R&S RF SMS Signal Generator relay oscillations, help needed. - Page 1
Forum section: electronics / repair

author: RichardM
author_rank: Frequent Contributor
timestamp: December 20, 2022, 10:53:07 am
content: Hi All,Probably bitten of more than I can chew trying to repair a R&S SMS Signal generator,but enjoy the hobby and challenge. The unit just displayed random flickering characters on boot and a buzzing sound (turned out to be the relay discussed below). One Oil capacitor in Power Supply had popped its top so I replaced the main capacitors and the power rails seems reasonable although I do find measuring ripple a bit of a challenge. The unit has +15, -15, +5, +20 and +28V rails and ripple is generally < 50mV peak to peak, the worst being the 5V rail which shows 350mV p-p with a 50mA load.The main issue I am having at the moment is with the relay oscillating on Motherboard 2 which is connected to the Power supply with a 24 pin DIP socket and ribbon cable. The user manual indicates the little circuit that contains a comparator circuit to create a delay. from the manual:The +15-V power supply of the attenuator is effected via a control circuit which is provided on the motherboard 2 and has the following functions: The power supply of the attenuator is switched on via the relay RLI only about 45 ms after switching on the SMS. During this time the reset process t akes place in the microprocessor. The computer outputs controlling theattenuator have not yet assumed a defined state. This switching delay prevents undefined switching states at the attenuator. The relay control is accomplished by means of a comparator (B1), the inputs of which are connected to the operating voltages of +5 V and +20 V via delay lines. When switching the SMS on C101 is quickly charged via GLI while C100 is slowlycharged via R4. Thus the voltage at the inverting input is for about 45 ms higher than that at the non-inverting, the comparator output remains at "low"and the relay unexcited.I have attached the Power Supply schematic and Motherboard 2 schematic.I have pulled all the cards from the unit to begin with as this did not make any difference. When Motherboard 2 is connected to the power supply using measuring points MP 1 - 10 on the power supply (not the rinbbon cable as it is supposed to be) the delay circuit works fine and I see C100/102 charge and when they cross and the relay turns on through the B1 Op Amp PIN6. However, if I connect Motherboard 2 to the power supply using the 24pin DIP ribbon cable interconnect and try to power the unit I get rapid oscillatins in the relay in response to a sharp drop in the +15V rail. An image of the +15V rail, B1 PINS 2,3,and 6 are also attached. Note, when I probe PIN3 with my scope it stops the oscillations hence why the image looks fine. I have replaced all the resistors and several diodes and tried another UA741 but the problem remains. A swapped the relay but that also is not the problem. Transistor also ok on curve tracer.This has me completely stumped. Is it the ribbon interconnect thats the problem ? Or some other issue on the power supply perhaps ? Any help very much appreciated. Sorry for the long post.RegardsRichard

author: khs
author_rank: Regular Contributor
timestamp: December 20, 2022, 11:16:05 am
content: I had a similar problem with my SMS2.It was a problem with the +15V power supply.After power up all relays are switching to reset the bistable relays.This requires a lot of current.The big capacitor (C8 in the SMS2) after the rectifier had lost it's capacity.After replacing it all works fine.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 20, 2022, 12:17:10 pm
content: ThanksI have replaced C8 with a brand new capacitor already ? Also C1 22,000uf and two other 4700uf capacitors.Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: December 22, 2022, 12:31:21 pm
content: Solved. I am another victim of those bad TI dip sockets. I think I may replace them all or at least re-seat all the affected ICs. Unfortunately the signal generator just shows random characters so my repair saga continues.