Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pastortom on March 25, 2010, 05:25:44 pm
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Well, it's been a while since I posted on this problem, but I have a nice Bandmaster Reverb Amp (TFL 5005D). I've checked and rechecked all the tubes, and changed a few to new ones. All tubes check out nicely. NO REVERB. :embarrassed:
I checked a rechecked the reverb unit, and even bought a new one (I need an extra one around anyway....)........The unit checks out fine, and swapping it out with known good units makes no difference. (I have several Fenders, and have tested all the parts I speak of in various amps for trial and error surety). Still NO REVERB. :sad:
One of the guys pointed me to the actual reverb transformer. Well, mine looked a little stained, so what the heck.......I bought a new one here at the store, and finally got around to installing it today (yes, the wiring is exact....no mistakes or cold joints). With the new transformer, still NO REVERB. :huh:
All I get from the amp is "noise" (hum, actually) when I crank the reverb intensity. Really frosts me now. I thought for sure I had it licked, but with these amps, nothing seems simple.
The previous owner installed a variable "ohm" selector for the speaker output. Seems benign enough, but I can't afford to overlook anything at this point. It's not installed anywhere near any pertinent wiring for the reverb that I can see, so I can't see what it could have to do with it.
I just don't get it. Makin' me crazy.......... What do I try next guys? I'm out of ideas! HELP!! :cry:
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I just went back and checked something I had forgotten.........I "banged" on the top of the cabinet to see if there was any spring play, and you can hear the reverb......no doubt.......BUT, still not responding to an input signal.......there IS an output, but NOT from a line input.
What to check?????? I checked the wires again, and their both fine......no shorts.....(reverb cables, that is).
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Please read this analogy as an educational comparison:
If you took your car to a mechanic, and you found out he never tested anything , but rather randomly replaced parts, you'd be very mad, right?
So what we're gonna do is use a simple trick for testing the reverb signal path.
If you were really on top of things, you'd have a little audio signal generator that put out a low sine wave and was isolated by a cap from a probe. You could then take that probe, touch it to a signal point in the amp (with no guitar plugged in) and listen to the speaker for an output. If you heard the tone from the speaker, everything after the point you're touching the probe to is functioning.
What we'd like to do is test the reverb chain for function, starting at its output and working back to its input. So you probably don't have the handy tone generator (I don't either; I have oscillators, but need to fashion up a probe). Take a small screwdriver with an insulated handle, like an Xcelite or plastic-handled jewler's screwdriver. If you touch a plate or grid pin on a tube socket with this screwdriver, you will hear a thunk through the speaker. You want a small screwdriver so you don't acidentally short a pin to another or to ground, and you need an insulated handle to avoid a hair-raising experience.
Start at the end of the reverb chain, where the signal gets mixed back into the dry signal. Touch a signal point, then repeat at an earlier point. I habitually start with the plate of the last tube stage involved, then if I hear a thunk, I move to the grid of that tube and expect to hear a louder thunk (amplification, right). I keep working backwards like that from the output, plate, grid, earlier plate, earlier grid, until I find the point where signal is not being passed. Then I stop and figure out if it is a tube issue (are power supply voltages good? Is the tube making good contact in the socket? is the tube working at all?) or if it is a component or wiring issue. You fix things in that area, then continue to work backwards to the input of that chain; when you get there, everything is working properly.
So you might have ruled out the reverb pan and reverb transformer. It's not ideal, but if you can bang the cabinet and hear the reverb crashing, then everything is good from the output of the pan back to the rest of the amp circuit. But you should be able to touch the scredriver to the pan input (maybe at the send jack) and hear some noise. The point at this jack is safe for you to touch your finger to the screwdriver and try to cause it to buzz. If it doesn't, then you have to wonder about the cable or jack wiring. If it does work, then continue with testing at the reverb driver tube (feeding the reverb transformer).
If you work backwards, you will certainly find the spot where you go from signal to no signal, and then you just have to figure out why no signal at that spot.
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HBP posted first, good teaching. It happens I pick up about where he left off.
Remove the PI tube. Disconnect the reverb transformer from the spring. Adapt a speaker to the reverb transformer output.
This "should" be a half-Watt amplifier. From your symptoms, it isn't. Yet the main amp works fine.
About the only thing in the old BMR (I don't know 5005) is a cap and a tube. This 500p cap, just short it-- it isn't blocking DC, it just shaves bass off the reverb path to reduce boingk/boom. If it plays with the 500p shorted, get a fresh 500p (470p).
Check the voltages on the 12AT7. Pins 2-7 at Ground (under 1V), pins 3-8 at several volts (6V on this plan, may be 3V or 9V in other plans), and hundreds of volts on pins 1-6. And of course 6VAC *across* the heater.
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I saw nothing in your original post regarding substituting a known good driver tube (12AT7). Cables are next, notorious for grounding out at the rca connection. Once you have ruled these out, then you need go deeper. Trouble shooting these isn't rocket science, you just need to follow a logical and methodical approach and you'll sort it out.
This is the order of testing I use when troubleshooting a reverb circuit/unit:
1. Tubes
2. Cables (including footswitch cables if used)
3. Tank
4. caps/driver transformer etc.
Good luck! :wink:
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+1 on the cables.
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WELL, I FINALLY found the culprit........(rather, the guys at Diversified Audio in Tampa found it)..........
As I said previously, the "no reverb, no matter what" was driving me absolutely crazy.........I've been tinkering on and off for months with this amp..........all new parts.........no go...........
They benched it for a while, and had to trace the entire circuit properly, one inch at a time........They discovered, much to their own surprise (as well as mine), that an RCA receptacle (NOT a cord plug.......the JACK) had just plain given up the ghost.....no apparent reason.......
IN all their experience, they've never had one go bad.......nor in mine........Incredible.
They got me the correct replacement, adjusted my bias and checked it out for any other problems, and voila'. :undecided:
Beautiful rich reverb once again. Thanks for everyone's help..........Appreciate it. :grin:
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I have had 2 RCA jacks go. As well as several cables. Tubes and connections, tubes and connections.
HBP/PRR method would have found it faster and cheaper than randomly replacing parts - a lesson for the next amp.
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HBP/PRR ? English for novices please.............. :embarrassed:
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Read the thread. "HotBluesPlate" HBP and PRR both posted methods by which you could diagnose the problem without having to do the random parts replacement or take it to a tech to get fixed. Hopefully a lesson learned for the next time something craps out.
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Thought you got the full 2x4 to the head treatment on your closet queen Deluxe Reverb a doizen or so threads down.
The most common cause of "no reverb" in my experience are the tiny connections to the RCA jack(s) at the ends of the reverb can. They just break, or lose a few eentsy conductors in the stranded wire. Yank on them gently, often they just snap off and can be resoldered. Next, the cables themselves.
Even though it seems to run quite hot, in many years of Fender reverb amp service, I've very rarely encountered a bad 12AT7 driver tube.
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I've encountered pretty much every reverb issue talked about here.
Getting intimate with your DVOM (digital volt/ohmmeter) will help you diagnose cables,RCA jacks and components inside the amp.
I've replaced a great many RCA jacks in many,many Fender amps.Had to repair many cables also.
Just because you've never had those problems doesn't mean they don't happen.
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Thanks guys this is some good stuff here , i have another poster for my wall, im both stupid and forgetfull so good stuff goes on the wall from time to time, It saves me head aches and It ends up getting read over and over when Im trying to find something else, which helps it stick . Bill :wink: