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Author Topic: Command Technologies 2500 High grid current no output  (Read 578 times)
K2ZJ
Member

Posts: 5




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« on: July 07, 2019, 07:32:17 AM »

I was wondering if someone could help me-

My HF2500 stopped working a few days ago.  I have owned it for about a year and its been rock solid. 

I was going to 40 SSB and keyed the amp - and there was no output power at all showing on my wattmeter.  I checked a few things, bypassed the amp and the wattmeter was OK, etc.

Plate voltage reads normal- but when I applied even 5 watts of drive, the grid current mtr pegged at full scale.  I checked several bands and antennas, same result- no output and pegged grid current. 

Since I have spare tubes I swapped in a fresh pair- no change.

The amplifier is very difficult to work on, but I took it apart and nothing looks obviously fried.  I checked the Grid Zener with a meter and it looks OK.

I'm not very proficient with amps, looking for someone who might point me in the right direction.

Thanks-

John K2ZJ
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AC2RY
Member

Posts: 726




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« Reply #1 on: July 07, 2019, 08:45:51 AM »

I was wondering if someone could help me-

My HF2500 stopped working a few days ago.  I have owned it for about a year and its been rock solid. 

I was going to 40 SSB and keyed the amp - and there was no output power at all showing on my wattmeter.  I checked a few things, bypassed the amp and the wattmeter was OK, etc.

Plate voltage reads normal- but when I applied even 5 watts of drive, the grid current mtr pegged at full scale.  I checked several bands and antennas, same result- no output and pegged grid current. 

Since I have spare tubes I swapped in a fresh pair- no change.

The amplifier is very difficult to work on, but I took it apart and nothing looks obviously fried.  I checked the Grid Zener with a meter and it looks OK.

I'm not very proficient with amps, looking for someone who might point me in the right direction.

Thanks-

John K2ZJ

As usually - start troubleshooting with checking that all power supply voltages are correct and regulators are working. Here http://www.vortexantennas.co.uk/hf2500bulbandregulatormod.pdf someone experienced the same symptom when 12 volts regulator got fried.

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K7RJB
Member

Posts: 185




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« Reply #2 on: July 07, 2019, 09:40:37 AM »

Give it a very close inspection for blown caps and burned resistors, burned switch elements, etc.

Check that the bias is switched (or whatever method applies).

Check that it behaves the same on all bands.

SSB will show no output unless the signal is modulated, use CW or AM at low power or supply a fixed modulation signal.
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K6AER
Member

Posts: 5690




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« Reply #3 on: July 07, 2019, 10:15:57 AM »

When you key the amplifier do you see any IP current. Acccess to the metering resistors and the power supply are very hard on this design.
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W1QJ
Member

Posts: 2945




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« Reply #4 on: July 07, 2019, 10:39:31 AM »

I have worked on many of these and have one here right now.  Older units that had open frame relays eventually need the contacts cleaned so that could be one problem.  Here is what you want to do.

1.  Turn on amp and verify that the HV is normal value.
2. With no drive power key the amp into transmit, observe "if" the HV dips down a little, and if you see idle plate current.
3. If the plate voltage does not dip down a little when you key up, you probably won't see any idles current.  If either are the case you have to follow the B+ over to the tubes and see if it has been interrupted.
4. If you do see idle current but no output then you have to investigate further.  It could be a problem in the tank circuit such as a loose connection at the blocking caps, which I have seen before.  It also could be a problem with the old style open frame relay.
5.  I have also seen the RG-8X coax go bad from the 4 to 1 balun over to the relay where it makes a severe bend and goes bad.

Well that is for starters.
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K2ZJ
Member

Posts: 5




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« Reply #5 on: July 09, 2019, 02:14:37 PM »

Thank you all for the suggestions!  But of course something different has happened !

I put the amp back together again so that I could try to observe the Plate Current etc.  At first - the amp seemed to work perfectly OK !  Everything seemed normal- good output power , low grid current etc.  So I turned it off and moved it back to the bench to secure all of the screws etc.

When I hooked it up again- it exhibited a totally new problem- as soon as I turned the amp on, the RX was attenuated, nearly cut off.  I watched my FT8 waterfall go blank.  I made sure that the relay cable from the rig wasn't shorted, its OK.  I left the amp on and when the timer light came on, the amp tuned up perfectly- but the RX was still gone.  As soon as I shut off the power, RX comes back.

This amp has the QSK circuit, with a Jennings RJ1A-26S vacuum relay.  It seems like the relay is being energized with +28 volts as soon as main AC power is turned on, but I have no idea how that might be happening?  Looking at the schematic it doesn't make sense ( at least to me!)

I am going to pull the relay and check it by applying voltage manually and using a meter to check the function.

Lou- any ideas?  You seem to have a lot of experience with these amps.

It kinda makes sense- something wrong with the T/R switching could have produced the high grid current readings I observed the first time.

73- John K2ZJ
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KM1H
Member

Posts: 5087




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« Reply #6 on: July 09, 2019, 04:11:57 PM »

I have worked on many of these and have one here right now.  Older units that had open frame relays eventually need the contacts cleaned so that could be one problem.  Here is what you want to do.

1.  Turn on amp and verify that the HV is normal value.
2. With no drive power key the amp into transmit, observe "if" the HV dips down a little, and if you see idle plate current.
3. If the plate voltage does not dip down a little when you key up, you probably won't see any idles current.  If either are the case you have to follow the B+ over to the tubes and see if it has been interrupted.
4. If you do see idle current but no output then you have to investigate further.  It could be a problem in the tank circuit such as a loose connection at the blocking caps, which I have seen before.  It also could be a problem with the old style open frame relay.
5.  I have also seen the RG-8X coax go bad from the 4 to 1 balun over to the relay where it makes a severe bend and goes bad.

Well that is for starters.

Im on the recommended servicing list for that amp and have had my share in here over the years.

As convoluted as that amp is it might even be something on the control board or a broken/pinched wire after your repair which was WHAT?

Carl
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W1QJ
Member

Posts: 2945




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« Reply #7 on: July 09, 2019, 05:07:02 PM »

Something external to the amp is shorting the relay cable perhaps.  Yank the relay cable out of the back of the amp next time the rx goes quiet.  If the rx returns then it’s a problem with the relay cable.  If it still quiet it’s on the amp.  Ever since Command Tech and QRO tech went out I’ve had many of born in for repairs.  Have a HF 2500 Commander in now along with the 2 meter Commander ll.  Also have a 3 tube QRO HF 2500.  Poor documentation on both types of amps is not unusual.  Try that simple test and yang the cable out the back.  Also try putting the amp in standby if the rx goes dead to see if that does anything.
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