Mid 80s Rat help -- burnt resistor

[sixstring531]

My buddy just found this at his parents house. It dates to the mid 80s, has the LM308 chip and looks great -- except the toasted resistor pictured. We plugged it in with a battery (prior to knowing about the resistor) and it passed signal fine but did nothing when switched on.

A few questions:
1. Can I just swap out the resistor or do I need to try the resistor and adjacent cap?
2. Off hand, does anyone know the values of the resistor and adjacent cap?
3. Do you know how to properly disconnect the pot from the body on this model Rat? I've pulled on the plastic knobs and they don't want to come off, and I don't want to break it. \
4. I take it this was probably due to the wrong power supply?
5. Is there any advice/and or anything I should else I should look to change when I attemopt to fix it?

Thanks friends.

[Govmnt_Lacky]

Oddly, the resistor itself does not look burned out (possibly it was swapped out already  )

1) Google "audio probe"
2) Build yourself an "audio probe" and follow this link: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0
3) Do the knobs have small set screws in the sides? If not, then they must pull off some how.

Here is a link to the schematic for that box : http://www.generalguitargadgets.com/pdf/ggg_rodent_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

Judging from the pictures you have provided, it looks to be the 47 ohm resistor coming directly from th epower source. Verify the color codes online to make sure.

Good Luck 

[sixstring531]

Thanks man! You can see the end of the resistor is somewhat pinkish (off colored) that may be part of the problem. Yeah, there aren't any screws in the knobs. I've been playing guit for 17 years and have never come across knobs this tough!

[sixstring531]

47 ohm or 470 ohm (or are they the same thing)?

[Govmnt_Lacky]

47 ohm or 470 ohm (or are they the same thing)?

According to the schematics that I have seen, there are NO 470 ohm resistors in the circuit.

There should be (2) 47 ohm resistors. Here are the codes for a 47 ohm resistor:

3 Color bands = Yellow, Purple, Black
4 Color bands = Yellow, Purple, Black, Silver (or Gold)

It looks, from your pictures, that the band colors might be incorrect. What are they?

[sixstring531]

It goes likes this (or vice versa) and looks just like one in the picture in the middle

Yellow - Pink(violet)-Brown - Gold    --- The brown could be a black, but it looks brown to me. From peering under the board, it doesn't look like it's ever been touched.

Your schematic may be a tad off, as this one has no LED.

[aron]

I bought a Rat, very similar to yours. It was non-working too. After debugging, I realized the chip was fried. So I put a socket in and put a new chip in and it fired right up. Check the voltages on the pins of the IC too.

[sixstring531]

What is the proper way to check the voltages on the pins? I understand using a meter, I just don't know what to set it on and how to properly measure it on the IC.

[Govmnt_Lacky]

What is the proper way to check the voltages on the pins? I understand using a meter, I just don't know what to set it on and how to properly measure it on the IC.

Apply +9V to the box. (Use a battery for now to avoid confusion)
Place the black lead from your meter to Ground.
Place the Red lead to each one of the pins on the LM308 IC (look up the datasheet for the pinout)
Place your meter on DC voltage (what setting depends on the type of meter you have)
Read what voltage the meter says and post them.

Good Luck 

[sixstring531]

Can anyone point me to what type of resistor/capacitor these are? Looking on small bear there are at least 10 types of caps and most don't have pics.

Thanks for all the info so far.

[Govmnt_Lacky]

Can anyone point me to what type of resistor/capacitor these are? Looking on small bear there are at least 10 types of caps and most don't have pics.

Thanks for all the info so far.

The brown/tan colored resistors are "carbon comp" resistors.
The blue/green resistors are "metal film" resistors. (these usually have a smaller resistance tolerance than carbon)

The dark green rectangular caps are "polyester film" capacitors and are NOT polarized.
The light green and brown round caps are ceramic.
The black cylinder caps are "aluminum electrolytic" capacitors and ther ARE polarized which means you have to insert them into the circuit a certain way. (look at the cap label and you will notice that one of the leads is labeled (-).

Good Luck... and remember... Google is your friend too 

[GGBB]

The brown/tan colored resistors are "carbon comp" resistors.

Carbon composition resistors are usually cylindrical and not curvy (google it), and dark brown not tan.  But there are no "rules" for body color like there are for value color codes.  These tan ones are actually carbon film resistors.

The blue/green resistors are "metal film" resistors.

Typically, but not always, and it doesn't follow that metal films are always blue/green.

The resistor in question is carbon film 47 ohm +/- 5% 1/2 watt (maybe 1/4 watt - can't quite visualize the actual physical size). Yellow-Violet-Black-Gold.  The five bar code would be Yellow-Violet-Black-Gold-Red (2%) or Yellow-Violet-Black-Gold-Brown (1%).  You can replace it with carbon or metal film, and 5% tolerance or better, as long as it's 47 ohm and 1/4 or 1/2 watt.

Since the pedal is true bypass, you will always be able to hear the guitar when the pedal is switched to bypass (off) regardless of whether or not the pedal is actually working.  If the resistor is shot you should be able to test it with an ohmmeter across it with nothing plugged in or turned on.  If it doesn't read 47 ohm +/- 5%, it needs to be replaced.  If it is shot, there won't be any power to the unit, so switching it on will defeat the signal.

If the resistor is fine, or you replace it and there is still no sound, you've got more serious problems requiring more sophisticated troubleshooting.  This resistor is responsible for protecting the rest of the circuit from drawing too much power and possibly catching fire from overheating or just generally frying itself.  The trouble with this sort of thing is that we don't know what caused the resistor to fry in the first place, and replacing it may just end up causing the replacement to fry because something else in the circuit is causing a power surge.  But it might just be that only the resistor is fried, so it's worth trying to replace it.