Marantz MA500 thread

[Bratwurst7s]

Installing the 35mm diameter main filter caps is a real challenge. Normally I do not drill boards if there is any other way around the problem. Normally I solder 1.5mm solid core leads to the lugs, bend them to the hole position and glue 35x4mm rubber o-rings to the bottom of the cap as a spacer. This usually works real well but in this case with the 63mm tall caps and a max of 66mm available space that didn't seem to work out.

Note that I am now going to jump around a bit between the amps because as it turns out both of the '97 amps had the original Elna For Audio caps replaced with Cornell Dublier CDM series 10000µF/80v parts, 35x51mm. So those boards had already been drilled, and not very neatly. The '95 and '99 amps had Elna For Audio caps installed, but the later ones were shorter than the earlier parts. Sadly I failed to measure their respective heights.

So really, with 3 of the 4 amps having been broken or drilled, drilling the 4th hardly mattered. But in general principal I don't like to do it.

Cheers,
James

 

[Bratwurst7s]

I thought that I had taken a lot of shots of my drilling, but don't seem to have done that, only one somewhat out of focus one of the first ('95) amp.

Basically you need to mark a vertical line through the centers of the original holes and drill new top holes 3mm up from them. No More. And not much less. This raises the upper cap right up against the bottom of the main connection socket. To high and it won't fit, to low and there won't be enough room for the lower cap.

On the first amp I tried to angle the holes a bit off axis from the original ones to get more room for the lower cap but it didn't really help. The 4th amp I found that with the upper cap raised in position be 3mm I could just barely use the original holes for the lower cap.



edit 18 Jan 2024: For more info concerning this issue see post # 62.

 

[Bratwurst7s]

 

[Bratwurst7s]

edit note: I forgot to mention that I tested all of the new SMH caps and they all pretty much fell in around 11000µF.

 

[Bratwurst7s]

Regardless of how well you install the caps there are 2 remaining problems. One serious and one not so hard.

To get more space horizontally the transformer mounting screws can be loosened and it can be pusher forward, as detailed on the other threads. There is a small trick that will give you 1 more mm also. Cut off the tabs on the forward end of the bottom frame that stop the transformer.

This is the easy stuff.

 

[Bratwurst7s]

The remaining problem is that the larger diameter cap will press up against the bottom frame. If you fully tighten the heat sink mounting screws the lower cap will not have enough room and press hard against the lower frame, putting pressure directly on the circuit board and flexing it. I saw no easy way to prevent this.

So I took the step of cutting a relief on the lower frame.

To remove that frame you have to remove 8 screws. 2 on each side and 4 on the bottom. Then unfasten the transformer and swing it up and out to the side. Pull the frame out.

Mark a line at the forward end of the large window at the rear end of the frame. Mark another line 23mm towards the rear from the first line. Mark a line on the sides between your upper lines, 3mm down from the top. This is the area that needs to be cut out to provide space for the lower cap.

 

[Bratwurst7s]

Since I am now retired and don't work at the hospital any longer I have lost access to a milling machine. So I blocked the frame in a portable vise and used my drill press to drill a series of holes through each side roughly in line with the marks. It looks crude, and it is, but it worked.

 

[Bratwurst7s]

From that point I used needle files to join the holes, hacksawed across the upper marks and filed it all clean.



Cheers,
James

 

[Bratwurst7s]

So here we are with the frame modded and re-installed.

 

[Bratwurst7s]

And the transformer re-installed.



Cheers,
James

 

[Bratwurst7s]

So now we just have to deal with the speaker binding posts.

I got lucky and found a strip of high density plastic in my stash, 31mm wide and 5mm thick. So I cut off 4 pieces 50mm long, marked and drilled them for the new posts. The posts have a nominal diameter of 9mm, with 2 flats on the sides, making the narrow area 7mm wide. So I drilled 7mm holes in my mounting plates and filed them to fit the 9mm dimension, the posts fit very tightly and have a good resistance to turning in the hole.

 

[Bratwurst7s]

For proper support for the mounting plate I drilled an extra hole in the rear panel of the case. It is centered 16mm from the left side of the panel and 34mm from the bottom. I used M3 screws with self locking nuts so the new hole is 3.5mm diameter.



Cheers,
James

 

[Bratwurst7s]

My first try at actually connecting the posts to the board was done using 1.5mm² solid core wire. This proved to be unworkable. Any stress on the wire or block is transmitted directly to the circuit board and there is too much risk involved.



I immediately discarded that plan.

Cheers,
James

 

[Bratwurst7s]

Next try. 1.5mm² flexible wire, in this cas some scraps of silver plated OFC speaker wire. This proved to be workable and was successful. I used a 70mm long piece of wire and stripped 15mm off each end.

After soldering the wire into the posts there was 55mm left exposed. I then pre-soldered the other end of the wire and used smooth duckbill pliers to squish the end flat and rectangular, 1mm x 2mm, the same size as the slots in the circuit board. This required a bit of trial and error to get right but it works. Once started into the slot I pushed the wire nearly completely through until the remaining insulation almost touched the board. The wire was then trimmed and soldered, leaving a little over 50mm of flexible wire connecting the posts to the board.



Cheers,
James

 

[Bratwurst7s]

Once I tried to fit it all together I found that I needed to trim the mounting block a bit to clear some mounting tabs for the rear panel. An area about 2mm deep was filed down the left side and a notch was cut out of the right bottom corner.



