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I have a HP 8566B as the title implies. The CRT tube does not turn on and I believe I have traced (at least one) fault to A1A3 high voltage regulator board. The HV oscillator does not oscillate and it just blows the 1A fuse. The Q7 bjt's collector is connected to the high voltage transformer making up the inductor part of the oscillator. The base is also connected to the transformer in a feedback configuration.
The display worked for the first 30 minutes or so of operation after I bought it and then when I inserted low (-10dBm) power at the input the display suddenly turned off, no smoke, no smell but the fuse on A1A6 was blown.
Have you had a high voltage transformer be damaged or do you have any good idea how I can verify whether it is working or not? I can't see any induction values nor turn ratio of the transformer in the troubleshooting manual so I don't know what are normal values.
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Saevar, ? I used to troubleshoot Solid State CRT power supplies. Replace the fuse and any shorted components that drive the HV transformer. Use a variable power supply and bring up the voltage slowly to the HV output semiconductor. Monitor the output waveform of the semiconductor(usually the collector) with an oscilloscope (max range setting). ?The waveform should have *no* small spikes or otherwise called ringing observed in between the higher amplitude ones. If you have ringing I would suspect the transformer as having shorted turns. With a bad transformer, as the voltage increases the unwanted spikes continue to grow. This condition usually causes the final semiconductor to fail(short) and then blow the fuse. ? Some HV output sections have a damper diode in this area that also fails. ? I don’t have the schematic in front of me but you can use this for a general approach. ? ? If this test appears to look fine, then possibly on the output of the transformer you will find a HV Tripler circuit that could be faulty. The Tripler semiconductor device could be self-contained or an arrangement of HV diodes. As the name implies the Tripler multiplies the DC voltage that connects to the CRT. Having a high voltage probe is the key to measuring the anode voltage on the CRT. ? Dave ?
>Have you had a high voltage transformer be damaged or do you have any good idea how I can verify whether it is working or not? I can't see any induction >values nor turn ratio of the transformer in the troubleshooting manual so I don't know what are normal values.
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Common fail in old TV receivers. Shorted main winding usually damages transistor driver.
Always diffucult to diagnose.
Try, with power off to excite the main winding with RF oscillator. May be several KHz frequency. Does it resonate? See a voltage waveform for the driver, it may help indicate the frequency.
In TV it was 15,750 KHz.
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Or 15.625kHz in the UK.
Dave B.
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Oh! Yes. PAL. Everything in the world is not according to U.S. standards (NTSC= naught twice the same colour)
There was an equipment " flyback tester" for such purpose but that TV tool might not suffice for this small CRT system if at a different frequency, it need not necessarily be 15 KHz, it could be many frequencies.
But due to relaive high value of test equipment it could be worth try to " ring" the system.
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Hi
From the information given and the method of failure it is almost certain that the A1A11 high voltage multiplier that provides the PDA (CRT Post Deflection Acceleration) voltage has failed. This is a common problem with CRT displays. It is just possible that the A1A3 high voltage transformer has failed but this is very unlikely. To check the high voltage transformer it is necessary to do a ringing test on it.
Be very careful around the CRT high voltage section as there is 4KV present in a number of places and the PDA voltage is some 18KV.
G Edmonds
On Wednesday 10 July 2024 at 15:57:48 BST, Dave_G0WBX via groups.io <g8kbvdave@...> wrote:
Or 15.625kHz in the UK.
Dave B.
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And 15,734 Hz in the US (NTSC).
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
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On 7/10/2024 7:57 AM, Dave_G0WBX via groups.io wrote: Or 15.625kHz in the UK.
Dave B.
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Hi all, Thanks for your valuable advice. George, what makes you conclude that the A1A11 is faulty? I am not that familiar with CRT circuitry and am therefore a bit lost as to how to test these various parts (without risking my health), i.e. ringing test and how to excite the main winding with an RF oscillator as Dave mentions. Any further advice is much appreciated. To make it more clear what is faulty is, the overview figure below shows the display circuits. On the far left is the A1A6 and in the center is A1A3 high voltage regulator. The HV oscillator does not oscillate and the fuse at the 23V supply is blown every time.?  The HV oscillator is shown in more detail in the images below with the Q7 transistor, who's collector and the 23V supply are connected to one of the primary transformer coils. The Base and the output of the OSC drive are connected to another transformer coil for feedback.  
