - some more obvious than others. Scratchy pots, thumping Vibrato, intermittent high pitch sounds, crackling background noise can all be caused by a conductive board - but the existence of these symptoms does not mean that you have a conductive board, only that is is a suspect. The correct procedure is to get your amp to someone who knows what they are doing* and let them analyze it. They should be able to call you with a diagnostic and a reasonable estimate BEFORE they do the work.For the record - ALL early fender bright switches pop. There is a 120pF capacitor with no ground path to discharge through. This builds up a static charge which "pops" when you engage the switch. A 10M resistor on the open leg to ground will/should tame this. The bias cap is another issue. As others have pointed out - it is either too low a voltage or it was installed backwards. Even the best techs occasionally reverse a cap. A good one will catch it as soon as it blows up though.
* Having a nationally recognized name, even if you wrote a book or two, does not mean you know what you are doing. Ask other musicians and local guitar shops, "who does the best work in town", or "who they would take their amp to"?.

RE

[gbono]

I assume we are talking about this "guru"? Gerald Weber shows you how to do a Quick and Easy Wattage Check - YouTube

Brilliantly simple measurement technique which is, of course, always repeatable.

[Enzo]

Caps that blow up have something causing it. They either face excess voltage or reverse polarity voltage. Under voltage rating can be one cause, put 60v on a 50v cap, it eventually fails. If there are mains glitches putting peaks at twice voltage on the supply, I wold think they would also put twice voltage peaks on the B+ supplies too. Power transformers don;t usually fail, and when they do, I can't ever remember one putting out substantially higher voltage, if anything the voltage drops. I guess it is possible an intermittant inter-winding short in the PT sends several hundred volts into the bias supply. Never seen it, but possible. If there is a bias tap on the HV winding, it is conseivable some open might result in an over voltage, again, never seen it.

Reverse voltage? Well that wouldn;t be the transformer, after all it produced AC to start with. So they we have to look to the rest of the circuit. Maybe an occasional arc in a power tube socket between the grid pin and the screen next to it. RUnning +400vDC into the -50v bias. Or someone is putting a tube in the socket with a broken off center post.

[Enzo]

Caps that blow up have something causing it. They either face excess voltage or reverse polarity voltage. Under voltage rating can be one cause, put 60v on a 50v cap, it eventually fails. If there are mains glitches putting peaks at twice voltage on the supply, I wold think they would also put twice voltage peaks on the B+ supplies too. Power transformers don;t usually fail, and when they do, I can't ever remember one putting out substantially higher voltage, if anything the voltage drops. I guess it is possible an intermittant inter-winding short in the PT sends several hundred volts into the bias supply. Never seen it, but possible. If there is a bias tap on the HV winding, it is conseivable some open might result in an over voltage, again, never seen it.

Reverse voltage? Well that wouldn;t be the transformer, after all it produced AC to start with. So they we have to look to the rest of the circuit. Maybe an occasional arc in a power tube socket between the grid pin and the screen next to it. RUnning +400vDC into the -50v bias. Or someone is putting a tube in the socket with a broken off center post.

[Dave Mac Farlane]

I've just had a look at the map and a) California is just 1.5 inches away from Texas, which is 1500 miles. A week off would cover that journey.
And b) what are you complaining about - I thought everybody in Texas and California were Millionaires.
You should be thinking about taking it back mate. When I ran a service unit, if someone brought something back, it had to have something completely different before I would think about charging them more - it simply meant it had not been 'soak tested' or proven.
Aside from which, as an auto mechanic used to say to me 'if people don't tell me it isn't yet right. I can't fix it.
This guy owes you - in fact, if it was me, I would pay at least some of your trip.
Dave.

[km6xz]

After seeing his video it is becoming clearer that he really, really, should not be working on other people's equipment.

[J M Fahey]

His own video shows displayed voltage swinging from 3.1 to 6.6 Volts. (check it)
Why he chooses 5 Volts is beyond me.
Since he despises signal generator , load resistors and scope, why does he bother to use a meter?
Whose readings he does not use !!!!
I have a better method, both more accurate and simpler than his, I do not even need a meter nor (ugh!) Math !!!:
The Fahey Guru Man method is .........................................
roll drums ..........................................
................
................
................
read what the manual says !!!
How's that ?

I'm going to the tailor to have a Guru uniform custom made

EDIT: although on second thoughts I think I do not want to be a Guru, they told me I would need to go through THIS:

[Steve Conner]

For apartment dwellers, there's an even simpler method: Wait till a Sunday afternoon so everyone is home, crank the amp up full and start playing your favourite Beano album riffs.

Time till neighbours start banging on wall... Watts
Never... <1
3 hours... 3
45 minutes... 15-18
15 minutes... 30
10 minutes... 50
2 minutes... 100

Cops show up... 120+
Building falls down... Original Ampeg SVT
Earth's crust swallows building... Fender 400PS with three cabinets
Amp blows up before neighbours have time to bang on wall... New Ampeg SVT

Make me a guru hat while you are at it, JM!

[km6xz]

It's evident that he does not care about minor details like measurable parameters based on his lack of test gear. He has a scope which he says he does not trust, and a pocket meter, which apparently he does, but nothing else. Sounds like a marketing huckster more them guru. I would not want him working on my mother's toaster let alone a prized amp.

Has anyone had an amp with an actual problem that required diagnosis successfully repaired for a reasonable price in a reasonable length of time?

[JoeM]

Weird video. GW is himself a dummy load.