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Author Topic: Alpha 91B Weird Issue  (Read 1945 times)
N4ATS
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« on: June 22, 2019, 05:51:19 AM »

Plug in the 2 pin HV connector , Soft start will not kick in. HV Caps new , Screen perfect , Diodes good. Both board HV and Power supply good. It is NOT the Hard Down relay , it is the most goofy alpha I have worked on and I fix quite a few. Different transformer installed as well, same issue. Both transformers measure 2040 VAC. Tube removed , HV unhooked going to the CAP board, it will come up. Using a HV power supply , I can charge the caps up..  WILD... Anyone have a similar issue?
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W1QJ
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« Reply #1 on: June 23, 2019, 03:24:10 AM »

I want to try and help you Bill but maybe I’m not understanding the problem correctly.  Anyhow did you change the bleeder resistors and if you did, did you use a different value bleeder resistor than was in there originally?  Does the meter read HV if you externally excite the supply?  I am thinking along the lines that there might be a HV-ok circuit that’s nor working or was upset somehow.  Lou
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N4ATS
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« Reply #2 on: June 23, 2019, 08:09:54 AM »

Bleeder resistors are original and they all checked just fine... Caps are brand new. Amp powers up fine without the 2 pin HV plug on the transformet connected. 
Capacitors tested to 500 volts, no arcing on any.  I have an HV capacitor analyzer.  It's not the hard down circuit either. Tried 2 different transformers.  if you look at the schematic you'll see that if HV doesn't start up correctly the soft start fuse will blow due to a short put across the input voltage to the comparator  that's not the issue I unhooked that circuit and the soft start still will not come on... the 10 ohm resistors smoke as expected. Alpha is also stumped..
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W1QJ
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« Reply #3 on: June 23, 2019, 08:23:42 AM »

The soft start circuit in the 91b was a weak point with that amp.  The second version of that amp the 99 improved on the step start circuit.  A friend of mine also had a problem with his 91b step start and with his,  there was a resistor that changed resistance enough to keep the relay from actuating.  I recall doping out that circuit with him.  He ended up rebuilding the entire circuit with new relays and resistors.  As I recall one resistor chews up 120v of AC voltage as it’s across 240 and over time that resistor drifts up in value. If it drifts high enough the relay doesn’t pull in or chatters.  With elusive problems you must expect the unexpected.
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W9IQ
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« Reply #4 on: June 23, 2019, 08:41:18 AM »

Have you checked D26 - D29 and C58 - C59?

- Glenn W9IQ
« Last Edit: June 23, 2019, 09:01:04 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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« Reply #5 on: June 23, 2019, 09:03:54 AM »

Also R6?

- 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 #6 on: June 23, 2019, 09:28:28 AM »

In reading your description in the first post, your SS description doesn't sound right. The SS relay is powered from the 28 VAC secondaries on the transformer. These go through a bridge rectifier (D26 - D29). This is then filtered by C58 and C59. It is the charge time constant of these caps forming the 40 V supply that delays the pull-in of the SS relay. R6 is the series current limiting resistor for the SS relay.

The HV circuit is independent of the SS circuit. If the HV does not come up, then the HV monitoring relay will not kick in so R18 will squelch the power relay hold-in circuit.

I hope that helps.

- 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 #7 on: June 23, 2019, 09:42:17 AM »

Yep , all diodes and resistors with in tolerance. D26, 27, 28 and 29 perfect. C58 and C59 Perfect.   R-6 is 200 ohms

PS referring to the SS above , I was talking about the hard fault relay circuit. That appears fine.
« Last Edit: June 23, 2019, 09:57:42 AM by N4ATS » Logged
W9IQ
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« Reply #8 on: June 23, 2019, 09:56:31 AM »

On the mains board PCB. Do you have a 40 VDC bus? This is what powers the SS relay.

- 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 #9 on: June 23, 2019, 09:59:47 AM »

Glenn , the SS works 100% as long as the 2 pin plug (HV) from the transformer is NOT hooked up. Let me make a video for you on you tube...
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KA4DPO
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« Reply #10 on: June 23, 2019, 10:03:06 AM »

The 91B is kind of an orphan, they were made in Bulgaria by the same guy who made Alpin amps I think.  Anyway, the soft start circuit was a known problem.  This may sound goofy, but are the tubes OK?  The Soviet era 800A's are known to have heat issues.
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W9IQ
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« Reply #11 on: June 23, 2019, 10:07:21 AM »

Did you check R1? The fact that the HV fault relay is not kicking out the mains relay when you disconnect the HV is telling.

- Glenn W9IQ
« Last Edit: June 23, 2019, 10:11:19 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N4ATS
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Posts: 1297




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« Reply #12 on: June 23, 2019, 10:10:57 AM »

Yes , R1 fine

Here is a video  https://youtu.be/Csgfv5LNXjM
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N4ATS
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« Reply #13 on: June 23, 2019, 10:14:54 AM »

R1 - 560 ohms
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W9IQ
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« Reply #14 on: June 23, 2019, 11:08:09 AM »

R1 - 560 ohms

R1 should be 10 ohms/16 watts.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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