Stage Line 500W amp repair project

[bikemech]

hi all,
just been given a stageline 500w power amp,
the story is this, the amp used to belong to me a few years ago along with a nice set of warrior speakers, they were used regularly with no problems aprt from the volume knobs (potentiometers) if i spelt that correctly being a bit scratchy, anyway i sold them to a friend who wanted them for a local pup dj gig every saturday, they worked fine for about two years, he asked me to look at the speakers for him the other week as one of the horns had stopped working, so i replaced both horns and both capacitors on the cross overs as one had blown, changing in pairs so both sounded the same, also changed potentiometers on the amp to get rid of the scratchy noise,

so i gave them back, left chanel started to play up after about half hour, as in distort when over half way, so i switched it to bridged so he could at least finish the night off,

after that the amp died and the protect light is on constantly, now hes bought a new one and said i could have it back to play with ,

so here i am, i have looked at the board physically and can see quite a large diode that looks to be fried,

i will get some pictures up as soon as pos so you can all see what i am talking about,

please bear in mind when reading or replying i AM A NOVICE!! and know the basics and possibly a little more,
i understand ac/dc and how to use a multi meter,

question is where do i start looking /testing, thanks in advance if anyone can help me as i am realy interested in electronics and would love to learn and repair this amp back to full glory,

if i do i also plan to build my own speakers to go with it ,
thanks all,
pete.

 

[davenn]

hi Pete

welcome to EP

wont comment specifically till you show the pics and particularly around the area of the blown diode

have you searched online for a service manual or at least a schematic ?

cheers
Dave

 

[bikemech]

hi dave,

done nothing yes this was my first stop, will get them pix umloaded asap

 

[bikemech]

 

[bikemech]

 

[bikemech]

 

[bikemech]

looking inside the amp with it switched on, the fault left led is lit,
but the burned resistor seems to be on the right side,
the plugs where you connect the speaker cables are left and right chanel as your looking at the photo,
right so where do i start,??

thanks

 

[davenn]

so you meant resistor and not a diode I assume ?

I don't see a burnt resistor as such in your pics
I do see the resistor leg has got hot, you will probably find if you look at the solder side
that that resistor ( maybe other components) have gone dry jointed and have been arcing
causing the burning

go over the board with soldering iron and solder sucker, remove the dry jointed solder from any crappy looking joints using a solder sucker or solder wick and then resolder with fresh solder


Dave

 

[KrisBlueNZ]

Wow, that's a pretty impressive looking amp!

And thanks for the details and history in the first post. This could be important.

Where is this blown component? Is it somewhere in the second picture in post #4? I don't see anything obvious.

Dry joints are a common cause of problems. Also, possibly, deterioration of the insulation on the output transistor heatsinks.

I think I would dismantle the heatsinks, measure the output transistors and driver transistors (marked in red and green respectively on the image below), check the driver semiconductors on the main board, resolder all suspicious joints, then go through a careful step-by-step reassembly and reconnection process.



I think that amp is using a strange two-layer construction for the output transistor heatsinks. There should be eight output transistors, not just four, and I think the other four are underneath the horizontal parts of the heatsinks for the other four. This will require some care and note-taking to disassemble and reassemble properly.

Repairing this amp properly with instruction via this forum would be quite a laborious process, involving lots of photographs and measurements. If you're keen, I'll give it a try.

I have SOME experience with audio amps of this vintage. I don't know of anyone here who specialises in them. I've attached a PDF of the schematics for you to print out and mark up, if you want to go ahead.

 

[bikemech]

fantastic!!
and thank you for all the replys,

yes the only thing i can visibly see is the resistor leg in pick 2 post#4 that has discoloured,
i have has the board out and could not see anything obvious, ie burned out tracks,
i will take it out again and have a closer inspection and post my findings,
i will print the schematic and have a look at it, wont be untill monday tho,
and yes i would like to try repair it,
is there any more photos or info that i can give to help you guys out?
thanks again

 

[bikemech]

also dont know if its of any help but noticed the cooling fan does not spin up when the unit is switched on?
or does it only come on when it starts to get hot??

 

[davenn]

fantastic!!
and thank you for all the replys,

yes the only thing i can visibly see is the resistor leg in pick 2 post#4 that has discoloured,
i have has the board out and could not see anything obvious, ie burned out tracks,
i will take it out again and have a closer inspection and post my findings,
i will print the schematic and have a look at it, wont be until monday tho,
and yes i would like to try repair it,
is there any more photos or info that i can give to help you guys out?
thanks again

do what I suggested in post #8 ... That is your first important job

also dont know if its of any help but noticed the cooling fan does not spin up when the unit is switched on?
or does it only come on when it starts to get hot??

it may well be temp controlled, many higher power amps are that way

Dave

 

[KrisBlueNZ]

I think it might not be a good idea to just fix the dry joints then power it back up without doing any checks. In audio amplifiers, especially high-power ones, all of the semiconductors in the output stage are tightly interconnected and any fault often causes failure of all of them.

The Fault LED is already telling us that there's a problem in the left output stage. This could be (and probably is) because the left channel semiconductors have already lost their magic smoke, in which case one of the fuses will be blown. If that's the case, it will blow a fuse every time you power it up and further diagnosis will be needed. If that's not the case, fixing dry joints may solve the problem, but if it doesn't, powering it up may be risking the output semiconductors. Your call At least the output transistors are common parts.

