My faithful Tektronix 465 scope failed again. This was a repeat of the B trigger holdoff problem, that mysteriously fixed itself a few weeks ago. Of course problems like that usually return and it didn’t take long. This time around, it didn’t fix itself.

I didn’t know much about how this scope worked and I had to spend a considerable amount of time over several sessions before I finally tracked down the root cause. It turns out the triggering circuits of this kind of analog scope are made up of a number of transistors that are often configured to operate over very specific voltage ranges that are set up by resistor divider networks. Frequently diode or gates are used to combine signals, which makes it hard to determine exactly why a signal is behaving the way it is. When something goes wrong, it’s not always obvious to the casual investigator what it might be.

Anyway, I eventually tracked the problem down to a bad transistor in the B trigger holdoff circuit. This is transistor Q1052 in the schematic. I verified the bad transistor by swapping it with Q1054, which is the same type. The nature of the problem changed, indicating that one of the transistors was bad.

The part number listed in the service manual is 151-220-03, but I found another TEK manual with cross references that listed a 2N3906 as a “similar” part. I had some of those on hand, so I swapped out the bad part and B Trigger started working again. Since this seemed like a pretty basic PNP transistor application, I was planning on trying the 2N3906, if the cross reference recommended something else that I didn’t have.

During this investigation I noticed an electrolytic capacitor that appeared to have some tarnish near one end. I decided to replace it before putting the scope back together. I looked up the value, and it was a 5 uF cap rated at 150 volts. I searched my stash and found I had nothing that would work in that application. I pulled the part and did some basic checks. It measured at 7uF. I found that resistance was infinity on my meter, although I know that means little. I have a primitive ESR checker, but it requires a working scope, so I couldn’t do that check. A search on Mouser showed that they had something in stock. However, the cost of two parts plus shipping was almost $15, so I decided that maybe the capacitor I had would do ok for now.

After replacing the capacitor, the scope refused to power up, with some apparent problem in the power supply. It made a ticking noise much like an Apple II power supply that doesn’t have a load. I double checked my soldering and could find no issue. At that point I decided to pull the cap again and order a new capacitor and do some research on the power supply before continuing…