music man hd130 intermittent what the??

[ilyaa]

hey!

long time no post -

fixing a music man tube/ss hybrid for a buddy

works fine sounds great for about 1 hour and then starts to lose volume and completely crap out, making some weird sounds on the way!

any ideas where to start with something like this? seems to be heat related...whats a good way to diagnoze/troubleshoot problems like that?

im guessing its the power amp or PI, because once it starts to crap out it doesnt matter which channel you plug into -

[phatt]

Turn off,, wait till it cools down,,pull out all Valves and wiggle as you go, Re-insert Valves try again and  if that fixes the issue you have just cleaned the pins on the valves.
If no go then you will have to look deeper but do the simple/cheap stuff before you re-build the space shuttle. :tu:
Phil.

[Roly]

Power loss after a period of operation can also be caused by output valves on the way out, 'tho one hour is a fairly long time for such a fault to show up, typically more like minutes to tens of minutes.

If you have a known good output pair you can swing in for a test do so and let it soak under drive into a dummy load and see if it still fades out.

[g1]

  Does it respond it you give it a whack when it is acting up?  Sometimes bad solder connections will only show up with prolonged heat.
  Also, a heatgun or hair dryer can help find heat related problems.
And if it has an effects loop, plug into the return after the problem occurs.  If the sound does not improve, then you have isolated it to the power amp or power supply.
What is the exact model/chassis number ?

[Roly]

a heatgun or hair dryer can help find heat related problems

Please - NOT a heat gun.  I've used one of these to bulk unsolder computer boards which it does in a couple of seconds, and consider it far too savage to use for faultfinding - the board will char and catch fire in only ten seconds.  Even a hair dryer should be used on low and with serious caution as it is very easy to overheat components.  The tip of your soldering iron can also be used to apply heat to particular components (but not electrolytic caps).

Another trick is cooling.  I dab some metho on the suspect component (normally a transistor) with a cotton bud and blow gently to force evaporation and cooling.

[J M Fahey]

Well, probably you won't believe this: when I was a poor young student (is there any other kind?) , one parts resource for endless experiments was to buy IBM mainframe boards, by the pound , (I'm talking weight here , not money) , which were about 8" by 12" , chock full of flip flops, meaning hundreds of NPN nondescript silicon transistors (probably optimized for switching speed) , close to a thousand resistors and capacitors, hundreds of 1N914 type diodes, plus lots of weird things (I guess now they were delay lines and similar exotic stuff) .

BIG problem was desoldering and pulling stuff for reuse.

Burn and slap [tm] technology to the rescue  .

What's that?

Well, you cover the kitchen table with newspaper (under Death Penalty by Mom's hands) , you grab the board with left hand, a propane torch on the right one, you melt solder on an area of the board and slap it hard against the table.

Hopefully most now free parts will be on the newspaper, without too much old molten solder jumping around.

Boards were destroyed, but who cares?

Not very different from:



That said, I'm not the only hot handed madman on the Net:
https://www.youtube.com/watch?feature=player_embedded&v=L8GTW_7GHc0

[ilyaa]

haha whoa those are some crazy techniques!!

well i cleaned the power tube pins/sockets and ran it for an hour into my dummy load and then played it some and the problem has not reappeared.

i might try the blowdryer idea for a minute and see what happens....

[ilyaa]

forgot to update this:

everything was working fine - heat tests yielded no results - it was all good

UNTIL the owner came by and i was proudly showing to him how everything was fine and then - the problem came back!!! low volume weird hum etc etc. i was a little bit embarassed because i had the guy over to return the amp but also relieved that the problem reoccured before i gave it back to him.

anyway, i opened it again took a closer look and found that one of the zeners in the power supply - there are two that regulate the +/-16V rails - was shorted. it didnt look back but the area underneath it on the PCB was a bit charred/brownish looking and it measured short. phew!!!

replaced that and the amp was all good - sounded even better than it had been -

my thought is: the junction in the zener was working but breaking down - once the amp was running for a bit the zener got hot enough from holding that voltage steady and the junction stopped working properly - maybe finally it broke down for good and measured shorted even when the amp was still pretty cold.......plausible?!

[Roly]

plausible?

mmmmm yeah.  Just remember that when PCB gets charred/carbonised it turns into a resistor.  A bit brown doesn't matter but when it's bad mostly you can "aeroboard" around it, however sometimes you have to cut the cancer out.

Yes, you got lucky it reappeared before it went out the door.  Failure on stage after a repair is not a good look - this is why we curse intermittents.

I always give "three months free return" on any repair, but with anything that is even slightly sketchy such as this intermittent I make a point of impressing on the client that the repair may not be certain, and that they must bring it back if it re-appears.  Standing by your work can sometimes be painful, but it's what builds your reputation.

[ilyaa]

what do you mean, "aeroboard"?

[Roly]

Sorry; wiring up components above the board, twisting leads together and making joints in the air above the (carbonised) PCB.