Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.
One resistor measures 108 ohms and the other measures .2285k ohms according to my lcr meter.
Bingo make new ones with nos carbon comp 2 watt resistors
Is it necessary to use carbon comp resistors I have some 47 ohm metal oxide? 3 or 5 watt resistors. And I have some 2 watt 100 ohm metal film resistors. I have new copper wire to rewind either with.
Due to their non-inductive properties, Carbon comp is definitely the best choice if you can find them. They're getting harder to find. The Metal Oxide film types are probably spiral deposited film. You can also use a pair of 100 Ohm, one inside the inductor, the other outside.
108Ω is barely usable; 228Ω is too high. The resistor needs to have low inductance (carbon composition meets that need). Though 2W cc resistors are no longer manufactured, I found eBay seller 'bigsmythe74' (http://www.ebay.com/str/bigsmythe74) sells NOS 2W carbon composition resistors. Since it doesn't have to be precisely 47Ω, I searched on price, and found 56Ω available for $1/ea + $4 shipping/order (http://www.ebay.com/itm/400880759592).
The datasheet for the Allen-Bradley cc resistors sold above shows Z/R ratio very close to 1. I've measured L = about 17 nH.
Definitely replace that as well.
As posted earlier check everything in and around the grids as problems there can also lead to oscillations.
You will have RF currents "flowing" into your power supply w/o a bypass cap!
You realize, a PA plate "choke" is really just a low-Q inductor that works to achieve parallel resonance (Hi-Z) in conjunction with the Plate tuning capacitor?
W8JI does a yeoman's job explaining this ... on the higher bands the design of the so-called "Plate choke" becomes a little more complicated (as the choke has some frequencies outside the ham bands where the PA plate choke can go SERIES resonant. THIS might be the core of the issue with this SB-200 maybe?)
It's worth taking a little time to review his webpage on this subject:
BTW, I *still* have RF getting by my RF bvpass cap on the 'cold' end of my plate choke on the 160 meter band on an old LK-500Za ... this is evidenced by the meter lamps glowing **brighter** when driving RF into the amp and the amplifier is ON, delivering RF to the output jack ... CLEARLY RF is getting into the 12V rectifier circuit, being rectified, and the bulbs get BRIGHTER when speaking into an SSB radio, OR using the amp with the rig in CW ...
I added a few thousand pF doorknob cap to the cold end of the choke, but, not much of a change took place, the meter lamps STILL glow brighter when the amp is outputting amplified RF on the 160 meter band ... the other bands (80 meters and up) are okay. Only on the 160 meter band do I have this issue ...
I’m gonna try and locate a suitable replacement for that missing cap today. I don’t have any hv bypass caps on hand. I didn’t realize it was missing until yesterday
What value is the "extra" one that was under the chassis? Could be some one moved it
The one under the chassis is part of a tuned network with a coil, it's not bypassing the B+: The network is installed across the LOAD capacitor, placing it in parallel with the RF output.
If there's no bypass at the bottom end of the plate RF choke, that is a big problem!