[Heathkit] HG-10 -VFO power supply, DX-60B, TR Switch

[KHeck73 at aol.com]

In a message dated 12/22/2005 11:49:12 A.M. Eastern Standard Time,  
rjmattson at hvi.net writes:

I never  found one specific to Heath, but there are a number of published T-R 
 
circuits that are tied between the PA plate and the pi-network. Since they  
share the pi-net filter, any harmonics are attenuated and afford the rcvr  
additional bandpass selectivity.


..
Thanks to all for input on the VFO and T-R switch questions. This list is  
great. 
 
I opened up my old DX-60B and, at the risk of being too wordy, I can  
describe the T-R switch modification. I bought this in 1975 from a radio club  
friend, who had purchased it at a hamfest at some earlier date. I don't  know who 
did the modification, but it looks rather professional... like part of  the 
original kit. 
 
I think this mod may have a voltage doubling supply in it  for the T-R 
switch? I haven't powered it up to measure voltage, but the 'half  wave transformer' 
is a clue.  Maybe I could also use the additional  supply voltage for the VFO 
(?), although I suspect there is a reason the  builder went to the trouble of 
isolating the T-R switch tube from the rest  of the transmitter.
 
There is a Stancor PA-8421 power transformer mounted on top of the chassis  
in the otherwise open area (looking at assembly directions) behind the RF cage. 
 It is secured on one lug by the same bolt that holds a 3-lug terminal  strip 
underneath (labelled "L"). I think I should add an extra bolt  hole on the 
other side of the transformer to tighten it down. The  transformer has a 117V 
BLACK primary, 6.3V GREEN secondary, and a  "125V 1/2 wave 50 ma - RED" 
secondary. The transformer wires go  through a chassis hole with a rubber grommet. The 
6.3V wires are cut  and not used. There is a 4-lug terminal strip with lug #3 
securing  it under the chassis on the stud that holds down the back  side of 
the RF cage (near V4- 6DE7). One side of the  transformer's 125V output goes 
to lug 3, which is chassis ground. The other  side goes through a 22 ohm 
resistor, and the other side of the  resistor goes to lug 1 of the added strip. The 
anode (triangle side) of an  old-style 'top hat' diode is connected to lug 1; 
the cathode (banded  'brim of the hat' side) connects to lug 2. Lug 2 also 
connects to  the positive side of a 25uF electrolytic and a 1K resistor. The 1K 
resistor goes  overtop lug 3 (ground) to lug 4, which is connected to the 
positive side of  another 25uF capacitor. The negative side of both electrolytics 
are grounded  together to a chassis bolted lug. A wire from lug 4 goes up to 
the T-R  tube mounted inside the R-F cage.
 
The T-R tube is a 6AH6. It is physically mounted between the final  tube 
(6146), and the loading coil. It has a spring loaded shield overtop.  The wire 
from lug 4 of the above described voltage supply runs to  an insulated standoff 
post mounted under the RF cage. A 220  ohm resistor is attached to the post; 
the other side of the resistor  is connected to pins 2, 5 and 6 of the 6AH6. Pin 
3 of the 6AH6 is  wired to pin 2 of the 6146 final. Pin 4 of the 6AH6 is 
grounded. Pins 2, 5  and 6 are connected together. There are two '103' disc 
capacitors;  one between pin 3 and ground, and the other between 2/5/6 and  ground. 
Pin 7 has a 220 ohm resistor to ground. Pin 7 also has a '114' disc  capacitor 
coming off to a standoff where attached coax runs  back to a BNC connector on 
the rear apron of the transmitter, marked  'to receiver'.
 
Note: I haven't fired this up in 30 years, but the T-R switch did  work, 
complete with ear-shattering receiver sound (an HR-10B). One reason I  stopped 
using this rig so long ago was because after some use, an electrolytic  blew, and 
it scared me to death. I see now that the one that blew was one of the  two 
in this added power supply. It was an old 'Beaver - dry etched foil' brand  
electrolytic. I'll have to replace both of them, and maybe deactivate the T-R  
switch before bringing everything else up on a Variac.
 
-Karl. WD4LEO.