Help checking a simple circuit on Arcade PCB - Page 1
Forum section: electronics / beginners

author: biglouie
author_rank: Newbie
timestamp: April 19, 2021, 09:30:35 am
content: hey all, first post, but have been reading for many months!I wonder if I might ask for assistance with a circuit check? I am learning how to fix early 1980s arcade boards. Lots of TTL, Z80 and 6800 processor stuff.I have a clock circuit, with a crystal, but I think results are strange, but half the battle for me is knowing what I "should" be seeing. So I am trying to learn the foundation of electronics, so I understand..(That's the goal!)I'm struggling to understand why the ceramic capacitors C12 & C13 are showing LO both sides (logic probe) when powered on. Every other ceramic capacitor on the board shows HI & LO. If I check continuity in circuit (with DMM) these capacitors do not buzz, ALL other ceramic capacitors on the board DO buzz, in circuit.I'd love to debug it, and understand what I should 'see' for this tiny circuit, and then check it and THEN I can understand what is actually happening.Its 3 resistors (R46 isn't there, or on any other of this board type)an LS042 capacitorsa crystal (12Mhz)Apologies for the ridiculously n00bie question. I'm keen, just lacking knowledge, despite youtube! :-)

author: MikeK
author_rank: Super Contributor
timestamp: April 19, 2021, 01:08:49 pm
content: A logic probe won't be useful in an oscillator circuit...use an oscilloscope.

author: biglouie
author_rank: Newbie
timestamp: April 19, 2021, 02:24:01 pm
content: Have it hooked up to a scope.I can see 12Mhz on C12 both legs and same at C13. Also same for R43 and R45, both legs.R44 has one side with no signal. Presumably thats because it connects to GND?Pins on the LS04 that are listed on schematic, show 12Mhz.Curious as to why all other ceramic caps on the board show a HI and LO and this showed LO on the logic probe.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 19, 2021, 04:10:46 pm
content: All the other caps are probably decoupling caps, they just help keep the VCC rail smooth/quiet against the individual chips changing load on the power rail as the chips do their thing. So of course they're always going to show high (vcc) and low (gnd) because that's what they're wired directly to.These caps you're asking about are doing some magic in the oscillator that I don't really understand well enough to explain. That's where your real question is though.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 19, 2021, 04:25:37 pm
content: Probe pin 12 of 2N and look for a square wave. If you get one... it works and consider the actual oscillator as a magic black box.Probing inside the oscillator is prone to issues where the capacitance of your leads/test equipment etc will cause odd behaviour.The output on pin 12 should look squareish... again your test equipment will affect what you actually see... square-ish is probably enough.If you are interested....http://www.z80.info/uexosc.htmYours looks like the 2nd example.

author: biglouie
author_rank: Newbie
timestamp: April 20, 2021, 09:00:38 am
content: Thanks everyone for your help.The link was really useful, I agree, example two is what I am dealing with.Good to know that this cap is different in operation to the smoothing capas, I had (incorrectly) assumed thats what all the ceramic 104 caps were doing.Pin12 3N, square'ish wave at 12Mhz. So that's fine. See belowI do want to understand what its doing. So I will read this 40 more times...The Figure above shows implementation of a series resonant oscillator circuit.The circuit is also designed to use the fundamental frequency of the crystal.The inverter performs a 180-degreephase shiftin a series resonant oscillator circuit.The 330 Ohm resistors provide thenegative feedback to bias the inverterin their linear region.I'll go and search up on the bold stuff! :-)

author: ledtester
author_rank: Super Contributor
timestamp: April 20, 2021, 02:12:19 pm
content: A recent thread discussing how crystal osc. circuits work:https://www.eevblog.com/forum/beginners/what-are-they-doing%21-xtalosccaps/

author: rstofer
author_rank: Super Contributor
timestamp: April 20, 2021, 03:26:54 pm
content: That is a lousy schematic, there are no dots where connections are made and this is particularly important when there is a 'cross' in the diagram. In any event, 3N2-1, R43, R44 and C12 are all connected together at the 'cross'. Connections are fairly obvious when there is a 'T'.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: April 21, 2021, 02:53:36 pm
content: Quote from: rstofer on April 20, 2021, 03:26:54 pmThat's true! If the OP could post the name of the arcade PCB we could play along at home with the real schematic.

author: Renate
author_rank: Super Contributor
timestamp: April 21, 2021, 11:24:06 pm
content: The short answer is you want to make a sine oscillator (out of logic gates) but feed it through a buffer as a square(-ish) wave.To make those inverters work as linear amplifiers you have to bias them like you wouldn't a logic gate.That's what those two 220Ω resistors do.The negative feedback biases them in a linear(-ish) region.So to keep your bias from being screwed up the two logic gates have input capacitors, making them AC coupled.