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The 2250B popping on power down fixit thread!

rBuckner

rBuckner

Luv 2 Restore
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Hi Everyone!

I thought I'd have some fun with this one. This is a fully recapped unit (by me) that adjusted fine and sounds wonderful. Plugging in short RCA cables into the Main In jacks kills the pop. The popping has been verified to be on both channels and looks exactly the same. Sounds like the preamp side, right? I'm also thinking it may be the speaker relay not going open fast enough as a possibility.

Could I have made a mistake or solder short? Sure! Backwards cap? Sure! Below is a scope picture, top yellow trace being the preamp output and the bottom green one being a speaker terminal signal. Follow along and I'll post things as I look into this.
DSCN4728.JPG
 
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Some diagnostics done.
DSCN4730.JPG
Couple of changes. The scope is now connected with channel 1 on top being a speaker terminal and trigger signal. The other trace is the other speaker terminal to verify that the waveform is very similar. Also of note is that I disconnected the speakers which got rid of the big ringing from the inductive "kick" that the speaker coils caused. The noise is concurrent with the fast rising part of the waveform as it returns to ground level.

DSCN4730.JPG DSCN4731.JPG
Here's a longer time picture with the green trace being the positive side of the speaker protection relay coil. The source is a raw dc supply that is dropped by a resistor to get the 24 volts across the coil. Looking at the other end of the coil didn't show anything worth posting. You can see the voltage come up to about 48 volts and then after the timeout where the relay kicks on it drops to a bit under 30 and gets "wide". I then turned it off and you can see the voltage come down rapidly and is only a couple of volts when the "pop" is on the yellow trace. So it looks like the relay control circuitry isn't holding the relay on too long.

DSCN4732.JPG

Here the same waveforms have been zoomed in on at the popping moment to get a better look at the relay "minus" side. This confirms the voltage was below the dropout voltage of these relays, about 15 volts IIRC. I do have the very similar 2235B on hand, also restored that does not pop and has the same power supply / speaker relay board. I may just hook that up for a look.
 
I didn't like how the second picture back's green waveform looked - like it was oscillating. That and the dc level is high, it should have been down around 1.2-1.4 volts. Suspecting the two driver transistors I pulled them, 2SC1318's and measured the gain with both being around 120. As they are "R" parts that's at the bottom end of the range. I replaced them with KSC2383's with gains around 160. The waveform cleaned up but it was still popping.

By running a 1kHz sine wave I can see that the audio cuts out before the largest of the spikes. But not before a negative excursion so that was what was audible. Of course it's coming from the preamp silly Randy! I knew that by separating the two earlier. I was working hungry which I guess had my brain confused. I just got back to it and thought I'd test all of the modes and it pops on all of them, even in the "off" position. If this board is like the 2325 one, there's a buffer at the output end of this board. Both channels pop. Time to look at the schematic and see what's in that buffer and what is common between the two.
 
Wellllll…. The tone control board, essentially the preamp in the 2250B is the same as the one in the 2275. I pulled a tone control board from a 2275 and it pops too!! Is it just the design then? I'm going to post the question separately but if anyone reading this within a day or two has a working 50B or 75, could you chime in as to any pop upon powering off? Thanks!!
 
This wasn't supposed to be such a hassle! Still haven't figured out the reason. I did have to quit before dinner last night and couldn't get back to it until 10PM. I was up until after 12 putzing around with it. I tried shorting the inputs right at the preamp board and it still popped. Just now I drove the main input jacks from a sig gen and powered down and no popping. So it definitely is the little glitch I see from the preamp. That glitch is at the output of the BA312 but not at the input. Hmm. I think I'll try a 47uF cap at the power pin of the BA312 to "hold" the power on, hopefully delaying the glitch until the relay contacts have opened. Also thought of putting a diode in across the timing cap on the power supply to make the relay shut off faster.

