Commodore 1084s Sound Issue

[kadird]

Hi all,

I changed mine 1084s' EHT few months ago and then monitor started blown away the TDA7052 sound amplifier/opamp chips.

I tried these list until now:

- Simply cleaned volume knob's potentiometer and all other pots with contact cleaner,

- Replaced potentiometer from two constant resistors, becuase i dont have stereo (dual) potentiometer for opamp's signal level input,

- Changed faulty opamps with new ones, they are blown again,

- Checked V+ of opamps, there are 9V of this point, its normal value from service manual,

- Changed caps around signal and DC supply path of 7052's, dont resolved,

- Measure voltage across thick steel like GND line cabling frame around CRT tube and wall mains N line and read around 42v AC, is this normal or not? (is this degaussing line?) I think this can be inducing wirelessly due to high voltage of CRT but not sure about this,

- Check other GND lines in PCB, some is continue to main GND point in AC mains socket, some not; some are connected betweeen, some not. I cant be sure about GND circuitry from service manual,

What should i do from now on? I think i should check the mains - live power supply part of monitor but i dont know about much these stage of circuit.

Can anyone help me please? Thanks.

[R_J]

Re: Commodore 1084s Sound Issue

The switching power supply Primary side has what is refered to as a HOT ground, This ground (with lightning arrorw) is connected to the (-) of the bridge rectifier. Everything on the secondary side of switching power transformer uses COLD ground as a reference.
If you use cold ground for your meter and check the voltage on the HOT ground you will measure close to half the a/c line voltage. This is normal.
The coil of wire that goes around the bell of the CRT is the degausing coil (See page 9 of service manual). One side is directly connected to the a/c neutral, the other side of the coil is connected to the a/c line side through a PTC thermistor.

[kadird]

Re: Commodore 1084s Sound Issue

Thanks for the explantation.

Where should i check now for audio then? When i put new opamp to the circuit, some high humming, some ("hissing") noise like random fm band we can hear from radio in no broadband station, and randomly occuring impulse-like high pitched noises hear from headphones and speakers for a while, opamp's heatsink become hot and then everything stop working and then only very very low output remains can hear from the headphones or speakers, which is at the input source, like playstation 2 or anything has analog audio output i am connecting as audio source.

[R_J]

Re: Commodore 1084s Sound Issue

The service manual I have is too hard to follow, it is very poor quality.

Where does the 9 volt supply come from? Is it from the EHT or is it from the SMPS (main power supply?
Replace the filter capacitor on the 9 olt power supply line going to the audio ic.

[kadird]

Re: Commodore 1084s Sound Issue

Here is my manual, it is good to follow.

https://1drv.ms/b/s!Al_2SS8NWtMWlYQC...ywiwg?e=wWwqgz

In this picture i marked 9v line goes opamps i think, because this is only 9v in supply side. Should i change caps numbered 2444 & 2447 seen here now?

https://imgur.com/a/B54l3ED

[R_J]

Re: Commodore 1084s Sound Issue

I would replace 2447 (2444 should be ok)

[kadird]

Re: Commodore 1084s Sound Issue

It is looking good.

https://imgur.com/a/PY2Z2DI

[R_J]

Re: Commodore 1084s Sound Issue

I would not trust the meter, I would replace that capacitor. There is nothing else that supplies that output ic. Maybe the capacitor 2291 is bad.

[kadird]

Re: Commodore 1084s Sound Issue

I do. Something blown too in supply part and dont know which one was. Monitor complete dead now, fuse is ok. And when i discharging sucktion cup of eht this time, very long arc jumped to the screwdiver. Normally it was only 3-5mm but this time it was minimum 5cm of arc occured between eht cap and the screwdriver connected to GND. What a mess Hope this is not EHT againg because it is very expensive to change again.

[kadird]

Re: Commodore 1084s Sound Issue

Maybe the capacitor 2291 is bad.

I changed this before.