Wireless of the Week - week 45

[David Dunlop]

After my initial reply to your post, I started wondering how on Earth I had ever rebuilt my CPP-2 after finding it in a completely disassembled state without references.

Did some digging in the basement Man Cave which eventually led me to a little used filing cabinet drawer. Some patient sifting through a number of ACCO Binders eventually revealed copies of both manuals. Happy Dance followed.

With regards to the issue of failed Rectifier Supply Units, a local amateur friend of mine, Don Trueman, has had first hand experience with this problem. Following are his observations and solution:

"Here's the poop on 6X5's...this is a rectifier tube with an indirectly heated cathode. Indirect cathodes have a breakdown voltage between the heater and the cathode, and can withstand the voltages differences on the negative half going cycle. The problem with the app in the 19 set supply is the bridge circuit used for the 540 volt side. The reverse voltages are very high and the cathode of V1c is well above ground potential, so super-imposing the plate voltage reversed (see PRV or PIV) on the tube floating already puts a huge stress on the heater cathode break-down voltage. V1c shorts and takes out V1d, the two that fail. Answer is to solid state the two tubes, ie. 1N4009 diodes in salvaged octal bases...you do get high voltage showing before the rest of the power supply warms up and the voltage is somewhat higher than 540 but seems to work ok...I made these changes a long time ago (2004!) to both my ac supplies and they've been fine.
The power supply is a regulated unit (noisy mechanically) and when keying up to transmit, it may throw a ripple in to the regulating circuit (windings) that kicks off a voltage spike that tips the cart…"

Hope this helps.

Cheers,


David

[Bruce MacMillan]

Originally Posted by David Dunlop

Answer is to solid state the two tubes, ie. 1N4009 diodes in salvaged octal bases...you do get high voltage showing before the rest of the power supply warms up and the voltage is somewhat higher than 540 but seems to work ok...I made these changes a long time ago (2004!) to both my ac supplies and they've been fine.
David

This is a good solution and many people add a dropping resistor in series with the diode to bring the voltage down to what it should be. This mod can also be done to equipment that uses selenium rectifiers. You'll need a gasmask when one of them fails.

I was surprised to find the problem with the 6X5 was known before the war. Zenith used them a lot and was well aware of the high failure rate. The same issue happened with it's little brother, the 6X4. It was used in the RS6 spy set.

The important thing is these mods are reversable with no changes to the original equipment.