Wireless of the Week - week 45

[Bob Carriere]

....according to my PS computer power supply my input varies from 112v in the day time and up to 123v right now.....

I use a variac and can dial in my input at exactly 110 or 112v..... when using the CPP 2

We also have a lot of 2 or 3 second blips on our power almost weekly.

cheers

[Bruce MacMillan]

The rectifier #1 uses several 6X5 tubes which are notorious for developing heater to cathode shorts. This grounds the B+ lead and burns out a winding. The power transformer & chokes are all encapsulated in one unit. If one winding goes open then it becomes a small boatanchor.

I always use a variac to bring the power up to 110V and no more. This ensures I don't exceed the input voltage but also allows the filaments to completely warm up. A line voltage of 120+ volts is a 10% increase in output voltage. The 540V B+ to the xmtr is now close to 600V and may stress the rectifiers resulting in a failure.

My unit has been in use for about 10 years with no problem and it's much quieter that using a CPP2 to power the dynamotor. (also much lighter )

[Bob Carriere]

My unit has been in use for about 10 years with no problem and it's much quieter that(n) using a CPP2 to power the dynamotor. (also much lighter )

....that you have a one of a kind power supply that feeds directly the HT and LT to the rec/transmitter without using the dynamotor???

Cool....... was it homebuilt?

cheers

PS..... Duh ...read your email again..... you are using the rectifier #1 which is supplied by your variac...... never actually ever seen one.....

....so, I assume, the regular power supply unit fed by the CPP 2 has the same 6x5 (3) tubes and susceptible to also being fried if the tubes go wrong........ will need to test my tubes and spares to make sure they do not have a grid short .......

[Bruce MacMillan]

The power supply I'm using is the one (photo #4) that Bruce posted. It is a psu that supplies the necessary HT & LT to the set. It was made for the military, mine is dated from the 1950s.

The CPP2 only provides 12V or 24V to power the normal vibrator/dynamotor psu for the 19 set. You don't have to worry about the tubes/valves. The normal psu uses a 0Z4 rectifier.

The specs of the Rectifier#1 states that it works from 90V to 135V using inductors to regulate voltage but I'm not sure how well it works, as stated there are a lot of dead ones out there.

The photo shows the rectifier #1 in use here.

[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.