Vintage Small Clone repair

[theundeadelvis]

Hey Guys! It's been quite a while since I've posted (haven't been behind the soldering iron much). A friend of mine dropped off an old EH Small Clone to see if I could fix it. Pretty much all it does is make a steady squeal when it's on. I checked the voltages of all the IC pins, and none of them read into volts, all are in the millivolts, so I assume for some reason it's not getting enough juice from the battery (checked the battery and it's at 8.8 volts). My inclination is to check the 47 ohm resistor that's connected to pin 8 of the lm358 (that pin is at 360 mV). Any suggestion as to where else I should look?

[KrazyKarl]

Not really sure, but it could be that the IC is burnt.

[R.G.]

Hey Guys! It's been quite a while since I've posted (haven't been behind the soldering iron much). A friend of mine dropped off an old EH Small Clone to see if I could fix it. Pretty much all it does is make a steady squeal when it's on. I checked the voltages of all the IC pins, and none of them read into volts, all are in the millivolts, so I assume for some reason it's not getting enough juice from the battery (checked the battery and it's at 8.8 volts). My inclination is to check the 47 ohm resistor that's connected to pin 8 of the lm358 (that pin is at 360 mV). Any suggestion as to where else I should look?

Ohm's Law.

If the voltage across a 47 ohm resistor is 8.8-.36= 8.44V, then the current through it is 8.44/47 = 179ma. If it's a 47 ohm resistor.

So it may be getting way more juice than it needs and the battery is just trying to keep up. Both the "wrong  resistor" and "too much juice" hypotheses need checked, and the value of the "47 ohm" resistor is where to start. Note that the "47 ohm resistor" means the resistor plus the solder joints and copper traces on both ends of it.

[theundeadelvis]

Well the schematic I looked at showed a 47 ohm resistor, but I can't for life of me find one on this board? So, I'm going to start measuring each resistor to see if the come close to what they are mark as w/their color bands. This thing does nothing when byoassed (no guitar gets through), and sounds like a swarm of bees when on.

[minatour87]

Vintage Small Clone should be through hole resistors. Check the color bands for 48 ohms.
this should help: http://www.elexp.com/tips/clr_code.gif
when i trouble shoot, i always visual inspection for cracked solder joints, burned components. 
Re-flow solder everything if in general they look suspect.
Those boards usually have excessive solder flux on them which drives me crazy. clean it off with isopropyl alcohol and q-tips.
i wear items is the foot switch. how is the power switched on?
peace
minatour87

[theundeadelvis]

Thanks minatour87, I hope to spend a good part of tomorrow digging into it.

[theundeadelvis]

OK, I've been looking at this thing way too long now. It's quickly becoming my nemesis. So, I appear to fairly normal voltages now (for what I can tell), but it doesn't seems to be oscillating. Here's my voltage chart.

2N5087
C=.1mV
B=3.4V
E=4V

2N5088 (Q2 I think)
C=8.74V
B=4.2V
E=3.68V

2N5088 (Q3 I think)
C=8.74V
B=8.69V
E=8.1V

MN3007
1=8.08V
2=3.9V
3=4.52
4=.1mV
5=.1mV
6=3.98V
7=4.27V
8=4.27V

LM358
1=3.97V (no oscillation)
2=3.63V
3=3.8V
4=.1mV
5=3.88V (no oscillation)
6=2.09V
7=3.66V (no oscillation)
8=8.75V

RC4558
1=4.2V
2=4.2V
3=4.2V
4=0
5=4.37V
6=4.23V
7=4.2V
8=8.74

CD40478CN
1=7.8V
2=189mV
3=6.97V
4=8.08V
5=8.08V
6=8.08V
7=0
8=0
9=0
10=3.98V
11=3.99V
12=0
13=7.8V
14=8.08V

I've bypassed the switch to be sure it's out of the equation. With the depth switch in one position it squeals (sounds like a swarm of bees), switched in the other direction it makes no sound. When I did have the switch hooked up nothing came though when bypassed (no guitar, nothing). Any ideas? What should I look at since I'm seeing no oscillation? Thanks again guys!

