Friends,
I was asked to look at and do appropriate repairs /upgrades for a Heathkit SB
221.
Photo album of As-found 221, and a W8JI photo, also, are at:
http://www.facebook.com/media/set/?set=a.2322627235213.2096379.1535563951&type=1&l=675e023e94
The 221 has had the 10 meter modification, but I am interested in reviewing a
step by step on what/how the upgrade is supposed to be done. Need a source for
that content.
There were owner comments of spitting, arcs, in the output network, and I found
evidence of such on the load capacitor, and will fix by burnishing the arc
burned areas.
Harbor freight has some nice, cheap diamond dust burnishing tools, one is a
thin flat variety. looks suitable to burnish pointy, rough things on the caps
rotor/stator plates. yes/no?
The load cap rotor plates are not centered within the stator plates, easily
seen by eyeball. Air gaps maybe 70 -30 percent instead of a centered 50 -50.
Measured with my Harbor Freight digital calipers, (hard to believe at < $20
on special) typical plate gaps stator/rotor; vary 20 mils to 40 mils, some as
low as 15 mils. What load cap gap does one need at a kilowatt level, for
antennas that are say, 2.5 :1 impedance, matched by the load cap? How much
RF voltage is between the plates, typically, with mildly mis matched loads?
Does the centering go off over the years, due to pivot bearing wear?
Brass pivot bearing and locking nut on end of cap...has anyone played with
rotor centering adjustments?
What are recommendations re redesign of grid grounding by direct grounding, vs
Heath Networks on the grids?
I am replacing the original Filter capacitor stack, and bleeders. Are there
new caps of the same OD diameter to match the I.D. of the plastic holders?
Vendors, Digi Key or Mouser, etc?
The plastic capacitor holder ends, are slightly deformed over the years, from
heat from the bleeders, which have already been replaced with 100K 3 watters.
A set of Electrolytics are available, on hand of 500 V 270 uF. 85 deg C;
are they a suitable/ideal replacement for the original spec 200 uF 450V? A
little better headroom on voltage, and a little more capacity, perhaps better
HV regulation.....
I have seen mention of a mod to prevent transformer damage in the event of a
grid to fil short. is that a recommended upgrade?
I have worked with other Amps, including 4-400/ 3-500Z home brews, with the
heavy Ceramic sockets. I think the old ones were made by EF Johnson. Same
design as Heathkit's. I carefully pulled (straight up and out, without,
"wiggle") the Eimac 3-500Zs and noticed the pull friction was quite light,
indicating possible weak socket contact spring pressure on the pins, compared
to what I remember, on other amps.. Have heard that can cause local heating of
the pin with resultant solder melting. Any suggestions on the possible need to
replace or bend socket tabs, etc to tighten pin contact pressure?
No front panel HV meter indication. Tested, HV Meter movement is OK, at
terminals. Voltage dropping res on rect board, are each OK ( 3 x 4.7 Meg,
plus a 5.6K to ground ) Odd... visual inspection shows no causes for lack of
B + indication. Discharge stick to ground at a bottom of Plate choke, after
a minute, maybe two, of OFF time, and Power plug disconnect, still produced
a pretty loud, strong "pop", What is "normal" B + bleed off time, after
turning power off? Thanks, 73, Pat AA6EG
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