Laserjet III DC power supply probs

Hi folks,

has anyone out there ever managed to troubleshoot the DC power supply
in one of these dinosaurs? Mine's developed a fault in the 24V supply,
killing the fans and main motor < 1 minute after powerup. Input from
the AC module remains stable (220VAC) , as does the 5V output since the
panel still functions and gripes about a "SERVICE 55". According to the
service manual, 55 indicates a DC controller error, but I've ruled
that out since the 24V output from the DC PSU still drops after
detaching the DC controller.

I'd appreciate any comments on diagnosing this baby on the component
level. Sure, replacing the entire module is a piece of cake, but hardly
worth the cost when I can get an LJ4+ for the same price (which of
course isn't quite as rock solid)...

Thanks 'n' stuff!

--GanjaTron

The LJ4, LJ4+ and LJ5 are just as rock-solid as the LJ3. They all have better
power-saving features. Retire the LJ3 unless you can get one for free to
scavenge for parts... Ben Myers

On 2 May 2005 05:43:09 -0700, "GanjaTron" <> wrote:

>Hi folks,
>
>has anyone out there ever managed to troubleshoot the DC power supply
>in one of these dinosaurs? Mine's developed a fault in the 24V supply,
>killing the fans and main motor < 1 minute after powerup. Input from
>the AC module remains stable (220VAC) , as does the 5V output since the
>panel still functions and gripes about a "SERVICE 55". According to the
>service manual, 55 indicates a DC controller error, but I've ruled
>that out since the 24V output from the DC PSU still drops after
>detaching the DC controller.
>
>I'd appreciate any comments on diagnosing this baby on the component
>level. Sure, replacing the entire module is a piece of cake, but hardly
>worth the cost when I can get an LJ4+ for the same price (which of
>course isn't quite as rock solid)...
>
>Thanks 'n' stuff!
>
>--GanjaTron
>

Go to www.anderson-tech.com and look for the toll free number. Give Mr.
Anderson a call. He may be able to help you out.

"GanjaTron" <> wrote in message
news: oups.com...
> Hi folks,
>
> has anyone out there ever managed to troubleshoot the DC power supply
> in one of these dinosaurs? Mine's developed a fault in the 24V supply,
> killing the fans and main motor < 1 minute after powerup. Input from
> the AC module remains stable (220VAC) , as does the 5V output since the
> panel still functions and gripes about a "SERVICE 55". According to the
> service manual, 55 indicates a DC controller error, but I've ruled
> that out since the 24V output from the DC PSU still drops after
> detaching the DC controller.
>
> I'd appreciate any comments on diagnosing this baby on the component
> level. Sure, replacing the entire module is a piece of cake, but hardly
> worth the cost when I can get an LJ4+ for the same price (which of
> course isn't quite as rock solid)...
>
> Thanks 'n' stuff!
>
> --GanjaTron
>

Let me know how you make out. I have several PS and remnants in the
shop which I can let go at modest price. Contact me
therepairman at optonline.net


On 2 May 2005 05:43:09 -0700, "GanjaTron" <> wrote:

>Hi folks,
>
>has anyone out there ever managed to troubleshoot the DC power supply
>in one of these dinosaurs? Mine's developed a fault in the 24V supply,
>killing the fans and main motor < 1 minute after powerup. Input from
>the AC module remains stable (220VAC) , as does the 5V output since the
>panel still functions and gripes about a "SERVICE 55". According to the
>service manual, 55 indicates a DC controller error, but I've ruled
>that out since the 24V output from the DC PSU still drops after
>detaching the DC controller.
>
>I'd appreciate any comments on diagnosing this baby on the component
>level. Sure, replacing the entire module is a piece of cake, but hardly
>worth the cost when I can get an LJ4+ for the same price (which of
>course isn't quite as rock solid)...
>
>Thanks 'n' stuff!
>
>--GanjaTron

wrote:
> Let me know how you make out. I have several PS and remnants in the
> shop which I can let go at modest price.

Assuming this is a thermal problem, I poked around with cold spray in
there -- to no avail. None of the suspect components (regulators, etc)
reacted to cooling. Without schematics, I'm pretty screwed anyway...

--GT

On 4-May-2005, "GanjaTron" <> wrote:

> Assuming this is a thermal problem, I poked around with cold spray in
> there -- to no avail. None of the suspect components (regulators, etc)
> reacted to cooling. Without schematics, I'm pretty screwed anyway...

