Yamaha S-300 Amplifier Flashing LED error - Page 1
Forum section: electronics / repair

author: tigerstyle
author_rank: Contributor
timestamp: October 05, 2025, 01:07:19 am
content: Hi,I have this amp I'd like to repair, a Yamaha S-300, the manual is here:https://www.manualslib.com/manual/1074982/Yamaha-A-S500.htmlI've seen a few threads around that are similar, herehttps://www.eevblog.com/forum/repair/yamaha-a-s500-amplifier-dc-protection/My fault is different though. When powering on the amp with no inputs/outputs connected, so no shorts or other faulty equipment, I hear a small relay latch on the 240V input PCB and the red power LED starts flashing. Amp will then do nothing but flash the LED. I have followed the manual and entered service mode and it shows 5 flashes of the LED, which is 'TMP Protection L/R CH' I read this as Temperature protection, but odd as it faults when the amp is cold and definitely not hot. The manual also doesn't use the wording TMP anywhere else, but a few pages down (Pages 23 and 23) it explains THM Protection, I thought maybe something lost in translation as in Japanese the same wording is used for TMP and THM protection.So I found from the manual there are 2 small PCBs that have the temp sensors on them and checked those PCBs, when I power on the amp I measure 5VDC for a second across VCC and GND but then it drops to zero, so I believe for a split second enough of the amp is working to power on, but it senses the fault immediately and shuts off.There is the 240V input PCB 'Operation 1' that I have removed the main transformer connections from to remove those from fault finding, and there is a single connection from Operation 1 to the the main PCB 'Function 1' On Function 1 I can still measure a steady +5VDC on the PCB at the connector, which makes sense as I guess that is the board with the main chip telling the power LED to flash.I have a DMM and soldering iron and basic skills, not sure where to go next, nothing looks broken, no traces shorted, no caps looks bad, hoping to be able to at least find what the fault is even if a repair is uneconomical.

author: David_AVD
author_rank: Super Contributor
timestamp: October 05, 2025, 09:18:53 pm
content: What voltage is on the output of each thermal sensor PCB? From the schematic it suggests they should be 1V.You can measure the THML and THMR voltages at pins 6 and 7 of CB506.Also, is there any corrosion on the boards? I've seen issues when a trace or via is corroded (broken).

author: tigerstyle
author_rank: Contributor
timestamp: October 06, 2025, 04:01:11 am
content: Thanks for the reply,The PCBs are all slightly dusty, but nothing looking too bad, I'll blow it out with an airline tomorrow just in case anything small is hiding in there.I've measured at CB506, I don't get anything at THML and THMR, I think this is connected to the same issue as I put in the first post, it's the same circuit, I only measure 5V at the THML and THMR PCBs for a split second when first powering on then it drops to zero. So with nothing coming in nothing will be coming out. On the output of the Temp sensor on THML and THMR I only measure below 1V for the same split second too, it isn't like there is a short there and it pins to 5V straight away causing the unit to fault.On CB506 I measure (to ground)PREMT - 4.12VSWMT - 4.89VNothing else there gives an output.I saw in the manual and with nothing to lose there is a 'Protection Cancel Mode' so I tried that. and got a good result.With this activated and measuring again at CB506 I got some voltages within spec (and some not!):Right channel - Pin6 - 0.0149VLeft channel - Pin7 - 0.9VSwapped the pcbs over left to right and now:low voltage followed the PCB, so something on that PCB must be dead/corroded/faulty.Good thing is it is a very simple PCB so can maybe jig something together from just bits of wire to fix.

author: tigerstyle
author_rank: Contributor
timestamp: October 06, 2025, 05:45:45 am
content: Only have basic skills here... But measuring with a DMM across the different legs of the temp sensor results between both of them seem the same, so I might assume the temp sensor is ok, is there a better way to check this? I looked in it's data sheet and I guess they don't have these type of rudimentary checks mentioned in them!So it could just be the capacitor that is faulty in the one circuit. Or some tiny corrosion in a trave I can't see. But will try to remake circuit from breadboard or similar test that.

author: David_AVD
author_rank: Super Contributor
timestamp: October 06, 2025, 07:16:16 am
content: The Yamaha temp sense boards do tend to attract dust (which wicks moisture in) and I've encountered corroded (and open circuit) traces on them a few times. Clean them up with some IPA or similar and check continuity of each trace from the connector to temp chip. Even if only one trace is open circuit, link across them all to solve future issues. Do this for both temp boards.