[Hallicrafters] SR-2000, SR-400, SR-400A and HA-20 Temperature compensating capacitors

[hallicrafterssr2000 at k9axn.com]

Good afternoon: 

Found a great replacement for the difficult to find dog bone caps in the VFO of the SR-400, SR-400A, SR-2000, HA20, and other radios. These caps are a superb replacement for the dog bone type.

Search for Ceramic Plate Capacitors using advanced search with the International attribute on EBAY --- yes EBAY. If you have a better source let us know. Ceramic plate capacitors made by Philips, Mullard, or BCE were used for circuit trimming and were probably the most exquisite temperature compensating caps manufactured.

Their attributes tell the story. They are rated at 100 volts but test by the manufacturer to 300 volts. I tested them to 600 volts with no damage or value warp. The N150 are PG, 2% caps and +/- 30 PPM. Before you disqualify the +/- 30 PPM, remember, even the C0G/NPO and all other commercially available class 1 temperature compensating caps are +/- 30 PPM whether they are 1%, 2%, 5%, etc.  The +/- 30 PPM is universal regarding the temperature compensating value. The 2% represents actual value of the cap.

The original caps used in the SR-400, SR-400A, and SR-2000 VFO were 5% caps, whereas the ceramic plate caps are 2%.  Another attribute to keep in mind is the reaction time of these caps. The original dog bone caps came in at least three reaction speeds. The reaction times of the caps used in the Hal VFOs were considered medium to fast. Caveat, The late model VFOs switched to ceramic disc temp comp caps that are slow to react. Is this a problem? No, it will slightly extend the time to stability. You can expect these VFOs to reach stability in any part of the tuning range in about 1 hour. After that they will wander +/- 10 of so cycles until you return from Holiday.

NOTE: The Ceramic disc caps used in the late model radios reach stability about 20 minutes later than the dog bone or Ceramic plate caps. 

What can you expect from the above VFO design?

Tune the VFO to 250Kc, turn the radio on from a cold start, measure the frequency after 15 minutes, then at 1 or more hours.  If all is perfect, the frequency will be +/- 10 cycles from the measurement at the 15 minute point and remain within a +/- 10 cycle range.  This is the drift neutral point.

Tune the VFO to 0 Kc then power on from a cold start.  Measure the frequency at 15 minutes then at 1 hour or more. You should measure approximately +140 cycles from the 15 minute measurement, with the majority of the drift occurring in the first 1/2 hour.  After the first hour, it will remain within a +/- 10 cycle range.

Tune the VFO to 500Kc then power on from a cold start.  Measure the frequency at 15 minutes then at 1 hour or more. You should measure approximately –140 cycles from the 15 minute measurement, with the majority of the drift occurring in the first 1/2 hour.  After the first hour, it will remain within a +/- 10 cycle range.


That is a perfectly compensated CTO VFO by design.  Read the spec for this VFO.  It can be made to be far better than the specs claim.  Remember it was mass produced and components were not hand selected.  

In the real world, due to 5% components and the +/- 30 PPM range for temperature compensation, you will rarely find the neutral/no drift point anywhere near the 250 Kc point, none the less it will be somewhere in the 0Kc to 500Kc range.

The +140 total drift at the low end and –140 at the high end is a result of the variable tuning capacitor having a temperature compensating value of +80 PPM.

If it is fully meshed, the positive temp comp drift is significant and if unmeshed it will contribute almost nothing to the positive temp comp drift. The temp comp in this VFO is designed to be neutral at the 250 Kc point.

The Halicrafters engineers worked with some very forward thinking VFO compensating designs to negate the effects of the variable tuning cap. A bimetallic end plate and series negative compensation was considered. They decided the conventional use of wide spaced copper plates was more than adequate and no further enhancement was necessary, even for the most demanding clients.  

NOTE:  They had considered using series negative compensating caps to balance the plus character of the tuning cap but found it not worthy of consideration for a VFO with a 500Kc range --- they were right.

If you consider a general coverage radio with a Local oscillator that covers several Megacycles, series compensation is unavoidable.  Case and point, the SX-100 design includes series compensation and has an exceptionally good drift personality. 

When properly compensated, it will be very difficult to detect any drift at any point in the tuning range. 

Don’t believe you will find a CTO or PTO VFO that will outperform these instruments. 

Hope you found this of interest.

Kindest regards Jim K9AXN