Boss CE-1 (original Chorus Ensamble) Repair/Mod
A friend of mine is having trouble with a Boss CE-1 which he wants to use with electric bass. I will be able to take a look at it in a couple of days.
As he described: The signal is certainly not altered, but passes through. It hisses on normal, but not on effect (though louder). It has been tried with a booster pre-chained, and with synth. Same thing, only the LED over normal/effect switch lights up and some slight clipping occurs.
After reading other posts on the CE-1 I suppose the trimpot at the base of Q7 could need adjustment.
Until I get to check it out,
Any tips or ideas are much appreciated
(as preferably it needs to be fixed within a week)
My tip is look at it to see what it actually is doing, otherwise we are just tossing ideas into the wind.
Do you have the schematic? There are JFETs used as signal switches. Are their gates getting control voltage as appropriate? You have a effect/normal stomp switch, are both throws on that switch making? You have a chorus/vibrato stomp switch. Are both sides of that working? There should be a LFO clock oscillator for the effect, and it would appear on that switch, so does it?
Yes I have the schematic.
This is what I measured at the gates of
(these were all 2SK30A Y)
On normal its OK. On effect the signal is still clean (slight clipping).
None of chorus/vibrato makes any difference. Still clean signal only.
You need to understand that those JFETs are gates for signal routing. SO they are turned on and off as needed. SO there will not be just one voltage reading at those gates, there will be at least two.
Imagine two branches in a circuit, A and B. A is through the effect and B is a bypass route. At the output, we can add a JFET at the end of each branch. Turn on JFET A and B off, we get the effects. Now turn off JFET A and turn on B, and we get the bypass.
SO you might switch the effect on and off and watch those gate voltages to see if they turn off and on.
We need to determine that the signal routing is switching or not. Then we will know if we have to fix the routing or the effect itself. My working assumption is that if the routing is working and the effect circuit is dead, you would get nothing rather than continued clean. But that can be wrong easily enough.
Okay I understand. I thought the voltages were supposed to match the schematic.
They change on normal:
Well, the schematic shows them as ranges, for example gate of Q11 says -7v to 0v.
It appears the gate controls are working. These are JFETs, so they are ON until turned OFF by a voltage at the gate. SO while it is running, use an ohm meter to verify the resistance between source and drain of each goes from high resistance to low when the gate has voltage.
My schematics have a row of waveforms along the bottom, your copy have that? They correspond to test points indicated on the drawing. Do you have a scope to check those? If not, you can make a simple signal tracer (google that if you need to) out of another amp. Essentially we put a series cap (to block any DC) at the input of an amp and touch it to points in the circuit. On the other amp we can then listen to the signal at points in our circuit. Almost good as a scope. Sorta.
If the effect part is not working, the central element is that 3002 IC, which is a bucket brigade delay line. The signal is clocked through it by clock pulses at pins 2 and 12. SO the clocking signal; must be present for any throughput.
Did we verify all sections of the stomp switches were opening and closing electrically? Does the LED come on when effect is selected?
I think we've got the same schematic.
Both peak level and effect LEDs work fine. I'm running a bass guitar and peak level lights up with the high input switch on. I can also hear the signal getting more low end when effect on. So that switch obviously works.
How can i verify the chorus/vibrato switch? What makes the LFO clock?
The source drain resistances range from about 30 Ohms to about 5K, so that looks good.
Point E between trimpot and base of Q7 reads -4.89V instead of -6 as in the schematic (might be worth noticing?). I traced it with alligator clips - clean sound.
Is this where the chorus sound should be introduced?
Also traced the ICs pin 3 (point D), got a little clearer sharper sound with less low end.
Point I - no sound.
Can't find any chorus sound.
Any suggestions where to start probing/what to look for?
A switch is a switch. Those stomp switches connect one set of contacts, then stomp on it, and it connects the other. By verify, all I mean is use an ohm meter to determine that indeed the contacts all open and close as they should. For example one set might never close.
The LFO is IC4, lower right, test points J,K,L. And I. That LFO signal is fed into four transistors Q3,4,5,6 which act like a phase splitter to make the two clock signals opposite phase from each other. Opposite polarity if you prefer.
Offhand i would not be too worried about the voltage at that trim pot being off a volt. Look at the circuit, all it is is a balance control for the two outputs of the 3002 chip. It blends them together, and we want them even.
The signal comes in through IC-1, and emitter follower Q2 puts that at the input to IC-2, point D. So I expect clean signal there. That signal also feeds Q8,9 - the peak light driver. So all that seems OK.
Point I is not audio, it is the clock signal from the LFO. I am not sure what the range is, but I'd expect something like 1Hz to maybe 20Hz, and it shows a triangle wave, so sub audio frequencies.
I expect to find a chorused signal at point E. If it seems dry, then important to find the two clock signals at IC-2 pins 2,12.
Lacking a scope, a signal tracer works for audio, but for the LFO, points G,I,J,K,L probably an AC voltmeter will pick them up.
I am trying hard to discover anything that will mean something other than a bad 3002 chip.
Ok the switch is fine.
Yes, point E has dry signal, that's what I meant.
Audio tracing IC-2 pin 2 or 12 (G), I can hear a low high-pitched oscillation that sounds like tropical birds.
Measured at IC-4 from ground pin 4:
From ground pin 11, IC-2, points G reads between 9 and 5 volts.
The text in the square at the bottom is very difficult to read.
Also not quite sure where to put the AC voltmeter probes.
According to the previous owner of the unit it should work. As in he knew the pedal worked well, but hadn't tried it in a while. And I doubt it's humbug. He's even offering getting it fixed at a local electronics workshop. I just don't see how anything could have suddenly gone bad.
everything works until it doesn't, you know? That is how something suddenly doesn't work.
On IC4, pin 4 is not ground. Pin 4 is the negative supply rail. That should be roughly -14v from ground.
We are looking for both AC and DC voltages at this point, so you need to specify which you are reading, especially at points like J,K,I. +16, means +16vDC, and -14v means -14v DC. AC has no polarity. SO if point K is 23v p-p, that means 23vAC.
SOme meters are fooled by AC waveforms with a DC offset, so by adding a series cap to your meter probe connection you can block any DC from the meter, if this is the case.
Your LFO runs all the time, and the audio is fed through the 3002 chip all the time. What turns the effect on and off is just the audio switching of Q11, 13. SO if you have good audio at pin 3 of the 3002, good clock at pins 2,12, about -14v at pin 11 and about -13v at pin 4, and also about +5v at pin 1. Then I expect to get good output at pins 13,14. Those two signals should be chorus-y. If not, then I start to think the 3002 has failed.
|All times are GMT. The time now is 02:12 AM.|
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio