Repair Question on SAE 2300 amplifier

The attached picture is an excerpt of the best schematic I have for this amp.

The amp is functional, fully working. I've re-capped it as well as replacing most of transistors on the driver board. I matched up pairs of KSA992 and KSC1845 to replace the dual transistor inputs, trying to get the DC offset lower. I've brought it down from high 60's mV both channels to 15mV and 24mV. Can anyone tell me what R28 does? It's circled in red on the picture. The boards had different values R vs. L., and I'm thinking it may have an effect on the offset.

Thanks!
Jason

Interesting, the resistor is marked SEL, meaning select I guess. It possibly is as you think, to adjust the offset. Hopefully someone who can read the schematic will respond.

I have a 330K in that position on one channel and a 470K on the other side. This amp has been worked on before and the work that was done doesn't look very good, so I was worried at first that someone had used the wrong value on one side or the other. Somewhat relieved to see that it's probably an adjustment.

If you have some resistors in those ranges laying around, it might be informative to try different values (not huge jumps, try next value up/down)
That said, your DC offset isn't bad. It might be best to leave well enough alone.

Those are for offset. If I get a good matched quad on the diff pairs, and its recapped, those are removed and offset improves.

The "sel" resistors are to set the quiescent current of the next stage, you measure across the 62 Ohm resistors, R17 and 18 to get the proper voltage. But what is the proper voltage? It's 1.6-1.7VDC on the 2400 and looks to be 1VDC on the 2300 schematic or that's what someone penciled in on the schematic.

Craig

Thanks for the replies. I experimented with those resistors: First I measured the voltage drop across those 62 Ohm resistors, R16 and R17. On the channel with the 470K resistor in position R28, there was only a drop of about .6Volts across the 62 Ohm. I stuck a 510K in R28 and checked again, and saw that it increased the voltage drop across the resistor, and therefore increased the current in the following stage. I settled on a 680K which put the voltage drop at 1.010 Volts. The other channel I couldn't get quite as close, but I ended up with a 510K in R28 and a voltage drop of .915 V across R17. The good news is that after doing that and readjusting the bias (changing those resistors did affect it) The offset came down to 9mV in one channel and 16mV in the other.

Not content to leave well enough alone I pulled the driver boards back out and matched up some resistors that I had already replaced with 1% metal films, for a closer match. I was able to get the offset down to 4mV and 6mV! I've nearly rebuilt this whole amp, and I think I'm about done with it!

Thanks for the help!
Jason

Congrats! May it serve you well for years to come.

You might want to revert back to the original resistor values.

1v / 62 ohms = 16.2ma current in the VAS transistor
16.2mv * 66v = 1.07 watts

The VAS transistor (2sa913/2sc1913?) probably only has a 1.5w to 2w rating at 25c. You might be pushing the limits of reliability of that part.

"Sel " resistors are Selected to adjust the idle current. I wouldn't change them without a service manual or reliable data about what current you need to set.

The target value should be detailed in the manual or schematic. It will tell you something like "measure xx mv +- X mv across xx resistor. "

I wouldn't suppose 0.6 V across R17 is a wrong value. Perhaps that's the proper value.

The instructions I gave are right out of the SAE 2400 service manual 1.6-1.7VDC and according to the 2300 schematic it should be 1.0VDC. It's a TO-202 type transistor and should handle some heat. I use a decade resistor when adjusting with fixed resistors and dial it in to the closest 1% value.

Craig

So, If the manual says "1v" across that resistor, you did right to change the resistors to get that value.

If the VAS transistors are heatsinked (I cannot remember if they are on the 2300) then things are fine. I don't remember them being heatsinked though.