Acoustic B600H is gainless!

[Sowhat]

Now that my shop is several miles away I don't have the convenience of getting an idea and going into the shop to prove it out right away. It's in the evening that I mull the day over in my mind and theorize. I have a little ponder I would like to share and see if the brain trust here at MEF has any good ideas on this.

I got an Acoustic B600H in that comes on but don't make any sound. I verified the supply voltages, power distribution, checked the output transistors which all checked out good steady clean big and small rails. I then did a preliminary signal trace from the input back. I made it through the first opamp IC1A just fine and through the soft switchable opamp pair IC2 responsible for the mute and through the switchable opamp pair IC8 responsible for the frequency notch. The output pin of IC8 (pin 5) gives me a nice good sized sine wave. Pin 2 of IC3A however show barely anything of the sine wave as well as it's output. I checked the parts between the output of IC8 and the input of IC3 which are C28, C5, R22 and R4… all check out good.

Per the attached schematic clipping, NetC36 is good and then there's the net with no name… let's call it NetIC8_5, that one's good too but NetIC3_2 is tumbleweed, as is net POST_GAIN!

Now, here's where the pondering comes in. Could VR1 the Gain pot be open which is in IC3's feedback loop? Could this cause IC3 to loose all gain? Could IC3 be I/O shorted somehow dragging down pin 2 of IC3? It would be great to get some opinions before getting back to the shop tomorrow and continuing on with this ticket.

[glebert]

I think if VR1 were open (and IC3 operating properly) you would be getting almost rail to rail square wave output from IC3, basically it would be in comparator mode.

[nickb]

IC3 pin2 is a virtual earth and therefore you would not expect to see anything there in any case. I'd replace IC3.

[J M Fahey]

Agree and add:
you *do* have signal at IC3-2 ... but it´s a current signal, no voltage (because, as nickb said, it´s a virtual ground) so a standard scope probe must show nothing there.

In fact, unless IC3 is a blob of molten silicon inside, everything shorted to everything, having zero voltage signal at pin 2 hints (may be wrong of course) at it working fine.

BUT: signal of any kind might not be even reaching that point.

From IC8-5 to IC3-2 there´s 2 paths: C28+R22 and C5+R4 .... plus connecting tracks and solder pads, any of which may be cracked/open .

No voltage on anything connected to Pin 2 as you already found, but check for audio voltage en both ends of C28 and C5 and on left ends of R22 and R4 .

Now that you are at it, check also for audio voltage at the right leg of R22 and R4 , if present it means that connection between them and Pin 2 is broken.

That´s why in principle I´m against shotgunning .... roughly half of problems are not cause by bad parts but by broken connections one kind or another and all the shotgunning in the World will not catch that.

You are doing fine, of course, by injecting signal and following it

Please also measure and post DC voltage on all pins of IC3a, we might find something abnormal there.

[Sowhat]

That´s why in principle I´m against shotgunning .... roughly half of problems are not cause by bad parts but by broken connections one kind or another and all the shotgunning in the World will not catch that.

Yeah, me too, it just makes a big mess. As for not seeing signal at IC3_2, I do see mini-microvolts of it but not enough to write home about. For signal tracing I use the scope in AC coupled mode which ignores DC components of a signal, or so I thought, so in that case... I should read signal at IC3_2 regardless if it's current or voltage, right? or is this just another of my many misconceptions? Anyway, all good advise and I will get right to it when I get to the shop, I'll keep you posted.

[J M Fahey]

I should read signal at IC3_2 regardless if it's current or voltage, right? or is this just another of my many misconceptions?

No, Scopes read/show Voltage only, not Current.
Hint: they have a selector switch labelled in V/mV , not A/mA .

As for not seeing signal at IC3_2, I do see mini-microvolts of it but not enough to write home about.

That´s the point.
You see a tiny tiny voltage signal there only because ground is never perfect.
Virtual ground too.

[Sowhat]

That´s the point.

Ahhh... yes! I should be measuring across R22 or R4 to see signal, Doh!

[Sowhat]

I thought I was onto something interesting with this but... How about dirty shorting contacts in the effects send receive connectors causing the problem? Yeah! that was it! Kind of a let down.

[glebert]

Switchjack strikes again!

[g1]

Then there must have been signal at the output of IC3A ?

[Sowhat]

Then there must have been signal at the output of IC3A ?

Oh yeah! Once I got my mind right and started looking at resistors like current senses for the virtual ground part of the circuit I got wave all the way through, that is until the effects in/out. I'm embarrassed, that should have been one of the initial checks, and it is, but that don't mean I always follow my own procedures.

[g1]

Ok. I asked because in post #1 you said there was no signal at input or output of IC3A, and I thought you might still have an intermittent fault that just coincidentally cleared along with cleaning the switching jacks.
I'm guessing I may have misunderstood that reference in the first post.

[J M Fahey]

Ok. I asked because in post #1 you said there was no signal at input or output of IC3A, and I thought you might still have an intermittent fault that just coincidentally cleared along with cleaning the switching jacks.
I'm guessing I may have misunderstood that reference in the first post.

I read the same:

Pin 2 of IC3A however show barely anything of the sine wave as well as it's output.

Hard to properly answer improperly written questions.