If you are happy to dig into component level, there are probably a
couple or so of 6-legged opto-couplers in the DC PSU. might be
worth monitoring voltages across the LED's and their o/p to see
if they are respondingt to drive. The LED's are known to die.
That was the problem with my HP (Canon engine) laser. Ended
up putting in a 6-pin skt, and a red LED in series with the opto
LED drive. Now red LED lights in sympathy with the fuser lamp,
if fuser doesn't light diagnosis is simple.
..
You will probably find a service manual on the internet, try
http://www.eserviceinfo.com/
for starters, though there are other sites. You might have to
do a bit of digging on the ESI site as it's indexing seems a
bit flakey, may be ok when you get the hang of it.

Other than that it's a case of following logic level signals
and strategic measuring of voltages. Like does the fuser
lamp come on at start up, if not why? Does it start to
cycle?

as old as this thing is, and the heat over the years, it wouldnt surprise me
that the radial electrolytics in the power supply is leaking or dried.

I know this is a far cry from a fax machine, but I worked on a fax machine
with similar problems, Recapped the power supply, and all went away.


"GanjaTron" <> wrote in message
news: oups.com...
> Hi folks,
>
> has anyone out there ever managed to troubleshoot the DC power supply
> in one of these dinosaurs? Mine's developed a fault in the 24V supply,
> killing the fans and main motor < 1 minute after powerup. Input from
> the AC module remains stable (220VAC) , as does the 5V output since the
> panel still functions and gripes about a "SERVICE 55". According to the
> service manual, 55 indicates a DC controller error, but I've ruled
> that out since the 24V output from the DC PSU still drops after
> detaching the DC controller.
>
> I'd appreciate any comments on diagnosing this baby on the component
> level. Sure, replacing the entire module is a piece of cake, but hardly
> worth the cost when I can get an LJ4+ for the same price (which of
> course isn't quite as rock solid)...
>
> Thanks 'n' stuff!
>
> --GanjaTron
>

ato_...@hotmail.com wrote:

> If you are happy to dig into component level,

Well... not exactly *happy*... :^)

> there are probably a
> couple or so of 6-legged opto-couplers in the DC PSU.

Optocouplers in a PSU? Didn't see those. I'll take a closer look.

> That was the problem with my HP (Canon engine) laser.

All old(er) HPs use Canon engines, AFAIK.

> You will probably find a service manual on the internet, try
> http://www.eserviceinfo.com/
> for starters, though there are other sites. You might have to
> do a bit of digging on the ESI site as it's indexing seems a
> bit flakey, may be ok when you get the hang of it.

Thanks for the link! Found some interesting stuff on there, though as
you point out, the index *is* pretty convoluted.

> Other than that it's a case of following logic level signals
> and strategic measuring of voltages. Like does the fuser
> lamp come on at start up, if not why? Does it start to
> cycle?

The fuser draws its power from the AC PSU, right? I doubt that's the
problem though, particularly when both fans are dead.

Thanks for the reply, mon!

> as old as this thing is, and the heat over the years, it wouldnt
surprise me
> that the radial electrolytics in the power supply is leaking or
dried.

The caps are definitely suspect, although they don't appear to be
leaky. I guess I'd have to get one of those fancy ESR meters. I've seen
a tiny cap develop a thermal fault in my Grundig VCR without leaking.
Took me ages to figure that one out. On the other hand, I've had 1/2
dozen leak all over an HP9000 apollo PSU, destroying the labelling on
board in the process! That was a no brainer (duh). In short -- caps
SUCK!

On 5-May-2005, "GanjaTron" <> wrote:

> The fuser draws its power from the AC PSU, right? I doubt that's the
> problem though, particularly when both fans are dead.

Yes, fuser (in mine) draws power from AC PSU. There is a power
board and a logic board in the AC PSU (Canon SX), and the DC PSU
feeds it with assorted signals. The logic board has a fattish resistor
of 1K2 driving the power boards opto-coupler, the one I added an
extra LED in series. Opto drives triac, with the extra red LED,
LED goes on/off, triac goes on/off, fuser lamp goes on/off. Now
it's easier to trouble shoot. AC PSU also has a relay in series
with mains supply to triac. I found I could short it out for testing,
so I put a domestic lamp in place of the fuser lamp, set it up
on a table, and drove it with a 9V battery and resistor, for testing.
Saved installing and uninstalling the PSU all the time.

DC PSU has opto couplers, there is an isolated switch mode,
raw AC in. It's txfmr drives all the rectifiers, logic etc, there
are output V level sensors, and their o/p feeds back to the
isolated switch mode. So we have a mains side, and
an isolated logic side.