-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Patrick Barthelow
Sent: Friday, 6 January 2012 2:45 AM
To: amps@contesting.com
Subject: [Amps] SB 221 Opened up, for upgrades, repairs
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&ty
pe=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?
%% I would replace the load cap altogether, saw some recently on ebay ,if
none found, Try contacting W1QJ, KM1h or nt8o@redbird.net for a
replacement.%
What are recommendations re; redesign of grid grounding by direct grounding,
vs Heath Networks on the grids?
%% I did it (GG gnd > strapped to chassis) to my original sb-220, amp tunes
smooth and gain increase noticed. If your grid chokes are burnt etc, I would
do it.
Fit a 50 ohm 80w (or more w) glitch resistor in the B+ line with a 1 amp hrc
hv rated fuse in series. This limits fault current, preventing other
component damage during a glitch. Your meters have pee-wee diodes fitted
across them for protection and are probably shorted. # Replace with BTB 6A10
diode's to protect meters and also another 6A10 forward biased across R3 on
rear of metering board to prevent B- > god separation during a glitch, which
can damage the plate transformer.. %
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.....
%% Yep they (270uf) will be fine, need 35mm diameter to fit snug. 330uf* is
around optimum as recommended by KM1H. I recently rebuilt a sb-220 here
using them *. %
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?
%% Yes if you get a tube filament grid short it protects the FT secondary
from shorting to gnd and melting down. East to do, procedure from somis.org;
- service note - The SB-220 and TL-922 use V-cutoff bias instead of R-cutoff
bias during standby. If a stock SB-220 or TL 922 is operated on standby with
a filament-grid shorted tube, the +110v PS is shorted to ground -- which
causes the unfused filament xfmr to shortly short and burn out. . Converting
a SB-220 or TL-922 from V-cutoff bias to R-cutoff bias: will prevent this
from happening. The conversion: .. 1. Remove +110v from NC bias contact on
relay without disconnecting the +110v lead to the coil. 2. Unsolder the lead
of the 100k R that connects to the NC bias contact, move it to the NO bias
contact and solder it . When finished, the 100k-ohm R should connect between
the arm and the NO contact. During Rx, the drop across the 100k R is c.
25v. During Tx, the 100k is shorted out by the relay. %
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?
%% Replace the sockets they are inexpensive. Eg;
http://www.ebay.com/itm/2pcs-New-5pin-Ceramic-vacuum-tube-socket-3-500Z-U5G-
/150730328562?pt=Vintage_Electronics_R2&hash=item23183a49f2 %
No front panel HV meter indication. Tested, HV Meter movement is OK, at
terminals.
%% Did you take those weeny diodes off? If shorted and since R1/R3 got blown
out, I would think they will be 0 ohms, i.e. preventing volt drop from the
shunt resistor(s) acting on the meter terminals etc. If not the 5.6k to gnd
you mention below is the HV meter shunt resistor. Temp tack solder test
leads across it, connected to a good Q DMM to check. If ok and meter diodes
are sorted then check the meter selector switch.
See # above. %
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
%% Adrian . vk4tux %
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|