Commodore 710

Here we have a Commodore 710, quite a rare beast, these were only sold for 2 years between 1982 & 1984. Listed on ebay as not working it also is sadly missing it's keyboard, these were detachable on this model of PET. Here is a few pictures taken just after unpacking. Significant difference on the back to other models of PET, there is no user port but there is an RS232 port and an audio output jack.

First look video here.

First look at the inside. Interesting how this model hinges at the front unlike my other PET. However just like my other PET the main PCB is covered in dust and crud, so lots of fun with cotton buds and iso ahead!

Here's the PCB removed from the case, i don't think it's quite as bad as my first PET but it is still very dirty. The ROMs appear to be in adaptor sockets so maybe they are not the original ones.

Well here it is, the SID chip, i must admit i didn't quite believe it had one until i actually saw it!

Had a bit of luck with the PCB, all it needed was a good brush over and it's come up quite well.

Next, the power supply. My other PET has a traditional linear power supply, large transformer with several secondaries then bridge rectifiers and voltage regulators to supply the various voltages. The 710 uses a switched mode power supply. Now before applying mains to any power supply it's always advisable to give it a good visual check over.

The first thing i discovered was that the mains filter capacitor had exploded. This is a bit of a red herring when it comes to fault diagnosis, the PSU will work fine without this capacitor it's purpose is to stop noise from the PSU getting back onto the mains. The filter capacitors in old PSUs are quite often paper and foil and over time they do go. The red circle shows where the filter capacitor was located. The rest of the PSU looks to be in good condition so now it's time to apply power. Well no smoke which is good but also no significant voltages on the output.

At the heart of a switched mode power supply is the switching circuit, this power supply uses an NE5560N. Red circle below. After poking around with the oscilloscope i discovered that there is no output from this chip. I have removed the original and replaced it with a socket. This aids in fault finding and makes it easy to replace the chip. Of course there may be nothing wrong with the chip, looking at the datasheet there are several pins which can be used to disable the output. Further investigation is required.

Turns out the NE5560N was working correctly but one of the outputs from the quad op-amp chip, TDE0124DP see red circle below, is permanently high which sets one of the NE5560N inhibit lines high. Disconnecting this output allows the power supply to operate. I will have to replace the TDE0124DP but for now i can continue to test the PET with this gate output disconnected.

With the PET reassembled and the power supply working it's time for the big switch on........It's alive!! Now i have to see if the adaptor lead i made for my Commodore 64 keyboard will allow me to type anything.

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