AMS DMX 1280 delay repair - logic chip question

hi all - these seem to have lots of old 7400 CMOS chips in it - specifically SN74LS1 and LS2 series.  I'm wondering if these are as failure prone and static sensitive as other CMOS chips of the period (like the 4000 series). I'm redoing all the soldering on many boards as it's all gone very nasty looking ...  grey and krinkled and cracked.  So i thought i might ask what the consensus was on 'maintenance replacement' ...

With AMSes, if they are working, dont muck around with them! You have been warned!
But check the battery on the processor card, these outgas and wreck the card.

leadpoisoning said:

hi all - these seem to have lots of old 7400 CMOS chips in it - specifically SN74LS1 and LS2 series.  I'm wondering if these are as failure prone and static sensitive as other CMOS chips of the period (like the 4000 series). I'm redoing all the soldering on many boards as it's all gone very nasty looking ...  grey and krinkled and cracked.  So i thought i might ask what the consensus was on 'maintenance replacement' ...

Click to expand...

74LS is bipolar technology not CMOS so standard anti-static should be adequate.

Perhaps share a photo of the suspect solder... I thought it started out grey....

JR

unfortunately in attempting to brighten the image the solder looks MUCH shinier than it is - it's a uniformly a very matte grey ... but this shot through a microscope (pardon the image quality please) seems to give a general indication of the solder condition.

is it  a mistake to try to re-work these boards without a grounding strap on one's wrist?

JohnRoberts said:

74LS is bipolar technology not CMOS so standard anti-static should be adequate.

Perhaps share a photo of the suspect solder... I thought it started out grey....

JR

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i thought maybe i could improve the chances of incidence of future failures by cleaning up the joints a bit ... i was emboldened by my old Sony D7 delay that i was able to bring back to life just by reflowing all the solder on the main board ... but yes i understand and believe me - i AM intimidated - but i was hoping this would be a good move ... i think there's a lot of voodoo associated with the AMS units - presumably simply because of their relative complexity

radardoug said:

With AMSes, if they are working, dont muck around with them! You have been warned!
But check the battery on the processor card, these outgas and wreck the card.

Click to expand...

That solder does not look that bad, but the wetting around several leads look marginal..(like leads were dirty or oxidised before soldering).  Perhaps a dose of liquid flux and reheating some of the joints wouldn't hurt. I am most suspicious of the the leads with a dark line appearing to separate the lead from the solder. Of course it is difficult to tell from just looking.

Handling while TTL parts are soldered into PCB should be hard to damage.

It never hurts to wear a proper anti-static strap while not many DIY do.

JR

JohnRoberts said:

That solder does not look that bad, but the wetting around several leads look marginal..(like leads were dirty or oxidised before soldering).  Perhaps a dose of liquid flux and reheating some of the joints wouldn't hurt. I am most suspicious of the the leads with a dark line appearing to separate the lead from the solder. Of course it is difficult to tell from just looking.

Handling while TTL parts are soldered into PCB should be hard to damage.

It never hurts to wear a proper anti-static strap while not many DIY do.

JR

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thank you - yes i was kind of looking askance at some of those joints too ... on the one board i already did (the one adjacent to the PS with the battery on it) i was just using a solder sucker and redoing the joint with 63/37 solder and trying to keep my heating time to around one second

Hi leadpoisoning,

As John said, 74LS is (usually Texas Instruments) low power Schottky TTL working between 0 and 5 volt rails and no special handling requirements are necessary as they are low impedance devices. 

If you have a chip that you know is definitely bust and have a replacement, I would suggest you cut all the legs on the component side with some side cutters,  use an iron to heat the legs in the component side of the board and pull them out carefully.  Then use a desoldering tool to clean out the holes.

