Eprom issue - scratching my head ...it seems to be using 230 volts on several pins!!

[Britguy1]

I am in the UK which as I am sure you know has 230 volt standard AC. I have an eprom chip and I am building a copy of another circuit board - a nice little light chaser from the 1980's, as far as I can see tracing the tracks on the original board, several of the pins on the Eprom chip are wired directly 230 volts on the actual chip. Thinking I was seeing things, I powered the original board up and metered across the pins on the chip as it was healthily doing its thing, and sure enough, while it was live, I have found 230volts feeding a number of the pins on the original Eprom!!!. I even then metered the pins on the Eprom rather than just the reverse of the chip socket and it was once again showing 236 volts across several of the chips. I am relatively inexperienced with electronics and thought Eproms used 5 volts - have I gone mad and having a late night or does it sound about right that certain chips on an Eprom do and can use 230 volts. Slightly confused. Any help much appreciated!

 

[markdem]

Does the EPROM have a part number? Quite strange to see such a high voltage going in to a logic part.
Is there a power supply in the device you are coping?

 

[Venkadesh_M]

post your copied circuit did you used any transformer or tapping directly from main voltage?

 

[bigdogguru]

Doubtful.

What did you use as the reference ground for your measurements?

BigDog

 

[betwixt]

Voltage is measure across two points, if the OTHER point had 230V on it, the meter may say 230 even though the EPROM was at or close to zero. The other odd thing is that the 230V supply is AC and an EPROM will be damaged if a negative voltage is applied between it's VSS (ground) pin and any other pin. Are you measuring AC or DC? Regardless, from the vintage of the equipment, the voltage should never exceed +5V.

Around that era, there were several designs of light chasers/sequencers/sound to light designs that did not use isolated supplies and were actually extremely dangerous to use, possibly this is one of them. They used the neutral side of the AC supply as ground for the control circuit and SCRs/triacs to control the lights and dropped the 'live' side using RC combinations and a regulator. If you can post the schemeatic it would help.

Brian.

 

[Britguy1]

Hey guys, I have attached two pictures - one of the front and one of the rear of the original circuit. Its frying my brain a bit. I don't want to risk blowing things up in a spectacular way on the copy board I am making LOL!

 

[betwixt]

OK, from that I can tell you it is an isolated supply design. Just check the isolation is still working though, with no power applied, measure the resistance between the L and N mains connections and the negative side of the big blue capacitor (the arrows point toward the negative end). If all is well, after a second or so for the meter to settle, you should measure a very high resistance, ideally several Megohms. If it measures a near short circuit, DO NOT USE IT !! it means the transformer is faulty and MUST be replaced.

If the resistance is high, the isolation is working and you should measure all voltages around the EPROM with the negative side of your meter connected to the negative side of the blue capacitor. For information, the MC14093 is wired as a slow speed oscillator and each pulse it produces will increase the binary count on the output pins of the CD4040. This count is connected to the address lines of the 2716 EPROM so it sequentially produces it contents on it's data pins and drives the corresponding triac on or off.

Brian.

 

[Venkadesh_M]

Hi Britguy1
It was a nice attempt and you wanted a old chaser, but how will you copy and program the new memory IC.. It seems to be 25V is for programing and is they are doing just reading or changing values...



And here is why your EEPROM getting 230V

look at this your phase comes and plays around the circuit You have to spend time for why they come to EEPROM and It will be a challenging job to complete tracing and programming everything.. surely there will be some reasons...

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you can avoid shock when touching EEPROM can avoid by giving correct phase and neutral.... You can make some changes and you can the phase out of the circuits by relay at the output ends or using SSRs...

 

[betwixt]

Looks like the neutral is wired to the EPROM GND so it isn't isolated anyway.

You should still measure all circuit voltages from the negative side of the blue capacitor. As far as the ICs are concerned, that's their ground reference point, even if it isn't true 'planet Earth' ground.
As mentioned in other posts recently, 2716 EPROMs are getting hard to find and as Venkadeshm points out, you have to program them before they will work. Their contents are very simple and you can change them to make the light sequence do as you wish but devices to place the data into the EPROM are not common these days because the 2716 was close to the last device to use high voltage programming (VPP voltage), after that most manufacturers went to 12.5V and it became the 'standard' for many years. If you are not copying the exact board layout, it would be worth considering the use of a 2764 EPROM instead. It has more pins but is otherwise pretty much the same in function and is much easier to work with.

Brian.