Cheers,
James

 

[Bratwurst7s]

After that I was able to re-install the board and heat sink assembly, and after double checking that everything cleared fasten it down. Note that with both of the '97 amps that had the CDE caps installed whover did it noticed they had an interference and clearance problem and simply turned the 2 heat sink mounting screws in a few threads and then left the whole thing loose except for the rear screws, thus putting a lot of stress on the board from the tight screws in the post and plug mounting blocks and the board and heat sink sitting with the lower filter cap loosely pressed against the bottom frame. Not nice.

Since I had 2 different configurations of amps, one pair with 4 of the ECW caps for the first amp in a pair, and one pair with only C701 having a poly cap, I needed a simple way to tell them apart. I used silver colored (nickle plated?) screws to mount the speaker posts in the "simple" pair, and black parkerized screws for the "fully worked" pair.



Note that I did get lucky with these amps in one way as they are all multi-voltage models. Since the mains voltage here is a steady 236v I set the amps for 240v.

Cheers,
James

edit 20 Jan 2024: I had previously stated above that I used Panasonic ECQ caps. That was incorrect. In fact ECW series were used.

 

[Bratwurst7s]

Unfortunately when I made my Mouser order I completely forgot to buy some multi-turn trim pots. The original trim pots were a bit touchy but it was pretty easy to tweak them to a steady 7mV after letting the amps warm up for 15 minutes. One amp was at 6mV, 2 at 7mV and one at 8mV before adjusting. Oh, and yes I did remember to bring up the amps on a dim-bulb-tester with a 100w bulb for the first power-on.



I love my trusty old first generation Fluke 77. Bought it on ebay and from the OD color and German rear text I'm guessing that it's ex German Army.

Cheers,
James

 

[Bratwurst7s]

So that's kinda it concerning the actual work done on these amps. They have now been in daily use for 12 days, 2 to 3 hours a day.



My very first impression after getting them in the system and warmed up was that my Speakerlabs respond well to the extra power. I had listened to 3 John Cougar Mellencamp cd's the day before and they were still filling my head so I listened to them again. Kick drums simply have more kick, the bass is simply tighter. Clarity is very very good, my concerns about the sound quality were put aside very quickly.

This is my first experience with monoblocks and the difference in soundstage and imaging is quite interesting and nice. I have been listening to these same speakers since the spring of 1983. Owned them 2 years longer that my wife has owned me actually. They have been extensively modded and tweaked over the last 10+ years but I am intimately in tune to their sound. And they are sounding very very good with these amps.

Time to change my sig I think.

Cheers,
James

edit: The amps seem to have no problems running my 4 ohm speakers, even in bridged mode. I listen at "robust" but not really overly loud levels and after 2 hours the amps are lightly hot, but not more so than the MX-1 got with the same runtime and volumes. I like to tweak the volume pot to just the point where it doesn't seem to get louder so much as "larger". That is the perfect point for me.

Also, I am using a Dynavox Amp-S switchbox and using the amps and Speakerlabs in both my 2-channel and 9.2-channel systems.

 

[Bratwurst7s]

I have a few thoughts about some tweaks that could be done to the amps, and after that I am finished for now.

The U shaped heat sink that Q701 is mounted to. I forgot to measure it but it looks like it's 1 to 1.5mm thick. I just might pick up a strip of 1.5mm copper and form it as a replacement heat spreader/heat sink.

As mentioned earlier, there are a few local filter caps that I would replace with higher values. 560µF or 680µF for the 470µF pair. 220µF for the 100µF that degrades so much.
And replace all ceramic caps with 5% tolerance C0G (NPO) types.
Repopulate the entire board with metal film resistors. Not that much work actually.

The NJM4558 op-amp doing the signal inverting is not the most loved or high quality op-amp on the planet. I might try replacing it with a NJM5532DD or NJM2068DD. I have lots of them on hand. Replace CS10 and CS11 at the same time with a 47µF/35v Panasonic FM with a 0.1µF MKS2 piggybacked to it. Those caps are on the supply rails of the op-amp and should help keep the faster parts stable.

These are some simple and not expensive tweaks that I might try. If I see a 5th amp for a decent price I might just pick one up and play around with it. And if I can find a parts unit with a good main board I would like to replace that damaged board.

One thing that I forgot to mention earlier is that with all 4 amps every single solder connection in the entire amp was re-soldered. Not touched up or reflowed but de-soldered, cleaned and resoldered with 60/38/2 solder. The /2 is 2% copper. I started using this solder at work 18 years ago and really like it.

That's about it. It has been fun and interesting. These are some nice little "Pocket Battleships". In about an hour and a half I will go out front and warm up the system.

Cheers,
James

 

[Bratwurst7s]

Oh. One more detail that I forgot to mention.

All of the amps have a 100k ohm resistor in parallel with the input socket and ground as a safety measure. I don't recall the position number at the moment. So when I opened both of the '97 amps I found a carbon film cap piggybacked in parallel to the back side on that 100k resistor. If memory serves it was a 22k ohm part. Which would substantially reduce the overall resistance. And they look very much original, including a bit of glue to hold them in place.



So the '99 amp did not have this mod. After I found this on the second amp that I opened I quickly checked the last 2. In the end I decided to remove these resistors.
It just seems to be an odd thing to do. It would seem that this was some kind of rush mod, but since the newest amp of the 4 had reverted to a single 100k resistor I kept with that.

Also, if you look closely you can see that the minus post of the input jack has a ring crack in the solder connection. Both of the '97 amps had a ring crack there.
That led me to consider another mod. Installing better quality RCA jacks without the block and connecting them to the board with some short sections of shielded 0.75mm microphone wire that I have. That would help physically unload the board from stress from the back panel.

Cheers,
James

Cheers,
James