Tektronix 475A power issue

[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]

Ideas?

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]

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

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.

[705jas]

Hi All

Thanks for the advice. The C1452 tests ok and the CR1452 is new (but still getting hot)  I have the 475 service manual which has full schematics and parts layouts but the 475A service manual, which looks complete, does not have any schematics.  I cannot see anything related to an unregulated 5v line??  In very short bursts ie: 1-2 seconds I can turn it on and I get some lights and all the voltages at the test points as I should except the +5v.  I have attached a pic showing the voltages I have tested around this part of the circuit if anyone has any ideas.  Also if anyone has the actual 475A schematic that would be useful also. 
Thanks in advance.

cheers...Allan

[bdunham7]

Thanks for the advice. The C1452 tests ok and the CR1452 is new (but still getting hot)  I have the 475 service manual which has full schematics and parts layouts but the 475A service manual, which looks complete, does not have any schematics.  I cannot see anything related to an unregulated 5v line??  In very short bursts ie: 1-2 seconds I can turn it on and I get some lights and all the voltages at the test points as I should except the +5v.  I have attached a pic showing the voltages I have tested around this part of the circuit if anyone has any ideas.  Also if anyone has the actual 475A schematic that would be useful also. 

Tek 475A schematics and manuals are all linked on the Tekwiki 475 page.

https://w140.com/tekwiki/wiki/475

Your current is obviously not going through the regulator transistor nor the 0.6R resistor.  You may need to track down that unregulated 5V line physically and see where it ends up.  It appears to have a dead short somewhere.

[Gyro]

Thanks @bdunham7, I'd never noticed the 475A schematic link on the right hand margin of the Tekwiki page, I must have looked at it a dozen times!

I don't think the +5V Unreg leaves the PSU schematic (there are no diamond symbols or other idents next to its arrow). The only place I can see it going on the PSU schematic is the Graticule illumimation lamps drive (bottom left). I don't see how a fault there could cause a short though - apart from maybe a short on the connections to the graticule lamp board, that would probably cause Q1482 to go short too (be careful of frying the 'Scale Illum' pot, don't set it near minimum or maximum while debugging).

My bet would have been on C1452 going short, you say it's ok, did you test it out of circuit? The PCB plated through hole is large enough that you can suck all the solder out of it and isolate the capacitor +ve pin (wooden toothpick works) for long enough to check it without physically removing the capacitor.

[705jas]

HI All

Still battling with this.  C1452 was fine when tested out of the
circuit.  I have almost removed everything in the PSU schematic regarding 5v but have not found anything.  check with a thermal camera and only the rectifier CR1452 gets hot (instantly).  Have taken the jumper off the gratical line near Q1482 but has no effect on the short. U1454 is shorted on Pin 6 but all others are fine.  Took that out but still no change.  Running out of things to check.  Cannot see where to go next.  Any ideas appreciated.

cheers...Allan

[Gyro]

It may seem silly, but have you checked CR1452 itself? Just because it is a replacement doesn't necessarily mean that it is good (particularly if from ebay / Ali). as it is the first thing the current hits after the transformer, and the only thing getting hot (apart from the transformer winding!)...

The only other thoughts are 'finger trouble' - solder splash, legs cut too long when inserting CR1452 and shorting to chassis etc.

Other than that, try feeding around 2A DC (current limit), with an 8V voltage (EDIT: better 5V) setting from a bench PSU into the output pins of CR1452 and probe around the relevant traces on the PCB with a DMM on mV range to try to localise the short. This needs doing with great care though as the +50V reference for the regulator won't be present. When you find the short, turn off the PSU before clearing.

[705jas]

HI

Thanks for the suggestions.  I have lifted and tested just about everything I can think of and the only thing I seem to have confirmed, by lifting the leg off the Collector of Q1456 is that the problem seems to be only on the 5v Unregulated line.  What I can't see is where that line goes?

cheers..Allan

[Chris56000]

HiI

The wassock that uploaded that grotty 475A manual to the TekWiki couldn't be bothered to scan the layout diagrams for the PCBs as well as the schematics either, and repairing Tektronix gear without proper and complete documents is very much a hit–and–miss affair!

I would pay particular attention to where Q1456 and Q1482 are mounted to make sure these two transistors  are not missing any insulating bushes or mica washers underneath them, otherwise the "+5V UNREG" line, (marked "+8.4V" on the diagrams I can find) will be directly shorted out, which would soon cook up and damage the bridge rectifier!

Another possibility is the wiring between C1452 and the collector of Q1482 – is it shorting to the metalwork anywhere?

It is unlikely that the scale illumination control has a direct short to earth in it as there's a limiting resistor feeding it, and in any event, this comes off the +15 V line, so a short to earth in the scale illumination control wouldn't cook up the bridge rectifier!

Chris Williams

[Gyro]

The wassock that uploaded that grotty 475A manual to the TekWiki couldn't be bothered to scan the layout diagrams for the PCBs as well as the schematics either, and repairing Tektronix gear without proper and complete documents is very much a hit–and–miss affair!
...

The schematics are on the TekWiki page (as Reply #6)...  https://w140.com/tekwiki/images/f/f1/Tek_475a_schematics.pdf  Granted, no layouts.