Electronics > Repair

Troubleshooting intermittent fault in an SRS SR530 reference oscillator PLL

<< < (5/5)

fenugrec:
Earlier you mentioned there was a strange 1V level on U305.14, on the RC input pin and I found that unusual. Have you looked more into this ? An easy test (before replacing U305 + C305) would be to clip, say, 100k in parallel with R309, see if it affects the voltage on that node and the frequency of the glitches.

timeandfrequency:

--- Quote from: DaJMasta on December 30, 2024, 05:52:58 am ---
I saw a strange waveform at pin 14 of U327 which really looked wrong, so I replaced U327.  Nothing changed, seems like the output of the LM311 is open collector, so the high impedance of the DG211 means that when the switch is switched off, there's basically no waveform on the output.  I then rotated some of the pulled chips out with the ones in circuit, the DG211s, the CD4046, and the LM13600s all got rotated to see if something would change and it didn't, so I think the ones I have all probably work normally.

--- End quote ---
IMO, the design around U328, U329 & 1/2 U327 is not that nice because of the open collector output of the comparators, as Kleinstein underscored.
SRS designers put the pull-up R366 after the switch and as consequence, the input of the lattter can be Hi-Z. All floating lines catch noise.
I would suggest to add two 22 to 47 k pull-ups (toward 5V) to the pins 7 of U328 & U329.


[ Update #1] U305 pulse duration should be 3.3 sec  (330 k & 10 µF). Pretty long for a reset pulse.

timeandfrequency:
Hello DaJMasta,

Let me first wish you a very happy new year 2025 and success in repairing the SR530.


--- Quote from: DaJMasta on December 30, 2024, 05:52:58 am ---
I haven't tried messing with the hysteresis capacitor on U315, so maybe that's still something, but since I've been able to "solve" the +5V rail noise problem with bulk capacitance and it hasn't made a difference, I don't know what help that would be.  It feels like I need to look off this page to find something, and the third PLL on the vertical board may be the leading candidate since Vvco is part of the feedback loop of the first and connects directly to it.

I say that it's the leading candidate, but I want to reiterate how little confidence I have in that, it just seems like the next most closely related thing to try after literally trying everything else I can think of.  I may be 50-60 hours into this repair at all and it is wearing on me, for sure.

--- End quote ---

As the noise seems now gone on the +5 VDC rail, are there still other signals/location where you can find a noise burst at the moment the loop unlocks ?

The VVco signal is pretty interesting : it is generated by the output of U308 pin 6.
From there, it is fed towards multiple Norton OPAMPS, most of the time via 1% resistors.
1/2 U307 pin 16  via R314 300 k 5% or  R315 680 k 5%  (selection switch is 1/2 U311)
2/2 U307 pin 1 via R322  249 k 1%
1/2 U309 pin 16 via R318 10 k 1%
2/2 U309 pin 1 via  2/2 U307 and then R323 6.8 k 1% ; there's also R324 6.8 k 1%  towards GND (*)
1/2 U1010 pin 1 via R1013 750 k 1%

If any of the sub-assembly connected to VVco alters it's value (back injection), the PLL around U306, U307, U308, U309, U310, U314, 1/2 U313 and 1/2 U312, goes bersek for sure.


(*) The reference VCO around 1/2 U309 + 2/2 U309 is directly inspired by the NS reference design at 'FIGURE 15. Triangular/Square-Wave VCO'.
The formula to calculate the output frequency is provided. But there's something weird in the SR530 schematic : if 3/4 U312 switch is open, Pin 1 of 2/2 U309 is set to GND via R323 (6.8 k) & R324 (6.8 k). So I assume that IA = 0 and Fosc tends to infinite (!?).


The fosc formula appplied to U309 also indicates that, if the rate of change of the triangle wave changes (as we see on the snapshots when the PLL unlocks), this means that C has changed (C307 & C308 connected via switch U310) or IA or IC has changed.
Hence, it would be interesting to measure the current injected into Pin 1 of 2/2 U309 (corresponds to IA) by monitoring the voltage across R323 (6.8 k) (**), at the precise moment when the PLL unlocks.


(**) To measure the current, a differential/floating mesurement is compulsory. Use 2 channels/probes of your scope and set a math trace that calculates ([CH A]-[CH B])/6800   so 1 V vertical division of the math trace should be equal to IA = 1 mA.
The A-B method is safe in this case because the whole board is powered by low value isolated DC voltages. It cannot be used on live/mains circuits.


#Update #1 & #2 : typos

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod