AVT-150 repair thread

[nak]

Hi,
Here's the story:
Drummer plugs amplified signal into FX send loop on the AVT-150, components inside burn up, now I am tasked with repairing the poor fellow.

What I've done so far:
Found the schematics (thanks to this forum!)
Replaced REG100 and REG 101(blown), as well as IC100 (op-amp hooked to the send/return, looked physically fine), R223 and R202 were blown, and replaced.

NOW, I've been following the signal from the input with an o-scope, and I'm not sure how I could check to see if an op-amp is functioning correctly (like IC1) should I be reading signal between pins 1 and 2?

If anyone has any ideas on what I should check, I have a multimeter, function generator and o-scope... I'm no expert, just an experimenter!

Thank you so much

[techineer]

Op-amps can be used for many different functions and may have different voltages present depending on the purpose for which they are being used, but....

In most cases in a guitar amplifier, op-amps are used as linear amplifiers. For this purpose, they are usually powered by a dual-polarity power supply. You can get a pretty good indication of their state by voltage measurement.

The AVT150 uses quite a few NJM072 op-amps, which contain two op-amps in each package.

Voltage testing: With the meter common connected to ground, you should normally measure +15V on pin 8 and -15V on pin 4. The output pins 1 and 7 will usually be zero volts DC, as will the input pins 2,3,5 & 6.

If you measure any DC voltage on the output, check the power pins first, then check the inverting and non-inverting inputs. Linear op-amp circuits usually operate by adjusting the output voltage until the inverting input (pins 2 or 6) equal to the non-inverting input (pins 3 or 5). On the diagram, look for a DC resistive path from the output to the inverting (-) input. If this connection is present, and the output voltage is less than about 12 volts, the voltages on the input pins should be equal.
If the voltages on the input pins are not equal, (any measurable value more than a millivolt or two) the voltage on the output will be +/- 12 to 14V.

If the output voltage is near the supply value:
Using a DVM, connect the probes between the inputs, with the black probe to the (-) pin and the red probe to the (+) pin. If the meter indicates a positive value, a good op amp will drive the output positive, and a negative value would drive the output negative. If the output voltage polarity doesn't match this, the op-amp is definitely bad. (It could still be bad the other way, too, but you need to find out why the input voltages are off before you decide the op-amp is bad.)

If you are following signals (with an oscilloscope), you should usually see a signal at the output if one is applied at the stage input. Depending on how the op-amp is connected, you may or may not see signal at the input pins. Again, the op-amp output works to keep the inputs equal... if the NI (+) pin is grounded, then you will not see a voltage signal at the Inv (-) pin.

[nak]

Thanks for the response, I checked some op-amps but the +15 supply was putting out about 21vdc. I finally figured out (after hours of trouble shooting) that when I disconnect CNB3 and CNB7 from the front PCB, I could read the proper +15vdc.

Any idea's as to why this is happening? I'll start checking components on the front PCB connected to those connectors I guess.

EDIT 1: I found that the regulator reads 18.9vdc with the headphones in and the CNBs disconnected. It's 15vdc without headphones in... gah! I'm not sure what to do :P

EDIT 2: When both CNBs are disconnected, I can hear a tone through the headphones, the signal doesn't seem to be effected by the knobs on the front, but once either CNB3 or 7 is plugged in I hear nothing...

[techineer]

Re-check R202 and R223. These are actually 1 watt resistors, but they look like much smaller ones. They are intended to act as fuses. If you put 1/4 watt resistors in, one may be bad.

You said the +15v was running +21v, but didn't describe the -15v line. Was it at -21V? Where did you measure this? This sounds like you were measuring the unregulated input to the regulator ICs. A good place to measure the op amp supply lines is at pins 4 and 8 of the ICs, with the meter common clipped to the chassis or the tab of REG101 (only).

Are the regulator ICs too hot to touch? How about any of the op-amp chips?
If an op-amp IC is hot, replace it. A hot regulator usually means that something is pulling too much current from it.

From your description of the cause of the damage, it is probable that many ICs got fried. If a high level signal (any more than about 10-12V RMS, such as the output from a power amp) was put into the SEND jack, I would expect it to overwhelm the +/-15v supply lines by way of diodes D104 and D105. This would behave like an AC power supply feeding the diodes. From there it would raise those supply lines to a much higher value, which would probably destroy every chip connected to them. The fact that the regulators and fuse resistors were blown supports this theory.

You could try 'shotgunning' it.... get a batch of the op-amps (NJM072BL) and analog switches (DG212CJ) and replace them all. Any that survived the blast probably won't last much longer anyway. Don't try to unsolder the chips intact... just cut the pins and remove them individually. Even this probably won't completely restore it. Expect to have to track down more problems such as blown switching diodes and low-value resistors afterward. Just hope it didn't blow the DSP board. Don't be surprised if you also need to replace the output ICs.

Sounds like the drummer was just living up to the stereotype..... they can make blondes look smart.

Good luck! I'm afraid you'll need it.

[nak]

Thanks again for the quick reply, I already have the parts to replace all the ICs, I'll try it tomorrow, and thanks for telling me to clip the leads, it'll make the job a little easier *wick wick wick*

I'll keep the thread updated

[nak]

I re-checked the fuse resistors and they are fine.

I replaced all the OP-amps/analog switches/output amp ICs, the v-regs seem to be staying relatively cool as well.

I found that when unplugging CNB10, I was able to get a signal out of the headphone jack in the back (I don't have a cab), the OD section of the panel seems to effect the signal, but the volume knob doesn't seem to do anything still. I'll take a look at the schematic sometime, must be getting pretty close to having it all fixed (hopefully >.<)