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Hi Saevar
Experience, experience and experience. I have been repairing oscilloscopes now for almost 65 years. Rule one, the higher the operating voltage the more likely to fail also frequently the high voltage multiplier fails when an instrument is put back into use after not being used for some time.
The high voltage multiplier takes the high voltage from the HV transformer which is probably about 3KVAC and multiples it up to the PDA voltage of 18KVDC. It is actually a Cockcroft and Walton multiplier, Google this.
I am not familiar with this SA but you should find that the HV multiplier has three connections one of which will be a thick high voltage cable that goes directly to the CRT, a ground and an input from the HV transformer.
Locate the wire from the HV transformer and disconnect it from the HV multiplier. Well insulate this wire as it will have 3KVAC at a minimum on it.
Replace the fuse and switch the instrument on, if the fuse does not blow the problem is the HV multiplier. You may even get a CRT trace that is defocused .
Let me know what you find.
G Edmonds
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On Wednesday 10 July 2024 at 21:38:06 BST, saevar <saevartj@...> wrote:
Hi all, Thanks for your valuable advice. George, what makes you conclude that the A1A11 is faulty? I am not that familiar with CRT circuitry and am therefore a bit lost as to how to test these various parts (without risking my health), i.e. ringing test and how to excite the main winding with an RF oscillator as Dave mentions. Any further advice is much appreciated. To make it more clear what is faulty is, the overview figure below shows the display circuits. On the far left is the A1A6 and in the center is A1A3 high voltage regulator. The HV oscillator does not oscillate and the fuse at the 23V supply is blown every time.?  The HV oscillator is shown in more detail in the images below with the Q7 transistor, who's collector and the 23V supply are connected to one of the primary transformer coils. The Base and the output of the OSC drive are connected to another transformer coil for feedback.  
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Hi again Saevar
I should add that if the HV multiplier has failed the oscillator will not be able to start and will draw a high current due to the load on it, it will then blow the safety fuse, just what you are seeing.
G Edmonds
On Wednesday 10 July 2024 at 21:38:06 BST, saevar <saevartj@...> wrote:
Hi all, Thanks for your valuable advice. George, what makes you conclude that the A1A11 is faulty? I am not that familiar with CRT circuitry and am therefore a bit lost as to how to test these various parts (without risking my health), i.e. ringing test and how to excite the main winding with an RF oscillator as Dave mentions. Any further advice is much appreciated. To make it more clear what is faulty is, the overview figure below shows the display circuits. On the far left is the A1A6 and in the center is A1A3 high voltage regulator. The HV oscillator does not oscillate and the fuse at the 23V supply is blown every time.?  The HV oscillator is shown in more detail in the images below with the Q7 transistor, who's collector and the 23V supply are connected to one of the primary transformer coils. The Base and the output of the OSC drive are connected to another transformer coil for feedback.  
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FWIW, I had a very similar problem with my 3585B spectrum analyzer. I’m my case the flyback had shorts. The circuits are very similar and the supply fuse (18V) would blow on power up. ?You can do a simple ring test with a signal generator and oscilloscope plus a few components (resistor/cap). You don’t need a dedicated ring tester for a one off test. Mine had shorts and failed the ring test. Ended up having to find a replacement flyback from a parts unit. It might be worth to test if you haven’t confirmed the HV multiplier and certainly before you buy replacement parts. You will have to remove the flyback from the unit to test it though.
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Hi George,
I couldn't wait so I tested the method you suggested immediately and you were absolutely right. There appears a small green "window" visible (like a miniature image) on the CRT in the center and the fuse is not blown. I am thoroughly impressed.
Thanks so much!
Regards
Saevar
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Idea. ? The flyback and HVM simply make a high DC voltage and low current (milli-ampere"?) ? Maybe this to replace a difficult to find HVM? Adjustable HV-DC output: ? amazon DOT com/Adjustable-Voltage-Electrostatic-Precipitator-Supply/dp/B0B4ZVG3B8 ? This would assume the sweep of the flyback is still working to drive the yoke - coils or deflection plates. ? 11.07.2024, 01:09, "saevar" <saevartj@...>:
Hi George,
I couldn't wait so I tested the method you suggested immediately and you were absolutely right. There appears a small green "window" visible (like a miniature image) on the CRT in the center and the fuse is not blown. I am thoroughly impressed.
