Help!!! Hafler se240 problem

[Z-Ya Later]

First of all hello to everyone from a n00b to this forum.

Hopefully one of you guru's can help me.

I'm having a problem with my Hafler SE240 amp.

I've searched the forum and didn't come up with much on the se240.

Amp has been solid since i've had it ( 9 years) Lately it has started to send intermittent feedback thru the speakers.

It's hooked up to a dual voice coil 12" Cerwin Vega sub and intermittently throughout the day (and night) I will hear a loud "duh" from the sub. Sometimes it's a single "duh" and sometimes it's a rolling "duh" ( sounds like thunder). There is no rhyme or reason to the timing.

When this happens at 3am the wife is thrilled!!

The amp is plugged in and turned on 24/7 whether the receiver is on or not.

First thing I did was unplug the RCA's to figure out if it was a source problem or not. Sounds continued.
(3am: Honey I’m TESTING the amp now, I'll unplug it tomorrow!!)

Next thing was to remove said amp from stack and hook up on bench (to reduce marital discord)

I visually inspected the board and components and didn't find any obvious signs of trouble. No burned components or bulging caps. I believe the mysterious blue smoke that runs all electronics is still inside the components.

Power Supply seemed to check out OK with 47.8 volts to both terminals of the bridge rectifier on the PS side. + and - rails both show 47.8 volts DC.

I'm thinking since this is an intermittent occurrence maybe the power supply is bad?

With the above tests I’ve about reached the end of my electronic prowess and what I could learn in 4 hours on this board.

Update:

The amp has been on the bench powered on for about 30 minutes. (no input, no load) and the right hand heat sink is HOT while the left hand heat sink is stone cold.

Maybe a thermal issues? What failing component could cause the heat sink to get hot with no load?

Any help (even wild speculation) would be appreciated.

PJ

[Zen Mod]

besides need for rebiasing ( probably booth channels ) , I can ask only one Q for now - is it thump on both channels ?

btw - have you at least schematic ? Hafler site have only user manual ;

I can check later if I have it ....

[d3imlay]

It may not be the bias, but instead it's oscillating that causes the heat. The bias procedure has you measure idling current. Make sure to change you're meter leads back to voltage after checking the current. Instead I use a .10 ohm resistor as a shunt and measure the voltage. A fet could be leaky causing the heat. Also it's possible that a front end transistor breaks over with slight increases in line voltage. Dried out electrolytics are another potential problem.

[unclejed613]

how about this question.... does anybody know if the SE240 and the SE120 are similar in design? if so, hafler has the schematics for the 120, and if they are of similar design, then once you know what the differences are, you can use the schematic as a general "road map". i've seen this a lot over the years, a company makes an "X100" and an "X200", and the only real difference between the two is that the 200 model has the output transistors doubled and a bigger power supply, and maybe a slightly different board layout. this is very often true within "series" models (the NEW XYZ series, the XYZ-250, XYZ-500, and XYZ-750).

[Z-Ya Later]

Zen Mod - Since the amp was driving 1- dual voice coil sub i'm not sure if the thump was in one channel or both. I will hook up 2 speakers and see if I can find out. I couldn't find a schematic anywhere. I did download the manual from Hafler's site but it's next to useless.


d3imlay- I'll have to do a search for the biasing procedure, i'm not familier with it. would it help if a separated the fet's from the heat sink and put some heatsink compound in there?

unclejed613- I think the 2 amps are similar except for output. I'm giong to download the scematic for the 120 now. I'll also try to get some pics up of the amp internals.

Work has been crazy so it will take me a couple of days.

Thanks,

PJ

[Stormrider]

Se120 is basically the same as the DH120, so the SE240 is similar to the DH220. The difference is the pcb layout and the outputs are plastic devices instead of TO3's.

Your power supply voltage seems off. It should be up around +/-60vdc across each filter cap.

[d3imlay]

As I recall the 240 had one board for both channels. I tested a bunch in the early 90's. Unless the outputs are loose, removing and regreasing won't help. Remember, when using the silicone grease, less is more.