Replacing transistors in amplifiers

[dherm8or]

I have been trying to repair some Altec 50Watt 1593A amplifiers. I have been using NTE 130 replacement transistors for the output and I am having problems. I need to figure out what the best replacement part is for the RCA40934 2 ones that altec uses. Are these still available. I had 5 years ago A SK/RCA semocinductor replacement guide but I can not find it now.
What I have done is on one amp it to replace all four. I had to replace the pre amp ones also to get rid of distortion. The amp behaved under low power but as soon after I started to run some music thru it driving some VOT's the amp smoked on the capacitor side. I had bought match paired transistors hoping for a little better performance. Today I tried to fix another one this one had one bad transistor. I started to push it under a 8ohm dummy load with pink noise and it was distorted at higher levels and then it smoked both on one side. I knew that this was partly to due to mismatched transistors and I wasn't watching for clipping on my analyzer. If I ran -16db 1k sine wave at full volume it would clip. I had a THD of -40 db on a side note. For those who haven't seen it Neutrik Test Instruments makes a the Minirator which is a handheld audio analyzer with LCD screen. This has been very helpful in equalizing speakers and repairing sound systems I think that the NTE transistor is not the perfect match to the RCA
Does any body have any tricks for me? Or is there a manual that may help me. Their is a pot on the preamp board and it would be nice to know how to properly adjust it.

[AllanCT]

I think you need the Altec service manual to fix those amplifiers. A big help for sure. I believe that pot. is for setting bias currents but it's been many years since I repaired one.

Manual should be your first search.

I doubt the problem is the NTE130 transistor.

I am sure you have checked the output transistors but always make sure any output load resistors such as 5 or 10 watt small value ones .1 or .33 or so ohm resistors are not open or shorted.

Finally if there are zeners on that driver board check them as well.

All I can remember with no manual here.

[Fitts]

I just checked my 3 replacement guides and the NTE 130 appears to be the right one. I think AllanCT has mentioned most every thing that I would have thought of and more.

In the past when I had no service manual, I always checked the bias voltage and worked to get it to .65 V or a little less. When the bias voltage is above .65, silicon output transistors will get hot quickly and burn.

Good luck,

[dherm8or]

That was great info. Now I need to find sources for a service manual or the steps I need to do to check the transistors for proper operation