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Negotiableterms's Marantz 2600

EchoWars

EchoWars

Hiding in Honduras
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The saga of getting this running really does NOT belong in Dollars & Sense (http://www.audiokarma.org/forums/showthread.php?t=86412), so I'll transplant a few posts here so it may have it's own thread...

First, although the unit was sold with a supposed bad transformer, it did not take a long time (once I dug into it) to see that the transformer was fine. Next post on May 30th.
Well, three days of battle, and I have straightened out the wiring (I think), and have all the output transistors replaced. As it was wired, the soft-start was non-functional, and the fan ran at full-speed all the time...very bad, as this is tough on the power switch, and once you hear the fan at full-speed, you'll be thankful that there are two speeds and the low speed is nice and quiet.

The fan was removed, cleaned, and I lubed the bearings with good synthetic motor oil. I don't really care for designs with active cooling...any time you are moving air, you're moving dirt. Not a big deal in a situation where you can clean, but that's about impossible on the 2600. Hell, most things are impossible on the 2600...this isn't your 'technician-friendly' receiver by a long shot (and the manual is the typical Marantz sucky waste of paper).

One channel had every single output device blown...I have no idea how that was accomplished. Both channels now have new ON-Semi MJ21193's and MJ21194's (sixteen total!), and new On-Semi drivers. The problem is the TO-126 pre-drivers. They are 250V, 50mA, Fuji 2SA939 and 2SC2071 (Sansui is fond of these too), and the ones that I have look like fakes...I can't trust them. Maybe if the amp section was easier to work on, I might chance it, but it is a major bitch to get the assembly out for service, so I want to do this once only and I'm substituting Sanyo 2SA1353's and 2SC3417's.

Gotta wait for the new transistors and the new soft-start resistor to come in, so I'm going to set this aside for a week or thereabouts till I get parts.
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OK...the resistors were finally installed, and the soft-start relay wiring straightened out.

Installing and removing the amp assembly is a big, serious PITA, and someone at Marantz deserves to get their ass kicked for this design. I had the amp assembly installed, and carefully fired it up in a current-limited mode. Soft-start worked properly, and after 10 sec or so the audio relay engaged. Awesome!! That means the amp is likely done.

But...trying to set DC offset didn't go well. With the offset pot centered, it read about 75mV on one channel, and about 175mV on the other. No big deal, except that the way Marantz engineered the pot the adjustment range is very shallow, and with the adjust pot all the way to the stop I still could not get the one channel quite to zero. Thanks Marantz. Luff ya...:finger:

So I get to pull the damn amp out again, and match new differential pairs, using Zetex ZTX694B's. Found a couple of pairs with gains at around 1100, and installed new ones in both channels. Then, put the pig back in.

This time, with the offset pot at center, offset is sitting at about 35mV and easily zeroed out. Awesome. So offset is done, and then bias is checked and set. Now, I ought to be able to hear something. Hopefully.

Connect the antenna, and dial in a station. Sound is very weak, but it IS there. Obviously there's still a problem, but I don't think it's the amp. To confirm, I feed a signal directly to the amp, and I run the output up to 50W easily, so whatever is going on is happening in the preamp. But while I'm here, I decide to see if the thermal switches for the high-speed on the fan work, so I let it heat soak for about 20 minutes at about 25W output, and I hear a 'snap', and (I swear) the room lights flicker as the fan, normally dampened by a couple of power resistors so it is all but inaudible, kicks into full-tilt F-18A-on-afterburners-mode as the thermal switch bypasses the power resistors. Two or three minutes of this and the switch resets and the fan goes back to 'sane' mode.

The stock resistors for both the soft-start and the fan speed dampener are too small, IMHO. Since the soft-start resistor was toast anyway, I bought these 25W 'hollow-tube' ceramic wirewound jobs as replacements. They fit great with the cheapo-style mounts, and I only had to drill two small holes to get them screwed onto the mounting plate. (see pic).

The stock value of the fan speed resistor was 680 ohms, but the best I could do for a 25W wirewound hollow-tube ceramic was 750 ohms. That slowed the fan down too much, so if you look in the pic you can see the 5.6K 3W resistor that I put in parallel to the 750 ohm job to bring the value down to about 660 ohms, which gives a fan speed just slightly higher than stock.

Next: see what's up with the preamp.
 

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Oh, and...
SoCal Sam said:
Interestingly, the power cord on your 2600 is about half the thickness of the one on my 2500.
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Not on this one...big, fat, and must be at least 16ga and maybe even 14ga...certainly doesn't need to be any larger.
 
