Orion 250SX question

[minibeast2150SX]

I am working on fixing an Orion 250SX amp. I took it apart and it is very obvious that I am not he first one to break into this little guy. There is plenty of tell-tell flux on the 8 outputs and they are ECG331/332 parts which I am pretty sure Orion never used. I plan to replace those with 2N6488/6491. So on to my real question, the power supply is using 8 Motorola D44H11 in the power supply, most are cracked and need replaced. They appear to be original based on the pristine solder. This is a gen-2 heatsink and revF board so this is very surprising to see bipolar devices. I was planning to use IRF3205's to replace these. Is that a bad idea?

[Perry Babin]

There were some amps that used BJTs AND had the TLx94 driver IC but they were relatively rare as far as I can tell. Soon after they began to use the driver IC, they changed to FETs.

I tried to convert one such amp to FETs but was unsuccessful (didn't have the amp long enough to experiment). The main problem was the B+ filter capacitors. They overheated and would have failed if the FETs would have remained in the amp.

If you try to convert it, you'll probably have to modify the drive circuit. The amp I worked on was not like anything I've seen in any of the other amps. Make a schematic of the drive circuit before making any mods.

[minibeast2150SX]

Perry, thank you for your always expert and professional opinion. Here is a pic, does it appear to you that this is the bipolar drive circuit or has someone mistakenly replaced the FET's with BJT's?


PS picture

[Perry Babin]

The large transistors near the MPSA06/56 drivers are very similar to the amp I worked on. I think this amp had BJTs originally.

[minibeast2150SX]

That is a Harris D40D8 and it looks to be the driver for 4 of the BJT's, with a duplicate on the other side. (I added more pics to the link in the previous post)

Ok, now that it is pretty certain that this has a BJT based PS, is there any other tips you can give me? All others I have had my hands on or have built were FET based.

Also there was no foam tape on the clamp bars, where can I get this? I am aware of the procedure you (Perry) posted on inital tighten down for burn in so you can skip that.

Thanks for your help, I am fixing all the obvious stuff before I even power it up, so much more to come on this thing. I can hardly wait to finally power it for the first time.

[Perry Babin]

Wal-Mart has the double-sided foam tape.

Check/replace the MPSA06/56 power supply drivers and check the 10 ohm resistor between the collector of the A56 and ground.

Make sure that none of the screw holes are stripped before you apply heatsink compound. It's annoying to find a stripped hole only after you have the amp entirely reassembled.

When you power it up initially, do so through a 10 amp fuse. Have the transistors clamped to the sink in case there are other faults that cause excessive current draw.

[minibeast2150SX]

I am slightly confused by your mention of heatsink compound. The factory gray tape is still intact. If not I would probably use kapton tape in its place. Do you suggest using heatsink compound between the tape and devices?

[Perry Babin]

Some types of insulators are made to be used dry and will work well that way when new. When re-using insulators, I always use compound. I always use heatsink compound with kapton or other insulators that can't fill gaps.

[minibeast2150SX]

Your experience speaks for itself, I just ordered your amp repair dvd based on the help you have given me. Thanks again!