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Author Topic: crate gfx 212 need repair help  (Read 9949 times)

ogeecheeman

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Re: crate gfx 212 need repair help
« Reply #15 on: May 19, 2015, 12:00:37 PM »
that is correct.no change

Roly

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Re: crate gfx 212 need repair help
« Reply #16 on: May 20, 2015, 01:00:24 AM »
...hummmmm...

Have you checked Q8, the bias transistor?
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman

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Re: crate gfx 212 need repair help
« Reply #17 on: May 20, 2015, 11:59:42 AM »
I did.replaced it anyway.with the original p/n. I know that this sort of malfunction (bridge installed backwards)can back up and screw up a lot but the only damaged components I initially found were  jfet transistor Q9 which literally exploded and R35- 4.7 ohm emitter resistor. even the bridge checked out although I did rebuild the power supply-caps,diodes,bridge etc.all other transistors checked good but at this point, ruling out nothing I have replaced them all with no change.the entire preamp stage up to the junction of C26 & R48 has good clean signal. from the power amp side of R48 nothing but wildly high negative dc voltage. strange readings here; read across collector and emitter of Q12 & Q13 is...+60vdc. across collector and emitter of Q15 & Q16...-5.6VDC. reading at junction of D18 & R56 is... -24vdc. junction of D20 & R51 is... -.820 mv
. ????

Vitrolin

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Re: crate gfx 212 need repair help
« Reply #18 on: May 20, 2015, 06:22:47 PM »
funny stuff...

What is Vb Q12 & Q13, it is a good idea to measure all three transistor pins when you´re at it.
if R49 or R76 is interrupted ther is no base current for Q12 & Q13, and they will be off.

Roly

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Re: crate gfx 212 need repair help
« Reply #19 on: May 20, 2015, 08:10:02 PM »
"????" indeed.  This one has certainly got me scratching my head.  ATM you are the front runner for my Dead Transistor Award for the most puzzling fault this year.   ;)


Quote from: ogeecheeman
the only damaged components I initially found were  jfet transistor Q9 which literally exploded and R35- 4.7 ohm emitter resistor

I think that we can put R35 to one side for the moment.  You have replaced it, and its associated output transistor Q12, D29, and I presume you have checked R74, but with -31V on the collector of the VAS it would seem that the output stage is only doing what it is being asked to by the VAS, Q14, which in turn is only doing what the diff amp is asking it to.

The exploded switch-on mute FET Q9 is interesting though.  This would have blown up when the nominal +40V was actually -40V and there is a direct uninhibited path from ground through the source-gate of Q9, and diode D14.  The G-S junction would have allowed a high current to pass from ground to the inverted supply rail via D14.  I would therefore expect D14 to be cactus as well, but that doesn't explain the negative voltages around the diff pair which seem to be coming from the -21V half rail via the NFB resistor R62 27k.

The voltages around the VAS suggest that it is turned on and pulling its collector down (and the rest of the OP stage is responding accordingly), but it doesn't explain why the diff pair is doing that.

The -24V at R56 and D18 will be reflecting the standing negative voltage on the half-rail.  This and the -0.8V at R51 and D20 are about what I would expect in the current situation - a result not a cause.

Quote from: ogeecheeman
from the power amp side of R48 nothing but wildly high negative dc voltage.

I think I can see what is happening, just not why, so I'll give you my take in the hope that it produces a lightbulb moment for you.

From ground via R44 27k the -10V at the base of Q19 will be due to the E-B reverse zenering out to the -17V on the common emitters, which in turn will be coming via Q20 E-B and the NFB resistor R62 to the -21V on the half rail.  That's the what, but I just can't see why ATM.

I know that you have replaced the diff pair Q19 and Q20 but my intuition is that there is still something wrong around here, and had you not already replaced Q19 that would be my prime suspect (followed by Q20).


{come on Brains Trust - I'm grasping at straws here   :-\ }
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: crate gfx 212 need repair help
« Reply #20 on: May 21, 2015, 04:57:52 PM »
I don't know that I trust my brain, but...

Q14 is what pulls the output stages up and down.  Currently it seems to be holding them down at a negative voltage.  In order the voltage to go up, or more positive I should say, Q14 has to have something to pull against.  So since the bias seems to work, I figure Q8 and circuit is probably OK, so I am left to wonder at the health of R49 and R46.  And of course check C62 that joins their junction.

