Deluxe Electric Mistress trimpots: help!!!!

[Fenderstratocaster0]

Well at this point a recapping is logical since 40 years is along time for these ecaps to go.

Are there any recommendations for replacing the 2 ecaps with specific ones.

Looks like the originals to me. ICC 220uF 25v and 250mfd 16v.

[Scruffie]

In those photos I can't see your black lead, you are grounding it to the enclosure when you're taking the readings aren't you? The one with 2.99 should be the output pin and showing 15V. If you are, you should check the AC readings from the transformer.

As for the caps, any old electrolytic with the same value (220uF should be fine for both of them, I wouldn't bother trying to find a 250uF as it's an obscure value) and equal or higher voltage rating will do.

[Fenderstratocaster0]

Thanks Scruffie.

Sorry for all the questions. Very new to this stuff.
Yes my measurements were taken by using the black voltmeter connection running ground to the outside of the enclosure.

I was reading about replacing caps and lots of statements about attempting to maintain as close to the original uF rating as possible so that's why I asked about the 220 and 250.

As for the reading of the output pin at 2.99 yes it was taken grounded as described.

If you wouldn't mind, how do I take the AC reading accurately from the transformer?

Thanks again for all your help! Will try to recap first and then continue since those should be done anyways due to age.

[Scruffie]

Multimeter set to AC, 3 leads go from the transformer to the board, black probe on the middle connection and red probe on the two outer, each should read around 12V.

If that checks out then get all the electros and tantalum caps replaced (the old EHX boards have very flimsy traces so you might want to practice de-soldering first if you're not used to it, a spring loaded sucker is pretty much a must followed by a little wick for the remainders) and then if it doesn't fire up right away you'll need to post up all your voltages.

[Fenderstratocaster0]

Okay so I first swapped the 2 ECaps out for new 220uF caps. Same symptoms. Nothing has changed.

I measured the voltage from the transformer. Using the black probe in the middle and the red probe on the right it was a steady 16.4V. Using the black probe in the middle and red probe on the left cable it was a steady 32.6v

Pic attached to confirm.

[Fenderstratocaster0]

If it's a bad transformer and plug I've read about converting it to a dc input jack and powering via a pedal power supply. I'm running a voodoo lab pp2+ so that would be ideal.

If there is a recommended simple swap I'd like to do it and just use a standard barrel plug. There is a doubler cable so I could do 18v or 24v if needed.

I'm not sure where or what to get for a boss style input jack.

[thermionix]

I don't know how the DEM is laid out, but looking at those wire colors (brown, brown, green), I'd guess that green is the center tap, and you would read ~16VAC from the green to both browns, indicating that the power transformer is okay.  Your 32VAC reading was across the whole secondary, skipping the center tap (16+16=32).

[Fenderstratocaster0]

Hmmm.

Well I guess if that's the case and it's not the transformer or plug then I try the tantalum caps next since it wasn't the electrolytic caps. 

I am starting to wonder if the hum issue  and the lack of any flanging is an unrelated issue.

For argument sake if I was to focus just on the fact there is no flanging happening that makes me wonder again about trimpots being a problem.

It almost seems like I have a mild chorus effect happening compared to a direct clean signal.

[Scruffie]

Your transformer readings are good.

You could start poking at the trim pots but you have a regulator that says it's putting out 3V... that's not a trim pot problem and needs to be taken care of before fiddling with the trimmers because the circuit isn't going to work correctly if that's the case.

Did the voltages on the regulator input or output change at all with the new caps? If not, my next step would be to disconnect the regulator output from the circuit to find out if something in the circuit is pulling it down or if it itself is to blame.

[Fenderstratocaster0]

Okay so my next steps today will be to:

1) Re-measure the output of 78L15 output pin to see if the new Electrolytic caps have fixed that issue.

2) If no improvement then swap the 1uF tantalum cap running off Pin 2 of the 311

3) If that tantalum cap replacement works, great. If not then disconnect the regulator (78L15) output pin only? Or remove the entire regulator from the circuit.

