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Thread: Peavey XR684 blowing fuses

  1. #1
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    Peavey XR684 blowing fuses

    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

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  2. #2
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    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 ?

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  3. #3
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    schematic for amp


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    Supporting Member Jazz P Bass's Avatar
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    You will need to "zip" the file for it to be viewed.

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  5. #5
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    Thanks m8
    will try again with a zip file
    Peavey XR684 schematic.zip

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  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Which power board do you have?

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

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    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.

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    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 ?

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  9. #9
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    Quote Originally Posted by guitarman51 View Post
    ...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.

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    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 ?

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  11. #11
    Lifetime Member Enzo's Avatar
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    You are using the bulb limiter, and I assume it lights up bright. That is why all your voltages are collapsing to near zero.

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    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 ?

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  13. #13
    Lifetime Member Enzo's Avatar
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    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.

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    Old Timer J M Fahey's Avatar
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    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.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Juan Manuel Fahey

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    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.

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  16. #16
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    Quote Originally Posted by guitarman51 View Post
    Hi thanks for that,
    I cut the track and the bulbs still bright. (100w)
    Did you also remove or cut the two jumpers right next to the rectifier bridge?

    I'm getting mixed signals here, you stated that the rectifier bridge was already tested with your meter, but now you are asking about reading across the two ac input terminals. What sort of level of electronics are you experienced at? Are you certain that all of the transistors and diodes have been tested correctly? It would help us to know what you know.

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  17. #17
    Old Timer J M Fahey's Avatar
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    Ok, then mark diode bridge pack for later proper reinsertion and unsolder/pull it, then test again: bulb *should* shine dull orange or less; if not I'm afraid we might have a PT problem.

    Measure the bridge "outside" , check you have the proper foreward/reverse diode measurements.

    Also solder a couple 4k7 1W resistors or 10k 1/2W across each large capacitor, or disconnected (as they are now) may hold charge for a LONG time and surprise you in a bad way.

    I guess that besides the track cut you also removed the links connecting caps to positive rail.

    As of paid advice, would you consider $ 320, like you paid earlier?
    .
    .
    .
    .
    .
    mmmmhhhhh , I also guessed not

    Don't worry, that's not the way we run things here anyway

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    Juan Manuel Fahey

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    thank you - bridge rectifier faulty
    I replaced the bridge rectifier today, reconnected everything (apart from the mixer unit) and i now have 7.6Vdc on the board. Something else dragging it down.
    checking the O/P transistors i have +7.6Vdc at the centre pin of the PNP transistors Q106, Q107, Q206, Q207 but 0v at the NPN's 101, 102, 201, 202
    now that we have something to see, whats the next step?

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  19. #19
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    A couple of silly questions. Did you reconnect the trace that was cut earlier? After replacing the bridge rectifier, did you prove to yourself that both 60 volt supplies were working?

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    Yes.. i did reconnect the cut track - and jumpers. but in my excitement to try it out i didnt check that both 60vac supplies were working under load. (they're ok measured straight off the trnsfmr) will check tmrw.

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    Cut the track again, disconnected the jumpers and have now checked voltages.
    bulb limiter does not light up
    P2 - P3 42Vac
    P3 - P4 42Vac
    116.2 VDC
    Feels like we are getting somewhere, Should i order a bottle of bubbly now ?

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  22. #22
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    Quote Originally Posted by guitarman51 View Post
    P2 - P3 42Vac
    P3 - P4 42Vac
    116.2 VDC
    From this, I will assume that you in fact have working plus and minus 60 volt dc supplies and not a single 120 volt supply.

    Your earlier post stated that you had +8 volts on the collectors of the PNP output transistors. If you look at the board and follow the trace, do the collectors in fact connect to the positive power supply line?

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    Hi Bill
    i measured the PNP transistors and could only get a voltage on the middle pin in situ. (the base). Nothing from the collector or emmiter pins
    looking at the scematic, i think the 'base' connects to the -60vdc so why would i get +8vdc ?
    In answer to your earlier question - your right i'm a newbie to electronics. so really appreciate your help
    I'm an electro-mechanical engineer and a musician i have always wanted to learn this stuff, thought i'd give it a go.

