Neil, you, perhaps you should subscribe the email address to the mailing list.
Main point for consideration:
The operation of the internal protection circuit is described in the section
‘main output’. Here, the voltage across c33/c37 triggers a zener, which drives
an opto-isolator, which drives Q22/23 as a bistable latch, causing tripping,
which is what you’re describing.
Can you get a scope onto the capacitors and validate the voltage?
Similarly, this latch is also driven from excessive Mosfet output current,
sensed by the 0.33Ohm R16. Nasty causes are the mosfets, any of the diodes
going short or ‘zener’, perhaps a shorted turn on the output transformer.
The fact the the PSU is being driven into self-protection is quite a good sign,
as it indicates that most of it is working, including the oscillator/PWM parts
and out to the driver stage.
Quick observations:
0. Do you know what caused the failure? This is often a useful guide to where
to look and consider.
1. things which go wrong are usually those which get hot. Power devices in the
SMPSU are a good place to look, so the power mosfets must be considered suspect.
2. the auxiliary power supplies are critical to operation, so check the volts
are present and correct.
3. close visual inspection - do any parts of the board look heat damaged?
Resistors can go well off value when over heated.
4. An electrolytics which are bulging must also be considered suspect.
5. The control electronics are very well described in the text; if you can
isolate them from the rest of the circuit in blocks, you can run them up on
independent supplies and validate that they’re working as expected.
I’d start with the power devices, though...
On 28 Aug 2015, at 20:28, keithlbird@xxxxxxx (Redacted sender
"keithlbird@xxxxxxx" for DMARC) <dmarc-noreply@xxxxxxxxxxxxx> wrote:
Dear club colleagues, I'm forwarding the attached on behalf of Neil, if you
have feedback or can help Neil please contact him directly at
neil@xxxxxxxxxxx <mailto:neil@xxxxxxxxxxx>
73 Keith
-----Original Message-----
From: Neil Bernard <neil@xxxxxxxxxxx>
To: keithlbird <keithlbird@xxxxxxx>
Sent: Fri, 28 Aug 2015 18:16
Subject: Help required - Bench PSU Help (TTi 1810R)
*******
Hi WKARS team
Bench PSU Help (TTi 1810R)
Ive been donated a very fine 18V 10A Bench supply, if I can fix it.
Doesn't power up, you get the familiar tick-tick of a switch mode trying to
start. TTI supplies are good supplies so worth fixing. The primary side seems
ok, and the mains input fuse hasn't blown. With the exception of the incoming
mains reservoir cap which is fine, there are only about 7 other
electrolytics, so i've ordered some low ESR replacements and we can give that
a whirl first by changing them. I did check a couple out of circuit and my
ESR meter shows them to be ok.. but its always hit and miss. All the main
rectifier diodes seem ok and are not short or open....a quick poke at a few
key resistors didn't show up much either..
Ive had a poke around a few key voltages and I do have a service manual
(https://www.dropbox.com/s/txkn4byzkdsvfiw/EX1810R%20Service%20Manual.pdf?dl=0
<https://www.dropbox.com/s/txkn4byzkdsvfiw/EX1810R%20Service%20Manual.pdf?dl=0>)
althoung the silk screen doesn't always match.
Getting stuck now, any thoughts or help… much appreciated.
Neil
M6DWU
neil@xxxxxxxxxxx <mailto:neil@xxxxxxxxxxx>
