Electronics > Repair
Troubleshooting intermittent fault in an SRS SR530 reference oscillator PLL
DaJMasta:
I've spent a dozen or more hours exhausting my options to the best of my ability and have started resorted to pulling and replacing ICs near the circuit in question, so I figure it's probably past time to ask if anyone else has experience with this.
I'm working on an SRS SR530 lock in amplifier, the manual (including schematic and theory of operation) is here: https://www.thinksrs.com/downloads/pdfs/manuals/SR530m.pdf , and I'm working on the circuit on page 85. The symptom is that the unit shows reference unlock intermittently, but only at lower frequencies - 100 Hz or 1kHz reference input are often unlocking, but 10 kHz+ is relatively error free and 50kHz I have yet to see an unlock on. Input is being fed directly from a signal generator, generally 1Vpp HiZ, but I've tried a few combinations. Looking at the PLL waveforms on the scope, they will stay stable and locked for a bit, then will jump slightly and sort of rebound back slowly towards the correct frequency (excursions on both sides of the intended frequency) and then sometimes keep being unstable and sometimes relock. The Unlock LED on the front panel reflects this, and the other 2 PLLs in the unit which are both referenced from the comparator at the end of the loop of the first show the same kind of instability and are time correlated to this one (so I think this first PLL is the issue.)
Best I can tell, the front end for the reference input is fine, a stable square wave of the right frequency arrives at the pin 14 input of U306 (CD4046), and I've verified that the pin 3 phase comparator input receives a signal and that the ouput pin 13 is adjusting with pulse widths that correspond to the phase difference between the two inputs (and are positive or negative from the output bias point), so I think the PLL controller is working. The VCO does sometimes get the right frequency, and the the triangle wave is consistent (pin 4 U309) and speeds up and slows down as the PLL controller is adjusting the loop.
I haven't super comprehensively checked, but it doesn't appear that the external (to this page of schematic) analog switch control lines are toggling at all when the instability happens. I've checked the comparator (U314) biasing and it is stable and consistent with printed voltages. Removing said comparator results in a much faster but stable PLL frequency with the controller not trying to adjust anything. While it's a loop and I know it's tricky to point a finger at a part when everything is feeding back on itself, I feel like I've checked as many signals as I can think of and replaced half the ICs involved already, so I'm wondering where the instability is hiding or if there's some kind of trick/test I can muster to figure out what's the cause.
timeandfrequency:
Hello DaJMastaTopic,
I've no experience on the SR530, but here are a few hints.
- Check your power supplies when locked and unlocked to see if there's a difference
- reflow each solder joint on the motherboard
- Use freeze-spray to check if it is a thermal issue that triggers the unlocked status
- replace all electrolytics
- measure all resistors and diodes
- replace the most error /partial failure prone parts unconditionaly (and use top notch DIL sockets, to ease replacement) : analog switches, opamps, logic ICs (in that order).
In tedious situations where locating to root cause is really difficult (like closed-loop systems^^), a succesful repair is not only a time investement, but it has also a financial cost.
This SR530 seem to use rather vanilla parts. Replacing all of the semiconductors, buy sockets, first tier electrolytics and two freeze-spray cans should be around 150-250 bucks.
It's up to you to decide if it's a reasonable investment or not.
DaJMasta:
Well I had hoped there was an option that was less brute-forcey than what I had already started resorting to - I am not interested in a blanket recap, or individually measuring every component. While I want it fixed, I'm seeking understanding too, and a substantial expense on a pretty common not so expensive unit doesn't sound appealing.
It's not working yet despite a few more replacements and a lot more investigating. I found an LM311 in the second PLL (U329) that had no real output, because while there was signal on the input, it was more negative biased than it should have been and never reached the threshold. The circuit was partially functional, but it turned out to be one of the output buffer transistors in U322 seemed to be malfunctioning, and replacing the chip has brought the waveform back (the phase controls on the front panel now maintain the right frequency instead of the PLL freerunning into a much faster mode). Now this didn't fix it, but the red unlock light is on less often.
