No Power to the 866a's on my Viking Valiant

[radionut]

I was warming up my Valaint last night and came back into the room to see that neither of the 866a's were lite up. I check the fuses in the line cord and they are both good. Any advice on where I should start to look for the problem?

Many Thanks,
 

[Jim, W5JO]

Be sure it is plugged in to the power socket then check the off/on switch.

[radionut]

The rest of the radio is getting power as you can see the other tubes filaments are energized.

[W3SLK]

How about the accessory plug?

[radionut]

A good thought. The accessorty plug is inserted has and not been disturbed. I had the radio on the air yesterday afternoon. At your sugestion I pulled it out and plugged it back in. Still no power to the 866a's.

thanks

[WQ9E]

The 866A filamentary cathodes are powered from the LV transformer.  Early Valiant transmitters were built with insufficiently insulated wiring for these (rectified HV also appears on these leads) and they would break down and short to the chassis and each other generally taking out the LV transformer in the process.

I suggest NOT powering it up anymore until you pull the chassis and physically check this wiring.  If the 5 volt winding of the LV transformer was damaged but everything else is OK then replace the 866A tubes with SS and disconnect the 5 V leads to the LV transformer.

It is a pretty simple part of the circuit so a few minutes with your eyes and a VOM will locate the fault.

[W2VW]

Prolly a cold solder joint on the first fil lead downstream of the xfmr.

[w1vtp]

Good advise not turning things on until some rudimentary checks have been done.  With the unit unplugged and the 866s out of the socket, check for resistance across the filament connections.  If it's open, then either a cold solder joint or an open winding. If there is low resistance, then is the socket(s) OK?

OK, I just checked the schematic and I cannot believe that the LV (300 volt) supply is on when the 866 fils are on (same transformer!!!!)  Someone tell me that I read the schematic wrong!!  If that's true, any live checks should be made with the Valient UNPLUGGED and the connection made to check for fil voltage premade.

Make that connection, set the meter for the right ac range, plug it in and turn on the SW7 (maybe called POWER?) - the switch that turns on the 866 fils AND the 300 LV supply.  AC voltage???

Update please   I like the suggestion of SS the HV rectifiers but if there's a problem, it should be diagnosed first.

Al  BE SAFE!!!

[WQ9E]

Al,

Using the LV transformer to power MV rectifier filaments (or any other type of vacuum rectifier used for the HV plate circuit) is very common.  The MV rectifier cathode has to be up to temperature before HV AC is applied so using an additional 5V winding on the B+ transformer is typical.

But in answer to your question, yes in the Valiant all of the voltages except for plate voltage are present whenever the main power switch is turned on.

[Ed/KB1HYS]

Shorts to ground on the 866 filament lines were common. when doing the resistance checks of teh filaments, check from the filament pins to the core of the transformer. 

Also, check the fils on the tubes?  Unlikely that they would both fail open at the same time, but ya know, stranger things have happened...

[AB2EZ]

I agree with Dave (W2VW)... that a bad solder joint in the daisy chain that connects one side of each of the two tube's filaments to the transformer is a likely cause.

I had a similar problem with my Ranger. A wire running from the jumper plug to the filament of the first tube in a string of parallel tubes was not making electrical contact to the socket pin it was attached to. I tried re-melting the solder on that pin with my 200W gun... I added some solder... but there was still no electrical contact.

So, I completely removed that wire and cleaned it with some steel wool. Then I resoldered it to the pin... and all was fine.

Apparently some sort of non-conducting oxide had formed around the wire... and even though it appeared to be nicely soldered to the tube's pin... along with other wires attached to the same pin... there was no electrical contact.

Stu

[AB3FL]

A few things.  The fil voltage to the 866As is 2.5V, they are in parallel.  On my valiant the 2.5V fil winding would arc to ground when HV was present to I had to disconnect that winding and I acutally put in a 2.5V fil transformer just for the 866As.  It works fine now.  I would check to see if you have the 2.5 volts on the fil winding.  I agree with the others that it is probably a bad solder joint.  If all else fails, find a 2.5V/10A tranformer like I did


Tom - AB3FL

[W2VW]

That fil winding is a liability as it needs the insulation rating to handle the high B+ and any transients.

Some of us remove it completely and use a string of 1N5408s or similar.

That winding can then be used to buck the primary of the transformer to make the rig think it's 1955 again.

The nice 866 sockets can be repurposed for a nice 811 modulator.