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 I was getting some excellent guidance on Reddit (link to original thread), but a key contributor's account is no longer active, and I've hit a wall.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.

 

[willinliv]

Thank you Nigel Goodwin I will give this a go and let you know how I get on.

 

[willinliv]

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.

OK! Some progress – injecting 5V into the secondary side of the standby circuit while also applying mains AC results in action from the rest of the set, with no apparent issues:

.

I'm very pleased that the fault now appears to be isolated to the primary side of the standby SMPS. Thank you very much for your help!

I guess the next step is to thoroughly test all the components on the primary side of the standby SMPS, probably out of circuit. I previously tested everything for continuity and with a component/ESR meter, but while they were in circuit. Hopefully, a more detailed inspection will reveal the culprit. I'm also considering the possibility of a diode failing under load, but I'll proceed with replacing IC609 first, as you suggested.

 

[Nigel Goodwin]

OK! Some progress – injecting 5V into the secondary side of the standby circuit while also applying mains AC results in action from the rest of the set, with no apparent issues:

.

I'm very pleased that the fault now appears to be isolated to the primary side of the standby SMPS. Thank you very much for your help!

It was a very likely possibility, but by using external 5V you've proved it 100%

I guess the next step is to thoroughly test all the components on the primary side of the standby SMPS, probably out of circuit. I previously tested everything for continuity and with a component/ESR meter, but while they were in circuit. Hopefully, a more detailed inspection will reveal the culprit. I'm also considering the possibility of a diode failing under load, but I'll proceed with replacing IC609 first, as you suggested.

A diode failing under load is extremely unlikely, as you've checked the rest of the components the chip is the most likely candidate. Transformers do very occasionally fail, but almost always if they do then it's a common fault (design flaw in the transformer), and Sony's have no history of it.

What they do have a history of the LOPTX failing, which blows the LOPT transistor - and if you replace the transistor it blows again instantly, with a loud 'crack' from inside the transformer. Even worse, depending on model, there's a good chance it blows the large SM micro-controller chip. LOPT issues were relatively common, PSU problems really very rare.

We never saw many of that model though, as it's a DTT model, and we were in an area that had no DDT coverage until DSO.

We did sell a relatively small number of then though, probably in low double digits.

 

[rjenkinsgb]

I've had dead TOPSwitch devices in other things, with no sign of overheating or damage & no other component failures; just old age.

 

[willinliv]

Okay, I've tried replacing IC609 with two new chips, but unfortunately, there's no change. I still have no standby power and no voltage on the secondary side of the standby circuit. I've traced the power path from the bridge rectifier through to the drain of IC609, and the voltage is present.

A couple of points in case useful to know:

IC609 Replacement: My replacement IC609 is a TOP209PN, while the original was a TOP209P. Datasheets online seem to point to the TOP209P so I had assumed they were the same.

Voltage at C639: I'm not seeing any voltage across C639, which I believe should show some activity, though perhaps not easily measurable with my DMM.

So Next Steps, I plan to start removing and testing components in the standby circuit, as all the tests so far have been in-circuit (except for the components I mentioned previously).

Given my joy at seeing the set powering on yesterday evening, I was wondering if it would be wrong (or potentially damaging) to replace the faulty standby circuit entirely with a separate, off-the-shelf 5V 1A SMPS module?

 

[willinliv]

Thanks All! Set now working again!
Nigel Goodwin last ditch effort paid off, checking each component out of circuit in the standby circuit (including the transister), also replaced switching IC TOP209P (in my case TOP209PN) a third time, and threatening a cheap 5v smps led driver seems to have kicked the set back into action. Now fully operational. Thanks for your help and encouragement

 

[Nigel Goodwin]

Well done, good result