Clock generator help - Page 1
Forum section: electronics / beginners

author: lazarusr
author_rank: Regular Contributor
timestamp: July 08, 2016, 03:52:30 pm
content: I am working on a circuit using an old Yamaha FM Synth chip. I have found the schematics for two Yamaha keyboards which used this chip. The clock generator schematic is essentially the same in both manuals (save that one of them doesn't use the two 1K resistors). The schematic is attached as 'two inverters'. The problem is, I can't get it to work.I have another circuit using four inverters, that works just fine. But I would rather use the other version, because I can then dispense with the whole hex inverter IC and use two NAND gates (which are spare from another part of the circuit) as the two inverters.Can anyone help with where I am going wrong? I can't believe that two service manuals from two different keyboards have fundamentally flawed schematics for the clock generator.

author: dmills
author_rank: Super Contributor
timestamp: July 08, 2016, 03:55:05 pm
content: Are you by chance trying to build this on one of those plug in breadboards?Regards, Dan.

author: lazarusr
author_rank: Regular Contributor
timestamp: July 08, 2016, 04:02:51 pm
content: Quote from: dmills on July 08, 2016, 03:55:05 pmI am indeed and I am aware of the potential for considerable extra stray capacitance. I have tried a variety of different capacitor values. The problem is, I don't get any kind of signal at all. Just a complete flat line at +5V.I realise I could also try the circuit which works without the last two inverters (which I assume are working as extra buffers to clean up the signal). The pulse isn't as clean, but may still be sufficient to drive the main IC.

author: MosherIV
author_rank: Super Contributor
timestamp: July 08, 2016, 04:13:04 pm
content: QuotePerhaps not if your solution is to just try random value capacitors.If you look at the first circuit and the capacitors either side of the Xtal, what value are they?You may find that the capacitance of the bread board adds so much to these values so that the capacitors cannot do their intended purpose.The capacitors are there to create a phase offset. It is this phase offset that allows the circuit to oscillate.I was searching myself about Xtals, I was researching building a DIY ovened crystal.

author: nugglix
author_rank: Regular Contributor
timestamp: July 08, 2016, 04:13:49 pm
content: Are you really sure that the ICs should be LS types?I did this kind of oscillator using CMOS variants of the 7414, these worked even on a breadboard.7414 are inverters w/ a Schmitt-Trigger input, this plays some role, afair.So try this w/ a 74HC14 (or 74HCT14 or 74AC14) and see if it works.Just an idea.Cheers

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2016, 05:00:53 pm
content: From Google

author: lazarusr
author_rank: Regular Contributor
timestamp: July 08, 2016, 05:41:15 pm
content: Thanks for all the helpful suggestions.The other ICs (which are involved in address decoding) need to be LS variants, as the expansion bus of the 80s 8-bit computer this is going to be connected to doesn't play nice with anything else. I wanted to see if I could use the two 74LS00 NAND gates that are left over from address decoding for the clock generator. If I could do this, I could ditch the dedicated 74LS04 (which I have managed to get to work) and reduce my chip count by one.Replacing the (working) LS chip with a CMOS chip doesn't achieve anything practical for me as I will still have the same chip count. I might as well stick to what is already working for me.I strongly suspect using a Schmitt trigger inverter (whether CMOS or TTL) would allow me to get away with two inverters, rather than four as it should result in a cleaner signal. But then I would still be back to the same problem of having a dedicated chip for the clock generator.

author: JPortici
author_rank: Super Contributor
timestamp: July 08, 2016, 05:49:02 pm
content: how are the other inputs in the chip? did you leave them floating?

author: lazarusr
author_rank: Regular Contributor
timestamp: July 08, 2016, 05:56:06 pm
content: Quote from: JPortici on July 08, 2016, 05:49:02 pmNo. They are being used in another part of the circuit.

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2016, 06:08:56 pm
content: Quote from: lazarusr on July 08, 2016, 05:41:15 pmHave a hunt through these Google images, you might find what you're looking for.https://www.google.co.nz/search?q=74LS00+NAND+oscillator&rlz=1C1ASUM_enNZ497NZ497&biw=1093&bih=566&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjnnam2uuTNAhXCipQKHVngCPsQ_AUIBigB