Power supply question... Rackmount delay unit repair

[Mackin]

Hi !

I'm a new member here.

I've had an Ibanez DM 2000 Digital Delay sitting on my bench for a couple of days already and it has got me scratching my head. There is obviously something that I've overlooked here and I was wondering if some of you could help me out.

Here's what's up...

From what I've seen inside the unit, it operates off three voltages which are +5V, +15V & -15V.

When I first opened the case, the power transistor (which acts as a voltage regulator) on the -15V side (which is located after the reservoir capacitor) was toast. I proceeded to replace it with an equivalent NTE part.

However, the new component is still overheating and giving me identical voltage readings as the burnt one that I took out.

I figured the problem was somewhere else.

Out of curiosity, I unplugged the entire audio PCB where all the op-amps using +15V & -15V are to see if the PSU would start behaving normally. That didn't do it, still overheating.

That power transistor is giving me low voltages such as 1.02V... 0.10V... between ground and E, B & C pins and it is HOT when I touch it... meaning it is passing a lot of current, correct ?

Now, I figured I should be looking for a short somewhere... or at least a defective component passing a lot of current, right ? Or perhaps to try finding the component that holds those power drivers at +15V & -15V ?

Thing is, I can't see any other component, nearby or not, suffering from or even showing signs of overheating / burning / etc... so where's all that current going ?

Any thoughs appreciated, I feel I'm stuck here.


Thank you !
Mackin

Here's a pic. : https://www.dropbox.com/s/wvj29kkq24...80221.JPG?dl=0

[The Dude]

Firstly, welcome to the place!

Do you have a schematic? Without that, my only guess would be a filter cap or bypass cap on the -15v output since you effectively removed the op amps.

[Mackin]

Firstly, welcome to the place!

Do you have a schematic? Without that, my only guess would be a filter cap or bypass cap on the -15v output since you effectively removed the op amps.

Hey thanks !!

No, I was unable to find either the service manual or the schematic. I have the service manual for the DM 1000, but the voltages and part numbers aren't the same. Bummer !

I'm pretty sure the filter caps on the +15V & -15V section are good as they both measure the same value when using the capacitance function on my DMM.

"Bypass caps" though... I don't know, you mean as in "ac bypass" right ? Like, some cap between regulator output and ground ?

I'd have to look again see if I can locate any of these. I assume they could be anywhere - both near and far away from the power section on the PCB ?

[The Dude]

"Bypass caps" though... I don't know, you mean as in "ac bypass" right ? Like, some cap between regulator output and ground ?

Yes. Usually .01uf. It can be difficult to find the little bugger that's shorted. Maybe see if any of them are getting hot?
At your own risk: I have at times, when it's getting down to impossible, inserted a higher current power supply to the suspect line and fired up. Often the defective part will explode or burn, most of the time without major damage to anything else. Of course, there is the possibility that traces could burn, etc.
Probably before you do anything so drastic, you should take a resistance measurement from -15v to ground and verify that there is indeed a short or near short condition.

Edit: Another thought: Have you verified proper input to the regulator and that it is clean? Sometimes excessive AC input to a VR can raise hell with the output.

[Mackin]

[...] you should take a resistance measurement from -15v to ground and verify that there is indeed a short or near short condition.

I am actually going to try this right away

[Mackin]

0.2 Ω between regulator output and ground.

Haha !

Thank you for your help so far, very appreciated !

I'll check back in tomorrow after giving the machine another look.

I have to make supper now or I am going to be late

[The Dude]

Another thought after rereading the thread. Apparently the unit uses transistors for regulators instead of IC regulators. Much of the time this is done, there are zener diodes at the regulated output to keep the voltage clamped to a specific voltage. Check your PS to see if there are zeners. If so, unsolder one end of the zener on the -15v output and see if it is shorted.

[Mackin]

[...] Apparently the unit uses transistors for regulators instead of IC regulators. Much of the time this is done, there are zener diodes at the regulated output to keep the voltage clamped [...]

That's correct, the unit does use transistors.

