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The Memotech MTX Series

Memotech Hardware Repairs

 

Update : subsequent to this repair, the same PSU developed another problem, rather than attempt further repairs, I chose to replace the PSU with an ATX PSU from a PC. Full details are on my FDX PSU Replacement page.

Read on for details of replacing failed suppression capacitors in the FDX PSU.

 

FDX Power Supply Repair

A word of caution: I not an electrical engineer, the information on this page has come from my analysis of the PSU in my FDX and reading of an Astec Service Manual (which pre-dates the manufacturing date code of my FDX power supply). This page is provided for "information only" in the hope that it may be useful - however, it should not be construed as a authoritative document on the FDX PSU or its repair. You use it at your own risk and if you are not competent to work with mains supply voltages, which can kill you, leave it to someone who is and put your equipment into an electrical repair shop. [This probably applies to me too, but, anyway . . . . .]

This page describes how I repaired one "common" FDX PSU problem, by common, I mean that of the very few FDXs that I know of, this component has failed in at least two of them, and given the nature of the problem, I am surmising that it is common to all FDX PSUs. To reiterate, this is the repair that I did on my FDX - not a recommendation that others follow the same steps.

OK, with the disclaimer out of the way, here we go......

The PSU installed in my FDX is a Astec AC8251, it uses a Rifa MP (metalised paper) line filter capacitor for EMI/RFI suppression on the input side of the PSU, the component ID on the PSU schematic in the Astec manual is "C2" and is rated at 0.1 uF, 250VAC.

Metallised film capacitors can repair themselves to some extent after a voltage spike but they degrade with age - and as 30 years is a long time, failure of this type and age of capacitor is common in many types of equipment, including radios, hi-fi systems, test equipment and computers. A common problem with the BBC micro for example, is also failing power supplies, caused by failure of the same type of capacitor - also made by Rifa. Failures of Rifa MP capacitors is discussed in the "Beware Rifa Class X Capacitors!" thread on the vintage-radio.net site. One theory put forward as the cause of the failure is that the epoxy case cracks due to age and/or temperature and moisture is absorbed by the paper resulting in corrosion of the zinc metalising which results in the capacitor going bang! In any event, this thread confirms the high incidence of failures of these components.

When these capacitors catastrophically fail, it is pretty obvious, in my case, a cloud of white smoke and a terrible smell resulted, this is pretty much the norm. Once the system was switched off though, the smoke dispersed, the smell abated and on powering up the system again, it appeared to be working fine, and continued to do so. Since the most likely failure mode of this type of capacitor is for it to fail open circuit, this would not stop the PSU from working, but would result in reduced EMI/RFI protection with unpredictable results on the FDX, the additional electrical "noise" could cause various problems, but the probability of their being any safety issues is small.

This leads me to believe that just because your FDX has no apparent problem, the capacitor may have blown sometime previously and you may be unaware of it - particularly if you are not the original owner of the FDX. Even if the capacitor in your FDX PSU has not failed, if it has not already been replaced, you should consider doing so before its seemingly inevitable failure.

As well as their RFI suppression properties, these capacitors in the Astec supply are "X" type capacitors, that is, they also have a safety function. Safety capacitors come in two types, "X" and "Y", type "X" are placed between the "Live" and "Neutral" lines, i.e., "across the line" and type "Y" are placed from "line to ground".

If a type "X" capacitor short circuits, there is no increased risk of electric shock, the risk is that self ignition of the capacitor or an over current situation that could result in a fire, although, in normal circumstances, a protective fuse or circuit breaker would open. If a type "Y" capacitor short circuits, there is an increased risk of electric shock to the user. Since these types of capacitors perform safety functions, they are tested and certified by independent safety agencies. The types are further divided into classes, depending on their target application and therefore required test voltage. Without detailing all of the classes, it is sufficient to say that for the "across the line" capacitors used in the FDX PSU, a type "X", Class "2" is required, i.e., an X2 capacitor.

