Acoustic 320 blows fuses. Expert advice needed..

[Jack W.]

Here's what came in...Heavy road use (worldwide) Acoustic head, power xformer, one screw holding it on, scraped chassis causing metal shavings all over output jacks. All Eq sliders road damaged. Blows internal 7A fuses. What I did: cleaned & wash chassis, remount xformer, heat shrink over xanny wires that were scraped. Check xanny insulation w/meg meter...seems ok (500M+) pri/sec/core. Replace all sliders. Check bridge, caps etc. Variac up, inject signal. Have signal on scope. Vcc 98Vdc, at OT cap 50Vdc, 0 Vdc on scope. Ran for about 1 minute. Heard a small arcing sound, small bang/click sound (like a big relay clicking in), fuse blew. Quick check output xsistors, seem ok, replace speaker jacks. (metal debris?) V up again, voltage at OT cap about half Vcc, lees than a min. bang...Disconnect Vcc from OT board. Power up, check signal at Pre out..ok all controls function. No fuse pulled. Measure about .120A AC at internal fuse link. Took out power PCB, removed & check all driver xsistors (all OK), replaced all E caps on board, redo solder joints etc. Took out all but one known good pair of RCA480060 power xsistors. Power up, no signal, loud bang at 110VAC current read before failure .150A ac. Rechecked all work. OT xsistors still ok. Note: the sound seems to be coming from inside power xformer? Could it be arcing from more current drawn from output PCB? I never encountered such a thing. Any help is appreciated. And of course, the band needed it weeks ago...
Also could use a READABLE schematics. Thanks guys!

[Amp Kat]

You should get a lighbulb limiter in a big way. What are the rail voltages reading. Use you're variac and verify they are both coming up together in a linear way. Something is shorted pulling one or both of them down. Sorry don't have the schemo but Enzo may.

[gbono]

Link to schemeatic here: Unofficial Acoustic Control Corporation Homepage -- Schematics

I would test the PT on its own - disconnect it from the bridge.
Are you testing the output transistors with a DMM?

[R.G.]

Either (a) the road abuse has damaged the AC wiring or (b) the PT is faulty. Or both. A can cause b.

Disconnect the PT primary and secondaries. Make sure they're not connected to anything. Bring up JUST THE AC wiring on a light bulb limiter or variac while watching currents. If the AC wiring is not faulty, you'll be able to get full AC line voltage at the terminals that the PT primary connects to, and you will not blow a fuse. If it still pops, turn it off and go find and fix the AC wiring problem.

If all is well, leave the PT secondaries disconnected - and make sure they're disconnected! - then connect up just the primary. Again using a light bulb limiter or a current-metered variac, bring up the AC power. If the current is bigger than the AC fuse rating, or if the light bulb limiter glows full brightness, the PT is internally shorted. Throw it in the trash can and install a new one. If this does not cause an overcurrent, the problem is past the PT, and may be rectifiers, filter caps, whatever, including many of the things you've already worked on.

But your description is most indicative of a short inside the PT.

[Jack W.]

Thanks for the quick replies! I have disconnected all secondaries, the pri fuses don't blow. No arcing..all seems fine at 120VAC input. It's only when the power PCB is conneted, which to me suggests a certain current draw causing the sec to arc in the turns, but without PTX specs ???
To answer of few replies. I use both a DDM diode ck along with a simple shop built curve tracer that also checks breakdown regions. The xsistors are fine. Also it is a single supply Vcc and ground. The Vcc as stated is approx 98Vdc. Also installed new OT cap.
When I did the Variac power up as stated in the first post, I used a 2.5 AFB fuse in place of the OEM 7A. which of course blew. Can I safely run the amp with no power xsisitors installed and no load as once stated by Doug Forbes (acousticcontrolscorp) in a 4/10/2007 thread? Would there be a sub xanny availble if it is the PT?
Thanks again guys! I'm gonna rip into both the output PCB & PS again today....

[Amp Kat]

Can't see why you can't especially since you only have one voltage source. What is that preamp block in that thing and is there a send out or preamp out? Sometimes even though the transistors check good with diode drops they can still be bad when they start pulling current. We had discussed this the other day here when a poster recommended using transistor checkers instead of diode meters. I'm wondering when you take the load or transistors out if the fuses stop blowing.
Troubleshooting guide shows Q12-18 when blowing fuses. Lots of other transistors in that thing also plus the (8) TO-3's. I'm betting on transistors that are bad showing good with the meter.

[gbono]

Looks like you have transformer # 560027 if VDC is 98V and C102 = 7800 uF/100VDC. I have never had to replace a transformer in an ACC amp but there is always a first. You might ask around at unofficial acoustic control corp forum - Message Board

Doug Forbes is an interesting character he claims to have NOS for these amps but I have had very bad transactions with him (ePay) and he is impossible to get in touch with.

There is always the $$ Mercury Magnetics rewind route - maybe someone on here knows a better resource for rewinding X-formers?

[R.G.]

It's only when the power PCB is conneted, which to me suggests a certain current draw causing the sec to arc in the turns,

It's exceedingly unlikely that a certain current is causing the power transformer to arc inside. It's much more likely that something on the outside is doing the arcing. So the test has pretty much ruled out the PT. There are still some flaws inside the PT that could do this, but they're rare.

Next step is to first connect a light bulb limiter (you really, really, really need one of these for this kind of problem) and then connect up the rectifiers, but disconnect the rectifiers from the main filter caps. Power this up. If the limiter shows big currents are flowing, you have proved the rectifiers are bad.