And when I am working on something like this, just to avoid confusion, I usually disable the limiters.  In this case those are Q10,Q11.  To disable them, we simple unsolder one end and lift the diodes D13 and D21.

Roly

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Re: crate gfx 212 need repair help
« Reply #21 on: May 21, 2015, 06:00:20 PM »
Thanks Enzo.  I have to admit to being baffled by this one.

Quote from: Enzo
I am left to wonder at the health of R49 and R46

I think Enzo means R49 1k5 and R76 3k3 (at least on the cct extract I posted), the bootstrap resistors.

Confirm +ve rail on R49,
measure voltage on the bootstrap cap C62 (the join of R49 and R76),
and the half-rail
Apply Ohms Law.

Also; I asked you to check the voltages down the diff pair tail, D12, R66 and R50, same idea as above.  Please post both sets of voltages.

I'm wondering if a track somewhere has been opened by excessive current because I remember a situation where a few mm of track between two donuts had been so neatly blown off the board it looked okay to casual inspection, needed a bright light an lens to confirm.

As a sidebar, given the damage to the muting FET Q9 I'd be giving D14 a close look too.  If enough current passed to explode the FET I'd be surprised if D14 wasn't damaged too (not that I think this is the cause of your basic problem).
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: crate gfx 212 need repair help
« Reply #22 on: May 21, 2015, 08:04:08 PM »
yes, exactly, I looked at R76 and wrote 46, sorry.

And agree with possible open trace.  Always important to keep in mind we are dealing with circuits, not parts.  A resistor is important, but no less so than the connections to the rest of the circuit.  An open trace to a resistor is exactly the same as an open resistor, to the circuit.  So when we verify a resistor is OK, we want to measure it from the ends of the things it connects to, just so we are also verifying the traces in the circuit.

Vitrolin

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Re: crate gfx 212 need repair help
« Reply #23 on: May 22, 2015, 02:39:11 PM »
i was just looking back in the thread to see what had been tried, and...
ok.amp comes in-user replaced bridge rectifier.installed backwards.
but why, ?
last time i replaced a rectifier bridge it was because someone applied 380VAC to mains instead of 230...


DrGonz78

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Re: crate gfx 212 need repair help
« Reply #24 on: May 22, 2015, 04:17:35 PM »
Hey it could have been someone who had a fault in the amp and just start shotgunning the thing to death. Probably they were about to just order a new power transformer, but thought wait lets put in a new bridge rectifier first(backwards). Then our poor Ogeecheeman gets it delivered to his bench so to make the possibility of repair that much near impossible. Just like all the great experienced techs tell us, "you can always say NO to a repair". As beginners in the repair trade we want to repair everything we can get our hands on. Knowing when to say no takes experience too.
« Last Edit: May 22, 2015, 04:18:38 PM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: crate gfx 212 need repair help
« Reply #25 on: May 22, 2015, 06:35:44 PM »
Quote from: Vitrolin
someone applied 380VAC to mains instead of 230

Just out of personal curiosity whereabouts are you?  Here in Aussie it's nominally 240V single phase and 415V phase-to-phase (and yagotta wonder how anyone could even manage to do that).


Quote from: DrGonz78
it could have been someone who had a fault in the amp and just start shotgunning the thing to death.

Like Doc I've been assuming (there's that word again) that ogeecheeman has been lumbered with the outcome of applied idiocy, but perhaps there is more to the history and a clue there.  "Replaced the bridge"  "But why?" is a very fair question.  Perhaps there was mention of an original fault or symptoms prior to the bridge replacement.  {but then I'm sure we have all had clients who swear blind that "it just accelerated into the brick wall all by itself!"  Yeah.  Riiight.}

Normally we are dealing with a single component failure and maybe some associated components that got killed by the primary failure.  With over-voltage or reversed supply things are different because all sorts of things all over the place could have been taken out, and multiple faults (like in a new build) don't just add, they multiply the difficulty.


Quote from: DrGonz78
"you can always say NO to a repair"

As it happens I had one of these only yesterday.  A Deawoo Royale EG-1 PCM baby grand piano (I kid you not).  I'm apparently only the latest of a line of techs who have had a look at this, and I spent most of the afternoon probing its innards with a CRO on the free list for a local community group.  As far as I can tell it has a "CPU" that consists of a couple of Field Programmable Gate Arrays (for speed I presume) and one of them doesn't seem to be doing what it should, talking to PROM but not I/O, no signs of keyboard scanning or the like.

