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The Memotech MTX Series

Memotech MTX Electrolytic Capacitor Replacement

From the "upgrade" of David Kimberlin-Wyer's MTX500

 

System Description : MTX 500 Serial No.16153, 4000-05 computer board with 32k DRAM chips.

Problem Description : Upgrading capacitors to handle higher voltage MTX PSU

Background

David bought two MTX500s, sold on eBay as pair, but with only one power supply between them, David managed to obtain an MTX PSU from Tony Brewer, prior to choosing which of his MTX PSUs to part with, Tony had done some checking of their output voltages and found the following variations :

Measurement Point

Between PSU plug pins

(no load)

PSU-1 PSU-2 PSU-3
1 and 3/4 14.1 15.4 19.1
2 and 3/4 10.1 10.0 11.7
5 and 3/4 9.6 9.5 11.7

Tony suggested that PSU-3 was possibly an early PSU from the Memotech Test Department, untypical of production models and elected to dispose of this one.

MTX Power Regulation Circuit

This is the circuit diagram from the MTX Operator's Manual, although Tony has indentified that the values shown for C51 and C53 are incorrect and the values from the Service Manual are 4700uF 16V (not 1000uF 25V) for C51 and 100uF 25V (not 470uF 16V) for C53.

Tony also commented that "it is good practice to use capacitors with a voltage rating significantly higher than the peak circuit voltage and recommended values when replacing capacitors as follows:

  • C51 4700uF 25V minimum (PSUs 1 & 2) or 2200uF/3300uF/4700uF 35V (PSU 3)

  • *new* C56 4700uF 25V (PSUs 1,2 &3)

  • *new* C53 100uF 25V (unchanged) "

Given this advice, and with a view to using PSU-3, David elected to replace the large electrolytic capacitors (C51 and C53) on his MTX computer board. David posted a comprehensive set of photos showing the process that he followed on FaceBook, with his permission, it is reproduced here on the basis that it provides useful tips on the replacement of C51 and C53, which, in view of their age, are potential candidates for failure/replacement on other MTX series computers. The image at the top of the page shows an "exploded" electrolytic capacitor - hopefully, you won't see one like it!

 

David sourced new capacitors from Rapidonline.

Comparing the replacements with the originals in the photo above, you can see that the replacement for C56 is considerably smaller than the original.

Before capacitor replacement

The MTX Computer board removed from the case, C56 is the blue capacitor in the foreground, C51 is the black capacitor at the top of the photo.

Before capacitor replacement

View of C56 from the front of the board, after removal of the video board.

Before capacitor replacement

The solder points for the two capacitors identified on the solder side of the board before the soldering iron comes out.

View of the location of C56 after its removal, showing the mounting holes at the left and right.

The photo also shows a good view of the voltage regulators and power transistor installed below the capacitor which is usually obstructed by the capacitor.

View of the solder side of the board after C56 has been removed and the mounting holes cleaned up.

View of the replacement for C56, showing the heat shrink insulation fitted before the capacitor was fitted.
After replacement of C56
View of C56 from the front, showing the clearance between it and the voltage regulators below it.
View from the solder side after C56 has been installed.
View of the solder side of the board after C51 has been removed and the mounting holes cleaned up.

After replacement of C51

View from the solder side, just before the excess length has been snipped off the capacitor legs.

Capacitors installed and the video board refitted.
A successful conclusion

 - It still works !

Was this screen display prompted by surprise or relief ???

All photos courtesy of David Kimberlin-Wyer

 

Related images
As an aside, what you might get if you source cheap components from China

Another relevant photo - not from an MTX, but it shows what a blown electrolytic capacitor might look like.

 

 

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Page last modified 26/10/2014