Re: Tek 7904 help!
Hi Chck and gang,toggle quoted messageShow quoted text
I have a later? horizontal board - no tant - 2 small can type caps. My manual shows the tant caps. Did check the caps and they were ok.
Some more measurements.
1. PS disconnected completely2. Resistance measurements taken at Z axis board with Fluke DVM
VOLTAGE TP NORMAL NO PLUGINS WITH PLUGINS
+130 6.6K 9.5K 9.5K
+50 2K 4.6K 4.2K
+15 90 92 27
+5 65 49 19
-15 110 216 31
-50 2K 55K 55K
+LAMP 5 INF INF 55k
(red underlines from spell check)
3. Highlighted readings look strange. Could I have 4 plugins with loading. Inserting each module reduced resistance by a similar amount from 216 ohms to 31 for -15 volt source.
4. Checked power supply voltages out of mainframe with 120 VAC. Power about 10 watts or so.130 = 136+5 = 4.8-50= 12+50 = 25-15 = 2+15 = 10
I measured the crt anode voltage out of the PS using and old Pomona tv hv meter and the meter would kick a bit - maybe a few hundred volts..
I guess that low voltages would be low due to burst mode not allowing output filters to charge to normal value.
I am thinking PS is ok or it is shutting itself down. Just curious why the different resistance readings when plug-ins are installed. We had quite an electrical storm on Tuesday Evening. Wednesday morning the scope was not working. As you can see from my QRZ.com page I have a bunch of toys and they are all working - so far.
So - 2 questions - why do plug-ins resistance read low and are my voltages close when the ps is not installed?
Just remembered I do have some old plug ins and will plug them in and see if I get the same resistance drop.
I really appreciate the help.
Bill, WA2DVUCape May
On Thursday, July 9, 2020, 12:04:05 AM EDT, Chuck Harris <cfharris@...> wrote:
There are two tantalum capacitors that every 7904 I have had
has blown. One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly. They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall. One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.
You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage. 6.3V caps on 5V, and 16V caps
on 15V are especially bad. Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.
Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.
The power supply itself is usually pretty solid. Look
at the rest of the scope for a blown tantalum.
Bill Riches via groups.io wrote:
Thank you Raymond and Harvey. Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell! Will check them in the nether regions!