Vox AC100CPH repair

[pompeiisneaks]

Hey all, my next project, found on ebay, needs a little repair on the power input stage, the input caps, at least one voltage regulator and fusible resistors died.  I posted the pics in the album below:

https://goo.gl/photos/7xf4REp5VpiJH1BT7

I don't know yet that I'll need help, but posting here for any input ahead of time.  I found a forum where someone else did a similar repair already.  Here's the link: http://vintageamps.com/plexiboard/viewtopic.php?f=2&t=88335

Are these amps poorly designed for power overload?  Or is this just abuse? 

One thing that seemed odd, I've got a shot of the resistors I pulled that were fusible's that failed. The schematic I've found here and elsewhere: http://el34world.com/charts/Schematics/files/Vox/Vox_AC100CPH.pdf shows them as 2R2, 2.2 Ohm, but I'm not sure of the wattage, but the forum above indicated 1Watt.  Does that seem right?  The pictures do not seem to be the right wattage, but being burnt out may have discolored them. If I"m looking at the ones in the picture they look like 1.1 ohm not 2.2 so I'm wondering if they were just bad from the get go?  Letting through too much current?  AT any rate, any suggestions and pointers welcomed. 

I'll keep the thread posted of my progress.  I'll have to order hte replacement parts, and from Doug I'll be ordering tubes and tube sockets.

[PRR]

> shows them as 2R2, 2.2 Ohm

Where do you see those?

[pompeiisneaks]

R204 and R235 Fusible Resistors, also listed on the BOM it shows them there as 1Watt, now that I looked later ;)  I'm positive that's the right rating, but damned if those ones I pulled don't look wrong to me.

~Phil

[pompeiisneaks]

Welp, got all the parts ordered, I found a reverb that is by ruby that seems to be the right exact model, and a bag, I've got all the tubes, and tube sockets coming from Doug, and most of the components otherwise came from digikey, due to this being an odd  board based thing (and the caps were 2200uF 50V which Doug doesn't seem to have).  Hopefully within the week I'll have it all back together, get it biased and see how it sounds. 

[PRR]

Why would 2 Ohm resistors blow?

First: that amp has TOO many parts, it has a PCB, it has chips, and the schematic is way too hard to read. Bring it over here, I have a hole in the swamp that I am trying to fill.

I still can't find what the +/-28V go to. I see them "protecting" the XLR output, which is dumb. Do they deliver power to anything?

{edit} OK, 28V goes to a couple LEDs, 7mA each. Through 1% resistors, WTF? 

The resistors should not blow in normal use (obvious assumption). Disregarding the AC-DC conversion, it takes 0.7 Amps to smoke the resistors, and the LM3x7 regs won't pass over 1.5 Amps. The odds of an overload falling in that range are slim. It can't be load on the +/-15V legs, because the 330r resistors limit current to 28V/330r=0.085 Amps.

So I am looking at the rectifier and the big capacitors.

If a regulator quit, it could be because a short on the 37V side allows the 28V cap to dump *backward* through the reg into the short. (There's back-diodes which "should" protect it.)

[pompeiisneaks]

I can say that if you read the article that I linked he had one fail exactly the same way, and replacing roughly the same components resolved it.  It may be a shit design by Vox though, not sure.  they put the fusible resistors there because they were protecting the power input, maybe due to bad design, not sure.   Either way, I'll reread what you're saying while looking over the schematic and see if I can make sense of it (You're already over my head :)

I thought fusible resistors, like in this case, are to prevent overload coming in from say a surge etc.  The more I've read on them, the more the documents seem to indicate the same, its not a failure in the circuit design, but instead a failure in the power coming into the amp, and this protects against it.  something else I was reading indicated that sometimes the fusibles are slow to actually blow, the have to be over amp for a while before they actually short out.  This leaves time for things like the caps that are gone, and the filter caps/rectifier area aren't even involved at this point.  I also see visible signs of heat on the input area, but nothing in the later stages. 

Additionally from the linked forum post about the other similar problem he stated:


"Once inside I focused on the low-voltage power supply (since no front-panel LED indicators were lighting up). I found that a regulator output filter cap (47uF @ 50V) had exploded on the -28Vdc supply. This is usually a sign that it had been exposed to an over-voltage condition. When I attempted to measure DC voltages in the low voltage power supply I could find NONE... ZERO ...NADA! I was miraculously able to locate a service manual for the AC100CPH and examined the supply schematic to get a clue:

It turns out that there are 2 low-value FUSIBLE resistors in series with each side of the AC power transformer's low-voltage winding and BOTH of these were open! Something terribly wrong occurred here for BOTH of these to be blown open. Careful examination of the transformer wiring connections (and connection point nomenclature comparison between the schematic and the PC board silkscreen) yielded the answer to the question "How?".

