Could someone help me understand this circuit? - Page 1
Forum section: electronics / beginners

author: Toomasu
author_rank: Newbie
timestamp: November 30, 2023, 05:58:16 pm
content: Greetings!I'm messing around with an older Yamaha 7.1 amp witch intermittently would turn off. Capacitor C4 is apparently a common culprit so I changed it and amp is fine, so that's good but I want to understand why.After studying the schematic I struggle to grasp parts of this circuit, more specifically around T1, D10 rectifier, Q6 FET and IC3. IC3 is a flip flop. Is this some sort of standby toggle or something?Thanks in advance

author: pqass
author_rank: Super Contributor
timestamp: November 30, 2023, 07:15:59 pm
content: D10 bridge with Q6 FET is a means to switch an AC load (transformer T1) using a polarized part (Q6). Notice that Q6 (if on) shorts upper to lower bridge arms which allows each half-cycle to take right-upper diode down through Q6 to lower-left diode or lower-right diode through Q6 to upper left-diode.C4 is a capacitive dropper which creates a positive supply using D7 zener cathode -> D8 -> C5 bulk cap to smooth -> IC3 D-FF Vcc.TE2 black terminal -> C4 -> R13 -> D7 zener -> D10 lower-left diode -> TE2 white terminal.IC3 TC4013BP is a Toshiba CD4013 equivalent. It's a dual-D FF with set and reset; not used here since they are both grounded. Only one of the D-FFs are used; both D inputs and CLK inputs each are tied together and in one of the D-FF both Q and /Q are left unconnected. However, the CLK is tickled through R14 from D7 zener cathode, which I believe will be a square wave (+zener voltage to -0.7) on the zero crossing point of the AC mains.IC1 opto communicates to W1 connector when the D-FF has power (ie. mains is available).Since the D-FF D is pulled-up via R15, when the cord is plugged into the wall, the Q6 FET is turned on at zero crossing, which energized T1.However, if Q5 defaults to off, then IC2 opto would shut Q6 FET off via D-FF since this opto shorts D to ground.To sum up,the D-FF serves to switch Q6 FET only at [positive-going] zero crossing and Q5 is a soft power off.

author: Toomasu
author_rank: Newbie
timestamp: November 30, 2023, 08:03:26 pm
content: Thanks for detailed explanation. Q5 is marked "no use" so I guess that function is not applicable, I have actually never heard of soft power OFF, only soft start i.e. inrush, delay and such.So am I correct in thinking this is a sort of power saving stand by function? Amp front panel has both toggle main switch and momentary switch for stand by

author: pqass
author_rank: Super Contributor
timestamp: December 01, 2023, 01:40:36 am
content: Given that Q6 is energized only on the zero crossing, this is better than engaging at the very top of the sine (with an ordinary switch) but an empty C10 bulk cap will still cause an initial inrush current (but it's only 3300uF, so not very much).I called it "soft power off" similar to some RaspberryPi power supplies that, in the last stage of OS shutdown, toggle a CPU pin connected to a transitor base to kill power to itself. Upon reflection, this isn't the case here. Notice D9 is a 7.5V zener. So that makes the stack, T1 secondary +V -> D9 -> R16 -> IC2 LED -> 0V, an over voltage trip. That is, somewhere between T1 sec +V > 8.7V (7.5V D9 Vz + 1.2V IR LED Vf) and T1 sec +V < 14V (D42 zener), will light IC2 IR LED enough to flip the D-FF D to low to shutdown Q6, killing power to itself.But notice that Q5 base and Q2 base is attached to the same PRY line (to IC402 CPU pin 80); that is, when the RY1 (9V coil) relay is energized, IC2 opto LED is shorted; ie. keeps it from accidentally tripping the over voltage check? But this doesn't make sense since energizing a relay may cause a momentary sag in the power rail instead. It's upon release of the relay that may inject energy from the coil into the power rail (although that should be snubbed by flyback diode D1). If Q5 isn't fitted then they may have discovered it was superfluous after they designed the schematic. This over voltage trip is to protect the +5.8V voltage regulator (Vin max = 12V) fed from the S10 line which comes back in via +5M line (see bottom-left of page 106 in service manual).Wow, this thing is so over-engineered.It's a circuit to turn on (at zero-crossing) a pissant transformer-based power supply for just a 9V relay and 120mA 5V regulator. Wow!RX-V1700 service manualIC2 opto datasheetD9 zenerIC407 RH5RE58 5.8V regulator

