Amplifier with funny relays and occasional hum - Page 1
Forum section: electronics / repair

author: 230volts
author_rank: Newbie
timestamp: July 27, 2024, 10:30:18 pm
content: I have a Marantz SR7000 amplifier that has recently developed some problems.Normally when powering up the amp, there would be an initial clicking sound, a pause of 3-4 seconds, followed by a second click when the amp would start outputting amplified sound. In April 2024, when powering up the amp, it started taking more than the usual 3-4 seconds for the relays to click into place. Once powered up, the amplified audio was perfect and there was no hum or crackle. I disconnected the amp and put it aside for a few months.When I powered up the amp this morning (it's 12° indoors), it took a full 40 seconds before the second relay clicked and amplified audio started coming out. However, after a minute or so, there was another click and the audio stopped. A bit later, the relays clicked again, and a quiet 200Hz hum came out as well as a momentary white noise/crackle which disappeared after a couple of seconds. After turning the amp off then on again, the audio was perfect, however after a few minutes it stopped again, only to restart after half a minute or so.Apart from the speakers and AC power, the amp is connected only to a TV using a TOSLINK optical cable - nothing else is connected.The amp is is powered through an IEC male plug with no earth pin. It's currently winter, but in the summer it gets very humid.Service manual:https://www.manualslib.com/manual/405468/Marantz-Sr8000.htmlUser guide:https://www.marantz.com/on/demandware.static/-/Library-Sites-marantz_northamerica_shared/default/dwaaab242d/archive-downloads/sr7000_om_scan.pdfI have attached photos of the IC connected to the mains plug and some schematics of the same board from the service manual.UPDATE: The hum/intermittent audio is exactly the same when headphones are connected, i.e. it still takes a long time for the relays to click into place and for audio to be heard. The same hum heard through the speakers is also heard through headphones.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 27, 2024, 10:56:02 pm
content: Probably the speaker protection circuit, which controls the relays, is detecting a DC offset on the output of one or more channels and is shutting it down.

author: 230volts
author_rank: Newbie
timestamp: July 28, 2024, 10:51:32 pm
content: Any idea on how I could verify whether this is the case? My first thought was that the relay is faulty because once I have managed to get the amp started, it usually runs mostly fine, except for the occasional shutdown, which a restart fixes.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 28, 2024, 11:48:15 pm
content: Quote from: 230volts on July 27, 2024, 10:30:18 pmIf you are referring to the white 'donut' near the mains input, that is a ferrite ring core for RF interference suppression and nothing to do with your problem.First a good clean is needed to remove the dust/dirt. Use a new soft bristle paintbrush to gently dislodge the dust build up while holding a vacuum cleaner nozzle nearby etc.Next I would remove each connector and reseat in turn as this will create a wiping action on the contacts, in case there is corrosion build up.If there is still an issue, next would be to look for dry/cracked solder joints on the PCB's. Of course this means disassembly to get to the solder side of the PCB for inspection.What's your level of skill in electronics ?, think about what you are doing inside this unit and consider safety first. Lots of exposed mains terminals make this a dangerous unit with covers off (plugged in of course).Make sure the unit has been unplugged for a day to allow all large supply capacitors to discharge before checking for bad connectors or solder joint issues.

author: 230volts
author_rank: Newbie
timestamp: July 29, 2024, 01:47:23 am
content: Quote from: lowimpedance on July 28, 2024, 11:48:15 pmI was referring to the PCB PB54, which is the small board whose schematics I attached. The service manual contains the full schematics for all the different boards that make up the Marantz unit. Notably, the main power amp board P704 also contains relays to switch the different speaker outputs.I have a multimeter, handheld scope and soldering iron. I'm not an expert at clean soldering/desoldering, but I can get done in a pinch.Thanks for the advice. I've attached a few more schematics that may be relevant.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 29, 2024, 02:17:24 am
content: Quote from: 230volts on July 28, 2024, 10:51:32 pmRE: pg-11-P704.png (5ch Power Amp P.C.Board)Check that the DC voltage on each of the outputs of all 5 amplifiers settles to near zero volts after power up.J702 pins 1,3, & 4 (Left Front, Right surround, Center)J703 pins 1 & 2 (Left Surround, Right Front)If not, this is probably what's tripping the protect circuit relay.

author: coppercone2
author_rank: Super Contributor
timestamp: July 29, 2024, 02:50:18 am
content: Funny? Funny how?

author: jonpaul
author_rank: Super Contributor
timestamp: July 29, 2024, 11:06:01 am
content: Mute Dealy:230volts:The vinatage Marantz, APT, and other amplifiers have a time delay mute that uses a comparator andlong time constant R-C circuit.The timing lytics typically change cpapvcitance and leakage as they age in the hot environemnt.Symptom is slow mute release on turn on and then lack of mute relay release.Also the mute relay contacts get oxidized as they handle small signals as well as power audio.We go thru these classics anda/ Find the mute R-C, replace the lytic or tant.b/ Open the relays and clean contacts.Funny Relay: Coppecone: BRAVO for the fine quote from the 1990 Martin Scorcase film "Goodfellas"https://www.imdb.com/title/tt0099685/Tommy fakes out Henry Hill:"You think I am funny? Funny how?"Enjoy!Jon