I'll look around for zeners when I check it tomorrow

[Enzo]

NTE always scares me when I see people using it, what was the original transistor type and what was the replacement?

Since I don't know you or your skill level, I assume nothing. So, tell me if ANY of this might apply. Sometimes people see the three legged thing and assume it is a transistor, when voltage regulators we usually use are ICs. So a 7915 is not a transistor. A transistor in place of a 7915 wouldn't work. Have you had this unit in working condition and now it failed? Or was it in failed condition when you received it? Thinking perhaps someone else tried to fix it and installed something wrong.

Certainly if the output of the supply is shorted to ground, that needs to be fixed before anything else matters.

[Mackin]

NTE always scares me when I see people using it, what was the original transistor type and what was the replacement?

Since I don't know you or your skill level, I assume nothing. So, tell me if ANY of this might apply. Sometimes people see the three legged thing and assume it is a transistor, when voltage regulators we usually use are ICs. So a 7915 is not a transistor. A transistor in place of a 7915 wouldn't work. Have you had this unit in working condition and now it failed? Or was it in failed condition when you received it? Thinking perhaps someone else tried to fix it and installed something wrong.

Certainly if the output of the supply is shorted to ground, that needs to be fixed before anything else matters.

Hi !

The unit was broken when I got it.

The part that I replaced was a A794 transistor (that sat next to a C1567, its NPN counterpart, taking care of the +15V half of the supply). I say transistor because the PCB had E & B letters marked where its legs were, which I assume meant Emitter and Base. Using the search function on the NTE website, an equivalent part called NTE374 was suggested.

The +5V section actually does have an IC regulator. There's a 7805 next to the previously mentionned parts, all resting against the heatsink.

I'll be back once I give it a look for that possible short we were discussing yesterday.

[52 Bill]

I'll be back once I give it a look for that possible short we were discussing yesterday.

Be sure to check the silicon insulator pad for tears or compression holes. The Collector of the transistor is often connected to a metal back plate of those style transistors, which needs to be isolated from the chassis metal. If the transistor is overtightened, the pad can tear open and there can be contact between the chassis and the transistor case.

[Mackin]

Be sure to check the silicon insulator pad for tears or compression holes.

I did pay attention to that when I replaced the original part and it looked fine.

[Mackin]

zener diodes

I think we have a winner !

I lifted one side on what I assume is a zener diode (picture !) that's connected with the -15V output. Measured it and got 0.3 Ω ! Short !

With the diode out of the circuit, the -15V output is no longer shorted to ground when I do a resistance measurement. That's pretty much it I guess !

Although I still have to get a replacement for that diode. Would it be logical to replace both zeners so they are identical or that's pointless ?

Since I don't have the original parts list, is there a way to get me in the ballpark, specs wise ?

Thanks to all of you for your help, I'm having a lot of fun with this !

[Enzo]

The output of the supply was shorted, the zener that set the voltage was likely connected to the base lead. That diode is shorted, OK, it could be a zener or a plain diode, both types come in glass. So look where it is wired. If it is wired from supply output to ground, is it wired reverse bias or forward? Oh never mind, I guess both types would be wired reverse. A zener for say 18v across the 15v line would offer protection, but a simple diode there would serve to prevent reverse polarity when the supplies discharge. In any case, across the rail to ground, it is a protection device, and that should not get in the way of troubleshooting once removed.

If I misread and that is the base voltage zener, then a 15v or 16v would be the thing.

Note its orientation, remove the part, look at it with a magnifying lens. ANy numbers on it?

[The Dude]

Note its orientation, remove the part, look at it with a magnifying lens. ANy numbers on it?

Yes, that ^^^. There should be some numbers on the device. As far as replacing both, I wouldn't worry about the other one. If the +15 supply is working, I don't see a reason to mess with it. This voltage is not super critical (within reason).

Edit: If there are no numbers, you could measure the voltage drop across the working zener in the +15V supply and replace the shorted one with a zener of that voltage. It ought to be close enough. Just be sure to observe polarity.