Yamaha A-S701 Stuck in DC Protect but no DC at outputs when in bypass mode - Page 1
Forum section: electronics / repair

author: sn00p21
author_rank: Newbie
timestamp: September 21, 2025, 05:58:31 pm
content: Hi everybody,I've recently came into ownership of a Yamaha A-S701 for very cheap, under 50 dollars. I got it home and upon start up got a solid orange then after a couple of seconds a click and a constant blinking orange. I found out about pressing SW501 and doing this gives me 4 quick orange blinks for DC Protect. After thoroughly inspecting the board and not being able to find any obvious damage, popped caps, burn marks, I decided to try to bypass protect mode by holding sw501 for 3 seconds. This worked resulting in the flashing "Line 1" I had gator clips on the emitter resistors, Lch pair measured .4mv while the Rch measured .7mv(repair manual says .1-10 is in spec). After this I decided to go ahead and switch the speakers knob from off to A then B while taking measurements from the speaker posts. All of them were sub 10mv DC. I then decided to check power to output transistors and they all measured a steady +/-44V.Unless I'm missing something here it seems to have failed in a rather uncommon way? I'm having trouble finding anyone else describing these symptoms. Seems like somebody almost always finds DC in one of the channels when it is tripped. Perhaps IC502 or the DC detect circuitry itself? Something else that I cant wrap my head around is the repair manual seems to state that it will not allow you to bypass protect mode if DC Protect is tripped but holding SW501 definitely makes Line 1 blink and sends power to the output transistors.Right now my goal is to identify the DC protect circuit and see if one of the components have failed. Any help is much appreciated, I'm willing to drop a decent chunk of money and time to getting this running.Edit Also it should go without saying that I do not have anything hooked up to the amp besides the AC power cable. And never did. I also have tried to do the "factory preset" by switching speakers to A then powering down the device however this does not fix anything. but when doing it does take a few more seconds before the click and flashing begins. If anybody has encountered this or has any ideas, I'm open to anything. I can kind of read of the schematics, but am unable to trace back from pin 89 IC502 to where the DC detect circuitry is. If somebody is familiar with these and could tell me the component numbers to look for I'm sure I could find them. I am pretty handy with a MM and can also use a soldering iron decently well. Again any help is much appreciated.

author: David_AVD
author_rank: Super Contributor
timestamp: September 21, 2025, 08:49:24 pm
content: If it's like any of the other A-Sxxx series I've repaired, it may have resistors gone bad (high in value) in the fault sense lines.The sense for DC protect is called PRD in that model and comes into the board with IC502 on it via pin 1 of connector W505.The PRD line comes from pin 1 of CB109 on the amplifier board.The manual says it's value should be between 0.947V and 2.517V DC (nominally 1.5V DC) when operating normally.What voltage do you measure there when in diagnostic mode?

author: sn00p21
author_rank: Newbie
timestamp: September 22, 2025, 01:40:11 am
content: Hello! Okay I measured both from CB109 and W505 both are 0.08 volts in bypass mode, triple checked. Is PRV also related? It’s between W502 and CB14 in the repair manual. Also a jumper for it from CB13 to W104, if this is relevant(reads protection voltage to me, thought I found it before your comment) the voltage is 2.93. Did Yamaha label one PRD and then PRV? Should they be off this far?Should I disconnect w505 and then measure PRD from each board?

author: David_AVD
author_rank: Super Contributor
timestamp: September 22, 2025, 08:38:28 am
content: As per the manual, PRV is the power supply voltage monitor line and is not related to the PRD line which monitors for DC on the amplifier outputs.Your reply is a little confusing. Did you measure the voltage between 0V (amplifier chassis) and pin 1 of connector W505?