4) assuming that none of this corrects the situation I will then post voltages for the ICs


Anything I am missing in the order or should do differently. I appreciate all the hand holding through this :S just trying to get this back to life!

[Scruffie]

I would go in this order;

1) Re-measure the output of 78L15 output pin to see if the new Electrolytic caps have fixed its voltage issue.

2) Test all the electrolytic/tantalum capacitors on the board for continuity to make sure none are acting as shorts, if they are, replace them then repeat step 1

3) If the issue is not resolved then disconnect the regulator (78L15) output pin from the PCB and measure its input and output voltage, if its input is at least 18V and the output reads ~15V then reconnect the output pin to the PCB, if the output is still too low then the problem is most likely the regulator or failing that, the rectifying diodes.

4) Post voltages

[Fenderstratocaster0]

Thanks. I'm going to have to visit my local electronics shop to pick up some pieces.

Would you mind taking a quick glance at the board and seeing if any visual things stand out as suspect?

Photo is before I swapped the 2 Large Electrolytic Caps. For the 1uF tantalum is that the little orange capped one on the immediate bottom left of the 311 chip? You mentioned pin 2 but when I follow that trace it goes to components above the chip (when referencing the photo)

[thermionix]

For the 1uF tantalum is that the little orange capped one on the immediate bottom left of the 311 chip?

That looks like a ceramic to me.  The orange/red one due north of the LM311 is probably what he was referring to.  Those two blues might be tantalums also.  Tantalum caps are polarized, usually have the + side designated, though often in very small print.

[Fenderstratocaster0]

Well. Not so good news.

I replaced the 1uF tantalum cap (it was the red one above the 311)
I also had to replace the 78L15 due to the continued poor reading on output.

New components in. And now there is NO audio when engaged.

What could have happened here?

[Scruffie]

Are you getting 15V out the regulator now it's been replaced? If so, voltages of all the ICs are needed.

[Fenderstratocaster0]

No now I am getting 3v out of BOTH the input and output of the regulator.

Also there is no sound happening when the pedal is engaged as if there is a circuit break preventing any signal. All that was changed was the 1uF tantalum cap and the 78L15.

What's the best way to troubleshoot the signal now that nothing is coming through.

[Scruffie]

You need to stop focusing on the signal and focus on the power first, if the circuit isn't receiving proper voltage, it isn't going to work properly.

So, looking at the schematic we can see there's the transformer, two diodes which rectify the AC power to DC, a smoothing capacitor and then the regulator.

So far you've replaced the capacitor, the regulator and confirmed the transformer has a suitable AC voltage coming out of it but that a suitable DC voltage isn't hitting the regulator and therefore a suitable one will not come out of it.

Which, unless something in the circuit is pulling the voltage down (which is why I had you test the regulator with the output leg disconnected from the circuit board to isolate it) means there's really only two parts left which could be causing the voltage problem, the 1N4001 rectifying diodes.

Did you test the regulators input and output voltage with its output disconnected from the PCB before continuing?

[Fenderstratocaster0]

With the 78L15 output removed it was reading correctly for input and output. But when it was reconnected both input and output dropped to approx 3-4v each.

[Scruffie]

Ahh, okay, now we're getting somewhere then

So that tells us something on the board is pulling the voltage down as it's gone bad.

As it's in a socket and it's easy to to try, try pulling out the 4013 and see what happens to the regulators output voltage.

If that doesn't bring it back up to where it should be you'll need to take voltage readings, start with the 741 and 4558 first.

[Fenderstratocaster0]

Okay so with the 4013 Out we have 3.6v in and 2.9 out.

With it back in 3.4v in and 2.7v Out.

741 voltages

1. 0
2. 7.0
3. 6.0
4. 0
5. 0
6. 6.5
7. 17
8. 0

4558 voltages

1. 5.1
2. 5.1
3. 4.6
4. 0
5. 6.8
6. 7.1
7. 7.1
8. 17