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Author Topic: Alpha 91B Weird Issue  (Read 1954 times)
N4ATS
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« Reply #15 on: June 23, 2019, 11:12:19 AM »

R1 is marked 560 Ohms and the R1 I am looking at is on the SS board between K1 and K2 connecting to C1. R4 and R5 are 10 ohm 15 watt

The other R1 on the Filter Board is 10 Ohms 16 watts as well
« Last Edit: June 23, 2019, 11:15:16 AM by N4ATS » Logged
W9IQ
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« Reply #16 on: June 23, 2019, 11:35:23 AM »

I am thinking the SS is not working in either case. It is only when you connect the HV that there is enough load to smoke the SS resistors. What do you think?

Can you monitor the voltage across the SS relay coil?

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N4ATS
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« Reply #17 on: June 23, 2019, 11:59:28 AM »

But the SS is working , I posted a video on Youtube of it closing during power up. Sure , I will monitor the VCC to the coil and try it..Stand by
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W9IQ
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« Reply #18 on: June 23, 2019, 12:06:39 PM »

I watched the video several times. I could not tell the SS was working - how do you know it is? Remember that all it does is switch out the series resistors. If the load is light, the resistors will still power most of the amp - like the blower that I could hear.

Here is what I would try. Leave the HV disconnected and power up the amp.

1.) Do you have ~40 VDC at the anode of D26 or D27?

2.) Do you have ~24 VDC across the coil of K2, the SS relay?

3.) Do you have AC voltage across R4 or R5 (the SS resistors) on the mains board? Careful these are connected to the AC mains.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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« Reply #19 on: June 23, 2019, 12:50:57 PM »

Also check that R17 (10k) on the HV filter board isn't open.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N4ATS
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« Reply #20 on: June 23, 2019, 12:59:20 PM »

Well both relays are pulling in but yes I did not verify their making conductivity so I'll check that out today
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VR2AX
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« Reply #21 on: June 24, 2019, 03:53:07 AM »

Is the board ok?
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N4ATS
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« Reply #22 on: June 24, 2019, 04:48:55 AM »

Resuming tomorrow.  On a plane to Dallas at the moment. Sometimes work gets in the way....
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W9IQ
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« Reply #23 on: June 24, 2019, 05:25:47 AM »

Bill,

My R17 question is based on the idea that you find the SS circuit is working with the HV disconnected. If R17 is bad, there will be high voltage injected into the control circuit. That could affect the SS when HV is connected.

If you haven't already, I would pull the finals to eliminate any sneak paths there.

Safe travels.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W1QJ
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« Reply #24 on: June 24, 2019, 05:41:35 AM »

Bill, have you confirmed that K2 (the SS relay) is kicking in even in the first part of the video?  It is possible that the step start resistors will pass even current to let the amp start without the capacitor load.  I could see one relay latch in the first part of the video but could not tell if the SS relay latched.
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W1QJ
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« Reply #25 on: June 24, 2019, 05:58:13 AM »

I looked at the video again.  I did see both relays engage in the first part of the video but could not tell what was going on in the second part.  Those step start resistors are getting hot real fast.  That tells me there is a huge current presented to the line at the very instant line voltage is passed the on/off contactor.  I would absolutely make sure that the tubes are removed from the amp.  What did you do about the HV shorting circuit?  I could not tell what you did with that???  Get those tubes out of the sockets fir sure. 

You said that you excited the PS bypassing the step start circuit and the HV came up ok.  When you did that did that did you use a Variac to bring the line voltage up slowly?  Often times you could have a problem in the PS and if you bring the line voltage up slowly it will be OK but if you hit it full blast it will trip out.  You can not take anything for granted.

Bill, also just for fun, please disconnect the screen supply from the circuit as well as the HV and see what happens.  That requires just removing the fast on connection from the screen board below the HV board. 

Have you looked under the HV board in the area of the HV connector?   You can't leave ANYTHING for granted.
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LZ1HD
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« Reply #26 on: June 24, 2019, 08:17:04 AM »

Hello Bill,

From the video you've made I see you're trying to start the amplifier with removed cover, respectively the lid crowbar made short contact for HV source. That's why it's this smoke. In my opinion, the initial problem was in one of the two tubes, but after that startup experience, problems can also be expected in the Mains Board. Also you should to check for damaged components in the Tube Deck.

73 Veselin /LZ1HD
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W1QJ
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« Reply #27 on: June 24, 2019, 03:11:33 PM »

Hello Bill,

From the video you've made I see you're trying to start the amplifier with removed cover, respectively the lid crowbar made short contact for HV source. That's why it's this smoke. In my opinion, the initial problem was in one of the two tubes, but after that startup experience, problems can also be expected in the Mains Board. Also you should to check for damaged components in the Tube Deck.

73 Veselin /LZ1HD

Veselin, yes, I thought same thing, but I think Bill is very aware of that.  if he isn't I would be very very surprised.  He is pretty experienced with amps, but it never hurts to be sure.  Bill, what did you do about the b+ SHORTING BAR?
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N4ATS
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« Reply #28 on: June 24, 2019, 03:15:51 PM »

On a plane headed home.  Yes of course the HV in not connected during testing. The single connector is pulled so the shorting bar is out of the circuit. I will check R17 tonight..also tube pulled...I fix many Alphas however this one is a winner
« Last Edit: June 24, 2019, 03:22:12 PM by N4ATS » Logged
W1QJ
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« Reply #29 on: June 24, 2019, 03:33:12 PM »

Bill, how did you excite the HV diodes and capacitors when you bypassed the step start circuit?
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