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crate gfx 212 need repair help

Started by ogeecheeman, May 07, 2015, 12:38:39 PM

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ogeecheeman

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.

Roly

{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.
If you say theory and practice don't agree you haven't applied enough theory.

gearhead63

#2
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.

ogeecheeman

#3
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.

Roly

{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.
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman

#5
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

Roly

{"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}
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman

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.

Roly

{"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:
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman

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.

Roly

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.
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman

#11
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

Roly

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..."

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?
If you say theory and practice don't agree you haven't applied enough theory.

ogeecheeman


Roly

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?
If you say theory and practice don't agree you haven't applied enough theory.