TTi EX354D PSU Repair

[TaylorD93]

So im trying to repair a EX354D PSU, currently it wont power up.

I have a copy of the EX354D Service manual, however it is not helped by the fact that the board is supposed to be a "Version 1" but the Version 1 GA diagram in the service manual, doesnt match my board. It shows 2x Bulk capacitors for the SMPS however mine has 1 and parts are located slightly differently.

It is a "Single Sided" board and doesnt have any of the PFC circuit added on later models, it certainly does seem to match the Version 1 schematic.

Anyway, the initial fault was, on power up it would immediately blow the Input fuse (10A).

My gut was that one of the main switching fets had gone short, they are an IRF740, so i bought 4 and did fined one dud, reading dead short.

Whilst there, i noticed that there were alot of circumferential cracks on most "large" solder joints so ive gone around tidying them up. As some were on the main bridge rectifier, i also buzzed it out, one diode was dead short. So i fitted a new BR.

RT1 was cracked, but the only marking i could find was SL15 15004, i could only find SL12 15004 online which is a 15 Ohm 4A NTC, i swapped it out with a 10 Ohm 4.4A rated NTC i had in my parts box.

At this point the board was looking ok to power up.

I refitted it to the chassis with all screws (this is required otherwise certain lands on the PSU are floating)

Powered it up with a 60W Lamp in series, the main Bulk capacitor charges to about 300V, but still no operation.

I probe around, and find that the "starter" circuit which is based on a 555 has about 1V Vcc wrt P0V. working my way back to the LM393 comparator with Q1 and Q24, Collector of Q24/Q1 is at 2.4VDC. According to the service manual, this point should charge to 29V wrt P0V then trigger the LM393 at which point 17V is generated on Q1 emitter (net name Vreg). Which its not doing.

With it powered off, D38, R98 and R60 all measure out fine, 33k per resistors and D38 is just a normal 1N4007.

I connected an isolated PSU of 22V across the Q1.Collector to P0V, and as i ramped the voltage up it then triggered at 29V and i had about 40mA current draw, Q1 emitter was infact at 17V as expected.

So I think something odd is happening here, the starter circuit seems to work, however when powered by the mains inlet it does not. The "feed" to the starter circuit via D38/R98/R60 seems to be intact, no burnt up traces etc

I have since powered it up directly from the mains and it doesnt blow the fuse, however it wont power up either.

[TaylorD93]

Update; just found D39 shorted, this diode provides a voltage back to the "starter" circuit once the PSU is actually on.

With this Diode shorted, it was loading the "supply" with R9, a 2k2 resistor, bit low when you have 66k up to approx half the AC supply voltage!

 

[TaylorD93]

Well thanks EevBlog! In writing my OP I rethought the circuit (good to bounce thoughts or explain things to folk)

And it now works after replacing the diode!!!

The 40mA load on the external psu was the give away. 29V on a high impedance cct shouldn't be that high a current. Penny dropped and I'm well chuffed!
 
See photo of it working on the dim bulb in the background!