Rohde & Schwarz - CMS50 Errors and DSP Timeout (Similar to CMS52) - Page 1
Forum section: electronics / testgear

author: TheBay
author_rank: Unknown
timestamp: June 30, 2023, 05:12:27 pm
content: I recently acquired a Rohde & Scwarz - CMS50 Radiocommunication Service Monitor, this is very similar to a CMS52.On powerup I get a message about Ram-data Invalid, I assume the nvram is battery backed up and replacing a battery will resolve this (SAFT 3V lithium Cell is 0V in the Digital Unit)Rebooting or resetting with power applied will show the Ram-Data is OK, it will stay OK for a short time after power off as some capacitors hold enough charge.Then when it goes through all checks I get errors on the following:Battery check - error 1AD-converter calibration - o.k.Peak-detector calibration - o.k.ModGen - filter calibration - error 2Modulation generator calibration - error 3Distortion-detector calibration - error 1Power-meter calibration - o.k.When it has booted up and I select certain modes or try to run a self test I constantly see DSP timeout; please <RESET>!I believe the errors are all due to the "DSP Timeout" issue.So far I have checked all the PSU rail voltages and all are in spec, I removed the "Measuring Unit" PCB which has a Motorola DSP DSP56001 IC And a Philips TDA1541A.I could see a DC-DC converter as part of the circut which generates -15VDC for the TDA1541A, so I soldered a wire to that circuit, checked it with a DMM And Scope and it's all within spec at just under -15VDC.I changed the capacitors in the DC-DC converter circuit as I could hear a high pitch hissing noise from the switching inductor, but it still behaved the same and after speaking to someone else online who has a CMS50 and a CMS52 theirs makes the same sound. So I believe that circuit is working correctly.I then removed the "Digital Unit" card as the person I spoke to with a CMS50 and CMS52 said that they have had a "DSP Timeout" issue with one of theirs and swapping that card resolved the issue for them, but they were not able to find out exactly what component was causing the issue.Again I checked that over thoroughly and tried to test all capacitors, diodes, zeners, resistors etc. And the basics all checked out fine and I could so nothing obvious. This card is heavily populated with IC's on both sides, a lot of them are socketed so removed what I could and put them back in again to make sure it wasn't a dirty contact.I soldered 2 wires to the TDA1541A (AO Left) And Pin 6 (AO Right) and put both cards back in the CMS50, looking on my scope with the probes connected to those wires I'm not getting any audio from the TDA1541A at all from the CMS50 Modgen, there is also noaudio on the Modgen output on the front of the unit (To be expected if nothing is coming out of the L/R channels of the TDA1541A)The CMS50 is generating RF and and the TX test is also showing it is receiving a transmitted signal, but I do not get any audio out on RF or AF connectors.It is quite difficult to work on as you cannot check anything with the cards in situ without extenders which I do not have, so I am constantly pulling cards out and soldering wires on to make measurements.There seems to be something wrong with communication between the "Digital Unit" and the DSP on the "Measuring Unit" I would just replace the TDA1541A as I have CD players here I can pinch one out of, but I would expect to just have no audio being generated by the Modgen and not the "DSP Timeout" error as well, which to me suggests something else is wrong in the chain. Also I believe the other person with the same fault had replaced the DSP IC and TDA1541A and it did not cure anything.I'm really hoping someone has come across this issue or someone here has experience with R&S Instruments that have a similar architecture. I would love to get this running as it looks a great bit of equipment.I have looked all over eBay for another unit to buy for parts/repair but there seems to be nothing on there and no "completed listings" either so I don't think this or the CMS52 was particularly common.Any help would be gratefully appreciated.