Author Topic: Flyback EHT too high, change cap value?  (Read 277 times)

Offline Overkill

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Flyback EHT too high, change cap value?
« on: March 30, 2024, 10:45:14 AM »
Hi all

I've been trying to repair some old Commodore computer monitors.

I've fitted a replacement Flyback but it's a slightly different model, and although it fits fine, I think the primary winding is different which results in a too high EHT.

I've measured the 15v output of the flyback and get 16v (about 8% too high).

I've been told that if I replace C207 or C208 with a higher rating it would lower the voltage. Well, the advice was to try keep adding 2200pf caps in parallel.

As a novice I see the logic, as this might be tuned circuit and changing a value of a component would change the output. 
Also, I've seen people say that if these capacitors lose their value, the voltage goes high.
Also, I see on slightly different board versions that they use different value capacitors, I've seen 7300pf to 9100pf.

So, does this sound correct to you guys, if I increase the capacitor value it would lower the flybacks voltage?

Diagram attached. 


Offline davekni

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Re: Flyback EHT too high, change cap value?
« Reply #1 on: March 30, 2024, 08:04:38 PM »
Quote
I've measured the 15v output of the flyback and get 16v (about 8% too high).
Quote
So, does this sound correct to you guys, if I increase the capacitor value it would lower the flybacks voltage?
Yes, likely to work, but no guarantees.  Presumes driver circuit's off-time is long enough, likely true.  If same transistor drives horizontal deflection as is common, will increase retrace time by same ~8%.  Hopefully that isn't an issue either.
As a starting point, capacitance will need to increase by 16%.  (1.08 ^ 2 ~= 1.16).  Looks like existing capacitance is 7300pF.  Add ~1170pF.
David Knierim

Offline Overkill

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Re: Flyback EHT too high, change cap value?
« Reply #2 on: March 31, 2024, 11:20:29 AM »
Thanks for your reply.

I'm looking for some caps, I see in the diagram the 3000pf cap is 600v but the 4300pf is 1600v, I assume if 2200pf was used in parallel that 650v caps would be OK?

Offline davekni

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Re: Flyback EHT too high, change cap value?
« Reply #3 on: March 31, 2024, 06:49:59 PM »
Quote
I'm looking for some caps, I see in the diagram the 3000pf cap is 600v but the 4300pf is 1600v, I assume if 2200pf was used in parallel that 650v caps would be OK?
I suspect "0.6kV" is a typo and the real voltage is 1.6kV.  Normally 0.6kV would be written as 600V.  600V would be low for typical flyback use.
David Knierim

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Re: Flyback EHT too high, change cap value?
« Reply #3 on: March 31, 2024, 06:49:59 PM »

 


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