SRS Stanford Research DG535  - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: September 28, 2025, 01:46:29 am
content: I picked up this DG535 for a few bucks and it was listed properly as being "For parts or repair."The system has been very unstable and getting worse. Constant CPU resets which appear to be related to high power draw in the -8vdc rail.I have the schematics and I've made some progress but testing was near impossible with the resets.So, I split the power top board and bottom and powered the bottom output board with bench supplies. That let me run the top (control board) and it is now stable. I'm pleased that I have been able to test all outputs. Only output B fails.I need to walk that circuit now and identify the root cause and likely the issue with power draw and instability will be fixed with that.I've read that the most common failure on these is the custom output stage transistor MRF965. SRS has a replacement 534 ASSY.I have seen what I think are these assemblies on photos I've found.I'm about to head back to travel so I won't be able to do the root cause for a week or two.Anyone know what the transistor is that's used in the 534 assy? I can buy one from SRS for 70 USD but that's a big purchase for what I have invested. I've attached a photo in case someone might recognize this transistor by markings.

author: fenugrec
author_rank: Frequent Contributor
timestamp: September 28, 2025, 01:32:23 pm
content: I don't know anything about this machine, but I see they use a modern SOT-89 part on an adapter PCB, which means (probably) whatever specs the MRF965 has, were not so critical as to be irreplaceable. Maybe SRS does some binning / matching as well.I would search for sot-89 RF transistors with similar specs on distributor websites (or directly mfg's like NXP etc), or maybe start with the NEC parts listed herehttps://smd.yooneed.one/code5153.htmlalthough I find NEC parts are less common in non-japanese designs and would not be my first guess here.Also consider searching for replacements for the more-popular BFR96, similar enough to be described in the same datasheet as the MRF965 (I'm guessing identical die, but different packaging affects thermals a lot, and the HF characteristics a bit).

author: Cyclotron
author_rank: Unknown
timestamp: September 28, 2025, 02:19:12 pm
content: Quote from: fenugrec on September 28, 2025, 01:32:23 pmPerfect! Thank you. I misidentified that package as SOT-223. The more I research it, the more confident I am that a modern replacement will be easy.I just need to spin up a board to create the adapters. I was going to order one of the SRS assemblies, but I can't find it for sale anywhere.

author: fenugrec
author_rank: Frequent Contributor
timestamp: September 28, 2025, 03:30:39 pm
content: Also, to state the obvious... have you tried asking SRS directly what transistor they use ? worth an email IMO

author: Cyclotron
author_rank: Unknown
timestamp: September 28, 2025, 04:17:58 pm
content: Quote from: fenugrec on September 28, 2025, 03:30:39 pmThanks! For whatever reason, I hadn't considered contacting them. I've done so with other companies on other projects so I'm not sure why I didn't think of that.Thanks again!

author: Cyclotron
author_rank: Unknown
timestamp: September 28, 2025, 11:52:29 pm
content: I've sent SRS an email to see if they will tell me what they use.Based on the clues I can find and the help of@fenugrecon the proper package in that image. I believe the transistor is a 2SC4536The markings and package matchNEC 2SC4536I will buy a few and some SOT-89 breakout boards.Edit:Maybe the NEC 2SC5336 would be a better fit.

author: Cyclotron
author_rank: Unknown
timestamp: September 29, 2025, 06:54:54 pm
content: I sent an email to SRS and asked if they could tell me what transistor is used on the 534 ASSY. I provided thelink to their PDFI got an email back from SRS:QuoteI have found the way these are purchased on their site so I'll buy one. I can then run some tests on it to see how it performs and match it. I'd like to replace them all with the same output transistor but with 14 of them that would be 700 USD buying the assembly from SRS. Not in my budget.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 29, 2025, 08:19:16 pm
content: mrf 965 infos / specshttps://www.littlediode.com/components/MRF965.html