DX90 Troubleshooting (and Repair?)

[guitjr]

Recently I was given an old DOD FX90. It wasn't in tip-top condition (no battery cover, for example) but it worked. And the little unit was impressive. After a few months it stopped working. I can still power it from battery or charger and it still seems to provide a signal that oscillates and can be made to feed-back, but it's not working as it should. When I plug in the guitar I can switch from Bypass mode to DX90 mode, but in DX90 mode it no longer Delays or repeats.

I thought this would be the perfect unit to learn how to troubleshoot and repair a piece of electronic equipment. Since the circuit is relatively simple, small, and self-contained I hope this might be an opportunity to learn how to go about troubleshooting audio equipment and fixing it. I have basic soldering skills and ability to recognize components. Also I have a relatively good multimeter to use in the process.

Seems I'm far (far!) from capable in this regard. I'm a bit embarrassed at having to post such neophytic questions, but I really would like to learn from this experience.

Here's the background on the box:

The model I have is the same as the one pictured in the top left corner of http://www.americaspedal.net/fx90/
Its schematic (and part layout) is the same as appears on http://experimentalistsanonymous.com/diy/Schematics/Delay%20Echo%20and%20Samplers/DOD%20FX90.jpg , a website dedicated to this stompbox!

Troubleshooting

1)   The first thing I did (after a thorough visual inspection and cleaning) was to clean all the pot controls. I then (under loop magnification) checked the circuit for bad connections and the components for signs of wear or burning (probably unnecessary because the unit doesn't involve any killer currents). Cleaning did not improve matters; I could not find any bad or broken solder joints; and it did not look like any of the components were visibly in bad shape.
2)   I searched this forum and found a posting http://www.diystompboxes.com/smfforum/index.php?topic=79292.msg654133
that listed the voltages that should appear on the pins of the NE570, 4007, and mc1458. The only chips that matched my unit were the NE570 and the 4007, but I figured I'd start there.

Immediately I encounter problems:

a)   The schematic (listed above) does not seem to contain the same chips it's accompanying parts layout diagram cites. The layout diagram lists a MN3005, MN3101, 4007, NE570N, and 423A. The schematic only shows four chips: a NE570, MN3101, MN3005, and NE570. Am I reading the schematic wrong?
b)   Voltage measurements:
    a.   The list in the forum posting cited above shows voltages as follows "(pin #) 1   0,9". What does 0,9 mean? (0 to 9 mv?. 0.9 mv? 0 or 9 mv?...)
    b.   When I attempt to measure voltages my (digital) multimeter refuses to settle down on a number. Is this how a digital multimeter is supposed to work? (It takes a good 10 seconds to count down to 0.0 on a zero voltage pin!) How does one ever settle on a voltage if the meter is constantly changing (ambient voltages???), even when the probes aren't connected to anything.

How should I proceed?

Thanks

[duck_arse]

hello and welcome, guitjr.

I know nothing of bucket brigade effects, unfortunately. there will be some real experts along shortly. but.

the mc1458 listed elsewhere is roughly equivalent to the TL022, shown as two halves of the one device (as is the NE570) on the diagram. I don't know what the 423A is that you refer to, perhaps the TL022? you can probe the voltages on your 4007 (they will change with the bypass state), the NE570 and the TL022, they should all be similar to the other thread. you will need all those volts for when you go here:

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

and follow the instructions.

as for your voltmeter, "0,9" "0.9V" "0V9" and "900mV" are all the same thing. they all mean 9 tenths of one volt, or 900 one-thousandths of one volt, if that is easier. if you have nothing connected to your probes, the meter may well "count", trying to find something to measure. mine does, it is auto-ranging and beeps ALL THE TIME as it counts from the millivolts range up. I recommend searching the utoobes for a video on how to multimeter.

and being as these circuits have internal clocks, some of the pins you probe may very well be changing, from 0V to 9V and back again, very quickly and very often. your meter will be trying to read a changing signal, and give you an idea of what it thinks the voltage is right at that moment. the AC volts range may provide a more stable reading in some places, and none in others.

over to you.

