Ibanez AD-202 troubleshooting

[beatnik]

I have this really nice analog delay and fx unit. It's an old Ibanez AD-202. Features two 3005 chips for delay and one 3007 for modulation.

Here you can find a service manual including schematics

http://www.lbquota.net/cms/sites/default/files/download/67/ibanez_ad202_service_manual.pdf

The problem with my unit is that developed a noise.

If I leave it powered off for some time, when I turn it on is ok, but after a while a crackling/popping noise start to build up, and after some minutes it becomes so loud that I have to unplug from the amplifier.

My guess was a power supply issue so I checked the +15V regulator and replaced C218, C222 and C224. That didn't solve the problem so I'm stuck on what to check next.

Any suggestions please?

[GibsonGM]

Bad caps (esp in power supply area), BAD/DIRTY POTS and/or JACKS, LOOSE WIRE somewhere (check first)...gotta eyeball it all first, man!  A bad solder joint could do that too.   Any humming along with the noise?  Chopstick is your friend...

If this is, as you say, related to heat as it warms up...I'd be suspicious of the power transfomer /     Any way to disconnect it and load test it?  A scope + load test might tell a lot.

Anyway, you might try to approach this from an audio probe perspective since it's non-invasive...see if ANY sections have no noise, and some do.  That will give you valuable data, or confirm power supply, I'd think.  Try to take it in logical steps, and to aggravate/mitigate the noise if you can.

[beatnik]

Do you think it can be heat related? It only takes a couple of minutes to build up the noise to max.

Do you think I should replace capacitors C237 to C246? They are between the 15V rail and ground in different sections of the circuit.

I am not able to test the power transformer but I could temporarily swap it for another one and see if that solves the problem.

[beatnik]

any idea guys?

[armdnrdy]

I would build (if you don't have one) an audio probe and go through the circuit to isolate the section where the problem starts.
Then check each component and replace if necessary.

Scroll down to audio probe:

http://www.diystompboxes.com/wiki/index.php?title=Debugging

[beatnik]

nice suggestion i try that. will keep you posted on the results

[beatnik]

i did audio probe and the crackling noise is in all the circuit.

replaced capacitors and regulator in power supply but still nothing.

i guess the most suspect is now the power transformer. i'll try another one and see.

[duck_arse]

you could try battery power by lifting a connection to the diodes at C2222, to rule out the transformer.

check and clean the jacks. also check for DC at the tip connections. you might also check C406, C405, C404 and C407, see if anything fishy thereabouts.

that's some circuit diagram!

[beatnik]

tried to power with external psu. still the same, so power transformer is ok.

i also disconnected all the meter circuit by lifting pad 11 (red) and 10 (yellow) to see if that maybe injected noise in the audio circuitry, but again didn't make any difference.

i thought it might be useful to record a small audio sample of the noise, so you can get a better overview.

here you can listen to it. be careful with listening volume because the noise is very low but then at some point there is a spike

https://www.dropbox.com/s/djk8awkfe4r7r96/Audio%201_01-02.mp3

do you have any other idea?

[beatnik]

one thing I didn't mention is that the noise is not present in the dry output jack. only in fx output.

this make me think more about a bad component in the audio circuit rather than something in the power supply. am I right?

[duck_arse]

please don't make me look at that diagram again! it sounds (over the immense whirrr of the laptop fan) like the run-off groove on a record. "not in the dry" eliminates only 4 opamp sections, there is still a lot to cover.

what were your conditions when the noise was recorded? did you have a sig in, move a pot, make any changes, or does it just do it? it definitately has "effect" on it. can you listen to the effected noise, and vary the various pots, to see what controls it passes through. we might eliminate some more stuff.

another thing you can try, short the inputs, so there is nothing but the internal noise in the audio. then try choppstikking the electros, the IC's, any large film caps. it might be a dry cap or a bad joint, and you might just get lucky.

[beatnik]

the noise has been recorded without any input signal.

yeah the noise gets effected and all the controls work as they should. in fact the effect works and sounds right, but there are these annoying cracks and pops.

i inspected many times for cold joints and re soldered al the suspicious connections. but i will try the chopstick test.

i am even thinking to take out all the ics one by one, and see if i can find the section where the noise come from.

