TDS-524A does not power up

[JVG]

Hello!

To fix complex power supplies like these it often helps to use a bench power supply to feed the control circuitry (make sure to fool the undervoltage lock out). The next step is to put something like 30V on the primary side instead of mains. You can also do the same for the secondary outputs. This will allow you to verify whether all the semiconductors and post regulators work without having to deal with lethal voltages.

Sometimes I think we have mental power

Guess what I did yesterday night without having read this message ?

I put a my bench power supply, set at 32Vdc, 0.25A max on the primary side and found a shorted transistor !!!

So, yes, I did on my own exactly the same you suggest!. I had previously used this same way to work on an Amateur Radio power supply (an Alinco 13.8V 32A SMPS) some weeks ago, as it was fusing main fuse. On that case, it was a matter of a faulty main diode bridge. But, as it had worked fine that time, I decided to give it a go... and, yes, it worked !

FYI, what I found is that Q9, a BU508A in my unit BIAS SUPPLY primary (note that this is different on the 520B, which uses U12 on a similar function) was shorted. I have ordered some spares from England (I am in Spain), as it is easier for me to order them online than to buy them in a shop (talk about time schedule conflicts!)

BTW, all caps checked fine on ESR tests done before the previous checking.

I will keep you posted on the final results.

Thanks a lot for your support!

Regards,

JOSE

[cap]

...easy way to trigger the POWER ON on it.

I think, the service manual tells you what two pins to tie together to turn it on, for module fault locating purposes. You may have to look at more than one manual to find it.

cap

[JVG]

Well, I have done some real advances !

I have replaced several items on the power supply: Q9 (BU508A) transistor, VR4 and VR5 transzobs and C15 capacitor. And, this time, power supply runs without burning again !

But my joy has ended soon... there is nothing displayed in the internal display nor in a VGA display connected to the rear panel. But there is valid video signal (checked with oscilloscope as service manual instructs), but a blank page.

So it seems as if the unit won't fully boot up... any hint regarding this?

In the boot procedure, the floppy disk is heard doing the usual seeking, there is a flash on the display, all front panel lights lit for about 20 seconds and DS1 display does some counting from '1', ending in 'e' then displaying a '.L' then some alternating blinking on the leftmost segments and ending with the upper left segment lit.

BTW, I have checked all power supply rails and they are perfectly fine. Of course, fan runs nicely.

Thanks and best regards,

JOSE

[nctnico]

The TDS520B component service manual also has a section on the diagnostics and LED display.

[JVG]

Hello nctnico,

The TDS520B component service manual also has a section on the diagnostics and LED display.

Thanks for the hint!. It is a bit hidden in the manual but I located it...

BUT I think I have a clue for my problem... D1 BUS BOARD could be plugged upside down!

I didn't know it can be plugged both ways and it may explain that the system is not even able to show a display, as some bus lines would be grounded!

I am eager to go back home

Regards,

JOSE

[JVG]

I had not updated this thread...to give it a proper closure, yes, my non-booting problem was related to an upside-down D1 BUS BOARD. Once it was properly fitted, the unit booted without a single error

As commented in other threads, all the repair, with plenty of pictures, can be seen in my WEB at:

http://jvgavila.com/tds524a.htm

Thanks and best regards,

JOSE

[irene88]

You posted a very helpful information. I could finally fix my challenge. Great work guys