B52 AT212 gain switch not working

[Jon Anderson]

I'm working on a B52 AT212. The gain switch in the overdrive channel is not working. It is stuck in gain 1. Engaging the switch doesn't activate the gain pot, or the volume pot for gain2. It does activate the contour for gain .

Based on the attached schematic, I'm thinking it's a problem with the CTL2 voltage that controls RLY2 and RLY6. When the gain switch is open, it measures about 11V. When the switch is engaged, it drops to maybe ten volts, so I don't think any current is going through the relay to flip it.

So I measured at either side of R91 that feeds the Q5 transistor. The base voltage is always very close to zero, even when the pin 4 of IC11B is at 9V.

I thinking that either Q5 or Q6 is burnt. Does that sound right? Do you guys see anytjing else I should check? I'm trying to debug as much as possible without taking the board out. There are a lot of jumpers and knobs on this amp.


B52 AT100.pdf

[MarkusBass]

Yes, it sounds right. However, there is always a chance that, for example, R91 is open, or there is a broken track, or there a cold solder join. Since you have multimeter, you can measure Q5 and Q6 and compare results with Q7, Q8 (which work correctly). You can also measure voltage on R92 (from Q5 emitter to ground). It's funny that similar resistor in Q7 emitter is also marked R92. This may be confusing.
You can conduct several even simpler tests:
- short Q5 collector to ground (with a wire). Does it cause relay click?
- connect base of Q5 with 100k resistor directly to +12V rail. Does it change anything?
For me it seems that Q6 is open but you may learn more but doing measurements mentioned above.

Mark

[Jon Anderson]

Awesome! Thanks for the suggestions. I'll give them a try.

[Jon Anderson]

I tried grounding the collector of Q5, and that did trigger the relay and switch to gain2. I also double checked the voltage going to the emitter of Q5. With the switch engaged, it was ~9V, where as Q7 was at 1.5v. I think you are right about Q6 being open.

I don't have a 2n5551 on hand, can I use a FET transistor? I've got a couple of 2N5457, MPF102, or J201. Would they work?

[olddawg]

Take it out and check it with the diode function on your meter. It's a general purpose NPN bipolar transistor. Don't use an FET.

[Enzo]

General purpose NPN. These are 160v, which is way overkill. I have a huge stash of MPSA06,, wwhich are 80v 500ma parts, and I would use them here, but even that is overkill. This is only a 1v circuit. The current rating is 600ma. Another go-to part in my drawer is the 2N4401, a 40v 600ma part. Your JFETs won't work. Most any NPN of 500ma or more and 30v or more will work. it is after all just a switch.

If I felt real creative, I might replace that pair of transistors with a single darlington like MPSA13. There are lots of good numbers, I just mentioned ones I stock.

[Jon Anderson]

I got the transistors out. Q5 measured ok with my DMM diode check, but Q6 measured open from emitter to base.

Since I had to order anyway, i've got some replacement 2n5551 and a few others. Thanks for all your help.

By the way, why are there two transistors paired here, anyway? How do they work together?

[Enzo]

Look up "darlington pair" or "darlington transistor". The combination increases current gain substantially, allowing the small current drive from the CMOS logic IC to control the current for the relays.

This is a darlington made from two discrete transistors. They make parts that have both transistors inside, like that MPSA13 or the common TIP142 we use in some power amps.

[MarkusBass]

Q6 drives inductive load (two relays), which may be a reason why it failed and why it is high voltage transistor. I would check protection circuit for each of the relays (diode and capacitor). If any of the components failed, the transistor gets very high voltage spike when the relay is turned off.

Mark

[Jon Anderson]

Tested the caps and diodes, got the replacement 2N5551 in (I replaced both, just in case), the gain channel switching is now working correctly. Thanks for all your help!