Cheers

Mike

If you are working on the processor board, it will have acid damage from the battery. The normal thing that happens is that the battery releases acid which corrodes all of the transistors and components below it which are related to the battery control. In severe cases it will corrode all the tracks underneath it. I suggest you remove that battery. There is a replacement made which is lithium I think, so doesn't have the outgassing problem. Clean up the board with a solution of baking soda and water, and then clean with methylated spirits.

radardoug said:

If you are working on the processor board, it will have acid damage from the battery. The normal thing that happens is that the battery releases acid which corrodes all of the transistors and components below it which are related to the battery control. In severe cases it will corrode all the tracks underneath it. I suggest you remove that battery. There is a replacement made which is lithium I think, so doesn't have the outgassing problem. Clean up the board with a solution of baking soda and water, and then clean with methylated spirits.

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i'm not sure the board was ever damaged - it has a relatively new NiMH battery from it's last service (of a few) at Kulka's place - so maybe if there WAS damage to the board at some point it was repaired - although there seems to be relatively little signs of rework on the board - and it was operating properly from what i could tell the last time it was running (until the power supply blew up - well it burnt a resistor due to what i think is a shorted ceramic cap).

sidenote: actually someone (i presume Kulka) put a few jumpers across the board so maybe there was damage or rework on it

I'd like to beg to differ with you on using baking soda for battery spillage though - having looked fairly deeply into the issue with the korg polysix i'm pretty familiar with the issue ... that might be good advice if one is trying to clean up spilled acids - but in the case of either NiMH or NiCd batteries i would think an ACIDIC solution would be best since the spills are highly alkaline ... just my two cents ... but for some reason many people seem to think that batteries always spill 'acid' .... i would think that 'cleaning up' with baking soda would only make things worse ...

JohnRoberts said:

Perhaps a dose of liquid flux and reheating some of the joints wouldn't hurt.

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I'm with JR also

I would carefully clean the tracks and solder joints, with PCB cleaner, and then reflow and clean again

Whoops said:

I'm with JR also

I would carefully clean the tracks and solder joints, with PCB cleaner, and then reflow and clean again

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thanks - that's kinda what i thought - i'll lightly 'redo' just the bigger/crappier looking ones

The original boards had lead acid batteries, baking soda is entirely appropriate.

radardoug said:

The original boards had lead acid batteries, baking soda is entirely appropriate.

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I think you mean nickel-cadmium (ni-cad), not lead acid.
-Charlie

VertigoRecording said:

I think you mean nickel-cadmium (ni-cad), not lead acid.
-Charlie

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yes and i think the ni-cd batteries leech alkali  ... so ....

Nicads-
"Since potassium hydroxide is a caustic agent that can irritate the respiratory system, eyes, and skin, you may want to take a precautionary extra step of using vinegar or lemon juice (both acids) to neutralize the alkaline build-up. Use a Q-tip or an old toothbrush, as shown here, and carefully wipe away."
Something I found using Google.
CB

VertigoRecording said:

Nicads-
"Since potassium hydroxide is a caustic agent that can irritate the respiratory system, eyes, and skin, you may want to take a precautionary extra step of using vinegar or lemon juice (both acids) to neutralize the alkaline build-up. Use a Q-tip or an old toothbrush, as shown here, and carefully wipe away."
Something I found using Google.
CB

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yes - i know thanks - i guess that wasn't directed at me (?)

When replacing TTL circuitry, note that AMS were quite notorious for designing "to the edge" when it comes to dependence on gate propagation delays. This especially in their StudioComputer and AudioFile, but to some degree also across their entire works.

I have had several issues where replacing a dead 74xx package didn't help at all, until I dropped in one from a batch close to the original (!!).

Do NOT expect just any chip family to work, even if "100% compatible" in theory

Jakob E.

VertigoRecording said:

I think you mean nickel-cadmium (ni-cad), not lead acid.
-Charlie

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Sorry, my bad, but the baking soda still works for me. Along with meths as a followup.

leadpoisoning said:

yes - i know thanks - i guess that wasn't directed at me (?)

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Correct!