Sony Trinitron SMPS Troubleshooting Help Needed

[willinliv]

Hi everyone,

I'm hoping some of you can lend your expertise to help me troubleshoot the Switch Mode Power Supply in a Sony Trinitron KD-28DX51U CRT television. The set is completely dead (no standby light, no response).

I've been working on this as a hobby project, and I have some basic electronics knowledge and experience with component testing, but SMPS is relatively new to me. I was getting some excellent guidance on Reddit (link to original thread), but a key contributor's account is no longer active, and I've hit a wall. I would really like to get past this problem - it's in otherwise great condition and I think it would be a shame to scrap it if it otherwise was working.

Problem Summary:

The TV is not powering on at all - it stopped working suddenly for previous owner. I've narrowed the issue down to the 5V standby power supply section. The primary side of the SMPS seems to be getting power, but there's no output on the secondary side. I suspect either a problem with the transformer (T602) or an issue preventing the switching IC (IC609, TOP209P) from operating correctly - and from what I read online it is most likely the latter.

Detailed Symptoms and Tests:

Here's what I've checked so far:

• AC Input: AC power is present and getting through the mains input stage.
• Bridge Rectifier: The bridge rectifier (D641) is working correctly, getting AC and outputting approximately 340V DC.
• Bulk Capacitors: Voltage is present across the main filter capacitor (C638), measuring around 340V. I've also replaced this capacitor.
• Startup Resistor: The startup resistor (R627) is intact, and voltage is present at pin 5 of the switching IC (IC609).
• Primary Side Components: I've tested the diodes (D628, D629) and other resistors and capacitors in the primary circuit, and they appear to be functioning correctly. The primary winding of the transformer (T602) shows continuity.
• Secondary Side Issue: This is where the problem lies. There is no voltage present across capacitor C641, which I believe should be around 5V for the standby supply.
• Transformer Secondary: The secondary winding of the transformer (T602) also shows continuity.
• Injected Voltage Test: If I inject 5V DC directly across C641 (with AC power disconnected), the standby circuit comes to life: a relay clicks, the 3.3V standby voltage is generated by IC608, and the standby light illuminates (with an error code, indicating the main power is off, which is expected).

Specific Questions:

1. Transformer or Switching? How can I definitively determine if the problem is with the transformer (T602) itself, or if the switching IC (IC609) is not being triggered correctly?
2. Feedback Circuit: I'm having trouble understanding the feedback mechanism in this particular SMPS. I see an optocoupler but I can't see how this communicates with IC609.
3. Troubleshooting Steps: Given the symptoms and tests I've performed, what would be your recommended next steps for further diagnosis?

Supporting Information:

• Schematic (relevant section with my mark-up in red of most recent findings): https://ibb.co/3mvDHCzM
• Photos of the Power Supply Board: https://ibb.co/zh745mWr https://ibb.co/VY0wTrnz https://ibb.co/h1XKfFQR
• Service Manual: https://www.manualslib.com/manual/825640/Sony-Trinitron-Kd-28dx51u.html
• Original Reddit Thread:
  https://www.reddit.com/r/crtgaming/comments/1jtulro/sony_trinitron_kd28dx51u_completely_dead_seeking

Thank you in advance for any assistance you can provide! Will

 

[Nigel Goodwin]

OK, bit of info - I was a TV service engineer for 46 years, I worked extensively on the worlds first domestic SMPSU, the Thorn 3000 series, and everything since then. I was also Chief Engineer for a Sony Service Dealer, and did Sony warranty (and out of warranty) repairs for decades.

First thing, Sony PSU's are EXTREMELY reliable, and as Sony use higher quality electrolytics, capacitors are almost never a problem.

T602 is VERY unlikely to be faulty, SM transformers are extremely reliable, and Sony ones particularly so - far more likely is IC609. R627 isn't a 'startup' resistor, it's a surge limiter and 'fuse' for if IC609 goes S/C.

There's really little that can be at fault, the main power supply is presumably OK, as it's not even powered until the relay kicks in, and that requires the standby supply to be working, and a command from the micro.

The photo-coupler is for feedback in the main PSU, there's no secondary feedback in the standby supply, it's roughly regulated from the primary side via D627 - and D627 failing S/C would kill the supply. You need to check all components in the standby supply, and if they are OK, then try changing IC609.

As you've already added a separate 5V supply, do the same again, but turn the mains on to the set and see if the main PSU (and the rest of the set) works - the standby supply is basically entirely separate, and just gives a 5V rail with VERY low mains consumption.

 

[starLED]

Hi everyone,

I'm hoping some of you can lend your expertise to help me troubleshoot the Switch Mode Power Supply in a Sony Trinitron KD-28DX51U CRT television.
Thank you in advance for any assistance you can provide! Will

I don't think you can find anyone better or qualified than Nigel for Sony TV's.