Trying to ReJiveinate my old MXR Microamp

[Mattysbrains]

It used to work fine and after sitting on the shelf for a year it only works in bypass. LED lights up but there's no sound when engaged.
There's a possibility I may have hurt it by using a reverse polarity adapter or something at some point.
With a battery for power the voltages on the pins of the TL071 are:
0.1 -1
5.3 -2
0.3 -3
0.0 -4
0.1 -5
8.4 -6
9.1 -7
0.0 -8

Do those numbers look okay? Do I have an issue elsewhere in the circuit perhaps?
Thanks in advance!
Chris

[ElectricDruid]

Schematic:

http://www.generalguitargadgets.com/pdf/ggg_mamp_sc.pdf

Pin 3 is too low. Yours is reading close to ground and it should be at half-supply.

Dead input cap C1 maybe, or a bad electrolytic C4 on the Vref divider? If your circuit includes the series diode as shown on this schematic, it should be fairly resistant to damage form a reversed supply - diode doesn't conduct, no current flows, nothing happens.


Edit: Oh! My apologies! Welcome!! Didn't see it was your first post.

[bluebunny]

Welcome to the forum, Chris.

Sadly, those numbers don't look great.  You've got most of the battery's voltage at the output (pin 6).  Pin 3 should be around 4.5V.

So you may indeed have zorched things.  Having said that, most schematics I can find have a diode to protect against reverse power.  I guess if that diode failed short (rather than open) then it's still a distinct possibility.

You could try snipping out the opamp and replace it with a socket (I'm assuming it's not socketed).  Then read those pins again.  There's also a 1uF electrolytic cap which may have failed with reversed power, which may in turn be affecting the pin 3 bias.  Replace that too.

Sorry, I'm shuttling between this reply and the schematic and starting to ramble.  I'll stop for now.   

Edit: while I was rambling, Tom got in there first with some wise words.

[antonis]

Pin 7 gets 9.1V so power supply configuration works..
Pin 6 measures 8.4 V indicating a possitive rail saturated amp..

Check R7/R8 values & R7/R8/R3 connectivity..
Start measuring DC from R7/R8 junction..

@Tom: Dead C1 couldn't dramatically affect Vref both for "open" (no connection to Vref) and for "short" (Vref about 3.1V due to 10M/22M voltage divider..)

edit: Welcome, also..

[ElectricDruid]

@Tom: Dead C1 couldn't dramatically affect Vref both for "open" (no connection to Vref) and for "short" (Vref about 3.1V due to 10M/22M voltage divider..)

No, fair enough. That pull-down at the input is *MASSIVE*. I was thinking 'shorted' but wasn't really accounting for that.

Still, that's a good thing! That means the problem pretty much has to be associated with the Vref.

[PRR]

Replace C3.

Or at least take one leg out and see if pin 6 comes right.

Poking pin 3 is fairly futile because 10Meg. A "proper" VTVM will read half, some popular DMMs will read 1/10th, and some will read less.

If C3 is perfect, then the amp is unity-gain and pin 6 will follow pin 3 with much less meter loading.

[antonis]

BTW, I can't figure out the use of 500k pot with 56k feedback resistor..
(most of wiper's travel is limited between 1.1 & 2 Gain..)

[Mattysbrains]

Thank you everyone for the welcomes and the assistance, I deeply appreciate it 

I'm just cracking into this thing. This is a 1994 version so it's very similar to the original design updated with an LED and mini phono power jack.
To start I reflowed most of the joints, skipping the opamp. I lifted a leg on C3 and no significant change on the voltages. It showed 4.6uf on the meter so I stuck it back in. I'm going to check R7/R8 now and then start poking around with an improvised audio probe to see if I can find where the music stops. Wish me luck!

[antonis]

Wish me luck!

Luck is accomplished by recaping 27 years old tantalum caps..

[Mattysbrains]

Update:
Checked values of the resistors and they were all okay. Voltage drop across R8 and R9 was 3.9 and 4.0.
I used my Boss looper to get a signal going and probed around with a 0.1uf capacitor stuck on an instrument lead to an amp. I could hear the loop from the input through C1 and R3 up to pin 3. Then it was no mas. From pin 6 out it was nothing but AM radio. 
Well I replaced all the capacitatoes and there was no change at the opamp but it got rid of the AM radio. I ended up chopping out the opamp and installing a socket and a 741 to test - Success!
Thank you all for the help!

[ElectricDruid]

Nice work! That looks ready for another 25 years now! 

I'm surprised it was the op-amp that was dead, but I guess it happens. Bluebunny was right! Getting rid of those old tantalum caps was probably a good idea from a long term reliability point of view, so I think Antonis was right too!

Enjoy the fruits of your labour!

[amptramp]

It looks like you have a DPDT switch where the schematic posted by ElectricDruid calls for a 3PDT switch.  Does the LED work and if so, what is it connected to?  It looks like it may be switched by one of the jacks.

[duck_arse]

I'm 47, I'm not old.

you seem to have replaced a 470pF ceramic with a 47pF mono ceramic, and a TL071 with a uA741.

which is nice.

[antonis]

I'm 47,

Weight  or aged..??

[GibsonGM]

47? Just a child still.

[Mattysbrains]

It looks like you have a DPDT switch where the schematic posted by ElectricDruid calls for a 3PDT switch.  Does the LED work and if so, what is it connected to?  It looks like it may be switched by one of the jacks.

That's correct. Take a look at the photo in my second post and you might be able to see the connections for the switch. The top pole handles the LED. The current limiting resistor is connected from the power jack straight to the switch and the LED connects directly to the center lug of the switch. The other leg of the LED goes to the output jack lug.
I gotta update that power jack so I don't fry this thing again!

[ElectricDruid]

I gotta update that power jack so I don't fry this thing again!

Add the protection diode from the schematic. I can't see one on the PCB. Then the board is safe if the power supply is reversed.

[Mattysbrains]

I think the protection diode came in when Dunlop updated the board layout. I'm tempted to install a proper TB switch along with that diode, but I'm debating if it's worth losing the battery space to make the change.