"click"....hum...."click"....louder hum....repeat

[Alex R]

low voltage on a supply rail is likely to suggest leakage or a soft short through a component rather than a psu problem. Maybe isolate and check the supply, then move downstream isolating as you go till you see the supply drop again.

I recently got one of those laser dot guided infrared thermometers that are cheap on eBay. good for looking for hot components. If there is leakage or a short the part may be getting hot.

[Jazz P Bass]

So check the Vac ripple.

[daz]

low voltage on a supply rail is likely to suggest leakage or a soft short through a component rather than a psu problem. Maybe isolate and check the supply, then move downstream isolating as you go till you see the supply drop again.

I recently got one of those laser dot guided infrared thermometers that are cheap on eBay. good for looking for hot components. If there is leakage or a short the part may be getting hot.

I have a temp probe that came with my meter and tried it on the outputs but nothing else. By the way, it's no longer doing that click, hum, click, hum thing, it's now just a constant hum. Does make me think a component. But man, i have tested EVERYTHING. I was thinking like enzo yesterday and going over every solder connection and hitting a lot of joints because certain things had me thinking that was the likely issue. But no joy. I still think it may be but this thing is a pain in that regard. Maybe i'll start sucking up solder and reflowing new in places because there are some giant mounds on this thing.

[daz]

So check the Vac ripple.

None on either. But i noticed when i turned it on today no hum for about 30 seconds then it abruptly started. Maybe speaks to a component heating up as alex mentioned, so i think i'm temp probe around a bit.

Aha....D109 and 110 in the upper supply are getting hot, about 20-30 degrees more than 107 or 108. I'll have to see if i have any of those on old boards. I have a ton of old parts boards so fingers crossed. Only thing is, thats the HF amp supply, right? This is happening with the LF. But i'll assume one can affect the other.

[daz]

i have some 1N5402's and thats what the schematic looks to say. Can't tell for sure, but it doesn't look like whats in there which are 1N5404. Specs sheets look like i can sub the 02 so i think i'll try it. I could wait for you guys to respond, but i like living dangerously.

EDIT:nope...still hums, tho it started that back and fouth low/hi volume hum again. So maybe something on the side of the PSU those 2 diodes are on, tho i checked most of it. Maybe not the discs tho. What a pita this thing has become !

[g1]

Go back to post #9. See if the supplies are ok with the power IC removed.
Whatever is pulling down that supply rail would also have made those diodes run hotter, so that is a symptom of the fault, not the cause.

[Jazz P Bass]

None on either. But i noticed when i turned it on today no hum for about 30 seconds then it abruptly started. Maybe speaks to a component heating up as alex mentioned, so i think i'm temp probe around a bit.

Aha....D109 and 110 in the upper supply are getting hot, about 20-30 degrees more than 107 or 108. I'll have to see if i have any of those on old boards. I have a ton of old parts boards so fingers crossed. Only thing is, thats the HF amp supply, right? This is happening with the LF. But i'll assume one can affect the other.

Slow down.
You are bouncing around like a ping pong ball in a box.

The 15 volt supply is NOT for the HF.
It is the preamp supply for the opamps.

Both TDA IC's run off of the lower supply.

If you can get it to not hum & then hum, that is good.
Now you have a chance to find out exactly what is changing.

For now, do not look for the cause.
Look for a change that will put you in the correct circuit.
Once you do that, then you can approach the root cause.

For the amp to hum, you either have an injected hum into the TDA inputs, the TDA's themselves are bad (or an associated component) or the power supply is breaking down (or is being made to).

Take five, grab a cup of Joe, then sit down and systematically monitor the power rails, the TDA inputs, anything you can.
But find what is changing.

[daz]

so remove the tda and see if the supply rises? But note that the diodes that were getting hot are NOT on the PSU you said should be 35v, the side where the issue is. Those diodes were on the HF side where voltages are correct you said, and no overheated diodes there. The only reason i checked those diodes was because i was just randomly checking all silicon parts. Anyways, i'll lift the TDA on the LF side anyways and report back.

[daz]

Yep....it's up, 42VDC now with the TDA out of circuit. But since i tried several TDA's in it what does this mean? I checked almost everything in the TDA's circuit except what i wasn't sure of due to the crappy schematic.

EDIT: ok, i missed your last post due to your posting timeline. So now that you say both LF and HF sides use the same PSU i know now i can check the components in the HF output section. I didn't bother before because i didn't realize that. granted, i'm an idiot when it comes to electronics but had that damn schematic been readable i would have known that but i can't even see where they get power from due to the illegible supply voltages.

[g1]

Which TDA did you lift that made the supply voltages come up?

[daz]

Which TDA did you lift that made the supply voltages come up?

What i assumed is the LF side, the one on the right.

[g1]

Then you don't have to worry about anything in the HF side.
But don't make any assumptions, verify that it is the LF chip that you removed.
Then you can start looking for whatever is causing that IC to conduct too much current which is pulling down the supply voltage.

[daz]

I'm sure it's the LF chip, that much i can say for sure. Partly from figuring out what parts are in that circuit from what i CAN read and also that on the board one is called UL100 while the other one is UH100. L for low H for high i'm sure. Plus its the woofer that when connected makes the noise and with that chip removed it stops.

[Jazz P Bass]

What is the exact voltage marking on the two 1000uf filter capacitors?

It looks like 35Volts on the schematic.

That 42 volt reading sounds odd.

There should be two different polarities.

KRK Power Supply.pdf

[g1]

Now you can leave the chip out and measure the voltages at the pads where the chip would go.
Post results and they can be compared to what the data sheet says.
I'm assuming the LF speaker or it's wires are not shorted.

Edit: Jazz makes a good point. Is there a plus and minus 42V?