Jailtime
Standin' on a corner
I'm working on a 2330 that had been previously hacked up. As a baseline, it did work and had acceptable DC offset after I repaired the preamp that was missing parts.
I rebuilt the amplifiers and part of the power supply today. Previous tech had installed MJE15030 transistors, a new relay, and some of the caps on the power supply. I finished that up, and put new caps, trimmers, diff pairs, and Q703/724/725 in the amplifiers. I used KSC1845 for diff pairs/Q703 (2SC1775), and KSC945C/KSA733C for Q724/725 (2SC945/2SA733).
I ran it up on a DBT (100W bulb) to get a ballpark on bias and offset, I was able to tweak bias in at 10mV, low but I like to be safe on the DBT. It did take a while to come out of protect, DC offset was 110mV on each channel. Tried to tweak it on the left, it would come down for a few seconds, then rise back up to 110mV (best I could do). Tried the right, same results, but it was a bit more unstable and tripped the relay. It hasn't come out of protect since.
Now both amps are showing around that 110mV offset on the pins before the relay, and both have bias of 37mV (!) which will not adjust, still on the DBT. They also pull down the 35V from the power supply, one amp will pull it down to about 26, both pull it down to 18. The power supply makes a stable 35V with the amps unplugged.
I rebuilt the amplifiers and part of the power supply today. Previous tech had installed MJE15030 transistors, a new relay, and some of the caps on the power supply. I finished that up, and put new caps, trimmers, diff pairs, and Q703/724/725 in the amplifiers. I used KSC1845 for diff pairs/Q703 (2SC1775), and KSC945C/KSA733C for Q724/725 (2SC945/2SA733).
I ran it up on a DBT (100W bulb) to get a ballpark on bias and offset, I was able to tweak bias in at 10mV, low but I like to be safe on the DBT. It did take a while to come out of protect, DC offset was 110mV on each channel. Tried to tweak it on the left, it would come down for a few seconds, then rise back up to 110mV (best I could do). Tried the right, same results, but it was a bit more unstable and tripped the relay. It hasn't come out of protect since.
Now both amps are showing around that 110mV offset on the pins before the relay, and both have bias of 37mV (!) which will not adjust, still on the DBT. They also pull down the 35V from the power supply, one amp will pull it down to about 26, both pull it down to 18. The power supply makes a stable 35V with the amps unplugged.
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