Vintage Yamaha PS-55 repair

[frisc0]

I recently acquired a very nice Yamaha PS-55 keyboard (from 1983 I believe). It sounds great, if it works .

Problem is, 9 out of 10 times switching the keyboard on, only the Rhythm section works, but not the Orchestra or Solo sounds. In the event the Orchestra and Solo voices do work, they keep working as long as the keyboard is switched on.

The keyboard has separate ICs for the Rhythms, Solo and Orchestra voices as can be seen in the attached circuit diagram.

Examining the circuit diagram I suspected the 'initial clear' circuit (connected the the reset-in line of the CPU) to be malfunctioning, resulting in the Solo and Orchestra ICs not getting initialized correctly at power on. I've replaced the cap in that circuit without effect.

From the CPU the reset-out goes to the Rhythm IC's Initial Clear-in (IC-in) which in turn according to the schematics produces the IC out for the Solo and Orchestra ICs. This IC-out line seems to be a direct connection without any components in between.

How should I proceed in tracing down this issue?

Your help is much appreciated!

[Swake]

You have certainly already checked the power supplies. Did you check the power directly on the chips in the non-functional area? They could be starved for juice in certain circumstances.

Rephrasing the issue: Are you writing that what is in PN1 always works, but the PN2 stuff only works infrequently?

Possibilities I see:
A- The issue can be within the controls/buttons board PN2, no LEDs working either? If this is the case it could be a bad GND issue in this area. Something like an intermittent contact, loose GND lead, an almost broken wire.

B- The issue is around in the sound generating chips -> your idea of the reset can be a good one, do you have a scope to follow the signal and to see if it is in line with the diagram near IC3/PIN35?

C- The issue is in the DAC after the sound generation -> Check if something comes out of IC14/PIN24 , there is a FET1 followed by some transistors Tr5/6/7.

D- The issue is in the sound path -> Follow the signal coming from the above FET over IC17 (does it receive SPO1/SPO1 signals to switch on?) till the mixer IC34.

Assumption is that you can always hear at least the rhythm stuff coming out of the speakers and that therefor the circuit is good from IC34 on till the amp and speakers.

[frisc0]

Thanks Swake for your extensive reply! Certainly a lot of leads for me to investigate further.

I do have a scope, but only an analog one, so more difficult to capture 100mSec pulses

The LEDs work all the time, so I don't really suspect the controls/buttons board PN2.

The rhythm section always works and get correctly amplified and audible through the speakers. However, rhythm and melodic section have their own separate DACs (one DAC IC for rhythm and a resistor ladder for melodic section).

The reason I think it's in the initialization is that, when the keyboard starts up without Solo/Orchestra voices, it remains that way as long as it is powered on. Tempering with controls/wires/connections do not suddenly enable the melodic section, only powering off and on again can enable it again. The other way around applies as well: if the melodic section is working at startup, it remains working even when tempering with wires/connections.

Is there anything that I can check / do around the initial clear function?

[Swake]

The diagram shown for IC(OUT) is not a pulse but a transition. It is my understanding that the signal will stay at 5V, this means you can see it changing at power-on with a multimeter or a scope.

RESETOUT coming from IC5/Pin3 is also going to DET (see J8 in the schematic). I believe DET is a separate board somehow from the main board. The output of the top section of DET is a signal named +5A that you can find back at IC8/Pin9 and Pin5 (I4 in schematic). That is a NAND that activates the clear enabled of the RAM IC10/Pin18 and IC11/Pin18 (J6 & 7 in schematic). Verify all the signals here. It seems to be generic RAM for all functions, but you never know.


 

[Swake]

Have you been able to progress on the repair?