Right now I have to prepare to meet with my accountant this afternoon to straighten out the QuickBooks debacle I'm in. Had to switch from the more friendly but lamer Online version to the "real" desktop program which is quite different. So gotta study how the new to me version works in preparation. Fun! :no:
 
Ha! We used the desktop version of Quickbooks for years in our dental practice. Intuit was constantly on us trying to get us to move over the web based version. We never did, and I'm glad we stayed with the in-house version. But my partner and I sold our practice about a year ago to a younger dentist because my partner wanted to retire and I knew I was a couple of years away from that. So I'm working back for the new owner on a contract basis and have zero responsibilities with the business side. I'm loving just dealing with dentistry and not having to bother with running a business, :D
 
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Okay, I had more time to fool with this. Hearing from good guys "thxdave" and Bob Speece that their 2275's didn't pop made me see what all 2275's I had around. It turns out two more! I tried the first of them and got the pop on power down. To be clear it's not real loud but somewhat at an annoying level. Unit number two did NOT pop! So I pulled that tone amp board and cleaned it up for a stare and compare session. I had the subject 2250B back together so looked at the before and after photos. I did spot a difference with that being the value of RE17 and 18. In the quiet board they are 5.1K and in the popper are 8.2K. So I have an experiment to do!
 
Experiment done. Still popping! While I was in there thought I'd try some other things. Changed the resistor more radically to 1K, pop! Lifted the ends of the resistors so no feedback network (with a series wired 10uF cap. The outputs were motorboating and it gave me quite a thump on power off. So I restored the 5.1K resistors and tried upping the 10uF cap to 100uF with no difference. Added 470uF more and still no difference. So I surmise that it may be that some BA312 ICs just do this and some don't. One way to tell would be to swap them between my non-thumper and a thumper unit. Sorry to say that's not going to happen for awhile. I also made some adapter boards to convert the single inline packaged BA312 IC to a modern 8 pin surface mount outline so any modern op-amp IC can be used but never tried them out. So that's another thing for later on.

Bummer I couldn't resolve this now!
 
Actually a discussion on cables was being considered...
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NAHHHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! :no::no::no: :p
 
To be more serious I'd love to try one out for this issue. In general the consensus is that the BA312 doesn't go bad often. I've robbed two units of theirs to make repairs and both were required. One was dead and the other as crackly as a pork rind.

I'm gonna dig out my adapter board and see if I have the pins needed to make one up. I know I have some quality SO-8 SMD package op amps in the LME sumpin' or another series. Danged if I could just find them in a 5 minute quick search!
 
Interesting problem... If the BA312 was the culprit would it be likely that they both fail in the exact same way with the exact same signature? What happens when you scope the power and ground to the tone amp board at power down?
 
One other idea.. If you take your Variac and rapidly set the output to zero from normal line voltage what happens?
 
Thanks for weighing in 'tx'. I hope ole Turbo is doing well these days!

I did look at the power supplies at the power supply (not at the board) upon power up and down and compared them to the one in the 2235B as it is exactly the same. I did measure ground to be very low ohm-wise. The waveforms were identical. I'll do the variac test as described when I get a chance and report back. I did switch it off with an external switch which did not help.

I did find the bag of op amp chips (LME49870) and looked at the adapter board I had made. It was for the TA7136 chip which is the same 7 pin SIP package but not the same function at the pins. I can mod the board and see if my idea works. Don't know when that will happen!
 
rBuckner said:
Experiment done. Still popping! While I was in there thought I'd try some other things. Changed the resistor more radically to 1K, pop! Lifted the ends of the resistors so no feedback network (with a series wired 10uF cap. The outputs were motorboating and it gave me quite a thump on power off. So I restored the 5.1K resistors and tried upping the 10uF cap to 100uF with no difference. Added 470uF more and still no difference. So I surmise that it may be that some BA312 ICs just do this and some don't. One way to tell would be to swap them between my non-thumper and a thumper unit. Sorry to say that's not going to happen for awhile. I also made some adapter boards to convert the single inline packaged BA312 IC to a modern 8 pin surface mount outline so any modern op-amp IC can be used but never tried them out. So that's another thing for later on.

Bummer I couldn't resolve this now!
Click to expand...
Did you ever fix your popping on power down? I'm having that issue on my 2275. Thanks!
 
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