开云体育

-----Original Message-----
From: tmillermdems <tmiller11147@...>
To: TekScopes <[email protected]>
Sent: Mon, Mar 12, 2018 2:11 pm
Subject: Re: [TekScopes] Tektronix 2465 capacitors etc. - Going deeper!


Here is what my schematic shows.






-----Original Message-----
From: tmillermdems <tmiller11147@...>
To: TekScopes <[email protected]>
Sent: Mon, Mar 12, 2018 1:10 pm
Subject: Re: [TekScopes] Tektronix 2465 capacitors etc. - Going deeper!

Not on my schematic. It is shown correctly with the positive pole connected to the junction of CR1131 and CR1132. It will have about 40 volts across it and of the correct polarity. Pay attention to the diodes and re-analyze the circuit.














But the schematic has C1130 drawn inverted.







-----Original Message-----
From: machineguy59 via Groups.Io <machineguy59@...>
To: TekScopes <[email protected]>
Sent: Mon, Mar 12, 2018 1:00 pm
Subject: Re: [TekScopes] Tektronix 2465 capacitors etc. - Going deeper!

Ed,

You are correct!! C1130 never sees a negative charge so a polarized capacitor capable of high ripple currents (low ESR) will work best. Non-polarized electrolytics have double the ESR of their polarized brethren. So a Voltage double should always use polarized capacitors. But the schematic has C1130 drawn inverted. I plan to refurb a 2465 series PSU in the next week or so and I will check the PCB markings to see if they have the same mistake. The wrong polarity on the schematic for C1130 is not the only error in the Tek documentation for the PSU. The PCB layout has C1115 and C1132 interchanged so many technicians following the PCB layout put a 25 Volt rated capacitor (C1115) in a 90 volt position (C1132).

I personally, would never use a 450 rated capacitor in a 100 volt circuit. My rule of thumb is nothing more than double the rated voltage. But I often violate that rule when working with low voltage circuits (say 15 Volts or less). The reason is low ESR over the lifetime of the capacitor. A high voltage electrolytic starts out with a higher ESR (because the plates are further apart and I believe the electrolyte is less conductive). Worse, a high voltage electrolytic capacitors ESR will rise and its capacitance fall if it is not properly energized. But, realistically, most of these scopes are not likely to be used more than another ten years before being refurbed for other reasons.