Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: ogeecheeman on May 07, 2015, 12:38:39 PM

Title: crate gfx 212 need repair help
Post by: ogeecheeman on May 07, 2015, 12:38:39 PM
ok.amp comes in-user replaced bridge rectifier.installed backwards.
replaced bridge,power supply caps, Q9-(J175), outputs- 2x TIP 147 / 2x TIP 142.
rail voltage on test lamp...+34vdc / -34vdc. collector and emitter voltage
on ALL output transistors = -21vdc.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 07, 2015, 03:02:31 PM
{An Irishman and a large crocodile walk into a bar...}


Hi ogeecheeman, welcome.


Quote from: ogeecheemanuser replaced bridge rectifier.installed backwards.

Bless their little hearts.


Quote from: ogeecheemanrail voltage on test lamp...+34vdc / -34vdc.  collector and emitter voltage on ALL output transistors = -21vdc.

This is contradictory, so you need to find why you are measuring +/-34V (presumably in the power supply) and -21V on both the output stage supply rails ("ALL collectors").  This will be an off-circuit problem such as blown tracks.

via's or plated-through holes from one side of the PCB to the other have been known to blow out like fuses.  Start with your + and -34V rails and trace them through towards the OP collectors.
Title: Re: crate gfx 212 need repair help
Post by: gearhead63 on May 07, 2015, 05:07:44 PM
What rail voltages were you getting before correcting the bridge rectifier problem and were the + and - voltages on the proper rails??
What voltage are you getting at your speaker output, volume down, no speakers connected? (DC)
You need to keep the speakers disconnected until you figure out whats going on!
Also, have you checked the speakers to see if they're still good?
Did you check your +/-40v rails with Q12,Q13,Q15,Q16 removed (at the transistors)?
(those are the OP transistors you replaced.)
I would check IC3 (NE5532) and IC5 (TL072).
If there was a reversal of polarity in the power supply chances are those IC's are toast.
I would definitely start with those IC's, they control the base voltage of those OP transistors. If they are toasted you better check all the other IC's in the amp.
They really hate reverse polarity! I'd almost bet your gonna find other issues because of that bridge rectifier.
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 08, 2015, 05:23:20 PM
let me be more specific-amp powered through test lamp-no load condition.this is the non-DSP version.Page 2 of the schematic.no IC's in power amp stage of this model
collector to ground voltages at outputs are + & -34 as indicated on schematic.
emitter to ground and base to ground voltages are -21vdc on ALL 4 outputs.(not good)
voltage at + & -  of speaker terminals is -21vdc .
oddly the bias voltage is right on the money. 7mv. and it responds when I turn the bias pot.
All transistors have been curve traced and checked good.even still, as a precaution I replaced the bridge,power supply caps,all 4 outputs, Q9- is a J175 jfet in the insert jack circuit that was exploded. also R59 - 4.7 ohms was open.
all IC's check out as well as their power supply.in fact I can run the pre amp stage fine with no issues taking the signal from the insert jack all is normal.channel switching works fine.it's in the power amp.there are no IC's there. this is the non DSP version. these voltages were the same before and after I replaced the output transistors and power supply components.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 08, 2015, 07:36:14 PM
{They walk up to the bar and the Irishman asks the bartender "Now would you be after serving Cat-o-liks in this bar me boyo?"}



Quote from: ogeecheemanit's in the power amp.there are no IC's there

You posted two circuits, <Crate GX-120, GX-212 Schematic(2).pdf> and <252SCH.pdf>.  Because of the thread title I have been using the former with the TL071/NE5532 op-amps, but because these two output stages are quite different we need to know which output stage circuit we are dealing with here, so we are all on the same page (literally).


Applying generally to both circuits; it seems that there is no distress, only that the OP stage is being driven to -21V out.  To me this implies that the power end is healthy and something is amiss in the differential input stage or VAS (IC5 and IC3, or Q19, Q20, and Q14)

{Attached is what appears to be the same output stage as in <252SCH.pdf> but in vastly more readable form. (Thanks to Jazz P Bass in http://music-electronics-forum.com/t16369/)}

You say "no IC's" so I'm assuming for the moment we are dealing with <252Xxc0.pdf>;

If you have -21V at the output then you should also have -21V at the base of Q20, the feedback side of the input diff pair, and it should be well saturated on.  This in turn should mean that the VAS, Q14, is turned right off, and that its collector should be close to the +ve supply rail.