[guitjr]

Thanks for the reply. I've done my best to respond (per "DEBUGGING - What to do when it doesn't work"). See next post.

Also, my Voltage meter did 'calm down' when it had some actual voltages to measure. (I guess when you're measuring millivolts, you're kind of working in an ambient environment.)

I hope this works, both as a repair to an actual piece of equipment, and as a learning process.

[guitjr]

Here is the checklist to fill out:
Project: Debugging and repairing a DX90
1.What does it do, not do, and sound like? Originally worked (months ago) Now there is a difference between when the switch is engaged or not (i.e., the switch works!) and when an instrument is plugged in I can cause it to go into some oscillation and feedback (but not like it's supposed to).
2.Name of the circuit = DX90
3.Source of the circuit (URL of schematic or project) = Project: top left corner of http://www.americaspedal.net/fx90/
Schematic: http://experimentalistsanonymous.com/diy/Schematics/Delay%20Echo%20and%20Samplers/DOD%20FX90.jpg (I believe, because the parts diagram accompanying the schematic is accurate)
4.Any modifications to the circuit? N
5.Any parts substitutions? No, working on original unit
6.Positive ground to negative ground conversion? Y or N (Negative ground, I believe)
7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? => 9.53v
Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:
Voltage at the circuit board end of the red battery lead = 9.53v
Voltage at the circuit board end of the black battery lead =  .1 mv

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages:



Because I cannot verify that the schematic (I found which claims to be accurate) is exactly the one corresponding to this unit I have taken a photo of the parts layout and done my best to measure and accurately list the voltages of all diodes, transistors, and ICs. Because I cannot associate each diode and transistor with the schematic, I numbered these components in top > bottom; left > right fashion; labeled the photo and used that nomenclature to provide the following listings.

I assume all the measurements that fell within the mv range (many of which fluctuated) are irrelevant. For that reason, I didn't indicate the fluctuations (which may, one time, have been fluctuating up; and another, have been fluctuating down).

Diodes         
d1:    A= .4 mv   K=5.96   
d2:    A= 8.66   K=8.44   
d3:    A= .3 mv   K=9.5   
d4:    A= 200 mv   K=7.98   
         
Transistors         
t1   E=5.3   B=.5mv   C=8.8
t2   E=84 mv   B=.546   C=5.67
t3   E=.3 mv   B=.2 mv   C=1.23
t4   E=4.41   B=4.41   C=200 mv
t5   E=.2 mv   B=.2 mv   C=1.81
t6   E=3.74   B=4.28   C=8.77
t7   E=199 mv   B=.71   C= 8.78
t8   E=.72   B=1.12   C=8.78
         
ICs         
MN3101         
1   8.26      
2   4.15      
3   ~ .6      
4   4.14      
5   8.1      
6   4.2      
7   5.75      
8   0.56      
         
MN3005         
1   8.88      
2   4.13      
3   5.98      
4   5.98      
5   .5mv      
6   4.14      
7   5.29      
8   5.57      
         
NE570N         
1   1.21      
2   1.82      
3   1.82      
4   .4mv      
5   3.38      
6   3.38      
7   - 23mv      
8   1.82      
9   1.82      
10   4.37      
11   3mv      
12   3.5mv      
13   8.95      
14   52mv      
15   1.818      
16   12mv      
         
HCF4007VBE         
1   8.84      
2   8.85      
3   .2mv      
4   .2mv      
5   8.84      
6   8.84      
7   .2mv      
8   .2mv      
9   4.02      
10   8.43      
11   .3mv      
12   8.03      
13   .3mv      
14   8.82      
         
TLO22CP         
1   4.45      
2   4.43      
3   4.43      
4   .3mv      
5   4.43      
6   4.58      
7   4.43      
8   8.82