Glenn, you're amazing.

I have this mental picture of you preparing to work on a piece of gear. You're sitting in full lotus position, wearing a white robe, gazing at a candle flame, opening your mind for the task ahead...
 
Fisherdude said:
I have this mental picture of you preparing to work on a piece of gear. You're sitting in full lotus position, wearing a white robe, gazing at a candle flame, opening your mind for the task ahead...
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That may not be a bad idea for this one. I've actually had to psyche myself up to get things done on this thing, since doing anything to it or finding proper substitutes for parts, or following the crappy service manual trying to sort through the wiring mess has all been an exercise in pain and frustration. Just getting the amp section running was a major victory. Sucks that the preamp has equally serious issues.
 
...what i really like are the pushbutton switches with that 90 degree lever mechanism, be careful with what you spray in them, or that mechanism will get sticky.....

Helmut
 
This is a really interesting story, I just wish I understood electronics much better so I knew what exactly you were doing or could picture what was going on inside the amp
 
Pulled the regulated supply, and recapped it today. After it was reinstalled, checked all voltages, hoping the preamp issue was simply a missing regulated voltage. No such luck. But the stablized HV to the scope because of the new caps has resulted in a marked improvement in the display.

OK...find the preamp issue. The input of the amp was shorted (so I could torture the preamp without pissing off the amp), and I fed it a sine wave and monitored the output. Turns out that the problem is no more serious than some very dirty switches. A shot of some DeOxit on the filter switches and it's sounding quite nice.

-note: Dirt in a unit with fan cooling, such as the 2500 and 2600, is more of an issue than with regular convection-type heatsinks. Whenever you're moving air, you're moving dirt. And getting the dirt out of this receiver is a serious damn job.

So...at this point I'm busy recapping the preamp. The tone amp has some 1/2W 750 ohm resistors that need replacing, and that has stopped me for now since that's a value I don't have. Order placed. Bummer...
 
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Glenn, don't know if they are the right size, but Mouser sells foam filter assemblies for those muffin fans, I put one on the fan that circulates air in my equipment cabinet.
 
Hello, the fan in the 2500/2600 is a standard 120mm x 38mm 115V ac fan. Would be no problem to mount a filter holder. But the fan sucks the air out of the cabinet! In fact the receiver has a kind of filter system, the metal housing has some coarse fabric or linnen? glued to the inner surface.

Helmut
 
Glenn, I love reading your descriptions of your work. I know next to nothing about the inner workings of receivers and amps, yet I'm still intrigued reading about it from folks like Glenn and Rob.

And how 'bout a pic of that 2600 when she's done...
 
ruesselschorf said:
Hello, the fan in the 2500/2600 is a standard 120mm x 38mm 115V ac fan. Would be no problem to mount a filter holder. But the fan sucks the air out of the cabinet! In fact the receiver has a kind of filter system, the metal housing has some coarse fabric or linnen? glued to the inner surface.
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No place to mount a filter, but the top cover does have a fine mesh material over it. However, judging from the dirt on the power transistor heatsinks it is only moderately effective.
 
I have two 115V AC muffin fans and running at full voltage I have heard quieter 747 takeoffs. You would have thought they might have considered a 12V DC fan. For my equipment cabinet I plugged a wall wart into the switched outlet on the CR-2020 so the fan comes on when the 2020 does, it is barely audible.
 
merrylander said:
I have two 115V AC muffin fans and running at full voltage I have heard quieter 747 takeoffs. You would have thought they might have considered a 12V DC fan. For my equipment cabinet I plugged a wall wart into the switched outlet on the CR-2020 so the fan comes on when the 2020 does, it is barely audible.
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If the fan bearing in a 2600 is in good shape, you won't hear it in normal operation. Normaly it runs at low speed, if you push the amp closer to its limits, the fan switches to full speed, you can hear it, but your speakers should be much louder in that moment.
The noise a fan generates is a function of speed and airflow not wether it is DC or ac operated. DC fans (with brushless motors) became popular with the IBM-PC, guess they didn't even exist in 1978.
One good thing, today you have a wide selection of AC fans you can find very quiet ones, if the old fan bothers you.

Helmut
 
Now, taking into account that EW's charges for his work are about as reasonable as you will find, I am still glad I am not paying the bill for this one! :)
 
You really can not hear the fan on my 500. I would think the 2600 fan should be the same...time for a new one.

jk
 
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