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7904 Power Supply Capacitors


William Himbert
 

Hello: I am in the process of fixing a 7904 power supply which is in the 'tick' mode. I found that C1354 (on the top right of the cap-rectifier board) is shorted. It says in the manual that it is a "CAP. ,FXD, ELCTLT: 47UF, 20%, 20V, Union Carbide T368C476M020AZ". It's polarized so I assumed it was a tantalum (especially since it shorted-out) but it doesn't look like the normal bead type - it's larger, thicker, almost square in shape and the color blue. It does look like a dipped part otherwise.


Does anyone think this is something other than a tantalum? Would it be OK to replace it with a regular bead tantalum with the correct capacitance and voltage rating?


Thanks you for your help. These power supplies are a bear to take apart, without the proper tools it would be impossible.


Bill


 

I have seen these blue roughly prismatic epoxy dipped solid tantalum
capacitors in old TM500 plug-ins before. In fact, the example I have
in front of me in my DM501 is the same 290-0529-00 T362C476M020AZ
47uF, 20V, and 20% part used in the early 7904 while you have the
later part.

I did a little research and Kemet came from Union Carbide's capacitor
division when it was spun off in 1987. The T362 series is long gone
but the T368 series is still made and there is data available on them
(pdf page 68):

http://www.kemet.com/Lists/ProductCatalog/Attachments/21/F3100.pdf

There is no reason to use a modern T368 part. I just dug up the data
so a suitable replacement can be selected.

In my opinion the 20V part has insufficient voltage derating for a 17
volt supply. I have a change notice showing a 100uF 25V part which
based on its 20% tolerance was also a solid tantalum capacitor.

There is more to the 7904 story then my service manuals show and I
know this has been brought up here before. Mine all show a 100uF 25V
capacitor for C1354 in the schematic. It might be worthwhile to see
what parts were used in later oscilloscopes compared to the 7904
(1973) like the 7834 (1977), 7854 (1981), and 7934 (1987).

I need to start thinking about rebuilding my 7000 power supplies. I
think I will start on a spreadsheet showing the parts used in each
model.

On 24 Nov 2016 11:41:24 -0800, you wrote:

Hello: I am in the process of fixing a 7904 power supply which is in the 'tick' mode. I found that C1354 (on the top right of the cap-rectifier board) is shorted. It says in the manual that it is a "CAP. ,FXD, ELCTLT: 47UF, 20%, 20V, Union Carbide T368C476M020AZ". It's polarized so I assumed it was a tantalum (especially since it shorted-out) but it doesn't look like the normal bead type - it's larger, thicker, almost square in shape and the color blue. It does look like a dipped part otherwise.

Does anyone think this is something other than a tantalum? Would it be OK to replace it with a regular bead tantalum with the correct capacitance and voltage rating?

Thanks you for your help. These power supplies are a bear to take apart, without the proper tools it would be impossible.

Bill


Jose Luu
 

On Thu, Nov 24, 2016 at 02:21 PM, David Hess wrote:


I need to start thinking about rebuilding my 7000 power supplies. I
think I will start on a spreadsheet showing the parts used in each
model.
I just fixed a 7904 where 3 of these capacitors had failed (2 in the suppy, one on the main board), I was wondering if you ended up doing the capacitor replacement spreadsheet you mentionned.

Regards
Jose


Jim Ford
 

Hi, Jose'.  I haven't seen Dave Hess post on TekScopes for a long time.  Try eevblog; he seems to be more active there lately.     GL.   Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Jose Luu <jose.luu@gmail.com> Date: 1/11/21 6:14 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 7904 Power Supply Capacitors On Thu, Nov 24, 2016 at 02:21 PM, David Hess wrote:>> I need to start thinking about rebuilding my 7000 power supplies. I> think I will start on a spreadsheet showing the parts used in each> model.>I just fixed a 7904 where 3 of these capacitors had failed (2 in the suppy, one on the main board), I was wondering if you ended up doing the capacitor replacement spreadsheet you mentionned.RegardsJose