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Friday, February 20, 2009

Dead Amstrad DRX400 Sky Box — fixed

Presented with mother-in-law's Amstrad DRX400 Sky Box, not working. Fault reported as "power button does not work".

Upon powering up, the power light came on red, and also the remote command light was on red continuously. Pressing the power button did indeed not make the power light go green, even after allowing a long time for boot-up. The remote command light being on continuously was not a good sign — it should only light up when an infra-red command is coming in from the remote control.

Sniffed around on the web, initially searching for "factory reset". This showed up some common problems with the DRX400:

  • Doing up the connector on the dish input too tight can result in the innards of tuner module breaking
  • A 2200μF capacitor can fail in the PSU module
  • A 3300μF capacitor can fail in the PSU module

Showing an invalid light combination seemed to tie in with a PSU problem.

Popped the lid and looked for burst capacitors on the PSU module, but could not see any.

The legends on the main PCB indicated what voltages should be coming from the PSU. Guessing that the end pin (number 14) was 0V, measured the potentials on the other pins using a DMM. Found various voltages, plausible for the indicated potentials but with significant errors (see table below).

Upon pointing out various parts of the innards to my wife, she commented that one of the capacitors on the PSU looked a bit funny, with a slightly bulging top.

The bulging capacitor was a 2200μF 6.3V 105°C radial electrolytic (branded "Licon") on the PSU PCB. This matched one of the common problems.

Maplin was showing a plausible substitute for under a pound, so I thought it was worth a shot. Procured a 2200μF 16V 85°C radial electrolytic from Maplin, a little larger diameter, but would still fit. Temperature rating 85°C rather than 105°C though if it gets over 85°C inside the box, I think we have other problems.

Replaced the capacitor on the PSU.

Upon powering it up, the power light came on red, and the remote command light did not come on. Pressing the power button made the power light go green.

Measuring the voltages again, they were much closer to the indicated potentials (see table below). Also I found that what I thought was the 3.3V rail was actually the 5V rail and vice versa!

Put lid back on. Connecting it up to TV and dish feed showed it was all working.

PinBeforeAfterDesired potential
131.3 V+4.3%31.1 V+3.7%30 V
229.8 V+49%24.9 V+25%20 V
38.11 V+21%6.42 V-4.2%6.7 V
43.59 V-28%5.08 V+1.6%5.0 V
53.59 V-28%5.08 V+1.6%5.0 V
64.32 V+31%3.39 V+2.7%3.3 V
74.32 V+31%3.39 V+2.7%3.3 V
84.32 V+31%3.39 V+2.7%3.3 V
9-5.12 V+2.4%-5.02 V+0.4%-5.0 V
100 V0%0 V0%0 V
110 V0%0 V0%0 V
120 V0%0 V0%0 V
130 V0%0 V0%0 V
140 V0%0 V0%0 V

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