attempt
a rewind job on the original. It has 15 separate windings, and 40 tappings.Mullard high speed valve / tube tester. The story of a huge mains transformer rewind.
The Mullard High speed Valve / Tube tester.
My precious Mullard high speed valve
tester has broken down. A shorted turn in the massive mains transformer. Giving
off the classic smell of burnt wax. There will be no chance of finding a replacement,
it's 40 years old. My only chance is to attempt
a rewind job on the original. It has 15 separate windings, and 40 tappings.
Removal from the chassis is pretty straightforward. Making a very detailed drawing of all the connections first.
The whole transformer has been potted ( set in varnish or shellac) this makes it very difficult to dismantle. Lever apart the laminations to remove all the side pieces, then using a hacksaw, cut off one end of the "T" shaped end. Now the bobbin can be hammered out.
I need the bobbin in tact so that I can count the turns off and measure the wire gauge. Over 6000 turns later the dismantle operation is complete. All the replacement wire is available from C.P.C. or Maplins.
Make a hand crank winding machine with a counter taken from an old tape recorder and set up the bobbin for the rewind.
To be continued................................
6,530 turns, over a mile of wire later the transformer is completed. Refit and test, using a variac to bring up the mains voltage gradually, first just the heater supplies, then with full H.T. Miracle! it's fine, except for a bit of a buzz from the iron laminations. I could not get them so tight as the original.
If anyone out there needs the circuit diagram and the specification of the transformer, then contact me and I will send them on to you.
I have most of the cards for the tester so if you are missing one, then contact me and I can send you a photo copy.
See the transformer bobbin half wound, with the makeshift hand crank.
In the second picture you can see the completed transformer, under test using a variac.
Many questions have been asked as to the best instrument for testing valves / tubes. AVO or the Mullard, they really don't compare, each instrument has it's different job to do. The Mullard will test a set of radio valves in a few minutes with results you can trust. In particular the grid current, or gas check, on the Mullard it is much more sensitive. On the Avo a test and check can take 10 to 15 minutes for each valve and great care must be taken with the AVO settings, It is so easy to wreck a valve or the meter movement with a careless settings. This can't happen on the Mullard, apart from selecting the wrong card it is pretty fool proof. The great facility of the Avo is the valve performance can be accurately measured and compared with other similar valves, so it is a simple matter to select valves for matching, in an amplifier for instance. The gas check however is very insensitive and a reading of 5 micro amps should be considered a bit suspect. Once you have your tester working well It is important to keep it in a dry atmosphere, as they are stuffed with transformers and chokes, which will all stay healthy if the moisture is kept out.