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Korg T2 PSU/Backlight Repair

 
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Old Sparky



Joined: 02 Sep 2017
Posts: 11

PostPosted: Sat Sep 02, 2017 12:33 pm    Post subject: Korg T2 PSU/Backlight Repair Reply with quote

Hi all.

I was gifted a T2 (yey) which worked fine for my first few practises with a band in 25years, booked a gig and the thing instantly failed (argg).
Fist the backlight went out, then the machine would crash/reset/hang intermittently (every 30 minutes or so).

Had a look and an area of the PSU pcb is scorched (around Q2).

After a search around the net, I purchased replacement electrolytics throughout, and replaced Q2(switching controller/drive?) and Q3 (backlight drive transistor).

Now the backlight works and so does the PSU, but were all rails were running low before(off load), they are all running high 6.588V, 13.6V, -14.1V (offload).

I am aware that the PSU is unstable/out of regulation without a load, but this leaves me at a dodgy point.
Not too keen to connect the suspect PSU to the CPU board, and would prefer to run it into a dummy load.

It appears that the PSU feedback path is through the 5V rail only, and I was hoping to load just this line to measure and adjust the outputs, but how much current is normally drawn on this line?
Also Q2 is running hot, is this symptomatic of running the PSU without a load?

Thanks for all help offered.
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Old Sparky



Joined: 02 Sep 2017
Posts: 11

PostPosted: Sun Sep 03, 2017 12:25 pm    Post subject: Reply with quote

A small update.
Firstly I forgot to include the service manual in first post, so here it is:
https://medias.audiofanzine.com/files/revue-technique-korg-t-series-t1-t2-t3-sm-476484.pdf

Managed to get the PSU on a load, to see how the 5Volt rail behaves.
Started small at 100mA and the rail dropped slightly.
Worked up to 500mA and the rail was sat at 5.2V, and Q2 was now running cooler.
Had to end the test then, Saturday night was calling.

Will run the PSU up through the range of current load, measure output voltages and temp of Q2. (bearing mind the service manual states T2 has 15Watt consumption... three rails and some headroom for the spec might point towards 1Amp on 5V rail (5Watt).
Thinking this will have a sweet spot of stability, and if so will leave the thing soak, see how stable it is.

If there's something I'm missing, please feel free to kick me up the proverbial to tell me. Knowledge does save time, and time is only a one way deal!
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synthjoe
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Joined: 21 Apr 2010
Posts: 1007

PostPosted: Thu Sep 07, 2017 11:32 am    Post subject: Reply with quote

Hi Sparky,

I would not power the synth partly. Do apply all voltages as there might be chips that will get an input voltage while not powered... That may damage the silicon which will be very difficult to replace. So do try to test the PSU with dummy loads fully for all voltages (I often use incadescent bulbs, like 12V halogens in parallel or series for the proper load, etc.), but then connect all the rails to the circuitry - make sure you always have all voltages connected to the synth.

That's the only piece of advice I'd give after a number of years in electronics... Smile
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Old Sparky



Joined: 02 Sep 2017
Posts: 11

PostPosted: Thu Sep 07, 2017 4:15 pm    Post subject: Reply with quote

Thanks Joe!

Must admit this T2 has been breaking my heart this last week.
Seems there is a fault with the backlight that blows the drive transistor.
So removed the transistor and powered it up without backlight.

Now it has a Low battery warning, but the battery is good.
Have traced battery volts to some parts of the circuit board (couple of memory chips, and a 4051) but have got stuck there.
The synth now just acts as though no battery and looses all data when powered down.
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synthjoe
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Joined: 21 Apr 2010
Posts: 1007

PostPosted: Fri Sep 08, 2017 12:43 am    Post subject: Reply with quote

Maybe you should replace the display with the much nicer LED backlight version - you have at least one thread (or perhaps more) here about that. Or, you can try to create a LED 'foil' out of thin opaque acrylic (that you can slide in replacing the original backlight) and a few bright LED's illuminating it from the side. Or you can get a replacement foil - but it is about the same cost as a new LED backlit display (and you can get rid of the noisy voltage converter driving the light emitting foil, if you go for the LED version).

The battery voltage is measured by an A/D converter and the 4051 does the switching between that voltage and some control signals. So a message does not necessarily mean that you're loosing data - depends on the component that fails. I don't have the schematic handy, but this one does not seem like a tragical problem - if your VALUE slider and pitch bend work fine, then just focus on whether the CMOS memory chips have the standby voltage when the synth is off? If not, then maybe a diode or resistor is dead. If they have the voltage, then the data should be kept while powered off. The 4051 has no actual function in retaining the data, just in the SW measuring and reporting the voltage when starting up (or running diagnostics) - and the two controls mentioned above (VALUE and pitch bend) use the very same components to operate.

I just got a pretty bad quality download of the schematic, so I cannot read component references with confidence. However, you should check R20, R19, D3 and D2. If you have about 2.5V on pin 28 of IC 28(?) and IC 29 (256k RAM's) when powered off, then you should be fine. If you measure anything less, then make sure the R20 D3 path is OK from the battery to the RAM. Problem might be caused by R19 (0 ohm, this is the bridge to the 4051 measuring the battery voltage when starting up - can be safely removed at the expense of constantly and incorrectly getting 'low battery' warnings), or D2 that is supposed to feed the RAM from the mains when the power is applied. If this diode fails, the backup battery fights to power the whole 5V rail when the synth is powered off, which of course it cannot do. Removing D2 to check whether a low voltage condition on pin 28 of the RAM chip can be remedied is a valid diagnostic step to take when powered off. If the battery voltage is present (around 3V on the common point of D3, R20 and R19) but not getting to pin 28 of the RAM's despite of removing D2, then most likely D3 or R20 is broken.
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Old Sparky



Joined: 02 Sep 2017
Posts: 11

PostPosted: Fri Sep 08, 2017 10:13 am    Post subject: Reply with quote

That is some brilliant information Joe, thank you so much.
I will have a look further at what is affected by that multiplexor and see if I can get away with ignoring it.

Will post back the outcome of course, what's a mystery without a conclusion!

Thanks again for the info and encouragement.
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Old Sparky



Joined: 02 Sep 2017
Posts: 11

PostPosted: Thu Sep 14, 2017 12:05 am    Post subject: Reply with quote

So an update of sorts...

Impending need for a keyboard, friends in need etc Razz...
Got a T2 donated!
No backlight, all the tact switches are shot...

The thing seems stable though, even the disk drive is fine!
Must admit that the sound reminds what the wife said about great efforts for small return, IDK.

Will return with more meanderings on the dodgy T series...
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synthjoe
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Joined: 21 Apr 2010
Posts: 1007

PostPosted: Mon Sep 18, 2017 5:54 pm    Post subject: Reply with quote

Good stuff - keep going! Very Happy

Combi 71 (I believe - called "70's Piano") is one of the sounds that makes me always smile when I play it - pretty much the reason I'm still holding on to my two T2's... Wink (to the point that I actually try to recreate that same sound on my newly bought 2nd hand Kross88 - if anyone has done that already, please save me the pain and send an export! Smile)
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