ibanez cs505 help needed

[parkverbot]

Hi!
I need help with a cs505 I recently picked up.
First, the diode on the DC input jack and the DC regulator were blown. Seems someone connected a high power wrong polarity power supply. With the regulator bypassed there's only chorus on one of the two outputs (white wire), the other output (grey and green wire) has only the dry signal with the effect turned on. Theres also a slight distortion on the fx signal. The LED doensn't light with the effect turned on. The bypass works fine.
I have the schematics fpr the cs9 and cs5, but they don't really help because the cs505 is quite different in the output section. The white-wire-output seems to be like on the cs9, but the grey-wire-output is not. (no green wire on the cs9)
Does anyone have a schematic for the 505? Ibanez experts out there?
Thank you
sebastian

[cjlectronics]

Does this unit use 18v or 9v? I've repaired a few of the older square button pedals and the only problem was the pedal requires 18v instead of 9v. In addition, someone wired it up with only a single 9v battery clip, making anyone who is not familiar with these pedals think they only require 9v.
If my memory serves me well, I think this pedal requires 18vdc...two 9v batteries?

CJLectronics

[Mark Hammer]

The question can be partly addressed by identifying the BBD chip it uses, since the various BBD generations were developed to use different supply-voltage ranges. If it's an MN3007, then there is a reasonable chance is uses something more than 9v. If it is an MN3207, then it likely wants something less than 9v. There may well be an accompanying 5v regulator for the BBD if that's the case.

I suppose the other way to tell is to check the voltage rating on some of the assorted electrolytics on the board. This is a much less reliable method since a large company like Ibanez would rarely waste its time and resources buying the lowest feasibly-rated cap for a given pedal. More than likely they buy one-size-fits-all caps for all their pedals (e.g., w/25v rating) and leave it at that. Once you get past 1980 or so, the voltage ratings don't have huge impacts on package size anyways.

[parkverbot]

i guess it's 18V

The battery clip was missing when I got the unit. The chip is a MN3007 (mitsubishi i think), the DC regulator is a 78L12 and two 9V batteries fit in very well. I already bridged the DC regulator and gave it 12V just to test: same thing happens, no effect sig on the grey/green output. If the output stage was like on the cs9, I would have guessed it's one half of the TL022 inverting the effect sig for the second output. I'll try to draw a schem..

[Mark Hammer]

One of the enduring problems/challenges for BBD chips is to maintain the stability of the bias voltage needed on the input pin in order to pass signal. Whether you use a trimpot, as some manufacturers do, or a voltage divider formed by a pair of fixed resistors, as other manufacturers do, you need to keep the voltage bias relatively stable, even as the battery voltage starts to decline. Otherwise, the delay signal starts to distort and eventually not make it through the BBD.

In the case of the MN30xx series, some designers let you use a 9v battery OR 9v wallwart, and just figured you'd switch batteries if sonic quality started to decline. Of course, that wouldn't be very long. Others took the route of starting you off with 18vdc so you could make use of a 12v or maybe 9v 3-pin regulator. 18v was way more than you need, but unless you pack the chassis full of AA or AAA batteries, there isn't a whole lot of terrain in the way of "normal" batteries in between 9v and 18v. Regulating at 12v would require at last 14v, so that would let your supply regulation and BBD bias work nicely, until your 9v batteries started to reach around 7v each. Not too bad.

Of course chassis size, and all the attendant costs, such as necessary packaging and shipping costs for different chassis sizes, is part of the cost of production, so you can imagine there was industry pressure (and good justification) to produce a BBD that would bias and run at supply voltages below 9v. The next generation of chips, the MN32xx chips, could be biased and run successfully at 5v. That meant that you could successfully power the circuit with a 9v battery, and the 5v regulator would continue to produce a stable 5v until the battery reached around 7v. The MN3207 will run at 9v. But as I say, the challenge is to have dependable biasing, even as the supply voltage from the battery is waning. As a result, the design objective became that of simulating a regulated supply voltage under all conditions, whether battery or wallwart. Happily, you can regulate batteries too.

Some folks swear by the earlier high supply voltage units, though it's hard to tell if it is the higher supply voltage in the BBD or the rest of the surrounding circuitry (or both) is the source of any audible improvement.

In any event, there's a lot you can deduce about a BBD-based design/circuit, just from knowing which BBD chip it uses. Thankfully.

[parkverbot]

ok, this explains the distortion

and it's 18V
But I'm still confused about the output section.
I'll replace the DC regulator first and let you know what I found out about the output stage.
The unit is 25+ years old, there must be a schematic somewhere out there..

[cjlectronics]

You can always buy the schematic from http://www.musicparts.com. They have made lots of money off of me with the many repairs I do.

CJL

[parkverbot]

It's tricky, but I'm getting closer

Do you charge your customers for schematics you have to buy? Or do you have a general 'schematic/expertise fee'?

[parkverbot]

the green wire

the green wire on the stereo output jack (switched to ground when plugged in) switches a FET in the output sections and kills the dry signal on the first (mono) output with fx switched on:
mono mode/green wire off: dry signal + chorus mixed on the mono jack
stereo mode/green wire to ground: chorus signal on the first (mono) jack, dry signal on the second (stereo) jack!
I have never seen this before. most chorus pedals have either mono outputs or phase-inverted stereo outputs.

[cjlectronics]

No, I do not charge them for schematics. I treat it like office supplies and deduct the cost from my taxes. You can do this in the US.

CJL