Marantz SR-18U receiver repair

[mikepukmel]

Hi All!
All my guitar amp projects are still pending (frown), work life balance is about 1/0. Maybe 1.2/0. Relaxing after dinner, browsing the web, found someone giving away an old Marantz SR-18U receiver, in non working condition, only about 45 minutes away. Being a sucker for such things, I picked it up. All I can say is: its not a Fender Champ.
Fished around the net, and downloaded the service manual. Also found a few youtube videos with clips of repairs.

Got a question for the electronics gurus out there. Luckily, this one doesn't have a switching power supply. So, plug in the unit, hit the "on" button, few seconds the display shows "ERROR PWR2", then it shuts off. OK looked through the entire manual, it has a fairly good diagnostics section for "ERROR PWR1" but not a thing for "ERROR PWR2".

Poked around, and it looks like these units are famous for small parts in the power supply/regulator going flooey. Since its got a honking CPU, it has circuits to check voltages and such and shuts itself off before it does something terrible.

I *think* what I need to do, is start at the transformers (there are two), and start measuring voltages. At the output side of the transformers (there are a few taps), then, around where the regulators are located.

Not sure, but I think I can unplug one end of the cable leading out of the power supply regulator boards and measure at the pins. One guy on youtube suggested to scope these as well since they could oscillate and you wouldn't see it with a voltmeter.

Looks like a fun thing to tinker with, and if I could fix it, even better, since the sound quality isn't quite to spec right now. (yeah, like I need another project).

Thanks!
MP

[The Dude]

Since you have the service manual, can you upload it, or is it too large?

[Enzo]

ANything can fail, but of all the parts in the thing, the transformer is THE most reliable one.

Does it have discrete transistor output, or does it use some chip amp? Should be snap to find the two main power rails. Is either shorted to ground? Is one shorted to the other?

SInce the display comes up and makes messages, I must assume 9working assumption nly) that your digital supplies are intact. +5v and likely +3.3v.

[mikepukmel]

Here's the manual I found. Its a beaut, ain't it? I can even read some portion of this one. Now all I have to do is work up the nerve to measure stuff.

Marantz-SR-18-Service-Manual.pdf

[mikepukmel]

ANything can fail, but of all the parts in the thing, the transformer is THE most reliable one.

Does it have discrete transistor output, or does it use some chip amp? Should be snap to find the two main power rails. Is either shorted to ground? Is one shorted to the other?

SInce the display comes up and makes messages, I must assume 9working assumption nly) that your digital supplies are intact. +5v and likely +3.3v.

Thanks Enzo, and this thing is a monster. I think 25% of the weight of this thing must be due to that.

This model, as far as I can tell, is a little more toward old school: linear power supply, outputs are discrete transistors, lots of discrete transistors on lots of boards.

The manual, as far as I can made out, is great. It has block diagrams, wiring diagrams, schematics, and great circuit board layout images, and complete parts list.

its the most complicated thing Ive ever seen. i think it has more parts inside than some televisions.

EDIT:
Last night I worked up the courage to open the case. Will grab some photos. The power supply boards look readable!

[mikepukmel]

Worked up the nerve to start measuring stuff. Started with the AC leading in to the two power supply boards.

On Schematic Measured
===============================
20vac 16.6vac
across both legs 33.2vac

Output of this board is a 6 pin connector. The connector is still "in circuit" since its soldered on this end.
On Schematic Measured DC
===============================
24v 21.0v
12v 12.2v
gnd
-24v -14.9v
15v 16.36v

Measured across the AC legs and DC legs on the bridge
33vac
42vdc

Nothing burned, no burn smell, 2 electro caps don't look to be leaking.
Maybe should try to find and disconnect the other end of the cable.

With my ,limited knowledge of this stuff, it looks like the 24v output is toast. But not sure if something downstream is hosing that up?
12v looks good afaik.
Not sure of 16.36 is in spec for 15v.

board solder joints look very good. Nothing disgusting in there, like a spilled soda or cigarette ashes.



On the other power supply board (looks like this is the audio amp supply board since it has gigantic electrolytics
On Schematic Measured
===============================
62vac 46.8vac
measured across both legs 93.5 vac

attached a page of the schematic for this board.
Hmmm.

[mikepukmel]

it has about 33vac into the bridge, and about +/-20v dc before the first two filter caps. Don't understand why 24V is marked on the schematic, since it has to be closer to +20v. The measured number looks ok. ?? But the negative side -24v which reads -14.9 that doesn't look right.?

[mikepukmel]

Anyone have any experience with cheap ESR meters like this one? https://www.ebay.com/itm/16420979409...MAAOSwb2hdaAzJ

A BK Precision is only about 40 bucks more. But, its 40 bucks more. The purple one looks kinda crappy.

[g1]

Anyone have any experience with cheap ESR meters like this one? https://www.ebay.com/itm/16420979409...MAAOSwb2hdaAzJ

DE-5000 will do more for less money.

https://www.ebay.com/itm/22425388929...Cclp%3A2334524

[g1]

As far as those DC supply voltages, check the ripple level with your meter set to AC volts.

[mikepukmel]

DE-5000 will do more for less money.

https://www.ebay.com/itm/22425388929...Cclp%3A2334524

Cool, thanks G1!

[mikepukmel]

As far as those DC supply voltages, check the ripple level with your meter set to AC volts.

Thanks, will do. Even looking for a reason to crank up my old scope, haven't turned it on in ages.

[g1]

There's a bit of discrepancy between the schematic and block as to 20 or 24V, but -15V definitely looks to be a problem.