author: Toomasu
author_rank: Newbie
timestamp: December 02, 2023, 02:10:22 pm
content: Thanks again! Not that I'm in a position to judge but seems a bit over-engineered indeed. Thanks for sharing attachments, although I already have the service manual.Might not be the best idea but I've started to strip this thing down. Plan is to build a pure 6 channel power amp with balanced inputs, since I have no use for HT but need amps for my active speaker build

author: donlisms
author_rank: Frequent Contributor
timestamp: December 02, 2023, 06:01:35 pm
content: I confess I haven't read this whole thread, nor understand all of the circuitry, but I just wanted to say that everything to do with starting and stopping a big power amp, or anything connected to it, inside or outside the case, is a huge deal. High power transients blow up speakers, and can harm humans as well! As an external example, unplugging a guitar can destroy many things.I just thought that might account for some of the "over-engineering."

author: Toomasu
author_rank: Newbie
timestamp: December 02, 2023, 07:56:30 pm
content: Good point. Not that I completely understand the circuitry either my understanding is that it is a means to protect the sensitive chips and processing circuits in the amp, for all the DSP and HT stuff. As for protecting speakers I intend to keep output relays and try to recreate the amp protection circuits with for instance power on delay, DC offset etc.

author: xavier60
author_rank: Super Contributor
timestamp: December 02, 2023, 09:46:16 pm
content: Yamaha have been using this arrangement for years mainly to regulate Standby supply and also produce Power Detect and Reset signals.The most common failure in the past has been the capacitive dropper, C4. As C4 fails, although the Standby supply might be ok, PDET and /RES can be affected.

author: pqass
author_rank: Super Contributor
timestamp: December 03, 2023, 01:56:33 am
content: See attached block diagram. The T1 low power PS complexity is the focus of this post.This PS does not supply the amplifiers. It only provides power for some digital logic (IC402 CMOS MCU, IC403 I/O expander), a relay (RY1) to turn on the main power transformer (with 6 isolated secondaries), the [non-mains side of the] reset+power detect logic and pullups. So why would it be necessary to switch it on at zero crossing? And if necessary, why use discrete D-FF+FET+diode bridge when you can just use a zero-crossing opto-triac+triac? That's why I called it over-engineered.I was also perplexed as to why the D-FF defaults to turn on T1. And the over-voltage trip logic that shuts down its own power source if the unregulated+rectified T1 secondary goes somewhere above 9V. Was it some elaborate technique to keep the 120mA 5.8V regulator (IC407) from exceeding its rating (Vin max 12V)? Or, was this zero crossing logic overly designed to stop spikes that may affect the MCU if an ordinary switch were used instead?Then the answer just dawned on me....the D-FF turns on T1 which fills the bulk cap (C10) above 9V, which switches the D-FF such that it turns-off T1 for the next sine cycle or a few, until the bulk cap goes below the trip voltage, which then the D-FF turns on T1 again, etc. Jeez! All this complexity to trickle charge a cap! All for <120mA of current draw (D-FF and MCU logic is always powered). Is there really a big standby power savings to pulse T1 in this way?When the main power transformer is turned on via relay RY1, relays RY250+RY251 switch from lower voltage secondary taps to progressively higher voltage windings. That's how the main amp bulk caps 2*12000uF@64V (C359,C363) are slowly filled. If OP strips this chassis of its amps, he should implement some delay logic to do same (R-C logic or 555s maybe).