Strictly speaking, you must use a safety rated capacitor to replace "C2". initially, as I had some difficulty in sourcing an "X" rated replacement, and had been suggested a possible (non-X rated) alternative, I was considering using a non-"X" rated capacitor made by a reputable manufacturer, such as the Wima MKP-10.  I decided that this was not really acceptable but you will need to decide for yourself if such a replacement is suitable for your application, in any event, I did find that X2 capacitors were available on the ebay UK store, from a UK vendor, so I used them.

Update: Mark Kinsey has pointed me to the Wima X2 capacitors on the Maplin website - I don't know why I couldn't find them when I looked, but anyway, if I was doing this again, these are the capacitors that I would use.

It is also worth noting that the same type of capacitor, though smaller (0.01uF), is used in two other locations, at "C1" and "C11". Given the difficulty in getting access to the FDX PSU and the fact that these are the same capacitors that frequently fail in the BBC Micro PSU, I chose to replace them at the same time as replacing the failed "C2". If you were really keen to minimise the risk of a future PSU failure, you should probably replace the electrolytic capacitors too, but I have not done this.

C2 Replacement

When "C2" is removed from the PCB, two pairs of mounting holes are revealed, the existing capacitor had a 20mm hole pitch but there were additional holes with a pitch of 15mm, connected to the same tracks on the solder side. This was handy as the replacement component that I wanted to use is slightly smaller than the original and had a 15mm hole pitch.

C1 and C11 Replacement

The two smaller, 0.01uF "X" capacitors are located close to the 2A, 20mm, fuse. Again, the PCB has two pairs of mounting holes, with hole pitches of 15mm and 12mm. The replacement capacitors that I obtained had a 15mm pitch, so fitted into the same holes as the removed components.

 

FDX Power Supply

The FDX used an Astec AC8151-01 to provide power to the 6" PCB card frame and the floppy disk drive(s).

There is a copy of the Astec Specification and Repair Manuals on the Manuals page

Specification Summary

+5 VDC

2.5 A

Input Voltage 115 to 230 VAC

+12 VDC

2.0 A

Input Current 0.85A (rms)

-12 VDC

0.1 A

PSU board solder side

   

 

C2, C1 and C11 Capacitor Replacement
A close up of the failed 0.1uF, 240VAC capacitor, C2, before it was removed from the PSU in my FDX.

You can see that the case has split, when this happened, dense white smoke poured out of the PSU and a film of brown material sprayed over nearby components as you can see on the adjacent component, C4.

A close up of the failed 0.1uF, 240VAC capacitor, C2, removed from the PSU.

You can see the case has completely split and the internals are protruding from it.

OK, the PSU has been removed, the shiny new components ordered off ebay have arrived, a quick check of the values and we should be good to go.

1. This is the replacement for C1 and C11, a 0.01uF, 275VAC, X2 -  that's  fine

(Manufactured by Iskra, you can open the component datasheet if required)

2. This is the replacement for C2, and should be a 0.1uF, 240/275VAC, X2 . . .

 

Wait a minute - what's a ".1K" capacitor?

In case you don't know (I didn't), as a general rule, if the number is an integer greater than 1, then the value is in pF, if it's less than 1, the value is in uF. the "K" is not a unit (kilo), it is the tolerance identifier, "K" means +/- 10%.

So, despite the odd looking value marking, this one is correct too: 0.1uF, 10%

(Manufactured by Tai-Yao, you can open component datasheets here and here.)

Replacement of any or all of these capacitors is a very easy task, so much so, that there is very little point in describing the operation in detail. The most time consuming part is having to almost completely dismantle the FDX to be able to remove the PSU. That part is common to just about every FDX repair job and it worthy of its own page - for details on how to disassemble and reassemble the FDX see the guidelines here.

Once you have removed the PSU from the FDX it is a simple matter of de-soldering the old component and fitting the new. As you can see from the photo of the solder side of the PSU PCB, it is a much less difficult task than working on, say, the MTX computer board.

 

 

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Page last modified 26/10/2014