Newbie requesting help with a dead ProCo Vintage Rat

[scrumper]

Hi,

I've acquired a free-but-dead ~1990 ProCo Vintage Rat, the re-issue of the large format original. The prior owner said it worked last time he used it a decade ago, but it turned out to be dead when I tried it out. Here's what I've figured out so far:

- it passes signal just fine when on bypass (switch off) whether there's a battery present or not
- it doesn't pass signal at all when switch is on, again whether there's a battery or not
- if you short out the battery connector (no battery, just a conductor across + to - terminal) then it does pass signal (albeit poorly) and the volume pot does attenuate the sound (obviously it doesn't distort).

That suggests that there is a complete circuit, right?

I can't see anything obviously blown or leaking.

Where should I start looking? Bit worried that the op amp chip might be fried, is that something I can test?

I have fair soldering skills and a multimeter and a desire to see this work, hoping someone can give me some pointers.

Thanks.

[TomCarlos]

Is this the pedal that you have?

https://www.electrosmash.com/proco-rat

[scrumper]

Not quite, it's this one:

https://www.ebay.com/itm/Proco-Vinta...8AAOSwzahdDScM

Edit: note that it looks like the original, but is actually a 1990s reissue. Identified with the word "vintage" on the serial number sticker.

[nosaj]

Not quite, it's this one:

https://www.ebay.com/itm/Proco-Vinta...8AAOSwzahdDScM

Edit: note that it looks like the original, but is actually a 1990s reissue. Identified with the word "vintage" on the serial number sticker.

TomCarlos link should work. Proco Vintage rat uses original circuit.

Check the voltage for the power supply section in the link, then check for voltage on lm308 pin7.

Then check this link for audio probe http://diy-fever.com/misc/audio-probe/
Maybe hook an mp3 player up to proco input and then use the audio probe to see where signal stops.

nosaj

[TomCarlos]

Scrumper:

Take a look and see if what you have matches (or comes dawg on close) to the schematic from the Original Rat. Check the Power Supply Voltages and voltages on the IC. Give us the pins and voltage that you measure.

Tom

[nosaj]

Scrumper:

Take a look and see if what you have matches (or comes dawg on close) to the schematic from the Original Rat. Check the Power Supply Voltages and voltages on the IC. Give us the pins and voltage that you measure.

Tom

Tom i used this for my determination of the circuit.https://www.diystompboxes.com/pedals...t_versions.htm
nosaj

[TomCarlos]

Nosaj... good find on that document. If nothing else, Scrumper can check to see if the Power Supply looks similar to one of the circuits and the IC with surrounding components.

[J M Fahey]

"First fix DC, then Audio"

Meaning: first measure and post voltage on all IC pins.
Then if normal start checking signal along the path from input to output to see where it gets lost.

[scrumper]

Thanks all for excellent advice. Will be tackling this at the weekend - honestly wasn't expecting such a quick response.

I'll report back then with voltages and other findings.

[scrumper]

Ok I had a look. It's not an LM308 Rat, this has an DPOP07 op amp, making it a bit newer. The circuit is different to those in the links above. It says "MULTI RAT" on it. Some capacitors are not included.

Voltage across the chip (pin 4-7 according to this: https://www.analog.com/media/en/tech...heets/OP07.pdf) is 0.48V using a 9V battery that reads 8.5V.
Voltage between battery negative and pin 7 is 8.3V - don't understand why that's the case.

I managed to get hold of an old oscilloscope, fed the pedal a sawtooth wave from a synth, and found that when the footswitch is not in bypass the chip is outputting on pin 6 a very low (millivolt) signal which is affected by the Filter and Distortion knobs. This signal appears to be a very noisy copy of the input signal on pin 3. There's also a copy of the input signal on pin 4 which is supposed to be ground. This goes away when the footswitch is in bypass.

This suggests to me the chip is damaged: am I right that the output shouldn't be a an orders-of-magnitude lower copy of the input, especially where the chip is intended to be overloaded to color the signal like here?
(The other thing maybe is there's maybe something wrong with the ground, a short somewhere or a broken switch? Why am I seeing the input signal on the chip's ground pin otherwise? I guess could be another symptom of the chip failure).

So it's a bit weird. I'm trying to trace out the circuit and figure what's what but it's a bit difficult with the circuit on one side and the components on the other - and the component side has space and labels for many things which aren't there. When I was 16 I studied and enjoyed electronics at school but that was 30 odd years ago and I've not thought about it since, so please bear with me while I scrape off decades of rust!

