Jump to content



Photo

Two problems with my 'new' GF-9696Z

APLD

  • This topic is locked This topic is locked
6 replies to this topic

#1 timbo-cgn

timbo-cgn
  • Members
  • 5 posts

Posted 22 January 2018 - 05:23 PM

Hi guys,

 

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.

 

My questions:

  • 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!

 

Cheers,

 

Tim.

 

 

 

 

 



#2 timbo-cgn

timbo-cgn
  • Members
  • 5 posts

Posted 26 January 2018 - 04:45 AM

Hi Guys,

some updates:

- tone issue fixed by cleaning and resoldering the rec/pb switch. so assumption was correct
- apld still under investigation - checked the scr and the resistors aroud it (all ok)
- ordered some M54834 in china - just in case it is really the IC
- waiting for the caps for the oscillator to replace the old ones - if this dows nit help, its the IC
- foud a new mech issue - one of the cassette pins is not turning smoothly. ordered a belt kit with the sleeve on the capstan (this seems to be the faulty component here)

keep you posted!

tim.

#3 timbo-cgn

timbo-cgn
  • Members
  • 5 posts

Posted 28 January 2018 - 08:09 AM

Hi guys,

 

just another update:

 

  • changing the APLD caps around the oscillator did not change anything. I tested the old ones when soldered out: perfectly ok. So I guess the IC is out of order
  • I checked all analog components to generate the music gap pulses for the IC for counting down - works as described in the service manual
  • Waiting for the IC now and hoping it is really the right one which I bought  :-)

Tim.



#4 Reli

Reli
  • Boomus Fidelis
  • 5,607 posts
  • LocationUSA

Posted 28 January 2018 - 09:47 AM

You are definitely more skilled than most people here  :-)



#5 hopey

hopey
  • Members (SA)
  • 398 posts
  • LocationMelb AU

Posted 31 January 2018 - 05:52 PM

Well written post how about some pic's?



#6 timbo-cgn

timbo-cgn
  • Members
  • 5 posts

Posted 01 February 2018 - 08:41 AM

Hi Folks,

 

pictures are actually a very good idea!

 

The front, dismounted:

 

 

IMG_0815.jpg

 

The main board with some other attached board.

 

IMG_0816.jpg

 

The evil IC. The red and the black cap near the IC are already changed:

 

IMG_0817.jpg

 

And the new one, arrived today directly from China:

 

IMG_0818.jpg

 

Just tested if it is really the right one (they stopped production in the late 80ties...)...and...yes. Working and the right one. Now I got to find some time at the weekend to rip out the old one on the main pw board and put the new one in.

 

Cheers,

 

Tim.



#7 timbo-cgn

timbo-cgn
  • Members
  • 5 posts

Posted 05 February 2018 - 06:26 AM

Hi guys,

 

just replaced the IC over the weekend: APLD is now working again. By changing all the belts in addition the core function of the set is now ok again  :clap: . Next step is to calibrate the whole device according to service manual which will need another day or so. Will post some pics as soon as the set is mounted again.

 

Cheers,

 

Tim.