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Tektronix 475A power issue

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705jas:
Hi All
I have a 475a that is cooking the CR1452 bridge rectifier (5v Rail).  I replaced the bridge but even with it out the positive and negative pads for the rectifier are shorted to ground.  Fairly sure that's not right but I can't find any shorted Tants etc.  Ideas?
Thanks
Allan

bdunham7:

--- Quote from: 705jas on May 09, 2023, 04:41:00 am ---Ideas?

--- End quote ---

How dead of a short?  If it is less than 0.6R, then it either has to be a shorted C1452, a wiring/PCB issue or something on the +5V unregulated output.  For the latter, I don't know where that goes, you'd have to stare at the schematics for a while.

If your 'short' is more like 1R, then maybe remove Q1456 and see if it goes away.  If not, then you are still looking at all the same things I just stated.

Gyro:
The short to ground on the negative pad of the rectifier is to be expected, the +5V rail is referenced to ground (chassis). It's just the positive pad short that you need to worry about. You say it was cooking the rectifier, it would have been helpful to know if the +5V regulator transistor (Q1456) was cooking too.

Shorted reservoir caps (C1452 in this case) aren't impossible. I recently had a reservoir capacitor fail short on the +50V rail, however the voltage rating on that one was rather close to the actual rail voltage. C1452 is a 25V part though, so about three times what it normally sees in circuit. A dead short at the rectifier positive pad does implicate C1452 (or a solder splash) though.

The 5V regulator has current limiting built in with R1458 being the current sense resistor. This is 0.6R so, as bdunham7 says this can help trace the fault. Check the resistance to chassis on both sides of R1458, if the dead short is on the Q1456 side then it may be a PCB short (solder splash from removing the rectifier?). If on the 'scope' side (the +5V TP) then it is somewhere on the regulated +5V rail.

You say you have checked all of the Tants but C1456 would be a prime candidate. A shorted CR1458 is unlikely but possible too. If not those then it is a matter of methodically tracing the +5V rail across the schematic pages and boards.

Fear not, it is almost certainly fixable. The Troubleshooting section - and particularly the Corrective maintenance part of the service manual are particularly helpful. Most parts can be replaced without removing the PCBs. Large reservoir cap replacement does require removal of the trigger board, but that isn't nearly as difficult as it initially looks.


P.S. I don't think the 475A has a +5V unreg rail(?).

bdunham7:

--- Quote from: Gyro on May 09, 2023, 01:25:29 pm ---P.S. I don't think the 475A has a +5V unreg rail(?).

--- End quote ---

The schematic shows one, but it may not go anywhere or perhaps it is referred to elsewhere as +8.4V, since that appears to be the actual expected voltage (if I read that correctly).  I didn't stare at all of the schematics long enough to see if either +5V unreg or +8.4V shows up anywhere.

Gyro:
Doh, you're right - I was looking at the 475 (non-A) Mil version manual schematic. Yes, that would be a prime candidate.


P.S. Any chance that you have a link to a 475A manual with schematics. I thought I had them in the Mil version manual but it was only earler today that I realised that it is for the 475 rather than the 475A. I have the paper copy but that tends to get a bit moth eaten over time.

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