SX-1250 Resistors blown after recap

[vlig-hifi]

I just finished recapping the four large capacitors and ran the DBT with no apparent issues. I then plugged it directly into the wall and after 2 min of being powered, smoke came from the Power Amplifier board (AWH-048). I removed the board and checked what looked to be a burnt resistor R24. From what I've been reading, a bad transistor could have caused this, but should I have done something before plugging it into the wall? Where do I go from here? I'm relatively new to this but have an engineering background and am willing to put in the time and effort to figure it out.

 

[BobbyN]

Years ago I had a bad power transistor cause this, same as you've read. I don't remember now if it was R24, but it did smoke the power amp board.
Bob

 

[vlig-hifi]

Do I identify bad components, replace and power up again? Is there anything that could be causing this issue or other components to test prior to power up?

 

[zebulon1]

Unplug the amp boards top and bottom Molex plugs.
DBT the set and check the big cap voltages.
The caps for the smoked amp board maybe installed incorrectly. The plus side cap mainly.
You want a good +-70 volts from the output side posts.
Double check your install.
As for the amp? Check all the drivers transistors for service and resistors for value from R24 and up. And all the diodes.
It might need only a few components but you have to check everything the +70 rail effects.
A good idea?:
It might be a good time to rebuild the amps anyway and the forum has a wonderful list to order from. Minus the resistors.

The big filters can take a beating. It's amazing how much reverse voltage they can stand. You might even find the reversed cap by feeling the temperature of the cap itself.
The best/safest way is to check all the wiring and compare to images on google.
I have a 1250 near the bench if you need any images for it.

 

[markthefixer]

The OP glosses over what was done to the power amp board before turning it on,

recap = caps, ?trimpots? ?transistors? Know what a thermal bias diode is and it's vulnerabilities? Sure that it's wire isn't broken?

No mention of a DBT to power up.
No mention of "tickling the dragons tail" on DBT to be sure idle current can be controlled.
NO mention of where to set the idle current trimpot - EVEN IF ORIGINAL - after ANY work (yes, just caps qualifies!!)

R24 burned up, R25 is probably damaged, as are Q7 and Q8, no less Q9 and Q10 and the output transistors.
That means the "idle current" was way too high, with the trimpot resistance excessive, or the current loop between Q7 and Q8 was open circuited.

OP: no - you have a lot of testing and interaction with us and a LOT of learning before the power cord gets ANYWHERE NEAR A POWER OUTLET.

This thread embodies my worst fears about releasing any of my recap lists. NO due diligence.

 

[vlig-hifi]

I apologize that my first post to AK embodies your worst fears but I'm new to this, and willing to put in the time to figure it out. I only the four large filter caps. There was no work done to the power amp board. Should I have recapped or rebuild the amp boards and the power supply assembly at the same time as the large filter caps? I went over all of the wiring and cap polarity and everything is ok.

I'll check the voltage with the amps unplugged, but should I have turned down VR2 prior to power up? Also, the DBT was 'dimming' a 60W bulb - should I have incrementally increased/decreased the bulb wattage before determining the test successful? Thanks for the help in advance.

 

[zebulon1]

My story:
I had the same issue on a Spec 2 when I replaced the filters. They were installed correctly but the one connection at the screw terminal had very poor crimp on the ring terminal. I usually follow up with solder but was in a hurry. The symptom that manifested was the inability to set the Idle on that channel after replacing the big caps. The set was complete and had been running on the break-in bench for a week and was stable as the days long. All the amp adjustments, limiters, everything, were made and spot on. I was waiting for the large caps to arrive and install them before it was considered complete.
Now, it wasn't that I couldn't set the Idle, it was very touchy and hardly any CW turn from full CCW got to the .030v's idle. I thought - well it's stable, a little funky (It was perfect before ) - but it works. Go with it. Bad decision.
Guess what? After running a 1Khz signal and increasing the gain slightly while watching the scope. Pop and Smoke!!
R21 let go. Which is a Spec 2 amp resistor that shares a similar function as R24 in the 1250's circuit.

