This amp is so old it makes me blush to think I was working on them new that long ago.
The mod is where the PC board designer accidentally left out circuitry designed into the amp and when the PC boards were made < rather then toss them to scrap > they added the missing circuitry and used the reject PC boards that a designer screwed up... CAD CAM was still on the drawing boards when these amps were made so they got hand drawn and hand made at great expense. PC board like this use to cost about 2 cents a square inch back then but that was a lot of money back then also, especially when you can add stuff onto like they did so neatly LOL
As for what this add-on does well I would need more circuitry then you posted to figure that one out. An easy test is to pull the diode lead and disable it and see what happens. Its doubtful it will hurt anything to dynamically test the circuit in and out of use.
Or think of it like this, It was added so you would be scratching your head wondering how such a thing was needed in the first place LOL.. I like to think it was always overlooked in manufacture and a last minute add on to get the amp off the assembly line, just like the 6 pounds of silicon grease they used instead of machining the transistor surface areas truly flat... All to save a buck, and they never figured anyone would be looking at it almost 40 years later going what the heck is that ?
The mod is where the PC board designer accidentally left out circuitry designed into the amp and when the PC boards were made < rather then toss them to scrap > they added the missing circuitry and used the reject PC boards that a designer screwed up... CAD CAM was still on the drawing boards when these amps were made so they got hand drawn and hand made at great expense. PC board like this use to cost about 2 cents a square inch back then but that was a lot of money back then also, especially when you can add stuff onto like they did so neatly LOL
As for what this add-on does well I would need more circuitry then you posted to figure that one out. An easy test is to pull the diode lead and disable it and see what happens. Its doubtful it will hurt anything to dynamically test the circuit in and out of use.
Or think of it like this, It was added so you would be scratching your head wondering how such a thing was needed in the first place LOL.. I like to think it was always overlooked in manufacture and a last minute add on to get the amp off the assembly line, just like the 6 pounds of silicon grease they used instead of machining the transistor surface areas truly flat... All to save a buck, and they never figured anyone would be looking at it almost 40 years later going what the heck is that ?
Perry has a good answer, and I will tend to agree with him. Your shining example of this amp is pristine, and original, its value holds better as is. and if it fails then yes upgrade but as it looks in your photos I would carefully put it back together and leave it factory pretty as is....No one can say you messed with it..
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