The Sinclair QL Forum

Hi all
I have a QL and when I plug it in there is a quick spurt of the microdrive motors the power light is lit and thats about it.
I have tried to tune it in to a tv but all I get is a very out of tune colour blob.
I have had a quick test of the voltage regulator and get the correct voltages one side but the other which should be between 4.75 and 5.25 I get about 7.5 anyone got any ideas.
many thanks in advance

Hi, on the voltage regulator you must have 9-10 volt on right pin that is the output voltage of transformer and 5 volt on left pin to drive all chips on motherboard, also you can check pin 14 of video driver chip ( the one below modulator) where you must have 12 volt.

hi
yes i get over 9volts on the right but on the left instead of 5volts I get 7.5volts.
many thanks

and getting 12.10volts at video chip

Hi Traindriver

So, sounds like the 5V regulation is fried.

If that 5V is being generated by the standard linear 78x05 regulator (screwed to the black fin-heatsink), then they are easy and cheap enough to replace.

If you have instead one of the various switched DC-DC converters mini-boards fitted in place of the linear regulator, you could try replacing first with another 78x05 to test out everything else and then ask Tetroid or Dave (available elsewhere on the forum) for one of their switched DC converters...

M.

Hi
It is a standard one fixed to the heatsink but would this of caused any more damage elsewhere if it's putting out too much volts.

Hi - quite possibly - but replacing it with a working 78x05 won't cause any further damage and will give you a clue as to what might need replacing thereafter

Traindriver69 wrote:Hi
It is a standard one fixed to the heatsink but would this of caused any more damage elsewhere if it's putting out too much volts.

If you really had 7.5 Volts on the 5V rail, this has probably caused a lot more damage in all places - Some of the more robust chips might survive this for a few minutes, some others probably die within a few seconds. Expect to replace a lot of chips - Fortunately, most of the hardware can still be bought at very reasonable prices.

I would start fixing the power supply and find out what else has been fried.

Tobias

I am going to try and get a regulator today once installed which points on the board are best to test next
Many thanks

Hello,

If you don't want to create more destruction (and perhaps redestroy the regulator). The best solution is to have a regulated alimentation (laboratory). You can fix teh voltage and the intensity. The regulated alimentation prevent the over intenisity consumption (cut or regul). then this avoid to have o much intensity in the sensible circuit.

Best regards.
Dominique