Edit: Yes, the fan only runs when the amp gets hot.

 

[bikemech]

ok just to let everyone know there are no blown fuses, that was the first thing i checked,
as for dry joints i will have a close look over the weekend,
also the guy i got the amp off bought a newer stageline amp (higher power) and the fan is on instantly,

also with it being the left channel, am i right in asuming the dry joints if any would be on the output side of that channel ?

 

[KrisBlueNZ]

ok just to let everyone know there are no blown fuses, that was the first thing i checked,
as for dry joints i will have a close look over the weekend,
also the guy i got the amp off bought a newer stageline amp (higher power) and the fan is on instantly,

I said that the fan is controlled by temperature on THAT model, not necessarily on other models.

also with it being the left chanel, am i right in asuming the dry joints if any would be on the output side of that chanel ?

Yes, but check both channels equally carefully. If there are dry joints on one, there are probably dry joints on the other; the only difference is that they happened to get bad enough to cause a failure on the left channel first.

 

[bikemech]

Just had the board out and resoldered what I thought were some dry joints, I haven't accidentally joined more than one point at a time, as in soldered to together that are not supposed to be, however I also replaced a missing screw that holds the board to the chassis, now I have blown 4 fuses as soon as it's turned on, the 4 at the bottom of the first picture,
Possible combination of dry joints and bad (semiconductors were they called)

 

[KrisBlueNZ]

Let's hope you've just made some mistake in the reassembly, not blown all of the semiconductors in the output stage. Now do you want to do a proper, careful job as I suggested in post #9? This involves dismantling the heatsinks, measuring all of the semiconductors (on-board and off-board), making any needed repairs, and going through a careful step-by-step process when you first power it up.

If you want to continue down that route, you can expect the project to take quite a while.

Is it possible to remove the circuit board and output transistors from the case? I would like to see some good clear photos of both sides of the circuit board, as well as photos of the heatsink construction.

 

[bikemech]

Just looked back on some of the pictures I took before I removed the board, and yes my fault I have put the live and negative the wrong way round (doh!!!) will put them right later tonight and see what happens, going to have to stock up on fuses I think, will upload some more photos for everyone hopefully later tonight

 

[bikemech]

after rectifying my earlier mistake and replacing 5 fuses ( 4 on the board and the main one in the back of the case) i now have both left and right fault led's lit, and it blows all 4 fuses on the board, only tryed this once!

i now have fault left and righ led lit !!

under side of moroco tip30c

under side of main outputs

 

[KrisBlueNZ]

OK, as well as fuses, you should get some fusible resistors that you can use instead of fuses during power-up and adjustment. I suggest around 33~47Ω fusible resistors rated at 1/2W or 1W. Here's a suitable one: http://www.digikey.com/product-detail/en/FRM50SJR-52-47R/47DVCT-ND/2813217 and there's a price break at 25 so 25 of them only costs USD 2.40. That should do for quite a few tests. Or you could get 100 of them for USD 5.16. Yes I'm serious!

You can modify some blown fuses to make it easy to fit new fusible resistors onto them, and to detect when the fusible resistor has "fused". You need to make at least four of these. They replace the fuses, and protect the components against high currents that will make them release their smoke. Instead, the fusible resistor (that you plug into the sockets at the top) will sacrifice itself. When this happens, the LED will light up. Expect to see this happen quite a few times!


So you construct this around a blown fuse. You should have a few of those lying around! Solder the components to the fuse but make sure it will still go into the holder. When you've finished, insulate the exposed parts using hot melt glue or something similar, so they cannot rotate within the fuse clips and short onto each other.

The two bits at the top are just receptacles for the wires from the fusible resistor, which is the sacrificial part. You can use pieces from "machine screw" sockets such as http://www.digikey.com/product-detail/en/AR14-HZL-TT-R/AE10012-ND/821766. You carefully cut the plastic so the metal pins fall out, and use each pin as a socket to receive a resistor lead. The two 4K7 1W resistors are something like http://www.digikey.com/product-detail/en/FMP100JR-52-4K7/4.7KWCT-ND/2058929 and the LED is something like http://www.digikey.com/product-detail/en/MV5491A/1080-1116-ND/2675607.

How do you feel about this so far? If you have any questions or uncertainties, ask and I will explain. This is outside your experience so you need to follow instructions carefully.

Do you have a multimeter? You'll definitely need one. It needn't be anything flash - $20 will get you something quite adequate. It's very helpful if it has a continuity beeper and a diode test range, but the transistor tester some multimeters have is just a gimmick.

If you want to continue, I think the next step is to desolder the output transistors and measure them, then probably work through the other semiconductors in the output stage, measuring and replacing them as necessary. It's quite possible that all four output transistors are dead. They're still available for USD 4.23 each, which is pretty reasonable. See http://www.digikey.com/product-detail/en/MJ15003G/MJ15003GOS-ND/919449 and http://www.digikey.com/product-detail/en/MJ15004G/MJ15004GOS-ND/919450.

Is it worth the trouble? I would do it, personally, because I think it's a nice amp, but it's your call.