[theundeadelvis]

Could this be caused by an issue with the 2.2uF tantalum cap? I may go ahead and replace it for good measure.

[theundeadelvis]

Replaced the tantalum cap, still not working.  I may try replacing the LM358 (if I have one or a suitable substitute).

[theundeadelvis]

Pulled the LM358, and replaced it with a socket. Tried a couple IC's with no luck. I guess I don't understand how the oscillation is achieved, so I'm sure where to  look now?

[theundeadelvis]

Replaced the CD4047 with no luck. Aaaaarrrrggghhhhh!

[Gus]

First thing you do with older EH and some other effects is check/redo every solder connection.  Also check the wires to find out if they are broken.
Then check the switch.

Then check if someone else tried to repair it and made it worse changing stuff or soldering wires to the wrong place.  I have found reversed LEDs, wires in the wrong place etc in effects I repaired when I ask if someone worked on it before the answer is yes.

Then start to troubleshoot.
Do you have an oscilloscope?

[theundeadelvis]

I feel like I might be getting closer. I broke out the audio probe and everything sounds good up to tpin 3 of the MN3007 (I've tried multiple MN3007 IC's with the same result. Going into pin 3 the audio is nice and clear.  When I touch the audio probe to the following pins of the MN3007 i get the following:

1-Buzz, like bees
2-High pitched squeal
3-Clear audio
4-Nothing
5-Nothing
6-High pitched squeal
7-Faint audio with loud buzzing (like bees again)
8-Same as 7

Now I'm sure what I should be getting out of the other pins, so I'm not sure where to look form here. Thanks again everybody!

[theundeadelvis]

One weird thing of note, there's what appears to be a transistor, that's not on the Tonepad schematic? It's black and domed on top, with white around the sides, has 3 leads, and "SCOTT" is printed on it. What is this little guy?

[theundeadelvis]

Me again. This is really starting to drain me. I've changed all the IC's, replaced all of the the electrolytic caps, repalced Q3, D3, and I still can't get it working. I don't have a scope, but I have checked and double checked voltages, and I have used the audio probe (results above). I know I should have some voltages that oscillate, but I do not, and I don't understand how that works enough to figure out what components I need to look at? If anyone can offer some insight on where I should look next, I would be extremely grateful.

I apologize for so many posts on my own topic. I use it as a map for myself, and hope that it may help someone else later on down the road. Thanks!

[theundeadelvis]

I think my next step is to replace everything around the rate pot. My assumption is that if it controls the rate of the chorus, it must affect the speed of the oscillation, so maybe something there isn't doing its job.

[theundeadelvis]

Could the trimpot cause any of the issues I'm experiencing? I can't get any measurement from it on the board, but was curious if it could even be the problem before I desolder it.

[theundeadelvis]

OK, I was able to get a reading on the trimpot while it's in the board. One lead measures 27K and the other 36K. This doesn't match what the schematic calls for (10k trimpot), but my voltages seems pretty close to correct, so I don't think it's the trimmer. I keep coming back to the buzzing I'm getting coming out of pins 7 and 8 of the MN3007. The input pin 3 has clear audio, but coming out it has a bad buzz. I just cant figure out why?

[theundeadelvis]

Looks like this one has beaten me.

[theundeadelvis]

Hey guys, So I gave up, couldn't get it out of my head, and now I'm back to it and close to fixing.

I've gotten rid of the buzz, but the chorus isn't there yet. It appears that my voltages still are not oscillating like they should be. The weird thing is, if I put my finger on (or near) the junction of the D3, the 150pF cap, and the 39k resistor I get a slight chorus sound. My guess is the capacitance of my finger is causing this (or ground reference maybe?), but I'm not sure what this indicates as far as what the problem might be? If anyone can offer me any suggestions, I will be greatly appreciative. Thanks in advance!