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Author Topic: OM2500 Hard Fault.  (Read 643 times)
SM5GLC
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Posts: 291




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« on: September 15, 2019, 05:10:40 AM »

Trying to help a friend who has problems with his OM2500... This is phone support as he is far away, and I have no hands-on on the OM. It shuts down after 5-10 minutes, even if in stand-by, but seems to be ok if restarted, but then shuts down again. It looks just as if there is a spurious press on the "OFF" button. Now looking at schematics it seems to be a protection relay in parallell of the "OFF" that will trip if there is enough voltage over the 5 ohm resistor that sits in series with the capacitor string, down to chassie earth.  To me this looks like one of the filter caps are toast, but could this be one of the windings of the transformers breaking down? The OM amp has two transformers with 4 windings each feeding one cap, 8 in total, and producing 2,8kV. Anyone with experience with the OM amp care to comment?
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W9IQ
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« Reply #1 on: September 15, 2019, 06:03:35 AM »

I don't have experience with the amp but I interpret the schematic a little differently.

The two 10 ohm resistors in parallel at the bottom of the capacitor chain form the power supply for the relay by virtue of the impedance of the capacitor. The trip is controlled by the microprocessor through T6 on the control board. When the T6 gate is high, it places a 1k resistor, R22, in parallel with the series combination of R21 and the coil of Re11, preventing the relay from closing (a closure forces the trip)..

C18 provides a short power on time delay holding the relay off, presumably giving time for the uP to fully boot up and assert the desired state of T6.

If your friend can safely monitor the voltage on either side of the T6 gate bias resistor, R18, you can confirm this interpretation of the circuit logic.

For a hint of what would cause the uP to assert a trip, take a look at the input parameters monitored by the uP. You could probe these logic inputs directly and probably find the cause. Temporarily shorting the CE of T6 (for a short time!) will allow you enough time to catch the offending input or even its corresponding front panel indicator. It may not be the root cause but it will get you in the right vicinity.

- Glenn W9IQ

« Last Edit: September 15, 2019, 06:15:37 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K6AER
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Posts: 5743




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« Reply #2 on: September 15, 2019, 07:24:28 AM »

The OM-2500 has a temperature fail on the tube in case the blower is defective.  Any thing over 80 degrees C will cause the amplifier to shut down. Is the main blower next to the tube socket working?

Idle tube temperature should be about 35 degrees C. At 1500 wats out, SSB, the normal temp will be about 55 degrees C.

After 210 seconds from turn on, timer run down, can the amplifier be placed into the operate position?

The OLED display has quite a list of fault conditions. Has your friend scrolled through the listings to see if their are any other unknown conditions?

Has your friend talked with Jim at Array Solutions yet? Their phone number is 214 954-7140. If so, what was his suggestions?
« Last Edit: September 15, 2019, 07:34:20 AM by K6AER » Logged
SM5GLC
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Posts: 291




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« Reply #3 on: September 15, 2019, 07:52:55 AM »

Thanks for the information. First... we do not have access to a proper schematics, this is the early version with a non-cpu control board, and the only schematic available is a reverse engineered, that may contain several errors. The amp is bought and used in EU so doubt that Array Solution should be invoked. The amp owner has sent an e-mail to the factory and expect to hear from them, but as several jucy DX coming up on top band he is eager to get back on air, pronto.

He recently replaced a faulty GU84b and was happy for a few months before this started to happen. He had yet another NOS tube and tried that today, and it seems to work, no shut down. So it may very well be related to a faulty tube, or corroded tube socket. The owner will try swap back again to the "faulty" tube to see if the problem is still there. GU84 are getting hard to find so he is a bit worried. The tube we suspect to be a problem, is delivering plenty of power, and there is no flash nor bang when the amp shuts down.

Will be interesting to see the outcome Cheesy
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W9IQ
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« Reply #4 on: September 15, 2019, 08:07:40 AM »

I don't think the blower will come on when the amp is in standby and it has not been placed in operate.

If the owner did not do the required adjustments after replacing the tube, that is the problem.

Send me your email address and I will email you the schematic and adjustment procedure for a new tube: w9iq at arrl dot net.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
SM5GLC
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Posts: 291




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« Reply #5 on: September 15, 2019, 08:47:12 AM »

Glenn,
You got mail Cheesy
/Lasse SM5GLC
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K6AER
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« Reply #6 on: September 15, 2019, 03:08:53 PM »

Glen,

I did not know that the early OM-2500's were a discreet design. Do you think the tube/shut down is a heat issued due to improper biasing?
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SM5GLC
Member

Posts: 291




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« Reply #7 on: September 16, 2019, 12:57:19 AM »

I have my doubts that a slightly misadjusted bias setting would cause a hard fault. Still, bias should be set to a proper value.

This amp had an odd problem with the control board, and this was verified by substituing the board with a known good. As the schematic is not public, and I only hade the faulty board, not the whole amp, I simply swapped all active components. Returned the board and it worked! Still a bit bummed I could not pin-point what exactly was causing the error, but rather this than sending the board back and forth several times. Now almost a year later this hard error shows, he tried swapping control board once again with no change, hard error.

Hopefully OM Power will respond and have some useful suggestions, they have probably seen all faults that will occure.

The schematic that Glenn sent me is indeed the reverse engineered one, made by SM5CKI long ago when he had problems with his controlboard amongst other fault, and he had a dispute with OM. I think the root cause here were cold solder joints.

I am confident we will track down the problem and fix it.
Cheers
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SM5GLC
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Posts: 291




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« Reply #8 on: September 20, 2019, 03:05:42 AM »

An update... It turns out that the culprit is the tube! After reaching a certain temperature it develops an internal short. Replacing with known good GU84 solved the problem. OM Power sells NOS GU84b, but at a price €650 +VAT Ouch!
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