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marco antonio
Joined: 28 Jul 2004
Posts: 61
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 07 Mar 2009 19:28 beckman industrial |
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The problem with this oscilloscope is that I can not control the intensity with the front control but the focus is normal. It began when I was checking with the component tester and the resistor R504 was burning, I found the transistor T515 opened so I replaced the resistor an the 3 transistor T501,TT515, T502 but no help. I checked the values of VR565,VR564, VR566, VR562,R563, R561,R560, R559 all normal.
Continuty to G1 from VR566 normal. What else can I do.
I will appreciate your help.
rgds
Marco
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07 Mar 2009 19:28 Ads |
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Chris56000
Joined: 19 Jan 2009
Posts: 49
Helped: 4
Location: Sutton Coldfield, UK
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09 Mar 2009 19:42 circuitmate 9020 |
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Hi!
I've a few suggestions!
*** First of all you must take the utmost care when measuring in the CRT grid/cathode circuit as an accidental test-prod slip will destroy the CRT!!! ***
*** You need a testmeter with 2.5kV insulation or preferably more *** for the following tests!!!
Check first of all the voltage across ZD502 - this should be 100V approx - this is the *** Negative GB source *** for the CRT grid/cathode circuit, and if ZD502 is s/c or leaky you will get maximum intensity with no control over it. If you're doubtful about making the tests, then renew R559 - R561 *** and also *** R563 using VR37 - rated HV resistors (Farnell), then fit a new zener (BZT03C100) in ZD502 position, then renew the intensity min/max presets VR564/VR565 and make sure the Customer Intensity control VR566 hasn't a dud spot!
Next you need to check the 33V rectified supply across ZD501 from the pink/white/orange leads (6.3V heater/31V) secondary of the transformer - you might as well replace all these components if you're not sure about 'em - they're cheap enough!!! Use a BZT03C33 for ZD501.
It appears you've also got a fault in the EHT regulation system since R504/T515/etc has failed. Replace R516/R517/R590 as a set along with the three BF459s as a set - leakage in any of them will destroy the others! The EHT feedback resistors are R508/R509 - use VR37 HV types for replacement purposes!
Its unlikely the '741 IC505 has failed but for what it costs turf it out if you're not sure!!! The system works as a variable resistance formed by the three transistors between the +ve return of the EHT tripler D502-D509 and chassis under the control of IC505 and the EHT preset VR510.
Thanks for posting the circuit extract - it helps others answer your query!!!
Good Luck!!!
Chris Williams
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marco antonio
Joined: 28 Jul 2004
Posts: 61
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09 Mar 2009 22:41 beckman 9020 |
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Chris
thanks for your info I will take action on it and I will advice of results.
rgds
marco
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