Hello everyone.

So, i got a hb-55p and i am using the av output.

When i power on the computer, the sound outputs something like sparks, similar to when moving a dirty sound knob. This disappears after 3 minutes after powering on but a buzzing sound then appears.

I checked the schematic and just for the sake of it, i replaced the caps in audio circuit (right next to the ABC channels of the sound processor) but i get the same behaviour.

Meanwhile, i noticed that the video rams were burning hot. Checked the voltages and all are ok(-5V, 5V, and 12V). I then removed the rams, fitted sockets and tried the computer. Same "sparks" on the sound. I thought that there may be some short at the rams that could be affecting the sound.

Since the zx spectrum uses the same rams, i used a test interface to test them and it reported no faults at the rams.

My knowledge in the hobbyist way stops here, so i'm asking some help from the technical minded people.

Looking at the circuit, and since i am picking the audio signal before modulation, and i have already replaced the caps, what could be causing the "sparks" on the sound?

Is it normal for the video rams to get hot to the point of really not beeing able to leave the finger there for 3 seconds?

Thanks.

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Is it normal for the video rams to get hot to the point of really not beeing able to leave the finger there for 3 seconds? Thanks.

Video rams get hot, but general not that hot.

Did you check the power supply voltages? I would not be surprised if the sparks came from an osccillating power supply, and perhaps overvoltage even after the initial minutes makes the ram ICs run hot.

I remember I had a HB-55 with broken VRAM.... So, maybe it's a common issue?

Maybe it is a problem in the power supply.
These "sparks" and the buzzing sound look like there is some AC coupling. Try to check if there is some AC voltage at the power unit output. If so, try with another power supply.
Old power supplys usually get dried and damaged capacitors...

I had similar issues on my 25 years old Atari XL 800. I replaced all electrolytic caps with new, and it is working perfect again... Also, I replaced 470uF/25V in DC filter with modern 1000uF/25V, since they are the same size. The higher is the capacity of DC filter capacitor, the fewer AC noise is leaked to PCB. Some audiophiles even go as far as using a battery of big 10000uF caps, connected in parallel for their analog stereo pre-amps...

As for hot DRAM chips, it's pretty normal. I had 150ns 4464 chips that where getting very hot while working (about 80C), but they worked just fine. The bad thing about heat is that it makes case plastic brittle, and electrolytic caps dry up faster in hot circuits (like in 8 years instead of 10).

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Video rams get hot, but general not that hot. Did you check the power supply voltages? I would not be surprised if the sparks came from an osccillating power supply, and perhaps overvoltage even after the initial minutes makes the ram ICs run hot.

What makes me suspect vrams is that the user ram or main ram has the same exact ic's and they run pretty cool.

All the voltages are correct at the power connector that goes to the board.

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Maybe it is a problem in the power supply. These "sparks" and the buzzing sound look like there is some AC coupling. Try to check if there is some AC voltage at the power unit output. If so, try with another power supply. Old power supplys usually get dried and damaged capacitors...

Do you mean AC voltage in where it should be DC? How should i read that? Just put the multimeter in ac and take a reading?

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I had similar issues on my 25 years old Atari XL 800. I replaced all electrolytic caps with new, and it is working perfect again... Also, I replaced 470uF/25V in DC filter with modern 1000uF/25V, since they are the same size. The higher is the capacity of DC filter capacitor, the fewer AC noise is leaked to PCB. Some audiophiles even go as far as using a battery of big 10000uF caps, connected in parallel for their analog stereo pre-amps... As for hot DRAM chips, it's pretty normal. I had 150ns 4464 chips that where getting very hot while working (about 80C), but they worked just fine. The bad thing about heat is that it makes case plastic brittle, and electrolytic caps dry up faster in hot circuits (like in 8 years instead of 10).

I have examined the caps, specially those at the power board and none seems bloated. Nonetheless, i'll replace them to see if i can get rid of the sounds.

Well, the vrams are the same as the rams in the user memory, but the first runs at the temperature you mentioned. AFAIK, at that temperature, there should be something dodgy going on.

Hi,

I replaced all the caps and the sound problems reduced significantly but then again 3 minutes after having the computer turned on, i get a buzzing sound and some "sparks". Not like it was before, but still pretty audible. :|