Sometimes, fault-finding isn't the answer...

[Kat Manton]

Hi,

In September, I picked up a job-lot of faulty 49-key MIDI controller keyboards for £30.

The two Quickshot MIDI Composer QS-5836 keyboards were easy repairs, both just needed the keyboard dismantling and the contacts cleaning. One had suffered from a drink spillage, the other was just well-used.

The third, an Evolution MK-149, proved a little trickier. Everything worked perfectly except for a strange problem with velocity sensing.

When you hit a key harder and harder while watching the data with a MIDI monitor, all was well, up to a point. Velocity ranges from 0-127, but for very hard key hits somewhere over 100, it'd drop back to 1. Very annoying, particularly when attempting to program percussion sequences and go for that really hard kick or snare hit!

I'd given up on this one for a while but decided to have another go the other day. I poked around with a 'scope and tried to fathom out what was going on, to no avail.

Eventually, after a nice cup of tea and a think, I gave up and just resoldered everything. None of the joints looked particularly bad, but obviously one or more was as this cleared the fault.

Sometimes, "proper" fault-finding isn't the answer. If it doesn't move, resolder it

Cheers, Kat

[HamishBoxer]

Better still wear a head magnifier,its another world for checking dry joints.

David

[Steve_P]

Very True!

I remember one engineer who resoldered every joint on every set that came in, and many just went right. Also, cleaning too. He used to drop IC's into a cleaning bath and then dry them. While the IC was in there, he'd clean the board where they sat. Sometimes, the dirt that you can't see would be enough to make the chip not work.

Not always, but sometimes. He worked on those things that workmen use to determine where in the pavement to dig a hole. They would come back filthy!

Cheers,

Steve P.

[Robert Darwent]

Originally Posted by Kat Manton

Sometimes, "proper" fault-finding isn't the answer. If it doesn't move, resolder it

Hello Kat

I've been using this method to fix elusive 'faults' for many years now. When everything obvious has been tried just resolder no matter how good and shiny the solder joints appear to be. It works for me more often than not!

[mickjjo]

This reminds me of about 30 years ago when trying to fault-find Datel testers for BT, these things had 4 huge circuit boards full of ICs and were plagued with dry joints often taking weeks to locate by conventional means. In desperation various bodges were tried including wedging matchboxes between the boards to get the things working and "out the door" . Some bright spark came up with the idea of blasting the boards with a hot-air gun to re-flow the solder!, . It worked every time and hundreds of manhours were saved and a lot of expensive equipment escaped the scrapheap.... .

Regards, Mick.

[slidertogrid]

This reminds me of the Hitachi colour portable sets we used to see in the late 80s, I can't remember the model but most were white, 10" screen and a rotory tuner on the top of the set.
They suffered from various faults including spurious mains fuse blowing and could be a real pig to cure, sometimes the sets would work for weeks before the fault came back.
Many hours and fuses were wasted until we were given the tip of resoldering every joint in the set, this took some time but provided a complete cure (most of the time).One set which had been a bit of a trial was part exchanged and after resoldering it a second time it was used as a camera monitor in the workshop to see if it would last, it was on all day 6 days a week and was only replaced 16 or so years later when the tube had got too dim to see a picture in daylight!
Should have tried the "solder it and see" approach more often I think!
Rich

[Nickthedentist]

This technique worked for me on a Sony AE1 tube base panel.

I was on the verge of scrapping the set, as tapping the tube neck caused the picture to go all red with flyback lines, which really did suggest an internal short to me.

Nick.

[ben]

Originally Posted by slidertogrid

This reminds me of the Hitachi colour portable sets we used to see in the late 80s, I can't remember the model but most were white, 10" screen and a rotory tuner on the top of the set.:

blimey i remember them,all the guts packed into such a small space too, heat buildup may have been one cause. always bad joints and hard to get at. Last one i saw, the red gun kept dropping out.

[Andy Green]

I also use this technique to repair a Technics SP25 professional turntable, back in the mid 80's at a radio station here in Ireland. Back then, they were about £500 each, we bought two for £250. Great units, would get to full speed in about 1/10th of a revolution!

The fault was that there was a constant 'flutter' on the sound. Getting the circuit diagram and looking inside confirmed that I hadn't a hope of fixing it using conventional methods - there must have been about two hundred discrete transistors in there! (almost no IC's - later models were much more IC based - this was an '81 vintage)

Resoldering everything in sight fixed the thing!

[Skywave]

ISTR that GEC all-transistor CTV's frequently responded favourably after a thorough 're-solder any iffy-looking solder joints on spec.' session.

Anyone else experienced this with these sets?

Al / Skywave.

[Steve_P]

Had this with loads of sets. Resolder everything and then look for faults...

Cheers,

Steve P.

[flyingtech55]

There was a Sony small screen colour set (1430?) that was terrible for dry joints on the decoder panel. It was the one that had the little touch contact things on the top edge to change channels.

I asked Sony about these and their engineer said to resolder the whole PCB. He said that they had had problems with the solder bath on the production line. He said to divide the PCB up into matchbox sized rectangles, resolder every joint in the rectangle, check the set still worked after completing each one and continue in this way until the whole board was re-soldered. He assured me that that was what the Sony guys did and it worked in 99% of cases.

I did that on all the ones I got in and it certainly worked for me!

TimR

[Hybrid tellies]

If I remember correctly some of the large screen Sony's from the 1980's also had the same problems with dry joints around the decoder board. They also use to suffer the same on the IF board. That meant unsoldering it from the main board,removing the screening then just resoldering every soldered joint in sight. It was quite a tricky job extracting the thing from the main board and equally fiddly lineing up all the pins when refitting it to the main board, but this would clear many an intermittent loss of signal or patterning fault.
Certain Hitachi colour sets from the early 1980's use to use a thick film type module for the frame output. Removing this to carefully resolder all the joints on this module, especially around the frame output transistors, cleared many an intermittent frame fault.

[Welsh Anorak]

Hi
Unfortunately, the use of lead-free solder has made the 'traditional' dry joint a thing of the past - they ALL look dry, even when you've reworked them!
Glyn

[Norman Raeburn]

Hi folks, I always wear a magnifier and check for dry joints and swollen caps this is particularily true for modern equipment. If this fails it's time for the AVO and the thinking cap. Cheers Norman.