Anders K Nelson

For several years now, my Marantz SR8500 A/V receiver has randomly refused to power on from standby, and occasionally kicks out of operation. I think it has something to do with the ambient temperature and humidity, which has been out of control this year in NYC.

Let’s see if we can fix it.

Background

My only datapoint to work from is a brief flash of the words “CHECK POW5” on the receiver’s display. I found a blog post that describes this as when the receiver goes into “protection mode” which can occur for multiple reasons.

Procedure

The service manual:

I’m looking at the 5v power rail first because that’s what the error message on the display is complaining about. There’s a 5v regulator (LDO) on the standby PCB and the first thing I noticed is a flat ribbon cable was resting against the LDO’s heatsink tab. There was some soot around the contact point, but the conductor within the ribbon cable potentially making electrical contact is listed as “N.C.” so I don’t think that’s a problem.

The schematic shows three 5v supplies: +5VL, +5VD and +5VV. They all come up and hold steady. I then look for a higher-voltage DC supply in case the 5v supply on other boards is generated locally. Sure enough, there are +15V and -15V supplies on the power PCB and I can see local 5v LDOs on several other boards. -15V comes up (down) as expected, +15v makes a lame attempt to hit +1v and sinks to nothing.

A check of the +15V network in the schematic shows no obvious barrier to hooking a bench supply to the system here, so let’s put +15v on the +15V supply and see what happens.

Aw yea. Cool, we’ll just replace IC63: an MC7815.

IC63 is bolted to a big heatsink and directly in front are 4 huge capacitors plus a huge rectifier also bolted to a big heatsink so it’s nearly impossible to remove it. What DFM (Design For Manufacturing) troll decided this was a good idea? Was this done specifically to prevent repair? But wait; things are about to get much more difficult!

To get the power PCB out of the enclosure, we have to:

1. Remove the main transformer and disconnect its 3 cables.
2. Disconnect a front panel cable.
3. Remove the top bracket on the amplifier assembly.
4. Unplug 4 cables from the amplifier board. 1 is effectively inaccessible by hand.
5. Remove the surround back left/right board and disconnect its 3 cables.
6. Remove 3 large screws at the bottom of the amplifier assembly, 1 of which is occluded by several components.
7. Holding 2 cables out of the way, remove the amplifier assembly.
8. Pull the power PCB and standby PCB assembly out like 3mm. Using a plastic stick, lever the standby PCB off the small/fragile connector it sits on. Pull the power PCB out the rest of the way, but not too far because it’s still connected by cable to boards in the other half of the enclosure.
9. Flip the power PCB over and put a non-conductive barrier over the huge output caps so you don’t die when you drag your arm across their terminals.

Each step also requires us to snip a plethora of zip-ties. Anyway let’s order replacements for all the LDOs in case we have another failure down the road (they’re cheap):

• Power PCB, IC63: MC7815 -> MC7815ACTG.
• Power PCB, IC64: NJM7915FA.
• Power PCB, IC61: KIA278R05PI -> KA78R05CTU
• Power PCB, IC62: KIA378R05PI -> KA78R05CTU
• Standby PCB, IC85: NJM78M05.

To save a ton of effort, I attempt unscrewing the heatsink bolt with a pair of pliers.

Aw yea. Let’s toss in the new part and see what happens.

Aw yea, now let’s put everything back together!

Moral of this story

No matter how huge the task, no matter how insurmountable your fear – if you can take apart a Marantz product and remember where all the screws go, you can do literally anything.

Gallery

Resources

• http://larrynz911.blogspot.com/2012/02/marantz-sr7500.html
• https://www.avsforum.com/forum/90-receivers-amps-processors/604758-marantz-sr7500-protect-problems-amp-shutting-off.html
• https://www.youtube.com/watch?v=vA5il0m_has
• https://www.manualslib.com/manual/701768/Marantz-Sr7500-A1b.html#product-SR8500