want to help me help kids? (amp repair)

[methodofcontrol]

i took on a job repairing various SS amps (mostly fender) for a rock n' roll day camp that starts up at the end of june. i've got 7 amps here + 1 PA + 1 keyboard.
some may not be worth repair, but i told the camp i'd have a look at everything to help them out.
of the two amps i opened up this evening: the first one is a fender stage 100
with broken reverb. the second is a fender stage reverb DSP that shorts the wall AC when on.
the stage 100 w/ broken reverb i believe i've narrowed it down to the BA4560 chip near the RCA reverb jacks on the board. pin 4 reads -16v, and pin 8 reads 16v but the others are all near 0v. does this sound like my culprit? (the amp works well, but turning up the reverb increases hum w/ no effect. dry signal works fine all the while. reverb tank tests fine, as do RCA cables.)
next up, the stage 100 DSP. i put my meter on the AC in jack and found a short when the switch is "on". switch, AC cord and AC jack test well alone. short is on main board but i don't see any obvious solder bridges or burnt spots. main board has been isolated from secondary boards and problem persists. not sure where to go from here, any thoughts?

thanks for the help/advice guys. i'm willing to spend an hour or two on each amp before calling it quits. i'd like to get these first two out of the way before i move on to the next so i don't have amps guts strewn about.

[Enzo]

BA4560 chip near the RCA reverb jacks on the board. pin 4 reads -16v, and pin 8 reads 16v but the others are all near 0v. does this sound like my culprit?

No, those sound exactly right. it might be bad chip, but those readings are correct.

Isolate the problem. Either the reverb pan is bad, the cables to and from it are bad, or the reverb circuit in the amp is bad.

Look inside the reverb pan. ANy of the small wires broken off inside? Disconnect the cables to the pan. Now measure resistance across each jack. Is either end open? Whatever resistance you get will either be correct or it will be open. If the pan had an open end, replace it.

Now with the amp running, and the reverb control up half way, and the reverb pan still unplugged, touch the tip of each reverb pan cable plug with your finger. One should hum when you do. That one should plug into the OUTPUT end of the pan. If you get one to hum like that, then the return circuit in the amp is working. The remaining plug is the rverb pan drive. You can send that to some other amp and see if there is a signal there, or scope it.

************************

short is on main board but i don't see any obvious solder bridges or burnt spots. main board has been isolated from secondary boards and problem persists. not sure where to go from here, any thoughts?

Shorted parts often look the same as good ones. Isolate the problem. Ther is a short somewhere. It is before the power transformer, after it, or the transformer itself is bad. SInce it blows the wall breaker, check the main fuse inside. It should be a 4A fuse (if you are on 120v mains) if some dimwit put in a 20 amp fuse or something, replace it.

The black and white transformer wires are the primary. Pull the black wire off post P10, and white wire off post P11, and tape them for safety. Now measure resistance across the AC mains plug with the power switch in the ON position. If it still measures shorted, then check for shorted C114. ( near the fuse towards the corner) Also check the power switch itself for wires in proper places and the sides not shorted together. I mean there just are not that many things it could be if it blows fuses with the transformer disconnected.

If that stuff is OK, then we look on the other side of the transformer. Reconnect those primary wires to P10, P11. Don;t power up yet. There are six power transistors on the heat sink, TIP142 or TIP147. Three legs on each. Check them. any have two or more legs shorted together? (This is where I am putting my betting money) If yes, we have found the blown amp stage. if they are OK, then look at the main rectifier diodes D54-57. They are over near where the two red transformer wires plug onto the board and the two large main filter caps. ANy shorted?

For that matter, are the two plain red wires connected there to posts P1 and P3?

If the rectifiers are OK, check for a shorted main filter cap.

If all that seems OK, then we disconnect the power transformer secondaries. Please make notes where they go. Pull the two red wires off, and the yellow wire too off P2. Also pull off the blue wires from posts P4 and P6, and the brown one from P5. Tape them all so they don;t touch anything. Tape them individually, not all together, of course. We already reconected the primary, so now plug the amp into the wall and turn it on. If the fuse still blows, then the power transformer is likely bad.


Let me suggest something. You may not want to get 10 repairs all going at once. I am used to it, but it could easily be confusing to you. But I do suggest you assess all the amps first. Just get a sheet of paper and try each amp, noting what it does wrong. You don;t want to order parts for the Stage 100 and then have to order more parts from the same place later for the other amps. Noisy controls or cracked solder can be fixed right now and the amp put back together if need be. Get your poop in a group, so to speak.

[cbarrow7625]

Method,

What about the AC (signal voltages) passing through the BA4560 chip? are there any? If there is signal present at the input pin but none at the output pin then I would guess that you have your culprit. I would also guess that 0V DC at all of the other pins is normal (although I am not familiar with that particular chip); it sounds like the +/- rails are running to it OK. I assume that chip is the amplifier at the input to the reverb pan? Please clarify. I also assume tha there is no signal voltage present at the input of the reverb pan. Have you measured that?

The "AC short" sounds to me like a shorted primary on the power transformer. Agin, I am not familiar with the specifics of the amp but from your description I cannot imagine it would be anything else. Have you disconnected the transformer and measured the DC resistance of the primary and / or secondary? They should be somewhere under 1 ohm (but slightly higher than the reading you get by just shorting your test leads together). Have you tried applying AC to the transformer with the secondary disconnected from the rest of the power supply? I the problem persists in that condition then I would almost guarantee a bad power transformer.