I said you may of installed the filters incorrectly but the connections should be checked for good contact and make sure to use proper electrical connecting techniques.
But that's the small issue now. The Amp driver board will consume some time to sort out.
Mark (MTF) probably see's something I missed. He's all knowing in these matters.

Order up the rebuild list and ask questions if unsure. The set will be better for in the end. And you will have a smile knowing you messed up and then fixed it.
Plan on using good tooling. No reason to chimp out, because this repair thing is addicting and it may not be your last one. Lots of threads on what tools to buy, here on AK.

 

[vlig-hifi]

Thanks for the feedback. I realize now between zeb and Mark that I failed to set the idle current during the DBT. I'll get to work on the amp board repair and be sure to ask questions prior to frying any more components. Should I replace the capacitors on the power supply assembly board? It seems that my failure caused a couple to leak.

 

[zebulon1]

Just replacing to filter caps shouldn’t of changed the amp settings. If everything went well in the swap it would of been fine.

Order the parts and remove the amp board. Check everything and replace the transistors, diodes checking all the resistors for value. Ceramic caps should be fine. Then order any resistors that have failed or are out of tolerance.

 

[markthefixer]

Generally the brown snot at the bottom is assembly glue. The caps when they let loose look like puke and I'm not kidding.

You replaced four main caps, on a 1250. I can't imagine doing that without removing the boards right next to them to access the screw clamps.

First and foremost - identify the thermal bias diodes, connected to the amplifier board yet screw mounted to the top of the heat sink. Their leads are fragile and love to break off flush. They must be protected, and not bent within a half of an inch of their body. If they break it's like commanding infinite idle current - bad news. The casualties include the resistor you described. Things start racing each other towards destruction, and the losers wait until later to finish blowing up. Then fiddling with the amp board can irritate the old idle current trim pots, just plain moving the board can do it, putting the commutator wiper onto a non-conductive corroded portion of the commutator. Same result - infinite idle current - smoke - flames - blown drivers, output transistors, resistors wires fuses etc... etc... etc...
It HAS happened, even to me. (in the dim mists of the past)....

NO question is dumb - we ALL went through this. Dumb is not knowing exactly what to do next and just "trusting the force" and doing something.

You have a good unit that has been covered in many many threads, reading them is some of the best prep you can do.

 

[vlig-hifi]

So I've spent the last couple weeks testing components and reading through old AK posts on this amplifier - a lot of good information. I ordered all of the components to rebuild based on MTH parts list checked with the manual and original components:
AWR-107 Fused Power
AWR-106 Power Stabalizer
AWM-091 Protection Board
AWH-048 R/L Amp Boards w/ added blown resistors
My plan is to rebuild the boards in this order and test in between each one. Is this a good plan? Alos as I looked over the amplifier, it seems that it's not a virgin. Some of the wire wraps have been taken off and resoldered onto the posts, and it seems that some components on the stabilizer board have been replaced. Will this cause any long term problems? Here are a couple of my questions/observations:

My amp boards are AWH-048B. I couldn't find too much information about any differences in AK posts and the resistors all match the manuals values. Can anyone tell me if there is anything I'm missing on the difference between 048A and 048B?

When I removed the stabilizer board, I noticed one of the solder pads is raised (pic attached). What should I use to repair the stabilizer boards raised solder pad? Will 2 part epoxy work or is there a special compound for this application?

 

[rcs16]

one of the solder pads is raised

Scratch away some of the green solder mask covering the copper trace. Add/form a small piece of solid bare copper wire over top of the trace, starting right where the lead protrudes thru the hole, for about an inch, solder the wire to the existing exposed trace. You could then add some epoxy over that to stiffen it up and insulate

Soldering the wire to the wrap posts is okay, Pioneer did it too, in certain spots, without reason that I can tell. maybe they thought they had a lose wrap during ass'y and thought it would be better to solder it as well.

I do not know the diff between 048A and 048B, suggest to check your assembly,pcb layout and parts against the documentation to see what the diff maybe. just use the exact same values as original and i think you are okay.

it seems that some components on the stabilizer board have been replaced. Will this cause any long term problems?

Make sure the comps are the correct values/voltages, could change them as well, so that you have a fresh start.