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Old 19th Nov 2018, 9:43 pm   #1
mark_in_manc
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Default Weir 413D PSU (again)

Hi folks

A few years back I started this thread on this power supply; luckily i found a circuit diagram at the time, which I put up there:

https://www.vintage-radio.net/forum/...ad.php?t=95600

That time it was an easy fault, but now I have let some magic smoke out and I wonder if anyone would hold my hand while I scratch my head over it.

It appears to adjust to a desired o/p voltage, but as soon as I add any kind of load this collapses - even with very small o/p currents.

The 6.5v (relative to +ve meter output terminal) supply which runs the control circuitry is stable, whether there's a load on or not. (This rail sits on top of Zener MR11 on the circuit diagram.)

The +ve side of the bridge rectifier sags a bit when a load goes on, but not enough to (IMO) be a concern - still plenty of headroom there to drive the o/p transistors.

R29 and R30 (ceramic 0.5 ohm current sensing resistors) both measure ok.

Current limit preset RV1 doesn't seem to do anything at the moment.

Any suggestions where to look next would be very welcome - I'm failing to grasp how the feedback works (or should work) from the description in the manual.

many thanks
Mark
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Old 19th Nov 2018, 11:55 pm   #2
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Default Re: Weir 413D PSU (again)

As the application of a load causes the output voltage to collapse, it would appear that there is insufficient base current being fed to the series pass transistors VT10 and VT11.

I would first try to establish that a fault in the current limit circuitry around VT2 is not causing the voltage collapse. You could temporarily lift the diode MR10 to prevent VT2 from exerting any control over the current drive to the series pass transistors and then apply your load to the output and see if the output voltage still collapses.

If there is no change to the fault symptom, then I suggest, as a start, that the emitter to base and emitter to collector voltages of VT1, VT3, VT4, VT5, VT6 and VT7 are checked to see if there is anything unusual (e.g. incorrect Vbe or very low Vce). Note that VT3, the level shifter and VT5 are pnp devices.

Ron
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Old 19th Nov 2018, 11:59 pm   #3
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Default Re: Weir 413D PSU (again)

Are you able to test the health of the electrolytic capacitors?

These and the switches are a good starting point.

Then test the diodes, zeners and the Vbe drop of the transistors.
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Old 20th Nov 2018, 3:15 am   #4
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Default Re: Weir 413D PSU (again)

If it is the same problem I had you face a major strip down to fix the cause of the fault.
It is worth fitting a link between the layers on the PCB behind the terminals if you ever strip it down far enough to get to it because if the terminals get at all loose it blows the voltage sense transistor.
https://www.vintage-radio.net/forum/...ad.php?t=89688
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Old 20th Nov 2018, 4:55 am   #5
mark_in_manc
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Default Re: Weir 413D PSU (again)

Well, I lifted one end of MR10, and with no load on, it blew the main fuse as soon as I switched it on. So this is something... Unfortunately I then blew up my cheap Chinese component tester by poking it too close to a charged capacitor.

Well, time to buy some fuses and calm down a bit.
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Old 20th Nov 2018, 6:09 am   #6
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Default Re: Weir 413D PSU (again)

The output stage gets current from VT7 which acts as an amplifier. As the supply regulates OK (well, before post #5!) this seems to be OK to this point.

Next, VT9 is an emitter follower, driving VT10 and VT11 in parallel via current sharing resistors in their bases and emitters.

VT9 gives unity voltage gain, but boosts the available current.
VT10 and VT11 do exactly the same agani to further increase the available current.

If VT9 or VT10&11 lose the feed to their collector, or they fail with collector open-circuit they cease to boost the available current, but just pass the current from the stage before, acting like simply a diode (their B-E junction). You still get regulation as normal, but little current will be available before the supply collapses.

Pretty much the same happens if any of this trio fail with a base-emitter short.

Time to remove and test VT9, VT10, VT11.

Like a transistor audio power amp, you have a DC coupled multi-stage amplifier controlled by feedback. Each stage can destroy its neighbours if it goes outside its designed operating linear region and turns full-on or full off. To prevent this, it relies on the feedback loop operating. Lifting MR10 opened the feedback loop. The feedback loop had been working OK up to this point.

It's now difficult to guess what might have happened.

I'd now just remove all the transistors and test or replace them all, do an Ohms and discolouration check of all resistors (easy with the transistors out) and check all the diodes. I would scrupulously NOT try any power until the whole lot was back together with 100% known good parts.

This is one of those circuits which can lead you around and around a circle of repeated destruction if you try to replace only a single part at a time. It's why people used to AC-coupled valve amps and tellies have serious difficulties when looking at a transistor audio amp. There is no longer the isolation of DC conditions from stage to stage and the one-part-at-a-time approach can fail.

The original fault says you probably needed to replace power transistors and driver. The rest of the transistors are just small cheapies.

David

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Old 20th Nov 2018, 6:32 am   #7
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Default Re: Weir 413D PSU (again)

Thanks David, that's a story I can follow. Well, I'll go quiet for a little while, while I do what is suggested, and post back eventually when I have it all back together.
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Old 20th Nov 2018, 11:52 am   #8
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Default Re: Weir 413D PSU (again)

Congrats Mark, 1000 posts!
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Old 20th Nov 2018, 1:26 pm   #9
mark_in_manc
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Default Re: Weir 413D PSU (again)

Given the circumstances and that they reveal just how little I know, that fact is a little embarrassing. In fact, I might remind you that (thanks to your FCS help) I know where you live
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Old 22nd Nov 2018, 11:04 am   #10
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Default Re: Weir 413D PSU (again)

AVOs at dawn?
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Old 23rd Nov 2018, 10:03 pm   #11
mark_in_manc
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Default Re: Weir 413D PSU (again)

Yes, the AVO, my favourite (blunt) instrument

( I have one like Steve is discussing at the moment, in the steel case. Cheating really, like soaking your conkers in vinegar...)
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Old 23rd Nov 2018, 10:28 pm   #12
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Default Re: Weir 413D PSU (again)

Check VT9 first!



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Old 24th Nov 2018, 9:17 am   #13
mark_in_manc
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Default Re: Weir 413D PSU (again)

I really wish I had. Well, I'll have to have all the transistors off one by one and diode test them out - just a bit preoccupied for now!
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