I'd first check that you are actually getting something close to ground on the base of Q19 (to eliminate the possibility the amp is being driven correctly by a rogue voltage, which I doubt 'coz I can't see where it could come from).

My personal intuitive hit pick ATM would be the VAS Q14 gone leaky and the diff pair can no longer turn it off, but it could equally be either (or both) of the diff pair Q19 and Q20 themselves.

Please confirm that <252Xxc0.pdf> is the relevant circuit,
and measure and post the voltages at;

- the common emitters of the diff pair, Q19 and Q20

- the base and collector voltages of Q19 and Q20

- the collector voltage of Q14


attach: <252Xxc0.pdf>


Quote from: ogeecheemanoddly the bias voltage is right on the money. 7mv. and it responds when I turn the bias pot.

I = E/R

0.007/0.47 = 0.015 or 15mA idle (in each OP transistor).

This supports the idea that the power output end is actually working correctly.
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 08, 2015, 09:58:17 PM
252Xxc0rr.pdf is correct. there are no op amps in the power amp stage.

schematic is correct.only difference is mine has no DSP.
one development- found an open trace on  base of Q14.after repair voltages on all output emitters and bases increased to -29vdc

as per your request;  common emitter voltage of Q19 & Q20 =  -17vdc
                                                                  Q19 BASE= -10vdc                                               
                                                                 Q19 COLLECTOR= -37vdc
                                                                   Q20 BASE= -17vdc
                                                                   Q20 COLLECTOR= -32vdc 

                                                                Q14 = COLLECTOR -31vdc
                                                                         BASE -31vdc         
                                                                         EMITTER -32vdc

as I said previously the preamp stage is working perfectly

thanks much for your input and help
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 08, 2015, 11:23:45 PM
{"Yeah, of course we do" replies the bartender, nervously eying the big Croc by the Irishman's side.}



Okay, good, as long as we are on the same power amp circuit.

Quote from: ogeecheemanas I said previously the preamp stage is working perfectly

Not surprising because it would have been protected by the forward-biased zener diodes.

Quote from: ogeecheemancommon emitter voltage of Q19 & Q20 is  -17vdc
                                                                  Q19 BASE-10vdc                                               
                                                                  COLLECTOR= -37vdc
                                                                   Q20 BASE= -17vdc
                                                                   Q20 COLLECTOR= -32vdc

                                                                Q14 = COLLECTOR -31vdc
                                                                         BASE -31vdc         
                                                                         EMITTER -32vdc

Ah HA!  Lot's there that don't make sense.   :dbtu:

First transistor, Q19, SEVEN volts across it's B-E?  What does that say?  Must be open B-E, right?

Second transistor, Q20, you've got the same voltage on the emitter and base when it should be VBE = 0.7V different  :o  and moreover the base is being pulled down by 27k to the half rail at -21V.  Must be shorted B-E.

Your voltages around the VAS have a bit of a "spread"  ;) , but putting 31<->37 down to misreading the DVM or something, and it doesn't matter here anyway, because despite the -17V on the diff pair common emitters, neither of them appear to be drawing any collector currents worth talking about.  It looks like the common emitter voltage is only due to current passing Q20 (shorted) B-E towards the half rail.

With no collector currents (rather than differential collector currents as they should be), the reaction of the VAS stage by pulling the output south -21V isn't all that surprising (however it doesn't automatically mean that Q14, the VAS, is okay.  It may also be damaged, so remove and test, being careful to get it back the right way; or replace it defensively considering what else has gone down).


{attach: Crate gfx212 OPstage voltages 150509.jpg - power amp extract with voltages}
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 09, 2015, 11:59:19 AM
thanks for the input.I had been leary of Q14 all along. Q19 & Q20 tested good on the trace. gut I will replace them with a closely matched pair. I'm with you on the power stage.I don't believe there are any other damaged components,drivers or outputs.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 09, 2015, 12:57:02 PM
{"Arrg dat's good" says the Irishman, "Den it'll be a beer for me, and a Cat-o-lik for me friend."}



Correction:  ::)  the seven volts across Q19 B-E is most likely due to the B-E junction zenering out in reverse mode because its emitter is being pulled so far negative, not being open B-E as I said.

I still don't like no apparent VBE drop on Q20.

Please test each of the transistors you replace to confirm which one(s) is/are dead.


