3000A modification/service bulletin voltage, bias and offset questions

[jfabes]

Hi all. Just completed the mod for a 3000A with a kit from AfA on ebay. No issues installing everything, directions were very good but not understanding how to adjust the bias to get 0V offset, without affecting the bias, without affecting the offset. The directions just do not make sense to me.

Currently, with the mod and all new mj21196 outputs (one channel was shorted), the bias set no problem at 80ma across the fuse holders doing the + and - simultaneously for each channel (btw, i also checked mv across the emitter resistors and all are 24-29 mv)...the trimmers look to be about the same position since when you raise one, the other raises, lower one, the other lowers, etc. so it seems that you could almost not even adjust one of them and still get 80ma so that's confusion point #1 for me. #2 is how do i adjust both at the same time while still getting 0V across the + rail emitter resistor?

My output voltages currently are:
L+ --> B,C,E: 2.1, 37, 1.5
L- --> B,C,E: -36.7, 1.5, -37

R+ --> B,C,E: -1.7, 37, -2.3
R- --> B,C,E: -36.5, -2.3, -37

The red emitter voltages after the new cluster caps is +1.4v for both channels, then with the 10 ohm resistor (or 8 ohm speakers), drops to 0V like every other cap coupled receiver i have worked on. So can someone do a sanity check for me here or advise what i need to do to get the red values to 0V or as close to 0 as possible, if possible? I have triple checked everything and would expect much different results if i had a bad solder joint or component installed incorrectly. it seems it all starts at the 25V of the smaller transformer primary, the two channels are about 1.5V difference. nothing is getting overly hot or anything, and unit sounds fine.

any help or guidance is appreciated. thanks.

 

[Sam Cogley]

Did you read the old service bulletin?

 

[jfabes]

multiple times. my wiring is correct and everything i have read said not to follow the bias setting procedure so i've been trying to close gaps on a bunch of threads that really just do not explain what to do. "let me look at my notes" or "i think you have to do this" and give zero detail or just never come back to the thread

 

[jfabes]

update. since no one seems to recommend checking across the fuses, i tried it across the emitter resistors while also checking the output before the np caps. i got both channels to <100mv and all 4 emitter resistors are 22-24 mv, *EDIT that should equate to 70-80ma based on the emitter resistor readings i got yesterday when i set the 80ma across the fuses but what does not make sense is that these are 1 ohm resistors not the original 0.3 ohms so that should be 80mv across the resistors, which would skyrocket the bias. so i'm not sure how 80ma across the fuses is only 25mv across the emitter resistors. perhaps this is why you are supposed check bias across the fuses.

all that said, i still have 1.4v at the speaker terminals and here's why. it's less than 100mv into the np caps, then comes off the other side of the caps to the impedance switch. on the same terminal of the impedance switch is a wire going to 8N on the driver board, so that is what is supplying the 1.4v at the speaker outs, which go to 0V when i attach a 10 ohm resistor. see yellow line on schematic. I'm not understanding the 1.4v from 8N, and does it matter?

 

[Sam Cogley]

It’s been a LONG time since I’ve messed with this on my 3000A, but I don’t recall running into these issues.

Next question: have you checked the service manual to see if it has any troubleshooting suggestions for this issue? I’m guessing you have, but just checking. This feels like a problem with bad transistors or something in the bias circuitry, but I’ll be the first to admit that I’m much better at troubleshooting hands-on than trying to think things through at a distance. The 3000A is pushing 55 or so years old at this point, and it’s known for having some random semiconductor failures.

 

[jfabes]

i checked the manual, nothing that i could see. i would say that i'm happy with the bias and offset, just do not understand the 1.4v from the emitter of tr801, and i do have the 4.5V where it is marked in yellow, then 3.7 at the emitter, then drops to 1.3-1.4 through r813 to the impedance switch

 

[Sam Cogley]

It's worth noting that there is a serious error in the Figure 4 schematic of the 1971 service bulletin. The description is correct under the second instruction on Page 3, (adding four 10 ohm 1/2 W resistors), but the schematic shows adding two 10 ohm and two 0.3 ohm resistors.

Also, if I'm reading your description correctly, I'm not sure that you're following the adjustment procedure outlined in the bulletin. It's rather counterintuitive. You're supposed to turn the volume all the way down, place 10 ohm 1W resistors across the L and R speaker terminals, and pull all of the output fuses. Then connect the ammeter to fuse holder 1 with a fuse installed in fuse holder 2, and adjust VR801. Then swap the ammeter and fuse, and adjust VR802. Then install a fuse in fuse holder 4 with an ammeter on fuse holder 3, and adjust VR803. Then swap again and adjust VR804.



If you're getting 1.4V to the speaker output terminals and it's not tripping the protection circuit, you need to adjust it as outlined in the service manual. That's the threshold for the relay. And if you are getting that kind of voltage to the output terminals, I'm still pretty sure one or more of those transistors or diodes on the TRD-6B or TRZ-2C boards is blown and/or leaking, possibly something further up the line. You said that you had a shorted channel, I'd be looking for upstream damage.