Ampeg BA115-hpt amp. Mains hum + signal breaking up - but only when hot...

[AlsAmpeg]

Hi,
hoping for some help with an Ampeg BA115-hpt.
Already got the schematics, thanks.

Symptoms :

1) A few weeks ago, slight distortion/crackle appeared. Turning off the tweeter "cured" this (now suspect BIAS at fault, originally we assumed the tweeter faulty)

2) Amp plays OK for about 30 mins (with the slight "crackle", if the tweeter is up).
Once it's "warmed up" : LOUD 50Hz hum and bass sound terrible - VERY distorted / breaking up.
The hum continues even with all dials set to zero.

Switch off and the amp cool down for 30 mins, then it's fine again (until it warms up....)

Checks so far :

RESOLDERED the main board (as it was out + doesn't take long.... )

BIAS :

Bias across all 4 (R72 - R75) was ZERO mv.
I adjusted to get all 4 between 15 and 20.
Curiously, to get the right voltage, the pointer ended up back-where-it-started.
(I gave it a few wipes back and forth 1st - with the amp off - before adjusting. I noted some physical-resistance at first so maybe some dirt/corrosion in the pot ?).

Slight crackle still there - but definite improvement.

HOT COMPONENTS :
Bias Trim-pot / D9 / D10 and Q6.

These neg. rail components are getting much hotter than their pos. rail counterparts.

e.g. Q6 (MJE-350, neg. rail) quickly gets up to over 100 degrees celcius, whereas Q76 (MJE-340,pos. rail) is around 35 - 40.
I'm measuring the tops of the actual transistors, not the heatsinks.

The trim pot & D9/D10 also get up to 80+ degress.
- - This is regardless of load/no load - -

I already substituted Q6 (in case thermal-runaway - and they're cheap) but no difference.

Could the hot-components be relevent to the problem ?

I have electrical/electronic experience but I'm not a pro.
I have a good multimeter but no oscilloscope or signal generator.

I've scoured forums and seen much on the BIAS problem but not found our exact scenario (unless I've missed something !)

Any advice or recommended tests/checks much appreciated - - trying to fix on a budget - thanks.

[Jazz P Bass]

Can you post the schematic?
Or link to it?
Hot in an idling SS output stage is bad.
If you have a way to safely monitor the mains voltage, you will see that the amp is pulling excessive current.
All totalled it may pull 50-60 watts.
But the fact that parts are getting hot does indicate that something is amiss.

[DrGonz78]

Here is the schematic...

[Enzo]

Be prepared for quick readings. Check the voltages across D9,10 when cold and then when the problem starts. Also take readings on D20,D8 to compare, they SHOULD be about the same. I bet they are not.

Sorry, but I start to suspect the outputs, Q5,8.

This is what freeze spray is for. Something warms up and causes a problem? Hit various parts - like Q5,8 - with freeze spray and see if chilling them makes it go back to working. Don;t douse the whole thing, selectively chill parts looking for ones that affect operation.

[AlsAmpeg]

Hello again -

many thanks for the speedy replies.
Looks like I'll be replacing the output transistors.

In this thread : http://music-electronics-forum.com/t30448/
gwright recommended IRFP-240/9240's instead of IRFP-140/9140's.

Anyone else concur ?
If so - would other components need changing or bias adjusting differently [etc] or is it a straight swap ?

There's little difference in price but :

use 140's = replace 2x mosfets only.
use 240's = replace all 4 mosfets (2 x 9240's also).

[Jazz P Bass]

Not too sure what the gent was getting at in your link. (that the 240 is a better part in this application)
The IRFP140 & 9140 are 100V/ 33A parts.
The 240/9240 are 200V/ 20A items.
The rds On is lower for the 140's. (not that this really applies in a Class AB amp)
The trick with power mosfets is getting a pair that are, relatively speaking, closely matched.
Purchasing from a big distributor helps, as they move a lot of parts.
This can be checked by measuring each transistor , directly across there respective ballast resistors.