Keep following this blog as I reveal some details of the repair of an extremely rare computer. I think that this system is currently the only fully operational original system of it’s kind in existence.

After a bunch of effort and a number of replaced parts, I have decided that my Tek 465 might possibly have a bad CRT. I’ve decided to put this repair effort on the back burner and ordered a INSTEK GDS-1054B digital scope. It has 4 channels and 50 MHz speed which should be sufficient for most of my hobby purposes going forward. I’m not real excited by the 50MHz speed, but given the price point that I was shooting for, it was either that or drop down to two channels to get a 100MHz unit at a similar price. I figured that I would use the extra channels more often than the extra speed, so I went with the 4 channel 50MHz unit. One problem is that it doesn’t support Z mode, which means I don’t have a way to run my SCELBI CRT Interface until I either get the Ten 465 working again or get one of my Heathkit OM-14’s restored. Somehow I ended up with three of those Heathkit scopes. I intend to restore two and use the last one for parts.

Finally watch my demonstration of an operating SCELBI-8H with Digital Group Video card at the Virtual VCF that will be shown on youtube tomorrow: .https://www.youtube.com/watch?v=7YoolSAHR5w. I think that this shows that with some effort, the SCELBI-8H can be transformed into a pretty useful computer.

Getting my Tek 465 operating correctly again has been quite an involved exercise and to be honest, I considered giving up on it. As related in my previous post, after replacing a questionable electrolytic capacitor, it didn’t come back to life. What really made me consider giving up, is that the problem appeared to be in the high voltage section that drives the tube, as the power supply started making some buzzing noises and the display disappeared. I don’t have proper probes for handling those high voltages and didn’t want to get myself into a dangerous situation.

However, before giving up, I decided to take a look, as best I could. With power off and no high voltages present, I did some poking around with a meter, using the diode checker function to check out transistors and diodes and ohm meter to check out resistors. After a lot of probing, I found that 10K resistors R1421, R1422 and R1423 were all open circuit. They must have all blown like fuses.

After replacing them, I found that the scope still had issues. What could have caused all these resistors to fail, I thought. Eventually, after examining many more components, I found that one of the connections to a custom film resistor was open. This was near the capacitor that I had replaced. I think it must have had marginal contact all along and I must have broken it when replacing that nearby capacitor which connects near one end of this resistor package.

This resistor is used in the high voltage switching supply as part of the feedback circuit. I think this open connection caused the supply to run away and generate too high a voltage which caused the buzzing noise and must have blown out the resistors.

Fixing this contact and replacing the blown 10K resistors seemed to have restored the high voltage as the display now started working and didn’t buzz any more, however horizontal blanking didn’t work correctly. The blanking circuit was less scary to me, as it didn’t involve super high voltages and I dived in.

Probing the blanking circuit, I found two bad transistors, Q1466 and Q1472. It turns out that Q1472 could be replaced by an ordinary 2N3906, but Q1466 used a Tektronix part number 151-223-0. I found that equivalent replacements are either a 2N4275 or 2N5769, depending upon who you believe. However I didn’t have either in stock. I found the same part was used elsewhere in the scope and decided to try a swap to see if I could get further in the debug. It’s convenient that the transistors in this scope are socketed.

After swapping, I found that I still had a problem in this part of the circuit. After much digging, I finally found that diode 1464 was also blown. Well it was actually internally shorted and acting as a low value resistor instead of a diode. This was another part that I didn’t have in my stash, a Tek cross reference page called for a 1n4244. It turns out that this is a diode with a 750 pico second recovery time, rather fast, which I assume is necessary in order to switch the blanking on and off quickly.

At this point, I decided to call it a night and to find correct replacement parts before preceding further. I found that someone in Oregon had the exact Tek part number 151-223-0 for Q1466 and I found another vendor had the 1N4244, though I had to order some additional parts in order to make up a minimum order for this particular order. I have been meaning to build a SCELBI quick loader box and needed some toggle switches for that application, so I tacked an order of 8 toggle switches onto this order to make up the minimum amount.

Last, I swapped back the good 151-223-0 to its original location and set aside the scope. You don’t want to be swapping these parts around willy-nilly, as it can ruin calibration.

It will be interesting to find out if the scope will be finally be made good when the new parts arrive.

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…

My faithful old Tektronix 465 has recently been experiencing issues with the delay function for the secondary horizontal timebase.

For those not familiar with this feature, here is a quick explanation: the Tek 465 has dual timebases. You can set up timebase A for say 1 millisecond per division and timebase B for say 10 microseconds per division and switch the horizontal timebase from A to B as the raster scans across the screen. This is useful when you want to zoom into the details of a signal sometime after triggering as the raster is partway across the screen. This switch in timebases can be by trigger or by a delay feature.

It’s basically a controllable zoom capability. I find the time delay version of this feature most useful, but recently I found that when the change in timebases was set to delay, it would trigger immediately, instead of delaying.

After several debug sessions, the feature started working intermittently. This slight improvement in behavior started after probing in the area of several suspect transistors. I pulled the most likely transistor out of it’s socket and tested it using a diode tester. It tested good. After reseating it and some other transistors, the feature seems to working reliably again, though only time will tell for sure.

Anyway I took a good look at some relevant signals while it is working, so I’ll have a better shot at debugging it, if the problem crops up again.

There is nothing worse than intermittent sockets and connectors. I’ve had this scope for a long time and it has served me well, but maybe it’s starting to show it’s age a bit. By the way, as I don’t have a second scope, once again, I used this scope to debug it’s own issues.