I just finished repairing my old Dynakit Stereo 120 basic amp after a sudden failure.
The 120 was one of the first good solid-state amps (all discrete of course) and I built mine in the mid 1970's, which I've used ever since. It was a relatively simple kit to assemble, since all the circuitry was on prewired and tested PCBs.
It's an old amp, and I suppose I'm sort of nostalgic about it, but it sounds great (no apparent "transistor sound") and served me well, and I'd hate to have to junk it. The only previous failure was a bad electrolytic in the signal path.
It powers some good sounding HSU bookshelf horn speakers (along with a powered subwoofer) for music at my desktop. A couple weeks ago while listening to some music at a low level while on my computer, there was suddenly a loud crack/pop sound from one of the speakers, and then everything instantly went dead...except for the sub still thumping away.
Fortunately, being a kit, it came with a manual (which amazingly I still have), fully describing its operation with complete schematics, detailed parts list, and node voltages. I removed the amp's cover to check voltages and determined there was no voltage out of the regulator (the amp was well designed with a series regulator for the 75V supply). At first I thought it was a regulator failure since the voltage from the diode bridge was good at about 90V, but after removing an output connection to test the regulator I found the regulator was okay. The regulator has fold-back current limiting, so a short was likely causing the zero supply voltage, and the likely candidate for that was the two output transistors connected in totem-pole fashion directly between the supply voltage and ground.
Indeed, one of the two amps did have two shorted power output transistors (<1Ω collector-emitter). It's likely one initially shorted, which then put the full voltage on the other, causing it to also blow. Apparently it was just an end-of-life type wearout failure (after only 50 years ).
My troubleshooting was helped by having a Variac for the AC input so I could slowly bring the voltages up during testing and repair with less worry about zapping something else.
The original transistors were listed as 2N3055's in the parts list, but were apparently selected devices as they had a Dynakit part number stamped on them with a stated 90V Vcer rating, whereas standard 2N3055's are rated at 70V. It is thus quite likely that run-of-the mill 2N3055's (especially the cheap Chinese knockoffs) would be marginal. Fortunately I was able to find an upgraded version of the 2N3055 (MJ15015G) by a reputable vendor, which has essentially the same characteristics expect for higher voltage rating (Vceo=120V) and better safe area.
(It was a serendipitous find as I just happened to see it listed while looking at the 2N3055A ON Semiconductor data sheet.
Interestingly, they were about the same price as the 2N3055A, around $6.50US).
I was concerned that the transistors' shorting may have damaged something else, but after replacing them (along with a 0.47Ω emitter ballast resistor that I smoked during my initial troubleshooting), the amp works good as ever.
I also bought a couple spare transistors in case the other channel has the same failure in the future.
So for about 35 bucks in parts (including the spares, resistors, new TO-3 thermal isolators, and thermal grease) I got it working again.
The 120 was one of the first good solid-state amps (all discrete of course) and I built mine in the mid 1970's, which I've used ever since. It was a relatively simple kit to assemble, since all the circuitry was on prewired and tested PCBs.
It's an old amp, and I suppose I'm sort of nostalgic about it, but it sounds great (no apparent "transistor sound") and served me well, and I'd hate to have to junk it. The only previous failure was a bad electrolytic in the signal path.
It powers some good sounding HSU bookshelf horn speakers (along with a powered subwoofer) for music at my desktop. A couple weeks ago while listening to some music at a low level while on my computer, there was suddenly a loud crack/pop sound from one of the speakers, and then everything instantly went dead...except for the sub still thumping away.
Fortunately, being a kit, it came with a manual (which amazingly I still have), fully describing its operation with complete schematics, detailed parts list, and node voltages. I removed the amp's cover to check voltages and determined there was no voltage out of the regulator (the amp was well designed with a series regulator for the 75V supply). At first I thought it was a regulator failure since the voltage from the diode bridge was good at about 90V, but after removing an output connection to test the regulator I found the regulator was okay. The regulator has fold-back current limiting, so a short was likely causing the zero supply voltage, and the likely candidate for that was the two output transistors connected in totem-pole fashion directly between the supply voltage and ground.
Indeed, one of the two amps did have two shorted power output transistors (<1Ω collector-emitter). It's likely one initially shorted, which then put the full voltage on the other, causing it to also blow. Apparently it was just an end-of-life type wearout failure (after only 50 years ).
My troubleshooting was helped by having a Variac for the AC input so I could slowly bring the voltages up during testing and repair with less worry about zapping something else.
The original transistors were listed as 2N3055's in the parts list, but were apparently selected devices as they had a Dynakit part number stamped on them with a stated 90V Vcer rating, whereas standard 2N3055's are rated at 70V. It is thus quite likely that run-of-the mill 2N3055's (especially the cheap Chinese knockoffs) would be marginal. Fortunately I was able to find an upgraded version of the 2N3055 (MJ15015G) by a reputable vendor, which has essentially the same characteristics expect for higher voltage rating (Vceo=120V) and better safe area.
(It was a serendipitous find as I just happened to see it listed while looking at the 2N3055A ON Semiconductor data sheet.
Interestingly, they were about the same price as the 2N3055A, around $6.50US).
I was concerned that the transistors' shorting may have damaged something else, but after replacing them (along with a 0.47Ω emitter ballast resistor that I smoked during my initial troubleshooting), the amp works good as ever.
I also bought a couple spare transistors in case the other channel has the same failure in the future.
So for about 35 bucks in parts (including the spares, resistors, new TO-3 thermal isolators, and thermal grease) I got it working again.
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