The builder, Deawoo Electronics, was spun off when GM bought their car operation in the late 90's and apparently closed not long after, so it's 20 years an orphan.  The thing doesn't even have a serial number much less other ID and wasn't built in large numbers (which is hardly surprising given the limited market for an electronic baby grand that would take up half the average living room).  It's basically a $200 keyboard in a very fancy baby grand case and apparently originally sold for around five grand!

They bought it second hand for $250 "going", but it obviously wasn't, and it now seems to have consumed technician time worth at least twice that.  No documentation on the net, no sign of any spare parts supplier, and apparently dead custom silicon.  Up close the electronics are what I'd call a pretty average build, certainly not "quality".

Taken together I had to tell them I think it's a lost cause (and I'm a sucker for a bird with a broken wing) but they are clinging to a faint hope that it can be rescued somehow and don't understand that when Roly tells them it's hopeless that is the most optimistic diagnosis they are likely to get.


However this is an amp, we have a circuit, common parts, and ogeecheeman is obviously capable, no beginner, so I think that we can still collectively pull this fat out of the fire (and besides, I'm dying of curiosity to find out how this story ends - I hate unsolved mysteries).


Until we get the voltages asked for above I'm still of the view that the problem is somewhere around the diff-pair, that the VAS and downstream is only doing what it is being told to do by the diff-pair, but the "why" still plumb evades me.  I cobbled up the diff-pair and VAS in LTSpice but no lightbulb moment there, it only confirmed what we already know and started me wondering if the problem is off-circuit, e.g. a blown trace.
If you say theory and practice don't agree you haven't applied enough theory.

Vitrolin

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Re: crate gfx 212 need repair help
« Reply #26 on: May 23, 2015, 03:07:15 PM »
Quote from: Roly
Just out of personal curiosity whereabouts are you?  Here in Aussie it's nominally 240V single phase and 415V phase-to-phase (and yagotta wonder how anyone could even manage to do that).
I´m im Europe, it 400VAC not 380, it was a Studiomixer power supply, when it was fixed i returned it at the sudio to check i mixer was fried also, and i asked him what had happened... he was goin to put up some new lamps an then everything just went black... some how he had konfused the conductet to the lamp out with L2 and somthing with N, any how hed had 400VAC on all outlets in the listening room, apperently the PSU rectifier bridge protected alot of the equipment.
after the first try he disconnectet everything and returned the lights connections as they were to begin with..

often the truth about how stuff breaks is revealed once they are fixed...

Roly

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Re: crate gfx 212 need repair help
« Reply #27 on: May 23, 2015, 05:41:29 PM »
Thanks, thought so.

Quote from: Vitrolin
any how hed had 400VAC on all outlets in the listening room

AAAARRRRggggghhhh...   xP

One very lucky fella messing with stuff he obviously shouldn't.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: crate gfx 212 need repair help
« Reply #28 on: May 24, 2015, 12:03:02 AM »
Same happened here and even some of my amps were involved.
I live in a low income (that's the polite way to state it) neighbourhood by the old Buenos Aires port, still lots of wooden houses covered in corrugated sheet metal.
One of them caught fire and a dozen poor families became homeless.
Some hippie/tree_hugging/greenpeace/vegetarian people decided to make a Music and Arts festival, sell tickets and street food and drink to help them.
Artists would perform for free, of course; and they asked everybody to contribute something, in my case some amps and speakers, a couple microphones and microphone stands, etc. , others contributed amps and instruments, lighting stuff, etc.
So far, so good.

One of them, the less stoned one, said: "I can get mains power" , proceeded to open a public lighting column access panel and attached two wires, which fed the improvised stage.

Problem is, he got 2 phases from our 3x380V triangle / 3x220V star system (same as old European standard) so 380V at the outlets.

He turned one amp on ... it exploded .... the second, the same.
He shook his head side to side, said "this is wrong, somehow" and kept turning amps on , maybe expecting to find one which would *stay*  on.

FWIW my amps blew fuses but survived, others were not so lucky.

A Peavey 300W powered mixer had main filters explode like grenades, cracking the PCB, go figure.