I poked around a bit more and started suspecting these transconductance amp chips (perhaps there are some good reasons they were EOL'd?), and I've found another which may be suspect - U307. The output shows a little activity shortly after power on and while pin 1 shows a very small pulsed signal after that, but the output is just railed negative (though it gets there gradually and shows some response to the pulses in the first couple of seconds). Trouble is, I don't have a replacement on hand, the previous was an SMD chip on a carrier with bent pins to fit in a socket, so I'll have a few days before I can try and replace it.
DaJMasta:
It took some time for USPS to deliver the replacements, but I got them in, swapped parts... and it's still broken. So I checked with the thermal camera again, no outliers (though CA3140s are power hungry!), and probed around some more. I've isolated it to the first reference PLL (not the quadrature PLL or the off-the-main board PLL) by pulling the chip and board, respectively, so I reflowed every pad in the physical area of that first PLL (two rows of ICs with the surrounding passives) - no change.
So I probed around some more, using the quadrature PLL as an apples-to-different-kind-of-apples comparison, and I narrowed things down. The fault I'm seeing looks somewhat like switching of the triangle wave (pin 4 of U309), where it will be stable and will change frequency appropriately, but when the unlock light goes off, you see a vertical artifact - it looks almost like you switch from one triangle wave to another mid waveform, but only briefly. Farther upstream, at pin 6 of U308 (Vvco), you can see normal adjustment pulses, a couple volts or so excursions from a fixed offset, but then when the unlock light is on, sometimes there are brief pulses that rail to the positive rail, a behavior that doesn't exist in the quadrature VCO. I've checked the other ends of the Vvco connection, and while the same signal is apparent on the near side of the resistor, it's not present past it, so it is being generated by this PLL. I've also replaced virtually every IC in the loop, save U306, the PLL controller, since it looked like its output is normal, but I suppose it's the next suspect even though at a glance, its output is normal and it starts looking strange at U308. U308, U307, U311, U310, U309, U314, U315, and U312 have all been socketed and replaced, testing with other pulled and new ICs along the way. The small holes on the board make IC extraction really timely, so it's been an annoying slog so far.
So I'm truly baffled. It's still intermittent and brief, so I don't know how a passive or a short could do it. It doesn't change pressing on any part or flexing the board, so I don't think it's a cracked trace. I've replaced almost every active part and checked the pins talking to other portions of the board and ruled them out. I've monitored the rails for noise, abnormality, or intermittent issues and seen nothing.
I suppose in making this post, I've just convinced myself to swap U306 because it's the last active part in the loop, but at least without time correlating inputs and outputs (I don't remember if I did this with the quadrature loop or this one in the past, but I know I've done a little), maybe there's something about the timing of its normal switching behavior that's resulting in wonky outputs of U307 which is quickly discharging the capacitors generating the triangle wave.
DaJMasta:
I replaced U306, no difference. I probed around and captured some synchronized waveforms, and this is the troublesome one I'm seeing. Blue trace is pin 3 of U306, the comparator input, pink trace is pin 1 of U301, the !Q output, and the yellow trace is pin 4 of U309, the triangle wave. This captured glitch is the issue I see, and you can see, there are two distinct slopes going on, and I'm not sure what's initially causing it. I've now replaced every IC in the loop and the issue persists.
I wonder if C310 between pins 2 and 3 of U314 could be allowing too much signal through and causing improper triggering or what, but I've checked all the resistors in the loop now and the two integration caps attached to U310 - no problems so far. It's worth mentioning that as a result of the work in the last post, the unlock errors are less frequent, but their nature is the same. It could be that I fixed something in a secondary loop or something that was slightly contributing which is now working better.
It's the wrong behavior, it's not a behavior present in the quadrature loop, and all of the faults I can think of that could cause it would have been fixed by replacing the ICs. Any guesses?
Navigation
[0] Message Index
[#] Next page
Go to full version