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2275 pops when powering on, and off

Thanks for the encouragement!

The PT01 switch assembly board is all passive components. The Low Filter and Hi Filter switches circuits are separate from the other switches on the board, except for a shared ground.

Here's the circuit, and here's what I've replaced:

The signal comes directly from the PE01 preamp board to JT01(right), JT02(left) and JT04(ground).
The signal leaves PE01 at JT05 (right), JT06 (left), and JT03 (ground)and goes directly to the Pre Out jacks.

I replaced the switch, the two .047uF caps CT01/CT02, and tested the resistors RT01/RT02.
I confirmed all the ground leads go to ground.

2275 PT01 schematic labeled.jpg

While the Low Filter switch is where the issue seems to be triggered, I don't think it is the circuit itself. There is no change when the Hi Filter switch is on or off. Suggestions welcomed.
 

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  • 2275 PT01 schematic labeled.jpg
    2275 PT01 schematic labeled.jpg
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The surge comes from the PE01 preamp board, independent of anything else.

I set up the PE01 preamp board on the bench, powered by a bench DC power supply, with no input, and a DMM on an output.
  • On powering up, there is a > +1.1VDC surge.
  • On powering down, there is a > -3.2VDC surge.
  • It is the same in both channels. But it varies .5VDC - 1VDC between tests.
This was fun to watch on my speakers, before I switched to using a meter instead.

My 'spare' PE01, for those of you following along, also surges. It is from a 2250B, not a 2275. Its +/- surges are about 1/2 the other board. Still enough for a nice pop, and of course the noisy Low Filter switch issue is still there.

My next step is finding an unmolested 2275 PE01 board for testing and to help with fixing the original board. I may have to get an entire 2275.

2275 PE01 power up reading_resize.jpg 2275 PE01 power down reading_resize.jpg
 
I can loan you one to try out and we can go from there if you'd like.(or if you'd want a complete 2275 we can go that route also)


Bob
 
I would be looking at how you are getting DC voltage into the line level/signal paths. Swapping boards around is a terrible troubleshooting method, IMO, because it is very easy to install a new fault in the form of crossed wiring or causing a non-visible break inside the insulation of a fragile single conductor wire. IMO, you should be looking for a shorted/leaky capacitor, a solder bridge, or maybe an open ground. I am assuming that you have disconnected the preamp section from the amp and the problem went away.
Anecdote, a customer brought me a Sansui CA-3000 preamp that two or three other techs couldn't fix. It would send a huge 'pop' every time just about any switch was manipulated. I pulled my (what little remains, lol) hair out for a while on that one. Only upon very close inspection did I find where some previous person working on it had "installed" a solder bridge that was sending +B into the line level.
 
tarior said:
I would be looking at how you are getting DC voltage into the line level/signal paths.
Click to expand...
This makes sense. Now that I have the preamplifier on the bench, completely isolated from the receiver, I can start to trace the signal through the board.
 
I bought a 2275 so that I would have an untouched PE01 tone amplifier board to try, and to test against. When it arrived I eagerly installed it in the 2275 I've been working on. No pops!

Just for fun I put the original tone board back in. No pops!

Go figure.

I tested it constantly for a week after that with the original board and never got a pop out of it. I don't know what changed during the week it sat before the new 2275 arrived. It's been back in the hands of the owner for a couple of weeks now with no apparent issues.

Thank you to everyone that helped on this.

-Randy
 
In dental school, we called that “bench curing”. If you’re doing some lab work on something, and it just isn’t working, you put it on the workbench, tell you buddies you’re going to let it bench cure a while, and walk away. You hope that when you get back, things will be better. It’s mostly a joke, but everyone tried it a time or two. And once in a blue moon it worked — just like it did for you. :D
 
I ran into a similar issue awhile back in a 2325 and traced it to the BA312 chip itself but can't recall the details. That wouldn't cure itself though! Another tone control board in a 2325 had the same issue and one in a 2275 did not. I did not swap BA312s around though to confirm. I planned on making my own BA312 to modern 8 pin adapter vs. the discrete one on the market but didn't get to it.
 
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