Peavey XR684 mixer amp

[N_Cook]

With just the main PA rail AC supply connected, no load,sometimes at
switch on will draw sustained >2 amp , so quickly switched off.
At 50% mains , sustained 1 amp draw until switched off.
Mains Tx ok on its own.
DVM-D cold testing of PA devices and bridge rect etc check out reasonable.
What to look for cold initially?
I may try hot test again with the speaker output pcb disconnected as
speaker protector thtristors on there.
Initially one output channel failed soon after power up, which could be
sp prot failure presumably, mains fuse did not blow though but that

=500W would be going somewhere it shouldn't.
Perhaps another 50% mains half second try out and see if a thyristor is hot.

 

[Gareth Magennis]

"N_Cook" wrote in message news:nke4tc$ro5$1@dont-email.me...

With just the main PA rail AC supply connected, no load,sometimes at
switch on will draw sustained >2 amp , so quickly switched off.
At 50% mains , sustained 1 amp draw until switched off.
Mains Tx ok on its own.
DVM-D cold testing of PA devices and bridge rect etc check out reasonable.
What to look for cold initially?
I may try hot test again with the speaker output pcb disconnected as
speaker protector thtristors on there.
Initially one output channel failed soon after power up, which could be
sp prot failure presumably, mains fuse did not blow though but that

=500W would be going somewhere it shouldn't.
Perhaps another 50% mains half second try out and see if a thyristor is hot.

Peavey typically put a Triac circuit on the speaker outputs to short and
blow the amp/fuses instead of the speakers should it go DC.

I tend to use a dual bench current limited supply, and remove the Triac when
troubleshooting.
Mine only goes to +-30v, but I've never found that a problem, it will still
safely find a short etc.



Gareth.

 

[N_Cook]

On 22/06/2016 18:58, Gareth Magennis wrote:

"N_Cook" wrote in message news:nke4tc$ro5$1@dont-email.me...

With just the main PA rail AC supply connected, no load,sometimes at
switch on will draw sustained >2 amp , so quickly switched off.
At 50% mains , sustained 1 amp draw until switched off.
Mains Tx ok on its own.
DVM-D cold testing of PA devices and bridge rect etc check out reasonable.
What to look for cold initially?
I may try hot test again with the speaker output pcb disconnected as
speaker protector thtristors on there.
Initially one output channel failed soon after power up, which could be
sp prot failure presumably, mains fuse did not blow though but that
=500W would be going somewhere it shouldn't.
Perhaps another 50% mains half second try out and see if a thyristor is
hot.








Peavey typically put a Triac circuit on the speaker outputs to short and
blow the amp/fuses instead of the speakers should it go DC.

I tend to use a dual bench current limited supply, and remove the Triac
when troubleshooting.
Mine only goes to +-30v, but I've never found that a problem, it will
still safely find a short etc.



Gareth.

What seems odd about this , is it is intermittant.
Sometimes it would power up properly , even at 100% mains, ie nothing is
permanently defunct.
I'll have a closer look at solder joints, if a bias is dropping out.

 

[Phil Allison]

Gareth Magennis wrote:

Peavey typically put a Triac circuit on the speaker outputs to short and
blow the amp/fuses instead of the speakers should it go DC.

I tend to use a dual bench current limited supply, and remove the Triac when
troubleshooting.

** The Kook needs to disconnect the relevant triac crow bar and see if that fixes the problem - could be that it is being triggered by a switch on thump.

Also check the ESR and uFs of the electros in the trigger circuits - if any have lost capacity, triggering becomes more sensitive to large LF signals.