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  24. #24
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    Quote Originally Posted by guitarman51 View Post
    ...could only get a voltage on the middle pin in situ. (the base). Nothing from the collector or emmiter pins
    Based on the transistor part numbers and on the board layout, the transistors are plastic case TO-3P or whatever the other number is. In either case, the center lead is the collector not the base. Looking at the transistor from the front with the leads pointing downwards, the pins are base/collector/emitter.

    With this pinout in mind, please retest the output transistors in situ and see what you find. Test from base to collector and to emitter as well as from collector to emitter. If there are any odd readings, then you may need to remove the transistor from circuit to retest.

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  25. #25
    Old Timer J M Fahey's Avatar
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    Please download the datasheets of all transistors involved and look at the pinouts.

    Then draw transistor packages on a sheet of paper, label pins, and staple it to the wall, near the workbench.

    Proper part pins identification is essential for any troubleshooting.

    EDIT: in general terms, old "TO3P" , TO218 and TO247 are basically the same case/package, with slight variations but all basically compatible, so any of them if found on a datasheet means roughly same size and almost always same pinout.

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    Click image for larger version. 

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    thanks for that - already done.
    I need to establish that the power generation is ok, but
    I'm struggling to understand the schematic
    Looking at the board layout i see what i assume to be the
    positive and negative DC tracks as i've marked on the attached sketch.
    But when i look at the schematic i don't see the -60vdc or +60vdc connections onto the board ?
    Im obviously missing something, help pls. thanks
    Do i need to connect a resistance across the circuit to test it under load ? if so what and where.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  27. #27
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    Quote Originally Posted by guitarman51 View Post
    Looking at the board layout i see what i assume to be the
    positive and negative DC tracks as i've marked on the attached sketch.
    But when i look at the schematic i don't see the -60vdc or +60vdc connections onto the board ?
    Im obviously missing something, help pls. thanks
    You have everything correct as drawn on the pc layout. The ac voltage from the transformer will be less than 60 volts. The dc voltage will be about 60 volts (+&-).

    The schematic does not show lines connecting the 60 volt power supplies to the power amp circuit. There are points on the schematic that are marked plus and minus 60 volts. You need to connect the dots in your head.

    The test of the power supply would simply be to remove the two jumpers next to the bridge rectifier and cut the trace under the board. Now read the dc voltage from the + and the - outputs of the bridge rectifier as referenced to ground. You should get +60 volts and -60 volts dc (black meter lead connected to ground, red to each test point). If the voltages are correct or even a little high, then you can assume that the power supply is working. There is really no need to test it with a load unless there is some reason to suspect that it is failing.

    There is something that is drawing too much current in the amp. That is why the fuse was blowing and why the light limiter lights up brightly. You said that the bridge rectifier was shorted and replacing it has gotten the two 60 volts dc supplies working again. When the power supplies are connected to the rest of the circuit, the limiter is still lighting up bright, so there still is something that is drawing too much current.

    You stated earlier that you had removed the output transistors and tested them. Did you have the correct pinouts when you tested these transistors? When you replaced them did you use the insulator pads or mica washers to isolate the cases from the heat sink? Are you certain that you have replaced the transistors in the correct locations on the board?

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ID:	36075Amp now working - The fault was originally the rectifier - i eventually replaced it, after removing the output transistors for bench test. When i put the transistors back i hadn't noticed that i had bridged 2 tracks (see pic) the tracks at this point are very close together and now that i have sperated them both amps are now working albeit a little distorted?, need to investigate further as i suspect i may have inadvertently damaged other components. for info i am now getting 54vdc + & - : Schematic says 60vdc is this ok?

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  29. #29
    Lifetime Member Enzo's Avatar
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    That is fine, it is within 10%, which sounds OK to me.

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  30. #30
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    Is that reading with the light bulb limiter?

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  31. #31
    Lifetime Member Enzo's Avatar
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    Good answer, good answer!!!

    Survey SAYS...

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  32. #32
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    Thank you - amp now working - had to replace bridge rectifier, repaired a broken connection on a diode and replaced output transistor and biasing transistor that i had inadvertently blown whils checking with my multimeter probes
    thank you all for the help.
    i do have another query re the cooling fan - a new post

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