CDS thing is dead

[steveW]

My dearest 71T died - it suddenly stopped as though somebody switched it off. Everything seemed normal, i.e engine cranked over CDS made the usual electronic whine, but it wouldn't start.

When the friendly chaps from John Abbot Garage (the local Porsche boffins) arrived, Tim fitted a spare CDS unit and, pronto - it was back to normal.

What causes the CDS to die and how easy (and expensive) are they to fix?

Apart from that, it was another beautiful day in South Africa!

[Zendalar]

My cds-died and stuttered because of a bad solder joint. It was the solid core wire that went to the electrolytic capasitor. I re-soldered all connections and Voila, it works like a charm.

They are not expensive to fix, if you can find the right parts. Thyristors are the only expensive part inside, small transistors are hard to find. 3055? transistor that resides inside, costs about 1$. Then there are several different wattage resistors, they might be hard to find, but usually a higher wattage level resistor with right resitivity will do.

Well, if the transformer goes bad inside (very unlikely) then you might have to say bye bye to the CDS, unless you get yourself another cds unit with good transformer.

I recall that Warren was fixing these thingamobs and I bet he would gladly take your broken CDS.

[Targa77]

If you're a rich man, you can purchase a Bosch factory unit. I purchased a brand new Permatune for less than $300 that plugs right into the original harness. Comes with a two year warranty. Of course, Permatune recommends that you purchase a new coil (they sell them for around $50.00) at the same time.

[Early_S_Man]

Steve,

Yes, they are easy to fix ... typical point-to-point wiring mixed with a crude, single-sided phenolic printed-circuit board as used in 1950's vacuum tube radios and televisions ... several decades ago! However, troubleshooting them is rather a 3-D jigsaw puzzle/nightmare!

If your CDI-unit stopped whining or whistling, then it is probably the 2N3055 transistor used in the inverter or 'DC=to-DC converter' section. Relatively cheap component and easy to find, though getting to it requires unsoldering every one of the 14 solid-conductor wires to the printed circuit board! It is really no wonder that the people who repair them are few, and getting fewer, as they are very, very tedious to work on! None of the other semiconductors are standard parts designated with '1N' or '2N' numbers! The two transistors in the trigger circuit are made by Texas Instruments and designated 'T 4018' ... and various Zener and switching diodes are non-standard parts as well!

I am always amused by the use of the word 'boffin' ... ever since I saw it used to describe Jimmy Stewart's character as an eccentric engineer in the movie "No Highway in the Sky" ... an interesting story about the prediction of metal fatigue problems in early commercial British jet airliners!