The AM Forum

While finishing up alignment of a Hallicrafters HT-30 that I restored for a friend I used a receiver to calibrate the VFO.  I noticed the VFO note was a little rough and my scope showed some noticeable ripple on the regulated B+ to the VFO.  I was a bit surprised since I replaced both of the filter caps in this supply.  The low B+ supply in the HT-30 is a choke input with a 9 henry choke and a 40 uf capacitor followed by a 10 henry choke with another 40 uf cap.  However, the B+ feed for the regulator tube (and thus the VFO) is tapped off after the first choke which seems odd.

Coming from the first filter section there is a 2.5K wirewound resistor followed by another 1200 ohm resistor which then feeds the 0A2 regulator tube.  I tacked in a 47 uf capacitor at the junction of the 2.5K and 1200 ohm resistor and this cleaned up the residual ripple and gave the VFO a pure note.  At the same time I checked the current limiting resistor for the 0A2 and its current is in spec in both standby and loaded to full output.

So, am I missing something here or is it reasonable to expect that a single section choke input filter will be sufficiently clean to feed a VFO?  The transmitter didn't sound too bad with the original circuit but it was noticeable.  The later HT-32 and HT-37 switched to a single choke in the LV supply with a capacitor input filter.   I would have liked to switch the regulator feed to the output of the second filter section but the second choke is only rated for 80 mils so the additional drain of the regulated B+ could be a problem.

The HT-30 is an interesting early SSB rig, it uses a 50 Khz. L/C filter system with a pair of 807 output tubes.  It only puts 400 volts on the output tubes and is rated for 35 watts output SSB/CW and 9 watts AM. 

807's??  Mine had 6146's.

Had it in the 60's and 70's mainly to drive VHF/UHF TX converters and it would sometimes do duty at a M/M contest station driving an NCL-2000 to full power with its mighty 25W. Never had hum complaints that I can recollect.

Carl

Carl,

You are correct, 6146s.  I just rearranged my tube storage and I had 807s on my mind.

(and no, I have not been drinking any cold 807s)

Roger,

   I wonder if the power supply is solid state or tube rectifier based?
I ask because if SS, and using ordinary recovery time diodes (LIKE 1N4007/1N5408) the spikes (< 5 us wide) from the delayed reverse recovery time will come straight through a choke input filter and can cause a 120 hz buzz from low level audio stages, or possibly FM from a powered VFO with these spikes on the power buss.

I converted a Central Electronics 20A to SS, and changed the filtering from Pi C-L-C to choke input L-C. The B+ was now around 290V, but the sideband carrier null was full of 120 HZ garbage. The first solution was to resonate the choke (4H & .47 UF). That killed the buzz completely, but the cap I used was not up to the task; it later failed. Eventually I added another R-C after the L-C for the low level stages. That worked fine too.

In your case I think the added capacitor at the junction of the two resistors is an ideal fix so long as you have room for it.

Jim
WD5JKO

Jim,

I did switch over to plug in SS rectifiers but I temporarily replaced the tube ones after I discovered the problem but with no change.  I did want to rule out the change to SS.

The HT-30 is fairly spacious underneath so space was not a problem.  It is a fairly simple rig to work on since there is plenty of room and it isn't that heavy (compared to similar vintage mid-power AM gear).   The hum wasn't that bad but it was noticeable when listening critically. 

Tomorrow the owner will get to install it into his SR-500 console and I will pick up his other HT-30 which will restored and then paired with my SX-96.

Rodger WQ9E