465 tantalum smoke
Hi, I just received a 465 yesterday from Universal Radio, as another oscilloscope project. It was sold as not working at all and sure enough, all the voltages were low.
-8 was +.53v
+5 was +.57v
+15 was +.07v
+55 was +45.2v
+110 was 102.4v
-2450 was 0 v
As a note, I write everything down as I go thru the unit to help keep myself organized.
Next I was checking voltages at Q1546, Q1534, Q1566 and Q1556. The E and B voltages at Q1546 was very low, so I checked the voltage test points for resistance to ground, and the 15 volt test point went to ground.
As testing voltages again at Q1556, I "purposely" pushed my positive test lead to from one transistor lead to another. I started getting life into the CRT screen. More of a flickering and dull light, but an improvement over nothing.
So,I rechecked the voltages and here they are.
-8 now -7.15v
+5 now +3.05v
+15 now +5.30v
+55 now +49.4v
+110 now +102.9
-2459 now -2400v.....measurement taken with Simpson 260-3 ( I bought this from the original owner a couple of months ago. He asked me what I was going to do with the Simpson, and I told him I was working on Tektronix 453 and a 465. He smiled and told me that he used these scopes and then was talking to me about working on booster rockets at NASA for the Apollo series. Also he told me all the electrical connections in his timing boxes had welded terminal connections, not soldered.) Nice guy!.
Anyway I decided to turn the scope again and fool around with Q1546. The 15v transistor mounted on the side rail.
On Q1546 I touched C and B together and saw the CRT lightup and thick nasty smoke out of the side of the 465. I turned everything off and had to open a few windows. After the smoke cleared, I checked the voltages again. BTW...the smoke was intense.
-8 now -7.99v
+5 now +5.01v
+15 now +15.01v
+55 now +55.1v
+110 now +108.4
As I never owned a working oscilloscope or used one, its all a learning curve for me. I understand I have to replace that fried capacitor, but all the voltages are correct and I have two traces that I can move around.
On the side of the 465 right beside the on-off switch, there is three tantalum capacitors. The one that smoked was the one closest to the end of the board. (closest to the back of the 465). My serial # B321XXX shows its a newer one. I'd appreciate if someone could point me to the correct capacitor to order. The online manual for the 465 isn't as readable as the one for the 453.
I understand my oscilloscope skills might be in question, but it seems that one capacitor took the 465 down. I listen to older tube radios, like my SP 600 and HRO 60 and drifted to minor repairs and ended up interested in older test equipment. Just thought I'd pass my experience along, and if someone could tell me what tantalum to order.
You may be talking abouttoggle quoted messageShow quoted text
C891, or C893, or C897
All the same part number
You really need to acquire a SM
These caps you might say are a boot strap, just extra filtering, just my opinion
On Apr 21, 2020, at 6:30 PM, Rob Naulty <@ignatz> wrote:
I would not recommend replacing tantalums with tantulums. I've restored a number of 465's and 475's and use aluminum electrolytics. You can usually find aluminum caps of 2 or more times the capacity of the tantalums in a package that will fit in the available space. These caps are just for bypass and are not critical. When I couldn't find a suitable replacement given size constraints, I found no performance degradation when just leaving the space empty.
Thanks, I'm going to clean it up and replace the capacitor as to finish the project. I need the experience with the repair and then move on to the other 465 I have.