HP Draftmaster RX pen plotter needs love

[David Collins (AU)]

Is there 42 Volts on both sides of the 42V fuse?   Seems odd that there's 42V to the switcher but no voltage at the other end but the fuse is intact.  Normally when the switcher blows, the main switching transistor shorts and takes out the supply fuse. 

The power supply transistors are on the black heatsink near the transformer with the yellow label.  I'd be checking those to make sure they are OK using a transistor tester or multimeter on the 'diode' setting. I'd also check nearby diodes and transistors to make sure they are OK as well.  Tricky without a schematic that's for sure. 

I have an RX that powers up but don’t have access to it until the weekend but I could try to assist with voltage checks to allow you to compare if that helps. 

David Collins
HP Computer Museum
www.hpmuseum.net

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Philipp Pap via cctalk
Sent: Wednesday, 10 May 2017 4:30 AM
To: Jon Elson <elson at pico-systems.com>
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: HP Draftmaster RX pen plotter needs love


> Am 09.05.2017 um 18:10 schrieb Jon Elson <elson at pico-systems.com>:
> 
> On 05/09/2017 08:26 AM, Philipp Pap via cctalk wrote:
>>  Anyhow, now I have 41 and 83 Volts at the 42 and 85V test points,  
>> but always 0 Volts at the +5,-12,+12,+15V test points.
>>  The fuses are good and the secondary transformer seems good as well.
>> 
> Does this have a switching power supply for the logic (5, 12, 15V) or is it a rectifier/filter/regulator off the transformer?
> If rectifier/filter/regulator, it seems odd that FOUR voltages would quit at once.  So, I'm guessing it might have a switching supply for those voltages.  It sounds like the regulation circuit failed (display got brighter) and then a crowbar circuit tripped to shut it down.  Unless you are pretty skilled, switchers can be a bit hard to troubleshoot.  It might be easier to get a supply with the needed voltages and just patch it in.
> 
> Jon
OK, I just read in the service manual
„The power supple is a switching type dc supply incorporating a pulse-width modulator (PWM).“ but I don’t know the meaning on the drawing „+5V ref“ „vcc“ — POWER ON —„~RESET , HALT, RESET“ Could that mean that the problem could be in this „POWER ON“ block logic???

Thanks again!