The 3 transformer connections should have been "BROWN - BLACK - BROWN", with the black center-tap connection in the center Ground connection. Unfortunately, this amp was built late in the afternoon on a Friday OR early in the morning on a Monday. The wiring instead was "Black - Brown - Brown"! This meant there was NO positive low voltage power supply and a DOUBLE_VOLTAGE negative supply! There was no evidence of any tampering or previously attempted repair. This amp was incorrectly wired AT THE FACTORY and was never tested, or it was also blown up AT THE FACTORY just prior to boxing it up to send to us poor saps in the States!"

Edit: spelling, added questions/comments.

[PRR]

I had a Friday-Ford with U-joints which had apparently never seen grease.

Mis-wiring may be possible. You'd think something else would not work, but maybe.....

An over-volted cap really could do such damage.

Fusable resistors are usually a Regulations thing. Certain faults are sure to make something smoke. Many faults won't blow the line-input fuse, but could persist long enough to set something on fire. Fusable resistors are supposed to burn-up first, without emitting flame or shrapnel. This does not always work. But it usually means there is some other problem (assembled-in, or a later failure).

[pompeiisneaks]

Thanks for the input, I hope that the time and money I've invested in this won't end up biting me in the ass.  you can say I told you so, if it does.

:*(

[pompeiisneaks]

Ok to ressurect this one, I've spent a ton of time on the power input, and it still is being wonky.  After burning up two 2200uF caps trying to find the issue, i've found this much so far:

1. I can't go over about 9v on the input to the caps before the negative rail cap starts losing voltage, and things get bad
2.  I've tested the 317 vs the 337 and both seem to be in the same range on the pins (2 and 3 are inverted on them, but that matches)

I can't seem to find out what is causing there to be some kind of drain on that cap.  anything past it as well has inconsistent voltage with the postitive rail, sometimes  higher, sometimes nothing, but I don't seem to understand why. 

Any suggestions on what could be dead in this to cause the negative rail cap to never be able to fill up and start draining as I get it to about 9v? 

I've found I can test if I keep the voltages around that 9v level, and it doesn't kill the cap, but that may be just a slow burner and end up killing it anyway (the above mentioned 2200uf one).

I'm stumped as to what could be wrong. 

One of the biggest problems I've mostly overcome was that whoever worked on this amp before me really ruined some of the traces on the circuit board so I've been having to manually jumper between connections based on the schematic and that's been able to solve some of the problems, but not all.

What is the best way to find where I seem to be shunting to ground (or so I think that's whats causing that cap to never end up filling like the other and starts losing voltage instead).

Any ideas would be appreciated.  If you want me to get some specific pics, please let me kn ow.  That or voltage readings in a specific spot.

[PRR]

Re-re-check those munged-up traces.

Remove IC208 and see if the raw supply comes up to -37V.

Replace IC208 C222 C233 C231 C232.

Temporarily disconnect R210 R253.

[pompeiisneaks]

Thanks, I've already done all of those.  I've got all brand new caps, new zener diode, new diode on one of the two (Q207), except I've not tried removing R253 or R210.  They both test correctly though.  i'm starting to think it may be the bridge rectifier. BR200.  I'm going to go pick up a w01 rectifier from a local shop tomorrow, replace it, and see what I find. 

If that doesn't work, I'm going to try and find a replacement circuit board from Vox (Seems unlikely this is a 2009 amp and they've been since bought by korg). or I'm going to use the sockets, tubes and transformers to build a modded AC100 classic circuit with a turret board and say to hell with it :)  I'll need to find out if the transformers will be okay in that kind of setup though.

~Phil

[pompeiisneaks]

Meh  I've got the replacement circuit board on order, I will try and test out the rectifier, but at this point, I've had to jumper a few parts that were damaged on the board so it looks like frankensteins amp,  I don't feel comfortable selling it to someone looking this bad.  Its a good/bad learning experience at this point.  Good because I learned a lot more about reading schematics and solving the damage to the board as I got it (although I created some more damage in the fixes).  Bad because I didn't solve the problem directly, so I'll have to live with the replacement board and agree strongly with PRR's assessment, this amp is a shoddy design.  Maybe next time I'll share a potential buy/fix/sell with the board for input and "HELL NO DON'T" responses :)