Marantz Model 25 Project

[bdondo]

I bought a Marantz Model 25 last month and it actually sounds and looks great, so I've been using it as my main system while I finish work on a couple other receivers. But this thing is supposedly from around 1969-1971, so it surely needs to be re-capped at minimum. I'll document the process here in case it's of any use to someone in the future, and for when I inevitably need help troubleshooting.

My parts list, which might not have ideal choices in part due to already having some capacitors on hand. I.e., I didn't try to figure out if certain caps should be "audio grade" vs "low ESR". https://docs.google.com/spreadsheets/d/1E6sDrUQtPXEu3vryZJbCwK78BvqNSiAIxJc6FD-hLWw/edit?usp=sharing

A couple photos of the unit as it was received:

 

[bdondo]

First up, I replaced the lamps, which was super easy (despite my ignorance). The bulb is a standard 6V BA9. I might replace with LEDs since the lighting is still pretty dim. But I didn't check if there's a paper thing (forgetting the name right now) to replace as well. I'll check that later.

 

[bdondo]

Next up is re-capping the power supply board. Here's a before picture. The cap in the upper right has no marking on the board, and the one off the edge on the side isn't listed in the parts list.

 

[bdondo]

I taped up the sides instead of trying to remove the wood casing. Not sure if that's the best way or not. On a side note, this receiver has the nice advantage of easy access from the bottom for soldering. Or in the case of this unit, seeing which PCB traces were intentionally cut and/or jumped with wire. It's a bit of a mess.

 

[bdondo]

Power amp (P600) done and working. All the caps' capacitance measured way off. One small one (C602) was completely open, at least after desoldering and removing.

 

[bdondo]

Next up is the main amp board, which has zero electrolytic caps, so I guess I'm done.. unless I should replace any of the transistors:

2SA562-O
2SC371-O
2SC497-R
2SA497-R

Some of the big white resistors look a bit rough, so I might replace those.

 

[bdondo]

Next up will be re-capping the tone board (P400)

 

[Lavane]

Next up is re-capping the power supply board. Here's a before picture. The cap in the upper right has no marking on the board, and the one off the edge on the side isn't listed in the parts list.

I would alao replace those 24 ohm resistors. Those fuzzy green leads don't look very healthy. I've acually had that type crumble in a Marantz amp I worked on. <edit> I see you mentioned that in a later post. Those two resistors on the two main filter caps got pretty warm at some point. Maybe go up a few watts with new ones? You can replace any small value electrolytic caps in the line stage and phono stage and amp board with something like Wima polypropylene stack film caps. Except for the styrene caps on the phono board of course.
I have a soft spot for those early Marantz low power receivers.

 

[bdondo]

I would alao replace those 24 ohm resistors. Those fuzzy green leads don't look very healthy. I've acually had that type crumble in a Marantz amp I worked on. <edit> I see you mentioned that in a later post. Those two resistors on the two main filter caps got pretty warm at some point. Maybe go up a few watts with new ones? You can replace any small value electrolytic caps in the line stage and phono stage and amp board with something like Wima polypropylene stack film caps. Except for the styrene caps on the phono board of course.
I have a soft spot for those early Marantz low power receivers.

Thanks, I'll do that. There are some on the main amp board that are crumbling a bit as well.

 

[bdondo]

@Lavane - would wire-wound resistors work as replacements for the various crumbly 2-watt resistors? That's all I can find for the 0.24 Ohm resistors on the main amp board.

BTW, I did get some replacement resistors for the filter caps, and new filter caps (which will look funny since they're way shorter). I did bump the wattage on the new resistors.

I'm using Wima film caps in a lot of places to replace the small electrolytic caps.

As for "Except for the styrene caps on the phono board of course" -- are you referring to the caps like the one in the red box in this photo?

 

[Lavane]

Wire wound would be good. You could go %1 precision wirewound also, but not really needed here. I stupidity passed on one of these receivers for free years ago. I don't remember if it was a model 25, 26, or 27, but had the cheesy BSR turntable on top. I really wish I had snagged it thinking back.
It's easy enough to remove the crappy turntable. And yes, unless bad, leave those little silver foil in plastic caps alone.

 

[bdondo]

Wire wound would be good. You could go %1 precision wirewound also, but not really needed here. I stupidity passed on one of these receivers for free years ago. I don't remember if it was a model 25, 26, or 27, but had the cheesy BSR turntable on top. I really wish I had snagged it thinking back.
It's easy enough to remove the crappy turntable.

Thanks! Yeah, I got this for about $250 shipped on the 'Bay. Surprised how cheap it was. I plan on putting my turntable on top of this receiver's top board. It fits rather well.

 

[Lavane]

That was pretty cheap compared to the crazy prices being asked these days. A drop of fader lube in each pot after cleaning will help to keep them quieter longer and track more smoothly.

 

[bdondo]

Tone board (P400) done.

 

[bdondo]

Well, I spoke too soon about the tone amp board being done. I hadn't tested with speakers when I posted. I definitely botched something -- all I get is a loud hiss out of the right speaker and nothing out of the left. The hiss does not change when I move the volume control. It exists on all modes (Aux, fm, am, phono). If I crank the volume in AM, I get s faint signal.

Moving the treble control changes the tone of the hiss (makes it more trebly, etc.). Moving the bass control seems to affect it too -- I get a "microphone" sound when I turn the bass knob. The pre-amp circuit (P400) on this amp sends the signal into a tone board (P900) and then the tone board sends it back to the pre-amp.

DC voltages all look good on P400.

I certainly don't want to ruin this receiver, but I have to admit -- troubleshooting is the fun part. It was difficult removing solder in some cases, so maybe I overheated something. Or maybe a wire came loose somewhere.

 

[bdondo]

And.. fixed. Had a poor connection between C415 and J406 and a less-than-ideal connection in the other channel.

 

[Lavane]

Nice! I put a signal transistor in backwards in a Marantz 1030 once. The pinouts of the replacement transistors were backwards from the original and I put one in wrong. That's why it's a really good idea to replace parts on one board at a time, then fire it up to make sure its all good before going further. I've seen plenty of threads where someone just shot gunned parts on all the boards, then fired it up only to find something went wrong. It's way easier to fix the problem when you know what board to look at.

 

[Lavane]

I see that the model 25 has the internal phono jacks on a switch. A built in turntable must have been an option with this model too.

 

[bdondo]

Yeah, there were a few models of turntable that could be mounted on top and wired into the receiver. Kind-of cool I guess, but I'm glad that part is optional.

 

[egellings]

First up, I replaced the lamps, which was super easy (despite my ignorance). The bulb is a standard 6V BA9. I might replace with LEDs since the lighting is still pretty dim. But I didn't check if there's a paper thing (forgetting the name right now) to replace as well. I'll check that later.

Looks like a #47 bulb to me.