All of these s.s. power amps are DC coupled throughout and damage often propagates backwards from an output short, first the power transistors, then the drivers, then the VAS.  When I find any dead silicon on initial cold test I normally end up pulling all the transistors and testing them out of circuit just to be certain I've got everything first time.  An almost unique reverse polarity like this one?  Then it could be just about anything just about anywhere in the power amp.

Protection circuits aren't immune either.  They are intended to stop a device failing, but if a device does fail not all protection circuits are themselves protected against failure conditions should they arise due to a failure that the protection circuit couldn't prevent (like some nong reversing the bridge rectifier  :duh ).

We look forward to your early report of success and "loud noises off".    :dbtu:
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 10, 2015, 03:24:32 PM
well...no shorts on B-E of either Q20 or Q19. both collectors are -32vdc and both emitters are -17vdc. so it's onto Q14. I'll let you know what I find. R50 readings strange. +15vdc on one side.. -24vdc on the other.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 10, 2015, 04:43:50 PM
Quote from: ogeecheemanR50 readings strange. +15vdc on one side.. -24vdc on the other.

Indeed, one end of that goes to +ve rail via R66 47r and D12, should be close to the actual rail voltage.  If either of these have been damaged then it looks like it may have been on long enough to also damage C29 220uF.

The VAS isn't out of the woods yet, but we could be looking in the wrong direction and the actual fault is in the emitter circuit not the collector.  Whatever, the diff pair and VAS need sorting.
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 11, 2015, 03:07:10 PM
my bad...junction of R50 and C29 is +32vdc. the other side of R50 is -17vdc. I have already replaced C29 so I'm a bit ahead on those 220uf caps.waiting on MJE340 (VAS) and GI 752 diodes to arrive
thanks again
gordo
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 11, 2015, 06:27:10 PM
Quote from: ogeecheemanmy bad...junction of R50 and C29 is +32vdc. the other side of R50 is -17vdc.

"It's just a jump to the left.
And then a step to the right..."
(http://vector-magz.com/wp-content/uploads/2013/05/shoes-step-vector.jpg)
New York to Chicago via Budapest, but we'll get there.
:lmao: :dbtu:

Quote from: ogeecheemanI have already replaced C29

In this rather rare situation I think that is a wise defensive move.


Quote from: ogeecheemanwaiting on MJE340 (VAS) and GI 752 diodes to arrive

The MJE340 should do it, but ... um ... why the GI752?  Did you find one shorted?
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 17, 2015, 11:59:57 PM
replaced mje 340. no change
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 18, 2015, 05:29:25 AM
Quote from: ogeecheeman on May 17, 2015, 11:59:57 PM
replaced mje 340. no change

So you have now renewed both of the diff pair and the VAS and you still have the same voltages as in post #5?  Is that right?
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman on May 19, 2015, 12:00:37 PM
that is correct.no change
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 20, 2015, 01:00:24 AM
...hummmmm...

Have you checked Q8, the bias transistor?
Title: Re: crate gfx 212 need repair help
Post by: ogeecheeman 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
. ????
Title: Re: crate gfx 212 need repair help
Post by: Vitrolin 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.
Title: Re: crate gfx 212 need repair help
Post by: Roly 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: ogeecheemanthe 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: ogeecheemanfrom 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   :-\ }
Title: Re: crate gfx 212 need repair help
Post by: Enzo 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.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 21, 2015, 06:00:20 PM
Thanks Enzo.  I have to admit to being baffled by this one.

Quote from: EnzoI 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).
Title: Re: crate gfx 212 need repair help
Post by: Enzo 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.
Title: Re: crate gfx 212 need repair help
Post by: Vitrolin on May 22, 2015, 02:39:11 PM
i was just looking back in the thread to see what had been tried, and...
Quote from: ogeecheeman on May 07, 2015, 12:38:39 PM
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...

Title: Re: crate gfx 212 need repair help
Post by: DrGonz78 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.
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 22, 2015, 06:35:44 PM
Quote from: Vitrolinsomeone 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: DrGonz78it 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.
Title: Re: crate gfx 212 need repair help
Post by: Vitrolin on May 23, 2015, 03:07:15 PM
Quote from: RolyJust 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...
Title: Re: crate gfx 212 need repair help
Post by: Roly on May 23, 2015, 05:41:29 PM
Thanks, thought so.

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

AAAARRRRggggghhhh...   xP

One very lucky fella messing with stuff he obviously shouldn't.
Title: Re: crate gfx 212 need repair help
Post by: J M Fahey 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.