Peavey XR684 blowing fuses

[guitarman51]

I have a Peavey XR684 Mixer amp that is blowing the 4A fuses
The amp was serviced by Peavey in Corby around 3 years ago at a cost of £320
since then it has only been used twice
On the last occasion it ran fine for a couple of hours, apart from a loud 'crack'
at the speakers whenever it was switched on. I switched the amp off during break and when i
switched it back on again the fuse blew. Replacement also blew.
Its difficult to find anyone in my area that can fix amps so being inquisitive and with a little knowledge
thought i'd have a go. But no luck to date - can someone help me out?

No sign of damage anywhere, no burning on CB or components.
Checked Power supplies - all OK
Rectifier - OK
O/P transistors - removed and checked OK
Crowbar Triacs - OK
Meter checked diodes - No faults found.
The problem only occurs when i plug the 60v connector on to the power board.
I am using a light bulb limiter to save on fuses as recommended on this forum. Bulb lights up when i power on
and dont get any voltage at the J/P outs (D/C nor A/C)

Now I'm stuck - HELP pls

[guitarman51]

oops i forgot to mention - i have the schematic for this amp but it's a PDF file and i cant seem to insert it into this message ?

[guitarman51]

schematic for amp

Peavey XR684 schematic.pdf

[Jazz P Bass]

You will need to "zip" the file for it to be viewed.

[guitarman51]

Thanks m8
will try again with a zip file
Peavey XR684 schematic.zip

[Jazz P Bass]

Which power board do you have?

There is a 400SC (posted), a 450 & a 1200SC

[52 Bill]

Unfortunately this is a stereo power amp and the board doesn't easily allow the separation of the two amps from the power supply.

When you say that you have tested all of the outputs, diodes, etc., you are talking about both channels correct? What about the driver transistors? The bias diodes?

I would guess that only one of the power amps has a fault, so I'd try and first figure out which one is causing the problem.

[guitarman51]

Hi Bill Thank you for looking and Yes it is the stereo amp version - the driver transistors also check out ok as do all the diodes. no shorts or opens, how do i seperate the power amps ? i can't see any obvious way of doing it ? would removing the O/P transistors from the circuit be enough ?

[52 Bill]

...how do i seperate the power amps ? i can't see any obvious way of doing it ? would removing the O/P transistors from the circuit be enough ?

That was my point before, the two power amps are intermingled on the pc board and there is no easy way to disconnect one from the power supply.

There isn't even an easy way to disconnect the entire 60 volt power supply from the circuit. The positive side can be disconnected by removing a couple of jumpers, but the negative side is not jumpered at all. Have the two main filter caps been checked?

If you have tested all of the transistors, all of the diodes, etc., then removing them will probably do more harm than good.

If you plug it in with the lamp limiter, the fuse holds correct? Have you taken any voltage readings in the power amp at all? Even though the limiter will lower the power supply voltages dramatically, taking a few readings may help point the chase in a particular direction.

[guitarman51]

hi - thank you for the rersponce, I have checked the capacitors C1, C2 and C3 - my multi- meter does not check capicitance
so i checked for shorts or opens and all seemed ok, they also look good, no sign of distortion or burning

checked voltages as follows:

from the transformerWith the output board disconnected :
P2 - P4 87Vac
P101 - P102 40Vac

When i connect the board i get the following
Rectifier primary P2 - P4 1.0Vac
Rectifier sec 0.9Vdc

On the board checks:

Amp 1 CR103-CR108 0v
Amp 2 CR203-CR208 0v

I then checked from J220 which is the same as J110, J216, J210

Amp 1 J220 - O/P transistors Q186, Q187, Q206, Q207 = 1.0Vdc
Amp 2 J220 - O/P transistors Q201, Q202, Q101, Q102 = 0.0Vdc

Could this signify that the fault is somewhere in Amp 2 ?

[Enzo]

You are using the bulb limiter, and I assume it lights up bright. That is why all your voltages are collapsing to near zero.

[guitarman51]

yes Enzo - i am using the bulb limiter and it is glowing bright
should i be using a different bulb ?
does the slight difference in measurements help ?

[Enzo]

Not a different bulb, but a different interpretation of what the bulb is telling you.

The purpose of the bulb limiter is to take up the mains supply when the unit under test wants to draw excess current. It keeps the amp from blowing fuses. So when the bulb glows brightly, it means the bulb is taking all the power instead of the amp.

Something in the amp is trying to draw excess current, it might be one of the channel output stages or it might be a power supply issue. If the outputs themselves are not shorted, perhaps the bias string went open on one channel. Many possibilities.

So when the bulb lights up bright, all those tiny remnant voltages tell us nothing.

[J M Fahey]

I just downloaded the schematic, unfortunately they connect almost everything with solid tracks (it would be kind to Service techs to allow disconnecting different circuit/PCB sections for diagnostic) and when you find a jumper, it's there only to simplify routing and avoid a 2 layer PCB .

That said, I guess they took the "lazy" route and replace the whole power amp and supply module, only justification I see for a $320 repair bill

Morning coffee cup (free advice time ) is over, but later will trace the PCB tracks , showing the +V , -V , and ground tracks and suggest track cutting (heresy !!!!! ) for easier/useful measuring, but let me find the least invasive ones.

You can separate the main supply from both power amps with just a single cut in the -V line (+V has jumpers JP051/506 which can be lifted) but suspect amps more than PSU so I'll check them first.
Stay tuned.

EDIT: my Boy Scout conscience does not let me leave: start by doing this cut and rechecking: does your bulb limiter now turn off or barely glow dark orange/red? (use a 75/100W one)
What are +/-V rails now?
If not, then probably the bridge rectifier is dead.
As shorted cap is possible but so unlikely I never saw one.

[guitarman51]

Hi thanks for that,
I cut the track and the bulbs still bright. (100w)
with power off I checked across the rectifier 2 middle connections and they're open circuit. Should
there be a resistance here?
ps and i will gladly pay for expert advise, iv'e spent hours looking at this, getting nowhere.