[nosaj]

Hi,

I've acquired a free-but-dead ~1990 ProCo Vintage Rat, the re-issue of the large format original. The prior owner said it worked last time he used it a decade ago, but it turned out to be dead when I tried it out. Here's what I've figured out so far:

- it passes signal just fine when on bypass (switch off) whether there's a battery present or not
- it doesn't pass signal at all when switch is on, again whether there's a battery or not
- if you short out the battery connector (no battery, just a conductor across + to - terminal) then it does pass signal (albeit poorly) and the volume pot does attenuate the sound (obviously it doesn't distort).

That suggests that there is a complete circuit, right?

I can't see anything obviously blown or leaking.

Where should I start looking? Bit worried that the op amp chip might be fried, is that something I can test?

I have fair soldering skills and a multimeter and a desire to see this work, hoping someone can give me some pointers.

Thanks.

Ok I had a look. It's not an LM308 Rat, this has an DPOP07 op amp, making it a bit newer. The circuit is different to those in the links above. It says "MULTI RAT" on it. Some capacitors are not included.

Voltage across the chip (pin 4-7 according to this: https://www.analog.com/media/en/tech...heets/OP07.pdf) is 0.48V using a 9V battery that reads 8.5V.
Voltage between battery negative and pin 7 is 8.3V - don't understand why that's the case.

I managed to get hold of an old oscilloscope, fed the pedal a sawtooth wave from a synth, and found that when the footswitch is not in bypass the chip is outputting on pin 6 a very low (millivolt) signal which is affected by the Filter and Distortion knobs. This signal appears to be a very noisy copy of the input signal on pin 3. There's also a copy of the input signal on pin 4 which is supposed to be ground. This goes away when the footswitch is in bypass.

This suggests to me the chip is damaged: am I right that the output shouldn't be a an orders-of-magnitude lower copy of the input, especially where the chip is intended to be overloaded to color the signal like here?
(The other thing maybe is there's maybe something wrong with the ground, a short somewhere or a broken switch? Why am I seeing the input signal on the chip's ground pin otherwise? I guess could be another symptom of the chip failure).

So it's a bit weird. I'm trying to trace out the circuit and figure what's what but it's a bit difficult with the circuit on one side and the components on the other - and the component side has space and labels for many things which aren't there. When I was 16 I studied and enjoyed electronics at school but that was 30 odd years ago and I've not thought about it since, so please bear with me while I scrape off decades of rust!

Looks like your on you own, would've been saved some time instead of telling everyone it was a 90s Vintage Rat, when in fact it's printed with Multi Rat.

Good luck this is all that's out there.
https://www.youtube.com/watch?v=ETM8Rx2xKMM
nosaj

[TomCarlos]

From the datasheet

Pin 1 VOS trim
Pin 2 (-) input
Pin 3 (+) input
Pin 4 V-
Pin 5 No connection
Pin 6 Output
Pin 7 V+
Pin 8 VOS trim

You mentioned reading 8.3 volts from pin 7 to the battery negative terminal. But if you move the black lead to pin 4 you get nothing? Follow the trace from Pin 4 back to the negative terminal. Make sure you do not have bad solder connections anywhere along that trace. Maybe you can solder in a jumper wire from the negative terminal to a point near the IC? See if that does anything?

Let me add, check that switch once again. Maybe it needs to be cleaned or replaced.

[scrumper]

You mentioned reading 8.3 volts from pin 7 to the battery negative terminal. But if you move the black lead to pin 4 you get nothing?

Not nothing - I get 0.5V across pins 4->7. It should be ~9V, if I'm reading the schematic correctly. I will check that path back to the terminal as you suggested.

Could be a fault in the switch screwing up the ground - I'll check that too.

(And nosaj - apologies for wasted time. I didn't see it was a "Multi Rat" until I had the thing in pieces and flipped the board over this weekend. Everything else including the serial number makes this out to be a ProCo Vintage Rat. My guess is this someone's kit-built thing, like your video, stuffed in a Vintage Rat case. It doesn't have that selector rotary like in the video.)

[Dave H]

Voltage across the chip (pin 4-7 according to this: https://www.analog.com/media/en/tech...heets/OP07.pdf) is 0.48V using a 9V battery that reads 8.5V.
Voltage between battery negative and pin 7 is 8.3V - don't understand why that's the case.

It's most likely because pin 4 is open circuit from ground/battery negative. With the battery disconnected check for continuity between battery clip -ve and pin 4. If it isn't a very low resistance find out where the open circuit is and correct it then do as Juan said and check the DC voltages on the op amp pins wrt battery -ve. They should read 4.5V on pins 2,3 and 6. 9V on pin 7 and 0V on pin 4.

[The Dude]

Normal pedal architecture has the ground path completed when a plug is inserted into the input jack. You do have a plug there for testing?