Revox
Joined: 13 Feb 2008
Posts: 158
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| Posted: Tue Jan 10, 2012 10:26 am Post subject: Quad of a 909 gone - need Help to find a solution |
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Hi, i'd like to understand why some Quads are not compatible with some EHT's.
I think finding a solution would be nice for many of us!
For me that's a little bit like a mirracle, because the chematics from 700 to 909 are all similar.
For a short testrun, i installed a newlike 7xxxx91 Quad out of a 700 and the 909 went back to life.
In the technical bulletin, there is a short comment, that the Driver Mosfet and the "wrong" EHT begin to oszillate.
As a conclusion i tried to mesure the signal in parrallel with the Quod. Because many parts of the EHT are on Line Voltage, you must use two probes in the differential mode.
First thing is, that i don't know what kind of Signal is normal on the EHT.
What a was able to see, was a Sinuid oszillation (around 80khz) and a euler like envelope. This signal comes every 1/20 s (i think this time depends on the brightness of the picture).
Second is, that the inducticity of the Old EHT is 40mH more than the new one (140mH), but this is only "cold" and not in working condition.
If you take a look in the service Manual (appendix) there is a cap in paralel to the driver Mosfet. I think the intention is to have a Sinuid signal to drive the EHT, not a squarewave like in the switched powersupply. In the old 700 this caps was 6800p big, the new one was 4700p. I would be lucky if this is the only change!
If you know anything about the EHT, please let me know!
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Last edited by Revox on Tue Jan 10, 2012 4:24 pm; edited 1 time in total
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