Ursa Major Space Station - Voltage problem

[morls]

Hi,
I've inherited an Ursa Major, which needs a bit of work. Here's the section of the schematic I'm working on (the schematic dates from 1978, and it's hard to read some of the writing):



I've come across an issue in one of the voltages - the minus 5V line is only giving minus 0.9V. All the other voltages are great.
There is -15V coming into FB1 from the PSU. There is -15V hitting R15, but the other side of R15 only reads -0.9V. Here is the area on the board:



The test point I'm using is between R15 and D5. I've replaced D5 and R15. The schematic calls for D5 to be 1N5231, but all I could get at short notice was 1N4733, which is the same voltage but different reverse current:



After replacing, the voltage at the test point is the same, -0.9V. I've checked all the caps following, and as stated all other voltages are good. I'm a bit stumped now. All I can think is that the zener diode might not be installed with the correct polarity, but I'm not going to flip it around for fear of damaging other parts of the circuit. I've also uploaded the PSU schematic.

 

[L´Andratté]

Aren´t that tantalum caps?
They are known to die with age and could short out the -5V rail to GND?

 

[morls]

That’s a good point. I’m planning to replace C1 to C12, which I’m understanding from the schematic to be 0.1uF/15V

 

[Khron]

Aren´t that tantalum caps?
They are known to die with age and could short out the -5V rail to GND?

They are, and they are.

But-but-but... What about the "tone"?

That’s a good point. I’m planning to replace C1 to C12, which I’m understanding from the schematic to be 0.1uF/15V

The tantalum caps (which are the suspicious ones) are the yellow ones; the one on the -5V rail seems to be C60. The orange C1-12 are ceramic caps, far less likely to fail.

 

[morls]

C60 follows FB2 on the +22V rail into the voltage regulator U1. The +12V out of this is good.

C56 and C62 are on the -5V rail though, and these are tantalum...

 

[Khron]

C62 and/or C56 then; for whatever reason, i thought that wider ground trace was the -5V rail. I definitely haven't slept enough last night...

 

[FIX]

Contact Tim Curtis in LA. He is the guy.

 

[abbey road d enfer]

C56 and C62 are on the -5V rail though, and these are tantalum...

I hope it's as simple as replacing those.
IC's are also known to go short some times, and it's a pain to know which one when there are many sharing the same rails.

 

[mjrippe]

I hope it's as simple as replacing those.
IC's are also known to go short some times, and it's a pain to know which one when there are many sharing the same rails.

If you have a multimeter with a "low ohms" setting you can actually put one probe on ground and follow the voltage line with the other - the closer to the bad IC (or cap), the lower the resistance.

 

[Brian Roth]

I discovered that my "Blue" ESR meter is quite useful for walking along a PSU rail to find a shorted cap.

Bri

 

[morls]

If you have a multimeter with a "low ohms" setting you can actually put one probe on ground and follow the voltage line with the other - the closer to the bad IC (or cap), the lower the resistance.

I’m not sure my multimeter is that accurate, but I’ll give it a go

I discovered that my "Blue" ESR meter is quite useful for walking along a PSU rail to find a shorted cap.

Thanks, another good option.

There are tantalum caps all over these boards, what are good alternatives?

 

[Brian Roth]

There are tantalum caps all over these boards, what are good alternatives?

I presume the tants are for power supply rail bypass.....very popular Back In The Day. The little solid tants (look like a small piece of candy) are nowadays problematic. What uF values are we talking about?

When I find a bad "candy tant", I typically use a modern low impedance aluminum electrolytic made by a reputable manufacturer (Nichicon, Panasonic) purchased from a reputable distributor (Mouser, Digikey). Ebay and Alibaba caps are typically no name "OneHungChow" chinese junk.

Bri

 

[Khron]

When I find a bad "candy tant", I typically use a modern low impedance aluminum electrolytic made by a reputable manufacturer (Nichicon, Panasonic) purchased from a reputable distributor (Mouser, Digikey). Ebay and Alibaba caps are typically no name "OneHungChow" chinese junk.

* OneHungLow

I'd also add Rubycon, United / Nippon ChemiCon and Elna to the (very) short list of reliable electrolytic brands.

 

[jensenmann]

Don´t get too scientific with your Space Station. Recap it completely and replace the tantalums with electrolytics. I´ve owned two SST282s over the years and both got funky at some point. Recapping cured them both.

 

[JohnRoberts]

a shorted capacitor or a faulty zener diode are the obvious suspects. To troubleshoot you can lift one leg on each cap. Then lift the other leg and replace them after fixing the rail voltage.

JR

 

[morls]

Thank you all, I've found a failed polystyrene cap and a couple of suspect zener diodes that I'll replace. Fingers crossed this will be all, but in any case it's a step forward