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It is quite interesting to note how the fuse blows. When there is a short in the switch transistor or rectifier bridge the fuse blows in a quite spectacular fashion with metal fumes covering the glass. At least if the fuse is a standard or slow blow.

Yes. Indeed quite complex. I am just thinking out loud now: if the Q5 transistor is not driven into saturation it might get hot. Then it will not be able to drive the following stages properly either perhaps causing a lower output. The error amplifier will then cause the duty cycle to change...

I also installed the same zip package as you did. But NFS came from fsck.technology site. Unfortunately it refused to install. It claimed that the TCP/IP package or AIX base program had the wrong level. I do have TCP/IP installed though. I don’t have TCP/IP configured yet (mostly because I...

Nice! Did you install all the updates? Did you manage to install NFS and the Distributed Function packages? Have you seen the files on fsck.technology site? A lot of RT AIX stuff there. I am using 1 gig out of a Seagate 2 gig IDE drive. Seems to work very well. I don’t have a 6157 so I...

Difficult one. The lower voltage on Q4 emitter may indicate higher regulated voltage consumption. But there are variations on the zener diode voltage. To be honest I am not sure why it has two 555 chips there. 640kHz switching frequency sounds very high for a PSU this old. Maybe you could...

I have had the “instantly blowing fuse” in a few VT100 terminals. As far as I remember this has always been the main switch transistor that was short-circuited. Now it was supposed to work before tranportation so it could be something that has came lose in the PSU. If you cannot find any...

What kind of temperature on Q5 are we talking about? Sometimes it is hard to target one single component by spraying it. It might be some other component close to it that is sensitive. There is a voltage regulator circuit Q4, D2 and R1. Check the voltage on the emitter of Q4. Is it constant...

The method I have been using succesfully with around 100 TU58/DC100 and a few DC300 is to use a heatgun at lowest temperature. Perhaps 50 degrees or a little bit more. Aim it at the drive belt at some distance and move along the drive band. The driveband then shifts from a matte finish to a...

The 8/M (and 8/F) uses the same type of supply. +5 and -15 is switched PSUs. +15 is linear. The switching is hysteresis switching so the output will be a sawtooth overlaid on the nominal voltage. Before doing anything, check all big semiconductors. I have had 2N5302 that shorted. Good old...

Is it connected to the machine? Verified that there is no short-circuit in the machine? Use a bench PSU and try to feed it from. It should work quite well from 20 VDC. Use dummy loads. Not too much loading for initial testing. If it draws a lot of power with little loading I would suspect...

You should use a dummy load and check the supplies. It has been discovered that the power supplies behaves very badly if the output capacitor is bad/low capacitance. It is a switching power supply. But with low capacitance it starts acting as a linear supply. The guys at RICM as researched...

Here is the relevant part of the schematics. You need extenders to properly access the signals. I think it is a pretty good investment to ease the debugging. The 11/05 doesn’t seem to have the microaddress on the backplane for debugging purposes like the 11/04. I repaired a bunch of 11/05...

You have clock signals that looks ok about right. Now you need to look at the microaddresses. /Mattis

Right. Then there are clocks! check the signals on E10, E64 and E84. What is the state of CK OFF signal? In my experience 7440 buffers are prone to fail. Just curious, what is the bandwidth of your scope?

You have to change the timebase to something like 500 ns / division. I have no clue what I am seeing.

BTW. Sometimes it can be useful just to probe with the oscilloscope and find signals that are sitting on levels that are not really valid TTL. An input that has a level around 2V means that there is no output driving it. That is normally not a good thing. Unless it is supposed to be unconnected...

Looks like there i no clock. Now you need to figure out why.. First I think that you should check on the oscillator itself. At E39:8. Then if there is nothing here there are multiple signals that could turn off the clock. Check all those and see if anyone has a level that blocks the clock...

You are right. The front panel is connected to the datapath. There is a 74150 that selects a bit to serialize over the connection to the frontpanel. It could be just that one that has a problem. But it is not very likely. I think the problem is in the control circuitry since nothing at all...

Ok. Then the problem is not the display. You need to get a board extender to raise the control board so that you can probe it. Douglas Electronis used to have suitable modern extender boards. I am not sure if still is the case.

If I recall correctly reversing the cable gives all LED on. Since there isn’t power in the cable it isn’t that dangerous. Ok so PP is always 0. That could explain why you have all LEDs off. What a out the clock signal at pin CC and the 74193 counters? Is there a clock and are the two 74193...