Dead amplifier

[rrcrain]

Well, it's been a while, but one of my two amps died. My first thought, because I saw something glow red hot in the case when I turned the unit on, was the unit suffered a cascade failure. AKA not cost effective to repair.

Fall is here, my depression over the event has abated a bit and I decided to open the unit to see how bad the damage was before trashing. No burn marks, no scorched wires or melted or loose connections to be found. Checking further, both 5 amp fuses were blown in the right channel, the channel that failed.

I decided to check the TO-3 transistors first, and found two that are a dead short between B-C-E. Each was part of a pair, it's matched transistor was good, er, acceptable given it's age.

Checking further, I noticed whatever repair facility replaced 2 of the 4 NPN transistors in the channel, and used the wrong part number.
 

The original part numbers are now obsolete (MJ15024 and MJ15025) but two substitutions are available. The better choices for the TO-3 NPN and PNP transistors is MJ21193G and MJ21196G. Mouses strongly recommended replacing all eight if going with these options. They have a slightly higher gain than the originals, supposedly better linearity and a high SOA.

 

If this fixes the amp, this plus the new electrolytic caps I installed last winter should be a minor upgrade in performance and perhaps extend this old Heathkit AA-1800's life a few more decades.

[rrcrain]

Regarding the replacement transistors in the previous comment, ignore it, one will work, the other will not. I asked Mouser to verify their recommendation and a more experienced tech did a bit of research and found one of the two transistors has more gain than the other as well as a couple other specs being slightly different.

SO, they recommend using MJ12024G and MJ15025G as drop in replacements.

Overall, I've identified 4 failed transistors, two of which were the wrong type.

[rrcrain]

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[rrcrain]

Four failed transistors, all a dead short. 

But none the less, the amplifier is repaired and appears to be fully functional. I decided to replace all 16 of the TO-3 transistors (both channels), not just the two that shorted. All of them checked to be leaky but functional and I decided I was being penny wise and pound foolish if I only replaced the failed parts.

As for performance. I haven't decided if it's my imagination or real, but my perception is this old amp sounds more "solid state" than it used to sound. I've had to reduce the gain on my tube preamp a step to reduce ear fatigue.