Electric organ PSU query.

[Diabolical Artificer]

I've been asked to look at an old lady's Korg Electric Organ, symptom's no sound OP, no indicator lights.

Checked the mains tfmr, all AC secondary voltages are present as far as I can tell, the manual schematic has no voltages given. I couldnt find any DC voltages on the PSU board,, but manual says to check +5v rail, which makes sense as the main board has a lot of logic IC's.

In the attached schematic you'll see a PQ05RF2 LDO reg, this has 13v - same as IP voltage on pin 2, the OP pin. On desoldering the diode,( which is across the OP on the schematic, but between pins 2 & 4 (on pin) on the PCB) OP is at 5v.

The diode in question has "M2" on it, looked it up and all I could find is that it is used in flyback type circuits, it checks ok on a diode test - 0.5v forward v drop. could someone have a butchers at the circuit, there's an inductor after the OP, but I think the PQ05RF2 is a bog standard regulator, not buck/boost etc.

Andy.

[evingar]

Yes, it's a 2 Amp linear regulator. The inductor is just part of the smoothing arrangement.

[jimmc101]

The diode connected between input (1&4) and output (2) is there to protect the LDO from charge stored on all the decoupling capacitors discharging back into the LDO when mains power is removed.
It is reverse biased in normal operation and should have no effect on the unit.

From your measurements it sounds as if this diode is very very leaky, did you check the diode in reverse direction?

If the main board has had 13v on its 5v rail a lot of it may be fried.
Hopefully there may be some sort of protection on it e.g. zener clamp plus fuse which may have saved it.

Jim

[McMurdo]

are you measuring this 13V with respect to the correct ground, the diagram suggests the 5V is referenced to digital ground and the +/- split supply is referenced to 'analogue' ground. Are they both intact?

[Diabolical Artificer]

Thanks Chris, worries allayed.

"From your measurements it sounds as if this diode is very very leaky" My thoughts too, only tested at low voltage out of circuit, I'll test it with some proper volts on it." If the main board has had 13v on its 5v rail a lot of it may be fried" Hopefully not, but I'm going to take a small bench supply and power the board up with that.

"are you measuring this 13V with respect to the correct ground, the diagram suggests the 5V is referenced to digital ground and the +/- split supply is referenced to 'analogue' ground. Are they both intact? " I'm measuring the 5v from the ground pin on the chip, so pins 2&3. I didn't spot that Kevin, as you say there are different gnd symbols, the diminishing horizontal lines I've always thought of as ground, the other one - chassis or earth. There are some anomalies between the schematic and actual PCB, EG pin 4 (on) is shown as tied to Vin on the schematic, on the board it isn't and has a separate trace and goes to the main board; I suspect there may be some switching on of this 5v rail from the main digital board, just to make things complicated.

Andy.

[Outrun_uk]

Just a note concerning modern Korg equipment, my hardly used (maybe 6-7 times) Monologue failed with no power about 5 days left on the warranty. Was sent back via the retailer, first Korg said it had been tampered with and wanted to charge for repair but after an exchange or two with the retailer they agreed to repair it.
They had it for about 3 months(!) and eventually sent a brand new unit. Apparently there was a short circuit on the 5v line, surely a repair or even replacement PCB would have been more cost effective...?
Not impressed by Korg but the retailer (Dawsons Music) were very helpful and apologetic so can’t fault them.

Cheers,

Kev

[Radio Wrangler]

Originally Posted by Outrun_uk

surely a repair or even replacement PCB would have been more cost effective...?
Kev

Only if the PC board was set up as a spare part.

With some modern stuff there is absolutely no spares back-up. Ordinary off the shelf bits are bought from distributors as required, anything else means supplying a full new unit. Costs are kept down by getting out of liability whenever possible, which means trying it on every time.

David

[Diabolical Artificer]

After more investigation I suspect the reg IC. Yesterday I powered the organ up again from the PSU, I had 5v on the regulator OP's, connected main board up, 5v dropped to 0.8v. Checked for shorts and gave the main board a thorough visual inspection, all seemed ok.

I'd come prepared with a little bench PSU, I made up a lead with fuse and a 10r and 100r resistor soldered on. Powered it up via the 100r , 5v dropped to 3v ish, tried again with the 10r, organ came on, lights lit and got sound OP. I also shorted the 5v rails with a 390r, rail nosedived.

I've been looking at my reg stash, but only have various iterations of the 7805, which are only good for 1A and the pin out is different, though I could bodge one in. The PQ05RF is good for 2A. Yesterday the main board was only drawing about 800mA.

So, is it the chip, or something else? I suspect the chip, can't see a cap or any other component causing the issue. How do these reg IC's fail? I've usually found they go SC. Any ideas welcome.

Andy.

[Diabolical Artificer]

Unsoldered the chip to eliminate the possibility that something else on the board could be at fault, then powered it up on a breadboard with a 10k R from IP V to the ON pin, to make the ON high. Powered it from a bench PSU, no load = 5v, when I connected a 2.5r load on it, 5v rail collapsed.

Next I did the same with a 7805, but with a 5r load, all kushti, therefore it's the chip. If I can't get a PQ05RF2, I though about using a L78S05CV, it will just mean moving the legs a bit, as pin out is different. The PQ05RF2 is no longer made, can get one from Littlediode though, but at over a tenner a pop it seems a bit steep.

A.

[Herald1360]

LM340AT-5.0 has a slightly higher current rating at 2.4A. Same pinout issue though.