author: sn00p21
author_rank: Newbie
timestamp: September 22, 2025, 11:49:47 am
content: My apologies. Yes I measured PRD at pin 1 on w505 relative to chassis ground it was 0.08 volts in Bypass.I was saying I measured both from connector w505 and cb109 to chassis ground to make sure both gave the same value and there wasn’t an issue between those two boards or connector! And thanks! I guess I missed the part on PRV in the manual.Edit: I reviewed the manual, PRV is very clearly for the PSU nothing to do with DC protect, no clue what I was thinking.I measured PRD resistance to chassis ground with the unit unplugged and got ~500 OHMs. My understanding is this should normally be much higher i.e tens of kiloohms. So something in the DC detect circuit is dragging PRD low toward ground. Could this point to C133 beaing leaky or one of the three small transistors on the underside of the board sinking it? There's also the R185/86 193/94 and 210/211 resistors back there that seem to be apart of the DC protect line? Are these the high value resistors you mentioned earlier or am I mistaken?Again sorry for the confusion with PRV and not specifying I measured PRD to chassis ground in bypass mode. Thank you!editing again, From the schematic C133 looks like its just a filter/bypass cap to ground on the PRD line, meant to smooth the DC detect signal before IC502 reads it. When I check it in diode mode it holds a steady ~.28 volts instead of dropping to OL. Is that normal in-circuit? Or could it suggest that it is leaky? If I cut one leg of C133 would I still be able to read a PRD value? If I cut then PRD went into normal range that'd suggest c133 is in fact bad, no? Is that a valid test to do here?still editing, Pretty sure I'm correct in suspecting c133, a healthy cap should either do the initial jump then fall to open or should rise to .7/.8 volts relative to ground in diode test mode depending on how its wired in. c133 wired to ground should behave with the jump and fall, 99% here correct me if i'm wrong. I should be able to make a jumper from c133 and put a healthy cap on the other end then if PRD comes into range that confirms it, no?fourth edit, I did end up cutting c133. The results of this were PRD dropping to negative .32 from the aforementioned .08v. PRD to chassis ground resistance went from 500 to 680 after cutting c133, not sure if this is notable. No biggie I guess, I should be able to solder the leg back together or just add one further down before PRD makes it to IC502.Now frustrated I tried to see if I could force prd to pull up to ~1.5v, I did this by using a 2.2k resistor from pin 9 on cb109 to pin 1. This only managed to pull up PRD to +.32v. Something must be shorting very hard in the DC circuitry correct? I'm probably going to disassemble and begin lifting transistors to isolate sides and see if PRD climes into range when one is lifted. If I'm missing something or going in. the completely wrong direction please let me know!

author: David_AVD
author_rank: Super Contributor
timestamp: September 22, 2025, 07:22:43 pm
content: Make sure you do the resistance measurements with it powered down.Check for corrosion that could be shorting PRD to 0V.You should be able to trace which board the short is by disconnecting PRD at pin 1 of CB109.

author: sn00p21
author_rank: Newbie
timestamp: September 22, 2025, 08:30:21 pm
content: Yes all resistance measurements were taken with the device powered down and discharged. With CB109 removed I am getting 58.4K Ohms between cb109 prd and chassis, and 600 ohms between w505 prd and chassis. So this suggests problem is actually in the function board containing IC502 if the expected resistance should be in the tens of kilo ohmms?I was not able to remove only the PRD pin, it was not that style of connector, had to take the whole head off.Is this likely an issue with micro controller or a component on the board? I will begin tracing the PRD path back to IC502 and inspect traces and components, on the line. Thank you for the help.I've identified C532 and D502 as two components along the PRD trace and will attempt to take resistance measures relative to ground from both sides of them to see if I can further narrow this down.

author: David_AVD
author_rank: Super Contributor
timestamp: September 22, 2025, 09:04:49 pm
content: The PRD line goes from pin 1 of W505 via R515 (1K) to pin 89 of the micro.The fact that you're measuring less than 1K to 0V at W505 suggests that the short can't be on the micro side of R515, ruling out C532 and the micro pin 89.Remove and check D502 (5.1V zener diode). If the short is still on the board after removing D502 then corrosion to 0V is the other thing to check.

author: sn00p21
author_rank: Newbie
timestamp: September 22, 2025, 10:04:31 pm
content: Okay I just got the function board out. Do these analog signal cards come off easy? I cant seem to get the connectors to release and they wont slide out like a pcie card. Would be a lot nicer to solder on without those in the way. Anywhos I will work on getting D502 lifted and testing.

author: David_AVD
author_rank: Super Contributor
timestamp: September 22, 2025, 11:51:36 pm
content: The A-Sxxx series can certainly be a bit fiddly to work on.

author: sn00p21
author_rank: Newbie
timestamp: September 23, 2025, 02:15:33 am
content: Hey David, I'm back. And yes these seem rather complicated(over engineered?). I've repaired DVRs when working at an ISP those were much less complex. Mostly fixing HDMI ports on those.Anywhos, with the function board removed I once again measured PRD to ground. At this point I can no longer use the chassis screw I was using for ground as the board is removed, so the new ground or 0V reference is the pin labeled mg on w505. PRD to 0V read ~10k ohms this way. After removing D502 it jumped up to ~60K ohms. So I believe you are correct sir.I also went ahead and put the MM into diode and shot voltage throughD 502, out of circuit. One way reads the correct ~.7V but the other way reads ~.25V instead of OL so pretty much confirms it. Any other tests I should do for further confirmation? Anything else I should replace for reliability reasons while I'm in here? Should I order a generic replacement or is there a high quality supplier/manufacturer I should source from? Thank you!Edit if I’m understanding everything correctly this diodes function was to pull PRD down toward ground when 5V or more DC was at the speaker posts. It became leaky and was shorting to ground at low voltages thus the odd PRD readings? That’s basically what happened no?https://www.digikey.com/en/products/detail/diodes-incorporated/BZT52C5V1-7-F/717736is this what I want?

author: David_AVD
author_rank: Super Contributor
timestamp: September 23, 2025, 09:13:48 am
content: Sounds like you've found the culprit. That diode should be fine.