Laney VC30-112 Combo - hum problem

[Neshco]

Hi everybody!
First time here, and no so good english. But Ill do my best.
Have this (almost new) amp and try to fix hum sound coming from both drive and clean mode.
I measure both 7915 and 7815. 7815 have just clean +15, but 7915 have +6V
Semiconductors which could be measured around look fine. As well as resistors.
So, I have a sound (even sound just good), but with background noise more or else noticable depending of volume level and other sound adjustments.
I appreciate any help or pointing direction.

[Mick Bailey]

What is the regulator input voltage?

Are you sure the output is +6v? it's a negative regulator.

[Audiotexan]

Welcome to the forums! =)

For reference: VC30_service_manual.pdf

Check the 4 caps in that area (c27,c25, c29, c30) for shorts/partial shorts as well, could be just a blown regulator, but never hurts to check filter caps. Also, make sure you have 'ground' on the common pins of the 7815/7915's as well.

Regards,
AT

[Neshco]

What is the regulator input voltage?
Are you sure the output is +6v? it's a negative regulator.

Yes I am. Actually I replace that regulator and nothing change.
I was sure that it was bad regulator, but...

Audiotexan
Welcome to the forums! =)

For reference: Attachment 29532

Check the 4 caps in that area (c27,c25, c29, c30) for shorts/partial shorts as well, could be just a blown regulator, but never hurts to check filter caps. Also, make sure you have 'ground' on the common pins of the 7815/7915's as well.

Regards,
AT

Thank You for welcome message and thank You for advice. And for a (very) quick response.
Ill check that capacitors and ground, as well as other parts around as soon as I can and let You know about progress.

Thanks again both for answer me.

[J M Fahey]

What is the regulator input voltage?
Are you sure the output is +6v? it's a negative regulator.

Yes I am. Actually I replace that regulator and nothing change.
I was sure that it was bad regulator, but..

Can you please measure and post input and output voltage at 7915 regulator?
Or even better, voltage on all 3 legs.

[Neshco]

Thank You for joining this thread J M Fahey,
I measure it. Its for 7915 on "idle" cca +22V on input pin, and cca +7V on output pin.
On 7815 on "idle" also, I have +22V on input and stable +15V as it should be.
When i RUN device voltage goes down for cca 1-2V on all pins except on stable +15V on 7815 as it should be.
Between 7815 and 7915 middle pins are -22 on idle.
Now Im gona dissasemble device to check recommended caps, this is what can I do without dissasembling.
Thank again for participating.

[g1]

Do any of the IC's feel hot?

[J M Fahey]

Thank You for joining this thread J M Fahey,
I measure it. Its for 7915 on "idle" cca +22V on input pin, and cca +7V on output pin.
On 7815 on "idle" also, I have +22V on input and stable +15V as it should be.
When i RUN device voltage goes down for cca 1-2V on all pins except on stable +15V on 7815 as it should be.
Between 7815 and 7915 middle pins are -22 on idle.
Now Im gona dissasemble device to check recommended caps, this is what can I do without dissasembling.
Thank again for participating.

Ok, then first and foremost find *why* you have +22V on 7915 input.

Checking for bad capacitors, hot ICs, anything else, is wasting time and useless until you first find why a negative supply has turned positive.

I also wonder how reverse biased electrolytics (the ones filtering the -22V raw supply) can stand that much reverse voltage without exploding.

22-7=15V reverse voltage

I don't believe anything above a couple volts, unless they are bipolar caps (which I very much doubt they are).

And how a reverse biased 7915 can somehow pass current enough to feed a preamp???

[Neshco]

Ok, then first and foremost find *why* you have +22V on 7915 input.

Checking for bad capacitors, hot ICs, anything else, is wasting time and useless until you first find why a negative supply has turned positive.

I also wonder how reverse biased electrolytics (the ones filtering the -22V raw supply) can stand that much reverse voltage without exploding.

22-7=15V reverse voltage

I don't believe anything above a couple volts, unless they are bipolar caps (which I very much doubt they are).

And how a reverse biased 7915 can somehow pass current enough to feed a preamp???

I dont get it.
Everything just look fine, no IC too hot. Exactly as it should be. Capacitors are not bipolar, and I just wonder why they are still in place.
This schematic is little bit diferent, because this pcb Is 2012 model, but I find all relevant diodes, capacitors and give it a test. They are all just fine.
I unpin out of 7915 and have exactly same +7V. Never see something like this. Im sure I measure it well. I Repeat it many times.
Tru to reverse unimer sondas just to see that - sign work.
Any idea? Im stucked.

[g1]

Ok, then first and foremost find *why* you have +22V on 7915 input.

Checking for bad capacitors, hot ICs, anything else, is wasting time and useless until you first find why a negative supply has turned positive.

Oops, missed the part about +voltage on the neg. regulator input, thanks JM !

Neshco, check resistance from ground pin of regulators to main circuit ground.

[J M Fahey]

OK, you have to solve *this* first.
On page 3 of the service schematic, top right just by the dwg#2041 label, you have the small schematic of the low voltage supply.

It receives 17+17VAC nominal, rectifies them with diodes D5/6/7/8 and charges capacitors C29/C30 to +/-18V (I find your +/-22V much trustier, 18 is impossibly low) which later feed the LM7x15 regulators.
As shown, each capacitor is forward biased.

RE re re re re check WHY you get a positive voltage where you should have a negative one.

Not a psychic or mind reader but ONE possible scenario is that "somebody" pulled parts "to measure them outside" , the random aimless "repair" procedure we always warn against, and replaced them the wrong way, both the cap and the diodes (hey!!! I read that electrolytics always connect the negative side to ground !!!!!! ..... and ..... the arrows always point to the + side !!! ) so rererecheck that.

It IS possible that due to aimless messing with that amp you actually have 2 +22V positive low voltage rails.
And the backwards biased 7915 (breaking down and working like a crude Zener) lowers one to +7V , so your Op Amps actually work (horribly) tied to +15 and +7V rails which is doable.

As Enzo often notes, Op Amps do not really need a ground reference, they are happy as long as the pin8 is a few Volts more positive than pin 4 (in a typical DIP8 dual Op Amp).

[Neshco]

Checked its shortcircuit.
Ill take a HQ picture of both sides and post here. Maybe I just miss some obvious thing.

[Neshco]

Mr Fahley, its not possible that someone is messed with this amp, because it was virgin til yesterday.
I personaly brake all seals to access it. Also, I never ever pull-out whole component unless its nessesery for replacement.
So, every piece is on place 100% (unless factory do something wrong with it).
Here are both sides of pcb.


However, Ill recheck everything again.

[J M Fahey]

Ok, I was thinking aloud with an open mind.
As Sherlock says: "'Eliminate all other factors, and the one which remains must be the truth, no matter how weird.'
Fact is you seem to have +22V where you should have -22V and thatīs already so weird that it must be solved or explained first.
Hope you find it

[Neshco]

Dear all,
I need to say sorry to everybody for fulling You around (unintentionally).
I just check pin output and find that they are not same at 7915 and 7815. I just measure wrong without knowing that its a wrong pin out.
Stupid me. Sorry to all a tousand times.
Now I need to go further. Hope You will help me with it.