author: xavier60
author_rank: Super Contributor
timestamp: December 03, 2023, 03:34:45 am
content: Tap switching on the secondary side is usually involved with the speaker impedance switch.

author: Toomasu
author_rank: Newbie
timestamp: December 03, 2023, 09:19:28 am
content: Over engineered or not, at least a lot of thought obviously went into this. The complexity of these things is above my knowledge but I do appreciate the discussion. Every little bit of new information helps a mere hobbyist like myself learn.As pointed out the transformer secondary has three stages and indeed when changing speaker impedance in menu, amplifier drops rail voltage. This I have confirmed by measuring, but it only shifts between two levels. I have not measured on power on though so it might as well do a stepped power on sequence.Or is it possible that there is some kind of power reducing safety function. The amp input stage rail voltages is controlled by some regulator transistors - see attached. If Q277 base gets zero voltage, input rails drops to 17V or thereabouts. My thinking that if for instance temperature goes over some threshold that the rail voltages is reduced linearly on input rails and steps on VAS/power rails.

author: xavier60
author_rank: Super Contributor
timestamp: December 03, 2023, 09:31:59 am
content: Q277 and Q264 are a differential pair that compares divided +LB to the 4V reference on the Base of Q277 for the regulation of +LB.What controls the reference voltage?

author: Toomasu
author_rank: Newbie
timestamp: December 03, 2023, 09:55:20 am
content: Q277 base reference is named LMTCNT and goes to a pull up controlled by IC402 main controller pin 3. Pin 3 is simply called LIMIT. So naming of these pins gives away intent I feel

author: xavier60
author_rank: Super Contributor
timestamp: December 03, 2023, 10:07:08 am
content: Looks like the idea is to reduce the peak amplifier output to something much lower than rail voltage but for what circumstance?

author: Toomasu
author_rank: Newbie
timestamp: December 03, 2023, 02:27:14 pm
content: Se attached from service manual. Seems to be a link to the monitoring of total amplifier output where it saysOUTLVL (Power limiter output level)Total value of amplifier output of each channelThe voltage at 141 pin of IC402 is displayed with 5V/255as a standard.As a means to keep total power under "permitted" level but still keep amp operating?

author: pqass
author_rank: Super Contributor
timestamp: December 03, 2023, 11:12:37 pm
content: According to the MCUdatasheetpin 3 ("LMTCNT" or "LIMIT") can be a DAC output and pin 141 ("OUTLVL" or "PLMT") can be an ADC input.Assuming "LIMIT" is a DAC output, then a 0-5V will vary the output voltage of the +LB/-LB power supply from ±12V to ±62V.See simulationhere(use slider on right to adjust) and attached screenshot.FYI: I made the power transistor hFE=25 in the simulation but it's actually 500 as per the2SC3852 datasheet(not a darlington apparently). Yet this can't support more than 200mA still (changed the 100R to 5R). I guess simulation ≠ reality or maybe that's all the input stage of the amps need. I dunno.

author: xavier60
author_rank: Super Contributor
timestamp: December 04, 2023, 01:09:02 am
content: The amplifiers' VA stages would only draw a few mA.Maybe the limiting is for when headphones are used.

author: Toomasu
author_rank: Newbie
timestamp: December 04, 2023, 06:57:48 pm
content: Thanks for simulation! I actually confirmed function a few days ago when I made a voltage divider on that T1 circuit, so it works just as you simulated. I really enjoy those Falstad sims. I did the whole amp section a while back, mostly for funhttps://tinyurl.com/ywee6rkgCould this be a good time to make a new thread as a project? Seeing this thread has gone way past my original question. I really do appreciate all your feedback and knowledgeI am currently trying to find out what to do with protection now that IC402 is removed. I have a few unused Arduino boards so maybe that would work, for all the ON/OFF delay stuff, protection etc. But that means learning coding from scratch, since I have no experience