If no overcurrent, you connect the filter cap to the rectifiers, and disconnect it from the power supply board, making sure that there is a bleeder resistor on it to bleed it down from the test. Again using the limiter, you bring up the line voltage. If the rectifiers and caps are good, you'll get full voltage on the filter caps. If this does not happen, the caps are bad (based on the preceding tests having been OK).

If no overcurrent there, you have proved that everything from the filter caps back is good.

So leave the limiter there (these things pay for themselves with fuses you didn't have to buy!), remove all 8(!?) power output devices and leave the amp unloaded. The drivers Q10/Q11 can drive an open circuit by themselves, probably. The only possibility is parasitic oscillation, which could still happen, but removing the output transistors is more likely to make it more stable, not less. Bring it up and see if the board by itself causes an overcurrent.

If not, it's got to be something about the power transistors or their mounting. Given the beating this thing took, it could be a fragment of metal stuck to heat sink goo, or something equally hard to spot.

But the process is to divide and conquer: separate the AC power input from the rest of it and walk forward, adding one section at a time to the "powered side" until you find which section you add causes the overcurrent.

[R.G.]

Looks like you have transformer # 560027 if VDC is 98V and C102 = 7800 uF/100VDC.

98Vdc is obtained from a 70Vac power transformer. All you have to do is find enough current rating. 70Vac at 5A should do fine. As a note for economy, using a 60Vac output transformer would reduce the output power a little, but not terribly. So anything from 60Vac to 70Vac secondary, at 5-6A will power it just fine. Surplus PTs with similar specs can be had for as little as $20. Apexjr has a dual 41V (82V in series) 6.1A transformer that's too much voltage, but otherwise a find power rating. It's $20.00. The Hammond 167S64 is a 64V, 10A transformer that's plenty big enough; you'll get 90Vdc out of it, not 98, which is a trivial drop in output power. The 167N70 is rated at 4A, so it's a little light for being run at full power at 2 ohms forever. I'd put a thermal cutout on it if I used the N70.

A premium fix would be the Plitron 757018201, Output: 2 X 35VAC @ 5.71A, 400VA. This would be lighter and quieter. It's CAN$116.00. Or you can go cheap with an ANTEK one of same specs, for US$44.00. That's a great bargain!

[Jack W.]

You guys are amazing..again thank you for your time & skills!
Well I took another shot at the power supply, Using a 40w lamp limiter...diconnected the PT at the bridge - no fuse blow - 120Vac Primary, using a 2 AFB & measured about 70Vac across the secondary. Had about 95mA AC current thru primary. Hooked sec back up to bridge. Took all the pwr xsistors out
Fired it up again, no load just scope on output jack. Lamp lit then very low orange glow (seen only all other lites out) Measured +78 Vcc across PS filter cap, had 120mA ac reading on primary PT. Had approx +45 Vdc Plus to ground on OT cap. No fuse pull. Scope showed miniscule dc along with some small PS saw wave. Let it run 5 minutes or so..no change. Ran 10mVac 1Khz into front end..had spec signals out at preamp out jack(there was some imposed tiny oscillation...I kinda expected at this setup..). I'm going to run my fingers thru the output PCB again, remove all drivers etc & externally flog them. Double check the power resistors (all others as well) Maybe replace some of the other caps in there as well...if all seems to check out, I have some new MJ15003G's I'd like to use to replace all the OT TO-3's...still unsure of that PT secondary though..
Thanks for helping me keep my focus...I don't do enough SS to keep it fresh...

[Jack W.]

Oh yeah...I got silicon grease up the wahzoo on this baby...one of the first things I did..because there was some metal shavings everywhere....clean clean clean...wash off old grease...redo...man that stuff gets everywhere! I was thinking of using silicon pads but I don't have enough in stock right now...I think grease handles the heat a bit better...maybe not...

[Amp Kat]

Yeah that crap gets everywhere and picks up everything but it works. ha !
You're doing good just keep it up. I think the tranny is ok and all it takes is one stupid component to give it up and it can take the whole shebang down.
Make sure you get that 98 volts back before you put those TO-3's back in.

[gbono]

With heat sink grease more is not better - you only need a thin film to remove the micro "air pockets" between the device and the heatsink surface. Silicon pads are just slightly worse than HS grease and a THIN layer of mica - I've also used anodized aluminum "shims" with HS grease - pricey though.

[Jack W.]

Your quite right, a thin layer is all that is needed...that stuff has a way of getting on hands, tools, lunch....
I removed & retested all driver, FET's and whatnot...Q9 (2N4248) showed an odd reverse breakdown shape so I replaced it. Reclean the PCB along with sockets to near production clean. Put back in, No TO3's in it, No Vcc hook up (PCB source disconnected) 40wtt limiter...etc. Fired up & it tried to arc..small but noticeable...got me rethinking...tore apart wiring harness from output PCB & preamp in/out jacks on the back panel. Rewired Supply lines & made sure properly insulated. There's also a rear terminal strip that has power, grounds & some drop resistors (for LED) and a CD cap. It looked okay, but I got real critical at this point..so I resoldered & replaced the cap & resistors. Again made sure of insulation. This is mounted near the output jacks & the PT... so it may have been exposed to debris or trauma (?)..to simplify..all boards hooked up to supply no TO3's in..powers up no problem so far...
One thing I noticed when scoping output, no load...the waveform clips on Positive wave only at about 30Vpp. Messin w/bias control changes the shape a bit but that's it...if I power down then back up...sometimes the wave is fine & symmetrical....I am I missing something or could it be just the protection/limit ckts?
Anyway, so far it seems to no longer arc...will put the TO3's in tomorrow ...any tips on biasing this thing?

[Enzo]

Sure, like any solid state amp, apply a signal, start with the bias cold and turn it up until the crossover notch just disappears, then back off a hair.