I recently bought a GF-9696Z in very good physical condition - all switches, knobs there and ok, no scratches. Of course it has some issues as I found out when switching it on.
Issue 1: Loud tone when playing tape on one channel (I hope easy)
- The tone sound like a 100Hz - 200Hz sound, it is only in tape play mode
- When touching some knobs and switches, it disappears for 20-40 sec and then comes back
- Even hitting the GF slightly has the same effect
- Not in rec mode, not in any other audio source
- appears even when the tape is not spinning (just 'tape" selected)
I would clean the rec/pb switch (and all other switches) and hope that the oscillation is generate by dirty switches.
Would you guys agree as a first idea?
Issue 2: More complicated: APLD is not working properly
- When you press any APLD button, number 1 is turned on and stays on.
- No other button can be selected
- APLD function itself is not working (so no counting down)
I already tested the switches, LEDs and cables of the APLD unit in the front - everything ok. I also measured all voltages of the APLD IC 751 which are in the service manual - everything ok except pin 5/6/7. I checked the two voltage stabilisations Q751 and D752 - working. I also checked the SCR751 function and the Q753 which is used when the tape APLD key are pressed down. Even the volume reduction via Q771 and Q772 is working properly. I also checked the logic that pin 4 (/OD) goes up as soon as a APLD key is pressed.
When I understand the function of the M54834 correctly, it uses an oscillator to shift the selected "channel" up and down. Down is used when an music pause is detected. Up is not used in the APLD application. When an APLD key is pressed the IC uses a high frequent oscillation to find out which key has been pressed.
For me it seems that exactly this oscillation is not working. I tried to measure something on PIN 5/6/7 with my DSO, but there is nothing going on.
My plan is to change the C755, C754 & R761 in case one of these guys has been fried. Another solution could be a fried M54834 - but since the reset to APLD LED 1 is working, it would be half fried...very rare.
- could someone of you measure the voltage on PIN 4 of IC751 when one of the APLD key (not the active one) is pressed.
It is 1.6V in my case which also my LT Spice Simulation generated...but in the specs of the M54834 the want to have at least 3V for high state...so I am not sure if this is right.
- From my theoretical understanding there should be a 20kHz waveform on pin 5/6 or 7. As I said there is nothing in my case. So could someone of you measure the voltages / waveforms at this pins when a APLD button is pressed?
- Has someone of you seen an error like this - I guess the APLD building block is used in all GF searcher models
Long first post....I know....hope to find some help from you!