[duck_arse]

what I meant was, can you find a control that doesn't have an effect on the outputted noise? and is the input left open, or shorted?

you may have a hairline crack in the copper traces on the pcb somewhere. you better hope you don't. when the internals heat, expansion and contraction, etc. does the voltage regulator get hot, is mounted on a heatsink?

[beatnik]

i am almost sure is a capacitor issue instead of a heat issue.

because if i leave the unit powered off for some days, then the noise needs a couple of minutes to develop. if i just power it after some hours, the noise is there since the beginning.

i tried to chopstick all the components and traces but I haven't found anything suspicious.

i tried to take out the MN3005 which are the only socketed ICs and I noticed the noise goes away in delay and doubling mode, but still present on flanger and chorus mode

the regulator uses the chassis as heatsink so it does cool very well.

I also cleaned potentiometers and all the trimpots with deoxit.

even shorting the inputs to ground didn't make any difference

I'm completely lost with this! I really hope in your help

[duck_arse]

I also cleaned potentiometers and all the trimpots with deoxit.

I'm completely lost with this! I really hope in your help

eeek! we're sunk, then! please don't make me look at that circuit diagram again.

at least you've narrowed the search field. I was thinking about caps and pots. if I get time tomorrow, I'll stitch those 2 parts of the diagram together (I'm sure you know the frustration of following/scrolling over 2 pages, non?). I was going to look at the pots, to see if they were being fed by any electros. you can check to see if there is any dc on the pots where it shouldn't be, I dunno if this works like our noise source, but "it must be something", as an old tech used to tell me.

have you shown us pics of the innards yet? that's always fun.

[beatnik]

you are right, that schematic is crap! apart being split in two pages, it is so crammed!

anyway, I thank you again for taking your time to help me

following your next suggestion I measured DC voltage on the pots, with the unit in delay mode.

on the width pot there is varying voltage, the values range from 4 to 12 V, also depending on the setting of the speed pot, which also has voltage on all three pins. I guess this is normal cause these are part of the LFO circuit, right?

delay time pot has one external lug on 0V and the other +7V. the center lug range from 0 to 7 V depending on pot position. again, i guess this is normal?

the most strange is the tone pot: on all the three pins there is voltage that randomly oscillates around +6 / +7 V and sometimes even drops till +2 V!! It drops randomly in a similar way the noise behaves (constant crackling and sometimes big spikes), so I am starting to suspect the problem is originating quite close to that area!!

what do you think it could be: the pot? the ICs? capacitors?

I wait to know your opinion before i start replacing components

[duck_arse]

so, maybe you've solved this yourself, and didn't know it. I joined those two sections of diagram so's I wouldn't have to look at the original again, and then eliminated. the coloured boxes below tell the story (use the original for details) .....



you said "in the wet, but not the dry", and "not in the supply", so that's the green boxes on the left. "not the metering circuit" is the top right box. I'm saying "won't be the oscillator", so that's the bottom right. that left the expanse of pink.

"not in delay or doubling, but in chorus and flanger" is the key, eliminating all the commoned sections. from the input to where those 2 sections diverge, and then when they again mix, the pink lines rule out another load. the blue lines then take out the "not in" section, leaving the orange (and the white) section.

three transistors, all emitter followers, the two doing filtering connected to a single source ....... Q106. I rekon if you pull this and put in any old low-noise, mid-high gain transistor you have laying about, you might have a fixxe. as there is only R176//R177//R196//VR104//C168 common otherwise, you will at least be very very close.

I bloody hope so.

[beatnik]

man thanks so much for taking your time to help!

but i'm really sorry to say i replaced Q106 and again, nothing changed.

this is becoming so annoying...

be careful cause i said there is no noise in doubler and delay mode, but only if i take the MN3005s out. If they are in circuit noise is there.

the noise is noticeably more present in chorus and flanger mode

[beatnik]

i think that's because by pulling the mn3005 out there is no output at all in delay and doubling mode, right?

[armdnrdy]

i think that's because by pulling the mn3005 out there is no output at all in delay and doubling mode, right?

That would be correct.