Viking 1 HV filter cap

[n4wc]

I would like to change the 8ufd to a 45ufd that I have on hand.  The 5R4's have been changed to diodes (not sure what type).  Other than that it is stock.  Any thoughts?
Thanks  Bill   N4WC

[W1EUJ]

Go ahead. Tell us how it goes.

[K7NCR]

Is this a choke input filter? Whithou inrush protection, wont this = smoke?!

[WQ9E]

Bill,

I am not quite sure of the reason for this change but a few thoughts:

1.  As stated earlier, with solid state rectifiers and a big filter cap I would add inrush limiting to not stress your choke and transformer.

2.  If that is a 45 uf oil filled capacitor it is pretty big if it has an adequate voltage rating so I presume you are mounting it somewhere other than the original location?

3.  With choke input and a fairly steady load on the supply you don't really need a big filter cap in the Viking 1 HV.

Rodger WQ9E

[w4bfs]

If the VI has the same plate transformer as the VII then it is 1800 V (total) center tapped ... the no load dc output voltage will climb to 1250 to 1300 V so changes in the plate supply have to be evaluated ... going from the thermionic rectifier(s) to solid state will raise the output dcv considerably.... just be careful and go slow ...73 ....John

[WD5JKO]

Bill,  I have had two Viking I's over the years and in both instances I replaced that original HV oil capacitor and solid stated the HV and LV supplies. I'd like to add to the wonderful post just added by Roger, and I will add comments after **:


Bill,

I am not quite sure of the reason for this change but a few thoughts:

1.  As stated earlier, with solid state rectifiers and a big filter cap I would add inrush limiting to not stress your choke and transformer.
** Yes good idea, but if you add push to talk (key the HV transformer primary), make the inrush time no longer than 100 ms. Alternatively allow the HV B+ to remain on all the time, and key the transmitter with grid blocked keying (for AM), or use a relay to switch the  HV B+

2.  If that is a 45 uf oil filled capacitor it is pretty big if it has an adequate voltage rating so I presume you are mounting it somewhere other than the original location?
** I quick check with Digikey shows that 100uf @ 500v is cheap and readily available. Look at Panasonic P7448-ND as an example. Two of these in series will provide 50 uf @ 1000volts. Just bridge each one with a suitable equalizing resistor (maybe 100K 2w). These caps could be mounted to a small perf board and use less space than the original oil capacitor yet provide over 10X the original capacitance.

3.  With choke input and a fairly steady load on the supply you don't really need a big filter cap in the Viking 1 HV.
** Very good point when on AM. If Bill wanted to modulate to a lower frequency (less than 100 hz), then the increased capacitance would help minimize modulating the power supply. Also for CW changing the amount of capacitance will have a huge effect on the keying shape and sound. A choke input filter usually has big > 20% swings when the load is transient in nature (like CW or SSB) even if the critical inductance requirement is met. Increasing the capacitance helps here, as does resonating the choke (only works if the choke is a 'smoothing' type as opposed to a 'swinging' type).


** One other point to ponder is to take the LV transformer 5 volt AC winding and use it to 'buck' the incoming primary AC line voltage to lower it by 5 volts. This will lower the filament voltage such that it will be closer to 6.3v with today's higher line voltage.

Rodger WQ9E

** Jim WD5JKO

[WA1GFZ]

My VII plate voltage went to 760 VDC when I did solid state and added a second cap on an l bracket between the chassis and case.

[n4wc]

Thanks for the input.  During AM operation other stations say there is no detected hum on the signal.  I think I'll leave well enough alone. 

Do you think one of those ZIM inrush limiters will work?  The ad says 300 watts, which I do believe is right at the edge for this xmtr.  My filaments are at 6.8vac and th HV is pushing 800vdc.  A variable autoxfrmr will work but I don't have one and they probably cost more than $50 shipped.  My house ac is typically 123 on a fluke dvm.

Thanks

[W1EUJ]

Do a resistor in line in the primary with a timed relay - after charging, short the resistor out.

[w4bfs]

6.8 vac is too high for fils ... please consider taking 5 V fil windings and wiring them in buck config with the primary of the lv xfmr ...should cool it off some...73...John

[WA1GFZ]

I have 126 VAC here so put both 5 volt windings in series with the LV primary.
Now 6.3 and the added thing the transformer barely warms up after many hours of use. Before the change it got quite warm. I just ripped out the TVI filter and installed a terminal strip to do the wiring near the transformer. also replaced the line cord. The 6al5 was removed and a second fuse socket added for the LV transformer.

[WQ9E]

Adding a couple of internal inrush current limiters is cheaper and will work better since you want separate inrush limiting for both the filament/LV supply which is on all of the time and the HV supply which is only on when transmitting.  Inrush limiters are available from Mouser and Digikey.  I would suggest a CL190 for the low voltage supply and a CL70 for the HV supply.  Just wire them in series with the respective transformer primary and the only caution is to keep them away from other components as they do generate a fair amount of heat.   

Total for the pair from Mouser is less than $3 plus shipping. 

Rodger WQ9E

Do you think one of those ZIM inrush limiters will work?  The ad says 300 watts, which I do believe is right at the edge for this xmtr.  My filaments are at 6.8vac and th HV is pushing 800vdc.  A variable autoxfrmr will work but I don't have one and they probably cost more than $50 shipped.  My house ac is typically 123 on a fluke dvm.

[WA1GFZ]

That will not fix the heater voltage or the hot LV transformer problem.

[WQ9E]

GFZ,

I agree that surge limiting alone will not lower the steady run voltage, I was just quoting and responding to the question regarding an external ZIM limiter which would not do a good of separately addressing inrush limiting into the plate supply.

For curiosity I just measured the Viking 1 that is on my bench and at 121 volts in (my usual line voltage) the 6.3 volt line is either 6.46 or 6.48 depending upon whether I should believe my HP or Tektronix meter; if your line voltage is significantly higher some reduction would be in order.  This particular Viking 1 was rescued from CB service proving the basic toughness of the unit!

Rodger WQ9E