Repairing a curious failure of our

AS 60 transmitter

 

28 November 2014

 

Since we came back from our summer holidays in late August, I encountered a strange fault within our magnificent As 60 (M1K) transmitter, I could no longer switch it on. This exceptional apparatus operates since about late 1981 in my home without any problem.

After some considerations, I viewed first the status of the 'three phase mains lines' designated: R, S and T.

It was apparent, that line-phase R is not available. Indeed, in the 'mains centre' of our house, I found that a fuse was defect (blown). Replacing it caused instantly a new fuse failure. Thus somewhere there must exist a short circuit. Blowing up a fuse of 16 Amp. tells us that the load must be quite heavy.

My first thought was, maybe one of the three smoothing input capacitors before the main (internal) switching relay is faulty (a mains-filter). It proved, however, that there are no such devices incorporated.

 

As so often, it takes months before we really could start looking at it seriously. 

 

 

Confusing is, that the power connector has four connections of which one is heavier than the others

 

 

My second thought was looking whether these capacitors were mounted somewhere near to the place where the power cable enters the AS 60 housing

It was soon clear, that the capacitors in front belong to a different circuit section.

Then I considered seriously the schematic diagram of the entire AS 60 transmitter. It is quite clear, that there is no other way then trying to access the AS 60 via the front cover plate. This also confirmed, that there does not exist a means of pre-filtering in the mains lines.

Luckily Wouter Elzinga supported me, and we quite easily could remove the front cover plate.

Wouter is always just giving me the right drive (push) to start something which is actually long over-due. Like clearing and cleaning the basement in the Klooster premises, or reshuffling the MLK premises. Without him being there - quite much would not have been accomplished so successfully!  

 

Let us follow the course of thoughts and how I traced the fault eventually.

 

 

My main assessment was that the heavier pin might represent N, this stands for 'neutral', which also is the centre of the three phase line supply (the centre of star-transformer-circuit); and which contact should be about ground potential. Please notice the drawing lower on this webpage. 

My next action was to measure the voltages supplied at the various connector pins. Still regarding the heavier pin being N, I found rather confusing results. Something must be wrong, but what?

Now approaching it the other way around. Looking for capacitors around the section where the power lines enter the main chassis.

This proved soon to be a totally wrong assessment.

Time has come to approach the problem systematically.

 

 

  

The schematic of the AS60 power supply, being part of the big total AS 60 schematic diagram (total size probably A-0)

For better reproduction, you can click at the schematic as to open it (and/or print it), in PDF format.

To get it for the web reasonably right, I had to fit the three schematic drawings into a single one (our scanner allows only just over size A 4), but for practical reason this drawing altogether makes it having a DIN A-2 size.

 

Please consider 'Bu 1' at the top right-hand side. Pin '4' clearly is heavier than the other three. Apparently, this plug does not imply a ground connection (contact). I created therefore additionally (in the past) one by grounding (earthing) the metal connector cap (List-Stecker) and 'ground' being established by its fixing lock. However, for safety we rely fully on an additional earth connection onto the nearby Cu cold-water-tube (visible on the first photo down right of the power supply cable). In Holland, and I guess in most countries in Europe, these water supplying tubes (pipes) are generally interconnect onto an earth electrode. Actually, according the schematic '0 or N' is provided at connector pin-1.             0 might originate from the fact that the AS60 was often fed by means of a generator. However, technically it is know a 'N', because somewhere in the "transformer cabin" 'N' is connected onto a common ground- or earth-electrode. At the consumer site its potential is not regarded completely neutral against ground. Please bear also in mind, that the current through the 'N' conductor is also determined by the regular current (flow) related to the 230 V ac 'consumer' supply (often in practice about 240 V). Between the three outer star transformer windings we find between all three 400 V ac (formerly it was 380 V). 230 V is derived from a single transformer section and the common 'N' connection in its centre.    

 

 

A quick and brief explanation of such a 'star type' transformer, which at least in Europe, is common standard for public electricity supply (maybe, puritans may object that the R-S-T symbols should be given rotated) N being also interconnected by means of an earth-electrode onto earth potential.

Maybe, in some parts of the US it is maintained differently.

 

230 V ac being delivered between 'N'  and R, S or T. Whilst, 400 volt ac being provided between: R-S and S-T and T-R. (230 √ 3 = 398 V) In which sequence determines the so-called direction of rotation. (three phase motors will then rotate: clockwise or anti-clockwise, by only interchanging two of the three wires; it does not matter which wires you inter-change.

 

Let us continue with explaining what caused the failure.

 

 

When we follow the line of the number '4' power contacts, we finally will reach the main relay section above

The cables are all bound neatly together and should be left this way. My next approach was to access the relay section, as potential number 4 was also interconnected onto relay contact number 4.

Carefully following the lines of the schematic drawing. By the way, for me a rather exhausting job, as following a bundle of lines particularly very near to one another is only possible by cm for cm following it with the tip of a screw-driver or other means.

To be sure that all four wires of potential '4' are refit again correctly, I choose to bound them afterwards by means of a 'kroonsteen' (sorry, my dictionary  provides: connector black/strip?). Destroying nothing and always allowing to restore the previous state of affairs.

Please notice, that all photos were taken afterwards, thus after the AS60 did function again, but before the cover-plates being replaced.

 

Continuing again

 

My next move was pulling all four cables out of the contact-strip number 4. One by one measuring their resistance against ground. My following query was, whether the short circuit at pin 4 of Bu 1 was still apparent. It was not, anymore. Only a single wire still having short circuit against ground. It proved, however, that with some imagination the wire virtually interconnected onto ground, runs to the left-hand side, in contrast to the three others.

 

 

Then I instantly suspected - might it perhaps be that the faulty cable (found) was connected onto the electrolytic capacitor C21?

Please notice the canned Al capacitor box, just left of the grey/yellow Brown Boveri relays box shown in the previous photo

The red line is all the time directly connected onto phase 'R' of our three phase power supply (R-S-T and N). Please consider my explanation above.

Viewing the schematic from the electrical point of consideration, a short-circuit of the electrolytic capacitor C21 might never causing about 0 ohms versus ground.

However, I approached it and de-soldered the (top) wire (negative capacitor connection) directly connected onto line number 4. Oh wonder, all lines where now having considerable high resistance versus ground.

What caused the problem?

Almost likely, the electrolytic block capacitor suffered an internal leakage against ground (of about a few ohms only), its metal housing is fixed (mounted) at the metal chassis. I disconnected both capacitor contacts and interconnected the two wires onto a modern type electrolytic capacitor (22 F instead of genuinely 25 F, which might not causing a big difference). After having de-soldered the first (negative) contact, I discovered that its leakage versus ground was vanished. From experience, never trust such a coincidence, and disconnect it entirely from the mains line circuitry!

It is no real wonder that this fault happened, as all the time (for several decades), even when the transmitter being switched-off, mains stood at the negative electrode of C21, thus a 'potential force' existing all the time. This capacitor may be 60 to 67 years old or even beyond. The function of it is to supply (smoothing) the starting up voltage for the main on-off relay (according to the text at the Brown-Boveri relay - it should be fed with 220 V = (dc).  Maybe, for what ever reason, a pulse on the mains line R caused a break-down of the internal capacitor insulation against ground (metal box housing). Phase S and T are straight away interconnected from the main connector onto the according relay contacts and nothing else; as long as the transmitter being kept in the 'off mode'. 

 

By Arthur O. Bauer

 

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