.... Phil

 

[N_Cook]

All very odd. One of the crowbars is doing what it should do and heating
up a couple of degrees in the process.
Randomly about 1 in 5 times at switch on, this crowbar conducts and
drops about .9V as a rail voltage is going on that speaker line.
Oddly , at 50% mains, whatever is wrong , will drop out after 1 or 2
seconds returning mains draw to normal.
One of the output devices failing internally and correcting/"breaker"
going-o/c from current passing ? With the heatsinks off and manually
switching off the amp, IR thermometer should show which, the next time
of powering up.
There are bad design/assembly errors in this amp.
Firstly what does not at the moment seem to be the problem, TO220
pre-drivers , the legs have shoulders so can normal assembly take
advantage of thru-board anchoring of the pins , when soldered. But there
is 3mm or so of freeboard under the shoulders and 2 of the 4 such
devices are waggling loose , flexing the pcb traces with any
movement/vibration of amp.
The 4 h/s screws per h/s have no spring washers under the heads, so the
polyester of the pcb compresses with age and heat (1998 make), one of
the screws was not very tight, I suspect the precursor problem.
..

 

[N_Cook]

For the following designations I'm using PV "XR power amp" schematic as
only preA seems available for the XR684

Two base connected o/p 2SA1302 ,only, of the R ch are heating up for a
second,driven from Q213(E) .
At this stage don't know if an internal problem to one of those pair ,
or as quite accurate "timing" of excursion, I'll put a monitor on
Q213{B} as its probably a further back excursion in the splitting area ,
involving a cap perhaps to give the timing.
Enough for today

 

[N_Cook]

I swapped the speaker leads over on the outlet pcb and the same 2 output
transistors heated up , so verified not a crowbar problem.
First guess (1 to 2 sec timing) its a problem with or associated with
one of the 22uF 50V caps in the splitter section. At least with no minor
supplies connected , its not an preceeding op-amp problem.

 

[N_Cook]

Looks as though the "monostable " timing is from R254 and C230, set by
the output swinging fully negative and reset after the cap discharges.
Normal bias Q213(B) is -1V and goes to -3.3V in DC swing state.
Don't know what is causing the initial disturbance to the bias, but
previously with amps used or stored in damp conditions, I've found it
due to leakage in minor Qs. Owner said its stored indoors and never used
outdoors but rust spots on metalwork and some pcb CuCarbonate green
spots, say otherwise.
Looks like some suck it and see zener clamps to localise down.

 

[N_Cook]

I changed TO92 Q208 2N5400 for MPSA92 and no change in crowbarring.
Before wasting any more time on this I thought I'd remove the crowbar
caps, just in case both were equally bad, so nullifying swap-over test,
but only one PA had enough switch-on instability to trigger either of
the crowbars.
With hindsight these Nichicon VP(M) caps ,2.2uF 50V are much the same as
polarised 2uF 50V caps, suspiciously small.
Simple DVM-R test showed apparent charge time of a 100uF cap, so iffy.
ESR test showed reasonable ESR, but RLC test was interesting.
Initially , for both caps the same, read 1uF then over 10 seconds
dropped to .9uF and continued slowly decrreasing exponentially.
So something in the chemistry gone wrong?, presumably cooked or
something ,with only 50V rating.
Replaced with 2.2uF 250V polyester caps on the other side of the board,
cable-tied down. I know overkill but 100V ones not laying around and
plenty of space on the track side of the output pcb.
Tried 30 switch-ons and no crowbarring and repeated on the other output.

 

[Phil Allison]

N_Cook wrote:

I changed TO92 Q208 2N5400 for MPSA92 and no change in crowbarring.
Before wasting any more time on this I thought I'd remove the crowbar
caps, just in case both were equally bad, so nullifying swap-over test,
but only one PA had enough switch-on instability to trigger either of
the crowbars.
With hindsight these Nichicon VP(M) caps ,2.2uF 50V are much the same as
polarised 2uF 50V caps, suspiciously small.
Simple DVM-R test showed apparent charge time of a 100uF cap, so iffy.
ESR test showed reasonable ESR, but RLC test was interesting.
Initially , for both caps the same, read 1uF then over 10 seconds
dropped to .9uF and continued slowly decrreasing exponentially.
So something in the chemistry gone wrong?, presumably cooked or
something ,with only 50V rating.
Replaced with 2.2uF 250V polyester caps on the other side of the board,
cable-tied down. I know overkill but 100V ones not laying around and
plenty of space on the track side of the output pcb.
Tried 30 switch-ons and no crowbarring and repeated on the other output.

** I told you to DISCONNECT the crowbars & look carefully at those electros....




..... Phil