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Author Topic: Gonset GSB-201  (Read 1874 times)
KD8CEI
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Posts: 8




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« on: December 31, 2008, 12:10:38 PM »

gsb-201 has no output.  plate voltage is good.  cathode voltage is good, all tubes light. have tried several sets of known good tubes. tried 811a's and 572b's. keying the amp looks like antenna disconnected, infinite s.w.r. at radio. continuity check shows good test thru rf path.all components appear ok and test ok,  caps resistors , diode.  0 plate current, b+ at 1500v, during amp keying with drive applied. How about that -4.5 grid bias voltage? Having trouble measuring that voltage. The manual said that it isn't needed with the 572b tubes, however the tube reference shows them to need -2v in class B grounded grid.  It seems to me the exciter power is stopping at the tubes, and not flowing thru the triodes.  Anyone have input on this?
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KA5N
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« Reply #1 on: December 31, 2008, 12:38:00 PM »

Sounds like the R/T relay is not making good contact or is dirty.  Could also be a bad connection at the relay or coax connectors or the input capacitor at the cathode (filaments) of the tubes.  If cleaning relay amp should be unplugged from AC power and electrolytic capacitors discharged.  Use DeOXit(alcohol will work) and paper to clean contacts.  Use a small strip of paper (printer paper, kraft bag etc.) with small amount of cleaner on it and draw through contact while holding them closed with light pressure.
Check solder joints and all coax connectors including patch cords for shorts and continuity.
Good luck and Happy New Year
Allen
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WB2WIK
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« Reply #2 on: December 31, 2008, 05:06:39 PM »

I assume you know the GSB-201 is set up to use a separate transmitter and receiver, and not a transceiver, and therefore does not contain an internal T-R bypass relay??

It also was set up to have standby voltage provided by the exciter, it doesn't have any source of that internally...

What exactly are you doing?

WB2WIK/6
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KD8CEI
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Posts: 8




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« Reply #3 on: December 31, 2008, 06:43:15 PM »

I have a foot switch hooked up to the t-r switch terminals on the back of the unit. It uses a external 120vac power source. I rewired the original relay inside to do the proper change over functions. All of this is working fine. What I'm not sure of is the "external cut off bias" and confirmation of the -4.5 grid bias voltage inside. I am trying to get output from the amp.
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KA5N
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« Reply #4 on: January 01, 2009, 08:33:52 AM »

If you have zero plate current then either the tubes are cut off (by bias) or open cathode circuit.  P1 (called pigtail) has to be connected to J2 or there is no RF drive to tube cathodes.  When the amp is working normally then the bias voltage is developed by the cathode circuit (cathode becomes positive with reference to ground)and this causes the grids to become negative with respect to ground.  Normal operating bias for a 572b is close to four volts.  Since your amp is not operating normally there is no bias developed.  You either have an open circuit somewhere or the tranceiver is not connected to the tubes.
Your transceiver should show a high SWR reading because it is operating into an open circuit.
Allen
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N4VNZ
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Posts: 17




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« Reply #5 on: January 01, 2009, 05:53:35 PM »

If the amp has terminals for external cut-off bias, normally these should be jumpered. One of the terminals goes to a grid and the other is ground (probably, check the schematic). The grid connection was designed for negative voltage input from an exciter in order to cut-off the tube plate current during periods of stand-by (to prevent self-oscillation of the tube, and to lower heat). If you are not using this feature, then that terminal should be grounded. If you have a floating grid, it could explain why you are not getting output.

Good luck getting any info from this board...

73, Dave
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KD8CEI
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« Reply #6 on: January 03, 2009, 09:12:49 AM »

KA5N, you are correct! I noticed on the schematic that one side of the cathode circuit is at chassis ground and also there is a ceramic cap installed across the cathode supply before the choke.  My gonset had one cap on each cathode supply going to chassis, and also the ground connection had been deleted. I returned my gonset to the print specifications, and now I see with 60w of drive 500w out with 600ma of plate current. And the tubes arent so good.  now my new question:
Is 75ma of idle plate current exceptable? with four 572b tubes?
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KA5N
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« Reply #7 on: January 03, 2009, 11:23:55 AM »

Four 572b's should be able to do more output than 500 watts, however the HV on the Gonset is fairly low (the schematic I am looking at says 1500 volts at full load) and most amps run the 572b's at 2000+ volts under load.  Also the manual on Bama says that 75 ma idling current is correct for the Mark II while the Mark III does a bit over twice as much. So which do you have?  
Well 1500 volts @ 600 ma is 900 watts (DC) input so depending on your meter 500 watts doesn't sound too bad.  So is your meter reading average or peak?

Anyway I am glad you got it going.  Maybe a bit more work will improve things.
Allen
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KD8CEI
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« Reply #8 on: January 06, 2009, 07:30:11 PM »

 What is the purpose of the choke filter in series with the bridge to the filter caps? What will happen if I delete it? Will the voltage increase? I havent seen this device in other power supply designs.
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KD8CEI
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« Reply #9 on: January 06, 2009, 07:38:41 PM »

 Reply to KA5N:   My gonset is a GSB-201-3340. And the meter is a average meter. It is not of high quality. Just an aproximate measurement im sure. My volt meter only measures up to 1kv,  any sugestions on measuring higher voltages, say up to two or three kilovolts? Thanks for your useful information.
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KA5N
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« Reply #10 on: January 07, 2009, 03:14:56 AM »

OK Haskell,
All versions of the 201 have a model number ending in 3340.  With no letter it is the original, then A, B denote the Mark II, Mark III etc.  The Mark III has a filter choke in the power supply and 572B's (of course along the trail someone could have swapped the 572B's for the 811's.  So you must have a Mark III.
All this means that 75 ma idle current is too low and 500 watts average output is too low.  The tubes are run zero bias and there is no bias supply. Don't remove the filter choke.
I suspect that the tubes have been run out and you need a new set (and they don't have to be matched, but should all the the same brand).  Taylor brand from RF parts will work and many swear by other Chinese brands (all 572B's are Chinese except old Cetons).
There may be some other things that need replacement but I can't tell for sure with the info you have given.  Since you have the amp running, it might be
a good idea to use it as is for a while before springing for new tubes.  Just see how you like it.
Allen
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W6OU
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Posts: 186




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« Reply #11 on: January 07, 2009, 03:41:13 PM »

The choke reduces 120-Hz ripple and can improve static voltage regulation. The choke-input power supply was prevalent for plate-modulated amplifiers back in the AM days. If you short the choke out the no-load HV will increase.
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