SONY SMPS from BetaMax - Page 1
Forum section: electronics / repair

author: lucatz
author_rank: Contributor
timestamp: September 02, 2025, 12:54:47 pm
content: Hello all,Looking for some guidance on repairing a Sony SMPS from a Zenith BetaMax machine. Issues are on the Primary side of the SMPS, I have so far found to have a few components that are bad.NEC C2752 (Cracked Case)Zener Diode (SHORT)Glass Diode with no blue strip (SHORT) other one tests 0.6v(D602)Resistor (OPEN) off Pic (GRAY RED ORANGE) Gold 82K 5% ?Have tested in circuit Caps (Nippon Chems)and all seem fine. The Main Transistors are also ok I think, they are 2SC2769 and test as follows.RED on B, black on C =0.52 vRED on B, black on E =0.52 vIn the image the transistor with no markings is a D774, it was flipped on the picture. So from the Image the components that have shorted or failed are Q603, D603, D604 and resistor.Anything else I should test before going to digikey. Anything special about the Zener i should look for.Thaks all

author: coromonadalix
author_rank: Super Contributor
timestamp: September 02, 2025, 01:18:41 pm
content: change c610 and c612

author: lucatz
author_rank: Contributor
timestamp: September 02, 2025, 02:25:28 pm
content: Thank you and yes i will add to the list.The NEC C2752 is a 2SC2752 I think and has a Vbe(SAT) of 1.2v so should the Zener D603 which connects to Q603 Base be something little greater than 1.2V, maybe something like 1.4v?Thanks

author: lucatz
author_rank: Contributor
timestamp: September 03, 2025, 12:47:20 pm
content: Found a suitable replacement for the 2SC2752, it will be the 2SC4346.

author: lucatz
author_rank: Contributor
timestamp: September 03, 2025, 04:03:47 pm
content: Well my search for a schematic to help with diode selection has failed. D604 i may be able to pull another diode off the board to find its correct value but D603 is alone and has no markings whatsoever. I was able to find a service manual for a SONY SL-F1/SL-TT which would be same as Zenith VR9800/VRT9850 but unfortunately the PS in the sony is a newer revision and is completely different. My version is a SONY SR12A and the one I found is SR12E.Any pointers on making educated guess about what type and value diode D603 is, from the PIC it would be the one with no markings at all. The other i could maybe pull one from the board and test it out.Thanks

author: coromonadalix
author_rank: Super Contributor
timestamp: September 03, 2025, 05:47:47 pm
content: can you give more details on machine model(s) and the sony smps board number ...

author: fzabkar
author_rank: Super Contributor
timestamp: September 03, 2025, 05:51:58 pm
content: Is there a capacitor in parallel with the Zener? Its voltage rating would be the upper limit for the Zener voltage.

author: lucatz
author_rank: Contributor
timestamp: September 03, 2025, 06:26:36 pm
content: Quote from: coromonadalix on September 03, 2025, 05:47:47 pmThe Model is Zenith VRT9850 aka Sony SL-TT. The PS in the Zenith is SR 12A and I also see 1-606-315-21 and YKC437V-O or 0, not sure.I'll add that I found a Service Manual for the SL-TT but the PS in there is a SR 12E and is completely different.Quote from: fzabkar on September 03, 2025, 05:51:58 pmI do not have a CAP in parallel

author: lucatz
author_rank: Contributor
timestamp: September 04, 2025, 07:31:44 pm
content: Is my research correct...IF the VBE of NPN BJT is 1.0 and max is 1.5, since ZENER Diode is connected directly to Base of said transistor would it not be a Zener with a 1.2 volt value?

author: amyk
author_rank: Super Contributor
timestamp: September 05, 2025, 12:41:06 am
content: What output voltages and control signals does it have? If there are too many failed components, it may be easier to fit a new PSU with the same or better specs.

author: lucatz
author_rank: Contributor
timestamp: September 05, 2025, 12:53:01 pm
content: Quote from: amyk on September 05, 2025, 12:41:06 amThis is not a simple 5 v or 12v PS, dont think this is possible. I just need to bounce ideas with someone that is better at electronics tham me. I mean i can do the research but the lack of experience shows

author: coromonadalix
author_rank: Super Contributor
timestamp: September 05, 2025, 12:58:11 pm
content: without a proper service manual all of this is guesstimate ...i searched a lot and found nothing, even old zenith services manuals on archive showed nothingbest luck is snooping on ebay if you can catch one, there is a few betamax on sale right now ?

author: lucatz
author_rank: Contributor
timestamp: September 05, 2025, 05:31:49 pm
content: Quote from: coromonadalix on September 05, 2025, 12:58:11 pmAgreed, I figured someone with better electronics knowledge would be able to decipher a diode value from the layout...

author: coromonadalix
author_rank: Super Contributor
timestamp: September 08, 2025, 02:32:03 pm
content: sadly as other wrote you can guess things with parts around it, but it could blow again or create other problems

author: amyk
author_rank: Super Contributor
timestamp: September 09, 2025, 04:04:13 am
content: Quote from: lucatz on September 05, 2025, 12:53:01 pmWhat voltages does it produce? AC or DC? At what current? How is it turned on/off? These are basic specs and there are plenty of existing off-the-shelf products for the common voltages, and even adjustable ones.Quote from: lucatz on September 03, 2025, 06:26:36 pmAre the output voltages different between the SL-TT and VRT9850? Even if the PSU itself is designed completely differently, as long as the outputs and controls are the same, it can be used as a reference for a retrofit.