I was curious about troubleshooting the noise people report though, when I power my scope up it's always made this "screech" or "chirp" sound.
Hi Per,
I think with all your experiments, it deserves a topic of its own... it's starting to get messy/confusing with both our 2215's being fixed in the same topic.. people will get lost and confused. Getting hard to follow I think... Space is cheap on the forum, just create a topic for your scope and everything will be easier for every one to follow and help / comment on
As for the SMPS, the theory of operation is described in the service manual, have you read it ? Most of what you say is already explained there, and your remaining question are answered there as well. Please take the time to read it.
But in short :
But, and this is the part I don't understand: There is a resistor called R914 connected before both Q915 and Q917.
As the manual says, this resistor as you can see connects the output of that 15V PWM regulated supply circuitry, to the input. It's just a little bit of positive feedback to help lock it sharply/quickly.
So does that not mean even if Q915 is not conducting U920 can still be on via that resistor?
Well no, look : the left leg of the resistor is connected to the base of Q917, which is regulated at 6,0V via the zener. So 6,9V is not quite enough to power the PWM chip, it's only half the voltage. Plus, the resistor is high value, 100K, so only 60+ uA can flow thourgh it at best... not quite enough to power the chip either.
Because this is precisely what I'm seeing: I have ca 15.5v on the VCC pin of U920, and the preregulator is running (43v present on the output).
You said it, once the regulator is running, power comes from the 43V outpuit, via CR913 which you were wondering about ;-)
But this part is also confusing for me: CR913 will try to fight the power coming in the other way, via R911?
It's not fighting anything ;-)
It's just there to power the PWM chip once the regulator/PWM chip has started. If the diode weren't there, the juice the start-up circuitry is trying to produce to feed to the PWM to get it started, would go straight to the regulator output/scope which would draw all the very modest current that's available during startup (remember it all goes through the beefy 150K resistor, so not much current available), and the PWM chip would probably never manage to start...
Well that's my understanding anyway.
During this time, if I try to measure the frequency with a DMM, it tells me it's around 32khz, when the noise disappears it's stable at 38khz.
Yes as the manual states, the regulator frequency is around 40kHz, so 38kHz sounds about right...
In my 2232 for example (exact same pre-regulator design), the PWM runs a bit faster, at 60kHz. The inverter following it though, runs at 20kHz, same as 2215.
I figured that must mean that either C920 or R920 is bad, but I desoldered them and they measured fine.
I replaced them anyway, but it made no difference what so ever...
Probably just warm up related... I wouldn't worry about it ! You say the noise stops in just a few seconds from what I understand ? That's perfectly acceptable or any SMPS I think... when we refer to the "whining" 22XX scopes, we mean a very, very high pitch, really loud, and which last a lonnnnng time, like 5 or 15 minutes, or sometimes never goes away at all !
But Hakan on Tekscope (same chap you got the transformer rework procedure from) gave us an old Tektronix internal "paper" of the era, where they discuss precisely the whining 22XX issue. So even back then they the issue ! Tek wrote that it's a combination of things, but mostly a frequency tuning problem, so they advice to slightly tune the regulator frequency to put get rid of any resonance issue.
The thing that makes me think I'm not crazy is that if U920 really starts up before Q915 has conducted,
It does not, read SM again
Because that's also part of what Q915 does: it switches in the stable VCC provided by capacitor C913, it's the whole reason for it's existence...
No C913 does not "stabilize" anything. It's Q917/Q915/ Zener that regulate the 15V supply for the PWM chip. First this circuitry is powered at startup by the voltage building up across C913 via that big 150K resistor connected to the rectified mains, but then as soon as the PWM starts to regulate, power comes from the 43V regulator output, via that CR913 diode you talked about.
Anyway, please start your own topic if you wish to discuss this further, it's getting messy around here
You can just copy/paste your prior messages there so people know what you already have done so far. Give us the link to your topic once created so we can follow you there, if you have more questions or what not
As for me, back to my repair... making slow progress but making progress. Studied the schematics for hours, as well the theory of operation, several times.. with a few weeks in between each "round", so as to let the knowledge slowly seep into my tiny brain. I think I got the hang of it now, I am ready to do some probing around and produce some trouble shooting data, at last....
Hoping to start poking around and report this week.. holiday is over so time is become scarce again...