Sorry I can't just pinpoint the issues for you but I though I'd give you some things to look at. We can't let the kids down. Hopefully some fellow ampagers will jump in with more suggestions.

[Enzo]

4560 is just another in the long line of the 4558 op amp family. A 4558 would work just as well.

[methodofcontrol]

stage 100 (broken reverb):
duh...i forgot to check the input and output signal on the reverb pan. i had already checked the pan, the cables and the solder joints on the pot since it's board mounted. all of that stuff checked out ok. i turned the amp on and tried touching my finger to the output cable and there is signal there, so the problem must be on the send side. when i plug the send-signal cable into another amp i get a low volume dry sound that is not affected by the reverb level.


stage 100 DSP (AC short):
once enzo said " Pull the black wire off post P10, and white wire off post P11"
i saw the problem immediately. someone else was in this amp before me, (unless it left the factory this way.....) the primary wires were in the wrong places, as were the AC main wires. once the switch was flipped, the AC sides were shorted together and the amp never saw any voltage at all. i readjusted these wires and the amp works flawlessly now. thanks enzo!


one amp fixed!

i've been taking notes as to what each amp needs before i order any parts. but i'd like to get he diagnoses as far along as possible on the ones that are open before moving on. as such...the trouble shooting will continue with the broken reverb on the stage 100 before i get the next amp going.

[Enzo]

The reverb control is nothing more than a volume control for the RETURN circuit. It would not be expected that it would have any effect on the drive signal. Were the cables plugged into the right jacks on the pan? If you put the pan in the amp now and rock the amp to crash the reverb springs, does the loud sprring crash come out?

[methodofcontrol]

cables are good. hooked up correctly to the pan. "crashing" the reverb tank doesn't make any sound through speakers.
i just triple checked the reverb pan for impedance across each jack. input gives me 59 ohms, output gives me open or sometimes i can get a 1.xx meg reading out of it. and remember, when i touch the reverb return cable w/ my finger i get sound through the speakers. maybe the pan is bad afterall? is there a way to repair the reverb pan? i checked the wires from the offending jack to the transformer wiring harness in the pan, no internal breaks so simply resoldering the wires a little shorter is out.
can i insert an effects pedal into the reverb send/return to verify that the circuit is ok and just the pan is bad? wouldn't want to overload any delicate circuitry w/ an active signal. what do you think?

[Enzo]

The OUTPUT end of the pan is open, so the pan is bad. If touching the tip of the return cable makes hum, that is verification enough that it functions. If you want to connect a signal there to test it, go ahead. It is just an input like any other input.

[pontiacpete]

have you tried this pan in another amp?

[methodofcontrol]

hey guys, i'm back!
sorry for the delay. so....update:
fender stage 100 DSP - REPAIRED! (power supply wires hooked up wrong)
fender stage 100- REPAIRED! (bad reverb tank)
crate xt65r - REPAIRED! (MANY bad solder joints)

i just started looking at this roland E-09 keyboard. problem was noted as "something wrong with speaker". only problem is, i can't verify the problem
because i just learned that i wasn't provided w/ a power supply. according to the manual, i need a PSB-1U power supply from roland. and according to my web research, that means 9v DC somewhere around 2 amps.
only thing i haven't figured out is whether or not it's center=negative, or center=positive. the only 9v power supply i have handy is standard boss/ibanez style good for 200 ma, center=neg.
anyone know? center= neg or pos? thanks!

[methodofcontrol]

answered my own question, thanks to sweetwater.com for posting EXTRA large pictures.
( http://www.sweetwater.com/images/ite...B1U-xlarge.jpg )
doesn't change the fact that i don't have a power supply available to test the thing. for now i'll move on to the next piece in the pile.

[olddawg]

answered my own question, thanks to sweetwater.com for posting EXTRA large pictures.
( http://www.sweetwater.com/images/ite...B1U-xlarge.jpg )
doesn't change the fact that i don't have a power supply available to test the thing. for now i'll move on to the next piece in the pile.

If you have a variac on your bench you can make a temporary power supply out of 4 diodes (or a bridge rectifier) and a largish cap. (2200, 4700, 10,000 uf- whatever you have). Just put a dc meter or your scope on the output and bring it up tp 9v under load. You can even hot wire it into the circuit and skip the plug entirely. It's even easier if you have a regulated adjustable DC bench supply.

[Gtr_tech]

Use yer meter to determine power polarity. Gnd pin will read dead short to another known gnd, and you'll see caps charging on the pos pin.

[methodofcontrol]

hey guys! i sent everything back with a clean bill of health yesterday! well...all but two items waiting for parts. i have a kustom PA which i haven't even tested yet, and a fender princeton 65 DSP that i need a new IEC power jack for.
i wanted to thank everyone for their help so far, it's very appreciated! and i wanted to offer the busted reverb tank out of the fender stage 100 mentioned in the beginning of this thread. the tank is in good shape, it just has a busted output transducer. accutronics quoted me at $11 for a new transducer, but i don't need the tank and fixing it to put it on ebay will probably just break even after parts.
so it's up for grabs if anyone here wants it! i'm in CT, you pay shipping. i will keep you guys posted on the kustom PA when i get to it. i was told it's not going to be used this summer, so i have no rush to get up and running just yet which gives me time to focus on some other projects.