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Auteur Topic: vintage Ibanez PT3 (analog multieffect) - debug help needed  (gelezen 4206 keer)

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Offline AleNL

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vintage Ibanez PT3 (analog multieffect) - debug help needed
« Gepost op: 12 november 2015, 16:11:44 »
Hi fellas,

I am new on the forum. I am Italian, I live in the Netherlands. I will post in English if you don't mind. A forum user (thanks, bruina!) suggested me to post my question here, due to the amount of experts around here. Expert on how to fix a vintage stompbox I am not, nor I am in electronics in general for what matters... so here goes.

I am in the process of trying to fix a defective "vintage" analog multi-fx by ibanez from the early 90's, the pt3 Power Trio. it's basically a tubescreamer + analog chorus + digital delay in a box. all the switches work (I serviced and resoldered them), the pots and effects work as well.. problem is, the unit clips and distorts the signal heavily, also when bypassed (it's not ground hum, sounds more like transistor, opamp or diode clipping/saturation)

I guess it's either the input buffer, output buffer or power section which carries the issue. The fact that the clipping is audible both at the regular outputs and at the phones output makes me lean towards an input buffer problem.

I managed to find the schematic, a guy in an Ibanez forum mailed it to me. However, it's a 4MB pdf and I cannot attach it here. I attach it here just a low res .gif zoom of the input section, to which my following analysis pertains. Pm or mail me and I'll send it the whole schem to you if you require it.

here's the analysis I could do with a voltmeter with the unit powered up:

power rails seem ok, Vcc is about 8.9V. but looking at the input jack surroundings, Vbias on base of TR1 is kinda low. about 3.5 volts. even though the power supply voltage divider works fine and the "A" labeled voltage at the other end of R2 is 4.5V as it should be. I cannot understand which (faulty?) component could bring a 1V drop on R2 in resting conditions without any jack plugged in the input. c1? tr1 itself? c2? IC2a? should I just replace them all?

R1, R2, R3 and R16 have the correct values... low Vb on base of tr1 also translates to low resting potentials for inA and -inA pins of IC2, which I guess is the first bypassed input buffer. maybe that accounts for the lack of clean headroom and distortion in bypass...

makes sense?

If any of you has tips on how to go on... much appreciated, cheers! ;D

Offline Mattnezz

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Re: vintage Ibanez PT3 (analog multieffect) - debug help needed
« Reactie #1 Gepost op: 12 november 2015, 17:08:47 »
Welcome!
Maybe the multimeters impedance loads R2, so please check T1's emittervoltage, it should be around 3.8V.
While you're on it, check the buffer opamp output, (pin 1) it schould be 4.5V.
Those places are lower impedance and easier to read for any meter.
I think the input is a good place to start troubleshooting, since the bypassed sound has the same dist issue.

Offline AleNL

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Re: vintage Ibanez PT3 (analog multieffect) - debug help needed
« Reactie #2 Gepost op: 12 november 2015, 20:54:15 »
Hi Mattnezz, thanks for the help!

I just powered it up again (this time with a slightly hotter power supply giving out 9.4V). Base voltage at T1 is 3.8V, emitter voltage is 3.3V. So they are both lower than expected.

I checked both IC1 ans IC2 (the former being the od/dist opamp ic, the latter being the buffer opamp ic.
IC2 (buffer) shows wierd behavior indeed. Pins 1,2 and 3 are all at 3.8V (same as Vb on T1). Pins 5,6 and 7 instead are correctly at 4.7V (half of 9.4).
IC1 instead has 4.7V on all input and output pins, as it should.
(side note: all ICs on this board seem to have pin 8 at +Vcc, and pin 4 grounded... whereas on the schematic pin 4 is rated at -Vcc... is that simply a nomenclature mismatch?)

Another strange thing is that, even though on paper both ic1 and ic2 should be the same (M5218AP), when I look at the board ic1 is indeed a Motorola 5218A, but ic2 is instead an ST MC1458CP1.

here's the 5218A datasheet
http://www.alldatasheet.com/datasheet-pdf/pdf/906/MITSUBISHI/M5218AP.html

and here's the MC1458CP1 (couldn't find it from ST, found Texas Instrument instead...)
http://www.alldatasheet.com/datasheet-pdf/pdf/527535/TI1/MC1458CP1.html

from the solders shape and looks, it kinda seems that both ic1 and ic2 were substituted or resoldered by previous owners (the solders look bigger and slightly "dirtier" than the other ic's)

so the plot thickens... ???

Offline Mattnezz

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Re: vintage Ibanez PT3 (analog multieffect) - debug help needed
« Reactie #3 Gepost op: 17 november 2015, 12:19:08 »
This is not really weird, as the opamp and transistor seem to be doing what they should. Tr1's Ve is a small drop below Vb and the opamp is just buffering its input-voltage.
Maybe transistor Hfe/Beta is to low, so it might load R2. That's what you asked in your first post  ::) Maybe C1 is leaky, or Maybe R2 is replaced with too high value in an attempt to mod the unit for higher imput impedance.
I would replace TR1, measure/replace R2 and replace C1. And then measure voltages again.

Offline AleNL

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Re: vintage Ibanez PT3 (analog multieffect) - debug help needed
« Reactie #4 Gepost op: 17 november 2015, 12:28:33 »
Ah I see. Ok, I'll fetch the components around, dust-off my (rather low) soldering skills and do the mods you suggest.  ;D

One question though: would it be worth to replace IC1 as well? I thought maybe one half of it (pins 1-3) could be defective/fried in a way that lowers its input impedance, hence setting a path to ground... just wondering

Thanks for the tips!

Offline Mattnezz

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Re: vintage Ibanez PT3 (analog multieffect) - debug help needed
« Reactie #5 Gepost op: 18 november 2015, 16:45:02 »
Hmmm, I just looked at the M5218AP's datasheet and this device (having BJT input) might be quite current-hungry. It states 500nA max input-current and the min input-impedance might be as low as 300k ohms. The standard 510k might be a little high... It seems pin-compatible with most dual-opamps if it's in a DIL-case. Maybe try to socket it and swap with a TL072 or other fet-input dual.
Before you start soldering you might do a simple check.
Desolder(one pin of) R16(1k) and connect IC2A's pin3 to IC1A's pin3 with a simple wire bridge, and see if the problem persists. Maybe all wil bias fine at half supply then.