[HBR] Another Receiver Project -- HBR-4, Part 5

[waltah at earthlink.net]

I have only one of the crystals I need because Yaesu jiggered the 
conversion scheme for lowest cost and eliminated the crystals for 
80 and 20, changed the scheme to use a crystal that's no good for 
me on 40, and my parts set was owned by a CB'er who changed 
the 10 meter crystals for his band.   So all I've got is 15 meters.   
But that's enough to get the oscillator working. 

I have ordered the rest of the crystals for it -- five HC-49/U, ranging 
18 Mcs-42.5 Mcs for now, with space for a total of 11 bands -- FT-
101 bandswitch.  

I contacted ICM where I have in the past bought crystals for such 
projects.   Unfortunately the website is a lot more cumbersome for 
ordering than it was the last time round and they seem indisposed 
to fix that.  Basically you must simply email them with a list of 
what you want.  

Does anyone know a reliable supplier of '1 each' quantities of 
crystals who has an up-to-date ordering process?   Guess I'm kind 
of spoiled by places like AES and online metals.

Lessons relearned while working on the transplanted TEMPO One 
(Yaesu FT-200) VFO:  

1.  Even small 'jumpiness' of frequency -- like stable at one 
frequency for 10 seconds, then a jump of 30 cps, and stable there 
for 15 seconds, then a hop back -- means a bad part breaking 
down.  

Voltages in a vacuum tube VFO will be higher than those in a 
bypolar transistor one, even if there isn't any plate voltage on the 
tank circuit, because the circuit Q is higher.   (Losses in the 
biasing circuits for the transistor and in the transistor itself are 
higher than in a vacuum tube circuit.)   Never trust those small 
silver mica caps that Yaesu uses so freely.    It's best to replace all 
the SMs in the VFO box with the best NP0 ceramics you can find.  

(Four other frequent causes of 'jumpiness' -- (1) VHF or UHF 
oscillation -- all tubes worthy of VFO use will do this and one 
should start with a 100 ohm resistor connected at the grid with a 
1/8" lead.   (2) Inadequately filtered B+.    (3)  Tube with an internal 
short or loose internal structure -- very light tapping of the tube will 
expose this.  It is best to have at least three VFO tubes on hand 
for initial testing.   (4) For Hartley or Colpitts circuits, heater to 
cathode leakage -- the biggest disadvantage of these circuits.)

2.  Yaesu's scheme was to build a good VFO, populate it with 
inexpensive caps, then reduce the drift with temp compensating 
caps.  As a result, replacing Yaesu's silver micas with NP0 
ceramics will give you a VFO with way too much temperature 
compensation.   The original basic non-adjustable TC capacitor (7 
mmf, N750!) should be replaced with an NP0 at the same time the 
silvered micas are done.

They'd have been money ahead if they had started with good caps 
and used a fixed temperature compensating cap, value chosen at 
the end of the design process, rather than cheap caps, two TC 
caps, and an air-variable differential cap to allow adjustment.   
Especially since the differential cap was always set to mid position.

Walt 
KJ4KV