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Re: 465B Oscilloscope No -8V

 

Another example of why it ?makes no sense to measure ESR in circuit ( or you must be sure from the schematics there is nowhere, on any place in the scope an other cap or other conducting thing parralel to this one)

The most caps fail on low capacitance or DC leakage, ?most times in smps ESR can become a problem. But i just repaired the batterycharger of my heated motorjacket. This is a normal transformer psu and only thing I could find was to much ripple. Two electrolitic caps. Not leaking, good capacitance but a rather high D (so also higher ESR) according my GR1608 bridge. My peak esr meter thought it was fine, my home made esr meter however agreed with the GR. I replaced them and the ripple was gone. A bit odd, i think there was some chemical intbility in the cap. I have seen that once before. Monitoring the ESR that was good, and then suddenly starts to rise, wiggles a bit then suddenly falls back. I have done a lot of tests on that cap and see it o this several times. I can stil not explain what happens.

If you measure a value in the pF range or een nF for such a big cap the problem is probably electrode erosion. If you open the cap you will see the anode most times is corroded in two and you measure the capacitance between the remaining strip and the kathode ( outside can)

Fred PA4TIM

Op 15 jan. 2013 om 15:50 heeft "rwdplz1" <rwdplz1@...> het volgende geschreven:

?

OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which should be about right. However, after desoldering it from the board (which is a real pain with the two large mounting legs), and measuring capacitance, one of my meters couldn't measure it, and the other read 0.185 nano Farads. Both of these meters read the capacitance of other small capacitors I have on hand perfectly, so the meters are fine. ESR of the other large capacitors is about the same, but I'd rather not desolder them from the board if I don't have to, to prevent damage to the fragile traces.

--- In TekScopes@..., "Tom Miller" wrote:
>
> OK, good. I would think the next thing to check is C-4419, a 5000 uF can capacitor.
>
> Regards,
> Tom
>
> ----- Original Message -----
> From: rwdplz1
> To: TekScopes@...
> Sent: Tuesday, January 15, 2013 2:40 AM
> Subject: [TekScopes] Re: 465B Oscilloscope No -8V
>
>
>
> The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms.
>
> I replaced one tantalum, C4203, 33uF, with no effect.
>
> --- In TekScopes@..., "pdxareaid" wrote:
> >
> > you might want to check for shorted tantalum caps in -8 circuits.
> > see:
> >
> > phil
> >
> >
> >
> > --- In TekScopes@..., "rwdplz1" wrote:
> > >
> > > Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
> > >
> > > I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
> > >
> > > So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
> > >
> >
>
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