View Full Version : SB-200 SWR meter reading
06-01-2006, 11:23 PM
I'm sure something obvious is being overlooked, but, I get no meter deflection on SWR or Rel. pwr when feeding 100W through this amp. I have checked the diodes at the SWR bridge (replaced them in fact), and checked the continuity through the meter switch. Everything seems normal except it doesn't work. I can apply an ohm meter to the diode(s), (observing polarity) and I get meter deflection. The rel. pwr. sens. does adjust the voltage level in those cases. But it does not sense RF.
I have run 100W, 20m, into a dummy load through the amp both as feed-through and with the amp powered up and no meter deflection in either case.
What am I overlooking??
06-01-2006, 11:43 PM
Hi Dave and all,
Probably the terminating resistor(s) at the opposite end(s) of the sensing rods. The ohm meter should indicate 50 ohms. An SWR meter is a directional current transformer. More modern meters employ torroidial transformers bifilar wound (one winding in each direction (polarity) with a single turn (a wire through the center) carrying the RF.
Tech note; each time the winding passes through the center of a torroid it's counted as a turn. Once you complete a turn around the outside you have another turn, two, not one. For that reason turns are counted on the inside only.
06-02-2006, 12:13 AM
In this case Heath used 100 ohm resistors and they are good.
06-02-2006, 01:31 AM
And the calibration pots if any? This is a toughie by "remote control", sure wish I could get my Simpson 260 in there.
06-02-2006, 01:45 AM
The calibration pot is the "rel. pwr. sens." I mentioned in the initial post. It responds just fine with the ohm meter voltage source on the sensing element. In this case, it is a Simpson 261 and a Data Precision 3500 4-wire multimeter.
06-02-2006, 02:30 AM
With what type of diode did you replace CR-1 and CR-2? They are germanium (i.e. 1N34 series) and NOT silicon (i.e. 1N914).
Check the .02 mfd capacitors that come off of the diodes.
Check the sensitivity control both for resistance and any short to ground.
The SWR meter function in the SB-200 is basically the same that was used in the AM-2 and other SWR bridges that Heath made around the same time.
06-02-2006, 10:03 PM
The original diodes appear to be good, (reading the normal germanium junction potential of around .3v)but I replaced them just in case with NTE109, a fast switching germanium with same spec's as the original 1N91. I have measured resistance across the .02's and they read infinite. If they were open it wouldn't cause loss of signal, they being just filters. The 100k pot reads correctly in all respects. Plus the fact that the ohmeter connected to the sensing elements causes meter deflection. That should mean there are no shorts or opens from the sense element to the meter.
06-03-2006, 01:42 AM
Did you check the meter switch contacts. On one of my SB-200's there were times when I would get erratic and/or no readings in the forward or reverse positions. Then, in a blink of an eye, they would work. Found a broken switch contact.
06-03-2006, 01:50 AM
I believe when I get meter deflections when I touch the sense elements with the ohmeter it shows the switch contacts have to be good.
Maybe I'll just sell this thing to someone who doesn't care if relative power responds or not. I think I want an SB1000 anyway.
06-03-2006, 03:55 PM
Just out of curiosity, does the SWR bridge work when the amplifier is actually in line? That is when the amplifier is putting out power.
You indicated that you are not getting any indication when running 100 watts through the unit which indicates, at least to me, that the amplifier is being bypassed. If you do get an indication when the amplifier is engaged but don't get one when it is not then that indicates a wiring error in the change-over relay.
In fact, that may be the case, a wiring error, if you don't get an indication when the amplifier is engaged.
06-04-2006, 11:33 PM
I mentioned that in the original post, though maybe not clear. I run 100w in bypass and up to 300w FSK through the amplifier, into a dummy load in both cases. I believe the SWR is probably OK, just very low because of the good load match. The rel. pwr. doesn't even wiggle
06-05-2006, 01:39 AM
The problem is fixed. A short run-down.
The person I bought it from had modified it by adding a standby switch to the rel. pwr. sens. switch. A good idea.
When the pot was positioned, the hot end was aligned with the right front foot/cabinet screw. a Bad idea.
When I opened it to make measurements, everything was good. Put it back together to run it; everything was bad.
Thanks for all the help and kind suggestions, but this was obviously something that couldn't be diagnosed long distance.
06-05-2006, 01:39 AM
Have you turned on the amplifier and actually put out any power using the amplifier?
I believe that some of the earlier versions of the SB-200 completely bypassed the SWR bridge when in "standby". That is when the relay is not activated there is a direct connection between the exciter input and the antenna jack which bypassed the SWR pickup completely.
06-05-2006, 01:46 AM
I guess I still didn't make that clear, I was using the amplifier as an amplifier. I put power into it and took out more power than I put in (in RF Terms).
06-05-2006, 02:17 AM
At least you found the problem!
They way that you were describing your usage sounded like the amplifier was not actually in use.
Now why someone would want to disable the SWR function doesn't make any sense to me. The meter switch takes care of moving the meter around.
Frankly, I like to monitor the plate current for a number of reasons including if any problems develop in the linear the plate current is going to be the first indication of trouble.
06-05-2006, 02:38 AM
No one was trying to disable the SWR function. #What the guy did was to correct the most common complaint with the SB200 in that you have to turn the power off to by-pass the amplifier. #It is simply a switch in the antenna relay line so you can leave the amplifier powered up and still run bare foot if you want. #Most amplifiers have a standby switch, but the SB200 didn't.
None-the-less it is operating normally now.
Oh yeah, I like the plate current, but normally monitor grid current, as it can kill you much quicker than plate current can.