I unpacked a recently acquired SB-220 today and fired it up prior to
rebuilding for 6M.
All seemed to go well and power was 1KW out on 10M. After awhile my nose
told me something was wrong.
Turns out the filament xfmr was very hot and was even a Dahl replacement
unit. After some forced air cool-down of the xfmr I flipped the amp over
and finally discovered C-4 was very leaky and allowing lots of AC ripple
from the 125V PS to go right up the CT lead of the filament xfmr. The
prior owner had already buggered the circuit with a FWB package and what
looked like a filter cap snipped from some other source....not a Heath
original. I restored the circuit to the original HW configuration and the
ubiquitous Mouser 22MF/160V cap for C-4. Thankfully the xfmr survived
and delivers a healthy 4.86V at the socket.
Makes me wonder how many SB-220 series have lost the filament xfmr due to
the infamous C-4??
On a slightly different note Rich makes mention of filament xfmr
protection from a tube short on his Web page but it is a bit confusing.
Maybe you can elaborate a bit Rich.
Summertime suggestions for SB-220 series owners:
1. Check those HV filter caps, replace if showing leakage. If no
leakage, measure the voltage across each one. In the SSB position they
should be roughly 370VDC and a +/- variant of 10-15V is OK with the 30K
resistors. A wide range of readings and it is time to consider
replacements. The Harbach assembly is an excellent choice since he uses
1% matched CGS caps and 100K resistors. They still fit into the original
coffin with no air circulation but it is better than original.
2. Replace the 200pf red rectangular grid bypass caps with 500V dipped
silver micas. Later amps used 115pf supposedly to better compensate for
grid XL. I use 120-130pf since they are on hand...work fine. Those old
block style caps are known to crack and go bang.
3. Check the output bandswitch for burnt contacts.
4. Measure the value of the parasitic suppressor resistor. It should be
47 Ohms. If over 60 it is time to replace with a new one or alternative
offerings.
5. Check the 500pf 80M loading mica, C-56; they like to split open.
6. Check the plate blocking cap, C-29 for any signs of cracking. I
sometimes wonder if this is really a RF rated cap since I have found many
bad ones.
7. Install a pair of back to back diodes across the meter leads. Any
1N400x series is fine. Original meters are virtually extinct and the
diodes will protect from a tube flash over or short.
8. Burnish the relay contacts. I have yet to replace one due to hot
switching but a little PM goes a long way. Make sure it engages with a
good clunk. Remove and compress the spring a bit if necessary for good
action.
9. Most SB-220's and early 221's will have the small Johnson 154-9 Tune
cap or the OEP 26-131. This cap is prone to arcing since it is only
.075"/3KV spacing. Later amps and Heath replacements were the OEP 26-164
which is about .090/3500V spacing. It will still arc on occasion when the
amp is mistuned but not melt down as did the original. There is no
Johnson/Cardwell equivalent. If there is enough interest I will have the
26-164 reproduced.
10. Check the fan. After many years and no lubrication they slow down a
lot. Replace with the Harbach unit...spend the extra $1 for the high
speed version, particularly if you live in a 50Hz world.
That's 10 subjects....others can fill in the blanks I may have missed.
The SB-220 is still one of THE BEST amps ever produced. It just requires
a bit of TLC after 25 years.
I also have a small supply of original SB-220 RF deck parts that have
been removed from 6M conversions. (SB-200 series also) Bandswitches,
input coils, tune and load caps, micas and doorknob caps, etc. All in
excellent condx at reasonable prices. Private E-Mail for info.
73....Carl KM1H
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