ABI DDS-40 XP I must be blind!

[alan8086]

I just started troubleshooting my DDS-40 XP chip tester.

I had started a year ago after turning it on in the cellar yielded lots of fizzing, crackling noises. Opened up the PSU and according to me, could see no visible damage:

https://www.youtube.com/watch?v=HR7hgRJOH2c

I just opened it again and look - its obvious!



A Blown RIFA CAP...



I do wonder about myself! As can be seen from the video, the unit does power up sometimes. Hopefully if I replace all those RIFA caps with something less useless, my issue will be partly solved.

A bit of Googling tells me I need to replace with Poly caps - I'll take some of the bigger electrolytics out while I'm at it and see if they are bulging and test the esr etc (with my £8 ebay component tester for what its worth on esr)

If I can get this rare as hens teeth IC tester working again then - Hooraahh! - I can hopefully get my IMS 8000 going and maybe my 2nd Superbrain board (JonB is working on the first one - may God bless him!)

I also found a use for the TL866A Eprom Programmer I bought a while back - I have binary files of both EPROMs if anyone is looking? DDS V1.11

 

[alan8086]

PSU is made by Coutant Electronics LTD

The Maker Plate:



Here is my diagram of RIFA cap placement and values:



The Offenders!



Rifa Caps are:


C16 - 0.01uF 250V 85C
C18 - 4700pF 250V 85C
C20 - 4700pF 250V 85C
C24 - 4700pF 250V 85C
C27 - 0.33uF 275V
C32 - 0.01uF 250V 85C

They seem to have words to this effect:

40/085/56 or
40/085/21

I assume the 85 is the max temp? Not sure what the other values refer to?

Also:

250V ~ SH 125VAC

Is the 250 a DC rating? I will buy Poly caps with at least 250V max rating...?

 

[Chuck(G)]

There's more to it. Several of those Rifas are used for AC line filtering, so they must be non-polar, with an AC rating and the proper X (safety) specification. 85 is probably 85C; nowadays, the temperature premium is so small that I use 105C caps routinely.

Stick a polarized cap in a non-polar application and it will likely go kablooie. The X rating specifies how the capacitor fails--either short or open. It's for your own protection.

 

[alan8086]

Ok - by Polar/non Polar you mean they either have to be inserted according to correct polarity - like electrolytics.

These RIFA caps are non Polar so polarity not an issue?

I'm looking at eBay for poly caps - they all seem to be 85C. I'll try Farnel/RS/Maplin and see what I find. Spec sheets will be better too.

 

[Chuck(G)]

Exactly--nonpolar caps usually do not have polarity marked--there's no "wrong way". They're also often marked with their AC voltage rating, which is usually less than the DC working voltage.

 

[alan8086]

Ok - just read up on type x2 and type y2 filter caps - ordered accordingly the filter caps and all the main Electrolytics for the PSU off DigiKey

To get myself up to the free shipping limit, I also ordered all the electrolytics for my Superbrain PSU!

I also 'had' to buy a SATA to USB adapter to get me there too!

Just short of £40

Buying a ZD-915 vacuum de soldering station has opened up a new world!

 

[JonB]

(JonB is working on the first one - may God bless him!)

Too late Alan, I'm damned (to wander the halls of Silicon Hell forever)... as we see with all these flaky Superbrains. Mine has starterd misbehaving now. The CCH one is not working with B:, just when I thought I'd fixed it (for the second time). And yours is sick as a parrot with what I am currently thinking is bus request / memory refresh issues. Sigh.

But... I'm glad you have discovered the joys of desoldering stations. A life changing acquisition if you are working on old through hole electronics, as you can see. By the way, I think the RIFA caps are normally there to prevent RF noise getting out of the machine and as such it should run without them. As to the others, yeah probably necessary. The clue is to see which ones are bridging the 240v mains.

Before reconnecting to the IC tester, check the voltages. With a dummy load (I use a 12v bulb for the SB PSU).

 

[alan8086]

Ok, replaced all RIFA and electrolytic caps - as predicted John, no difference to the devices behaviour.

 

[alan8086]

Is it in any way normal to have a 700ohm resistance across the +5v and ground? Measured at the power entry to the board and across most of the little ceramic decoupling caps? I assume yes in this case or the thing wouldn't work at all?