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Previous Page SB-200 Restoration, Page 2
The completed SB-200 (above) was now ready
for testing. The top screening cover was re-fitted and the SB-200 hooked up on
the test bench. An HP signal generator and HP 8922H GSM test set were used to
make measurements.
The SB-200 was powered up, the tubes lit OK and the HT was good at just below
2400V DC. Transmit was engaged, the tube current rose to around 80mA on the
panel meter with a very small and acceptable drop in HT voltage. It was held in
this condition for a few minutes before switching off and checking for
overheating after the HT capacitors had discharged. Everything seemed OK.
The SB-200 was then hooked up to the signal generator and the HP 8922H analyser, The SB-200 was first tested in the 20m band. After tuning up the gain was measured at 10dB, with spurious emissions better than -40dB below the carrier (-60dB at low power). It was then tested up to the 10m band, all good so far ! It was also checked on 40m and 80m with a separate HP signal generator (as the HP 8922H has a lower limit of 10Mhz), all OK there. Now for it's first QSO since the rebuild !
The outer case was cleaned ('Cif' bathroom cleaner mousse spray seems to work well) and the SB-200 chassis re-fitted into it. It was hooked up onto my 20m folded dipole and the Kenwood TS-570D was used as the exciter. Tests into both the dummy load and antenna were 100% OK. To get 400W of output only 40 - 50W of drive power was required, confirming the just under 10dB of gain measured on the bench. Other tests were carried out to check for spurious emissions and TVI in the house, no serious problems were found.
So, for it's first QSO ! After listening on the SSTV section of the 20m band for a while I heard I0ZSG calling CQ SSTV. I crossed my fingers and keyed up with 10W of drive for a 100W output. Everything went perfectly. The power level was just over 90W for 9W in (I don't fully drive the TS-570D on SSTV). I0ZSG gave me and RSV of 595 'perfect' picture report (see below). The 'replayed' picture from him was fine too (see below).
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Later I tuned to the RTTY / PSK area of the 20m band and set up for RTTY mode. I heard Jim VQ9JC in Diego Garcia (Chagos Archipelago) calling CQ, split frequency. It was 17:02 GMT and the 20m band was dying due to darkness here. I set the TX power to 20W for 200W out and keyed up. I got a reply and a comment about the nice signal.
So, it was time for a well earned beer. It took nearly two weeks to restore this Heathkit SB-200 to full working order again. I'm certain there will be more I will need to do to it after I've run it for a few times. The intention is to run it for a while, then open it up and inspect it for problems before fitting the matched pair of "Golden Dragon" 527B's. I may carry out a design change to stabilise the -2V grid bias. I noticed a very slight 25Hz ripple on the RF envelope which I traced to ripple on the grid bias voltage. This seems to be due to the design as the supply to the grid bias resistor chain is only half wave rectified (by D17).
So if you hear me on the air running
75W or more, you are listening to this restored Heathkit SB-200 linear amplifier.
LATEST NEWS
15/12/2005
During use I discovered a fault on the 20m band, the input SWR was poor, showing over 3:1 on the Kenwood TS570-D. This was traced to an open-circuit capacitor, C33, 360pF 5% in the 20m band input matching circuit (L3, C33, C34). Replacing it cured the problem, restoring the input SWR to back to 1:1.
The amplifier was run for a week and
inspected, all looked OK inside. The new 33 Ohm 1W ceramic composition
replacement resistors arrived from Digi-Key. These were used to replace the grid
carbon composition bias resistors R21, R22, R18. They are made by Ohmite, part
number OX330KE, Digikey part OX330KE-ND. The SB-200 tested OK after their replacement.
17/12/2005
I ran it almost all day during the "OK DX
RTTY Contest", set for about 90-95W output (just a gentle walk for this amp). No
problems at all on 15/20/40m. The repaired relay must have done over 400
operation cycles, it worked fine. The amplifier worked OK on 15m and 20m for every QSO I had. Without it the
TS-570D on it's own would have been getting too hot during long duty cycle /
long period transmissions at 95W. No sign of the input SWR problem on 20m, the
repair on 15/12/2005 (see above) has successfully cured that problem. A minor
problem was noted on 80m where the loading control seemed to require setting
just beyond the CCW end stop. I think capacitor C26 or C27 may be the culprit
here. I will replace them soon.