Thanks so much!
Regards
Saevar
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The PDA supply MUST be stable and regulated or the size of the CRT image will keep changing, Also not a good idea to operate long term without load on high voltage winding of the HV transformer.
If it is impossible to find the correct HV multiplier it may help to look for a Tektronix CRT scope one, my guess and it is only a guess without measuring the HV transformer output is that it is a X6 one.
G Edmonds On Thursday 11 July 2024 at 15:52:48 BST, Dave Me <scope.project@...> wrote:
Idea. ? The flyback and HVM simply make a high DC voltage and low current (milli-ampere"?) ? Maybe this to replace a difficult to find HVM? Adjustable HV-DC output: ? amazon DOT com/Adjustable-Voltage-Electrostatic-Precipitator-Supply/dp/B0B4ZVG3B8 ? This would assume the sweep of the flyback is still working to drive the yoke - coils or deflection plates. ? 11.07.2024, 01:09, "saevar" <saevartj@...>:
Hi George,
I couldn't wait so I tested the method you suggested immediately and you were absolutely right. There appears a small green "window" visible (like a miniature image) on the CRT in the center and the fuse is not blown. I am thoroughly impressed.
Thanks so much!
Regards
Saevar
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BTW, I removed and still have the HV section from an 85662A when I upgraded to an LCD. If you need parts... J.Kruth
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Do you guys that have upgraded to an LCD found it worth the 400$+shipping?
I am torn whether I should go that route and cough up the cash for this old SA or buy a replacement HV multiplier. There are a few available for 30-40$
There are more things wrong with the SA (RF section, input signal showing very low amplitude, like 70-90dB lower than expected) which I haven't gotten around to try and fix since I needed a functioning display first.
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I upgraded my 8566B with the NewScope LCD quite awhile ago and it has been well worth the cost for me. The 8566B is a workhorse (in both performance and weight?) and the LCD has added enough life to the SA that it will probably outlast me!
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On Jul 11, 2024, at 12:34?PM, saevar via groups.io <saevartj@...> wrote:
?Do you guys that have upgraded to an LCD found it worth the 400$+shipping?
I am torn whether I should go that route and cough up the cash for this old SA or buy a replacement HV multiplier. There are a few available for 30-40$
There are more things wrong with the SA (RF section, input signal showing very low amplitude, like 70-90dB lower than expected) which I haven't gotten around to try and fix since I needed a functioning display first.
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Wow, since you are not sure the RF deck works, and you MIGHT have a bad yig tuned mixer, I would go the least expensive way to get the display running. The LCD kit is nice but requires mechanical modification to the front panel of the display (milling stuff away) and the installation is not super simple. (Not bad, but takes time).
In a message dated 7/11/2024 1:34:50 PM Eastern Standard Time, saevartj@... writes:
?
Do you guys that have upgraded to an LCD found it worth the 400$+shipping?
I am torn whether I should go that route and cough up the cash for this old SA or buy a replacement HV multiplier. There are a few available for 30-40$
There are more things wrong with the SA (RF section, input signal showing very low amplitude, like 70-90dB lower than expected) which I haven't gotten around to try and fix since I needed a functioning display first.
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Hey Jeff - hope you’re well! I don’t know if they still do it but SimmConn Labs used to offer a milled front panel in exchange for your old panel. Might be worth asking if one goes the LCD route.?
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On Jul 11, 2024, at 12:49?PM, Jeff Kruth via groups.io <kmec@...> wrote:
?
Wow, since you are not sure the RF deck works, and you MIGHT have a bad yig tuned mixer, I would go the least expensive way to get the display running. The LCD kit is nice but requires mechanical modification to the front panel of the display (milling stuff away) and the installation is not super simple. (Not bad, but takes time).
In a message dated 7/11/2024 1:34:50 PM Eastern Standard Time, saevartj@... writes:
?
Do you guys that have upgraded to an LCD found it worth the 400$+shipping?
I am torn whether I should go that route and cough up the cash for this old SA or buy a replacement HV multiplier. There are a few available for 30-40$
There are more things wrong with the SA (RF section, input signal showing very low amplitude, like 70-90dB lower than expected) which I haven't gotten around to try and fix since I needed a functioning display first.
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