Diagnosing a mute MC2300?

Well - this video seems to have solved it.

You can see each autoformer has a separate, shorter, white wire coming out of it:

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And this confirms that short white wire plugs into the plug terminals, NOT the 1ohm tap:

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So - it's actually the circuit diagram that is incorrect!

The circuit SHOULD show C207/R225 and C208/R226 coming off the 1ohm taps (in the same way that C205/R223 and C206/R224 come off the 16ohm taps) instead of the white wire itself.

Now I can toss these thick white wires in the bin and continue with re-assembly ...
 
I did a lot of work on a 2300 last fall. I remember that it was drawing .6 amps without the boards in it and .75 amps with both output boards in it. (Used a kill-a-watt to measure). My 40 watt bulb was not bright.
I did have a dead short in the input board at the beginning. I also had issues with the multi-cap and replaced it.
Not sure if any of that helps….
 
Well ... I am scratching my head again.

New ON MJ15003s went in (ordered from Newark, so I am assuming genuine), with the 4 insulators in the correct spots (Q121 / Q133 / Q122 / Q134).

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While doing that, I visually inspected all the resistors on the heat sinks, and all seemed fine - most still looked brand new.

Put them back in.

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This is a 2Y unit so it has thermo-controlled fans. The thermo relays had been shorted by someone, so I undid it and put it back how it's supposed to be wired.

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WITHOUT the driver boards installed, through my DBT w/ a 53W bulb, the bulb is dim and the power LED is on.

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Plugging directly into the wall, I see +/- 40V from the PS, all is good.

HOWEVER ... with either new driver card in either channel, the bulb is BRIGHT and the PS measures only around +/- 3V!!!

So ... something is seriously wrong, somewhere. The driver cards are brand new from McIntosh directly, so I am assuming they are OK.

Need to switch gears from assembly mode to diagnosis mode ... any ideas for the first things to check?

I did quickly make sure the heat sinks are not shorted to ground (they are not) and neither are the output terminal screws (they are not).

Having correct PS voltages without the driver boards makes me think it has to be a driver board issue ... but again, they are brand new. I guess I shouldn't assume they are OK, even if new ... but very puzzling.
 
I would measure your cement resistors on the heat sinks. The value you get will not be super accurate because of the resistors low value, but it will be repeatable from one sink to the next. You’ll know if one is way off.
Quite a few of the knowledgeable Mc guys recommend changing those resistors if they are questionable.

Can you measure the voltages coming out of the multi cap can? Should be roughly +95v (blu/wht) and 110v (blu).

Just a couple ideas….
 
How tightly do the edge connectors grip the boards? I had some strange issues on my 2105 before I gently bent the metal tab contacts in the edge connectors to help apply more pressure to the boards.
 
I have been following this post on and off for the past week and wonder where all the "so called" experts are on this one. For one, a dim-bulb tester is not a piece of test equipment. It is a current limiter and its operation to perform a task is determined by the Wattage selection of the 120V lamp. Get yourself a Variac. They are not that expensive. Secondly, you could do well getting yourself a split power supply on eBay. There again, decent ones can be found around a hundred bucks with +/-28 to 32V rails and possible over an Amp. Having one can let you test boards independently, and don't worry about not having that 39.5V available. Having a scope would be a nice item later on. Better to have two good digital Volt meters to get things done.

What did you expect when you put a pc board in and powered it up through the 53W bulb? Remember Ohms law? Do the math. You have an amplifier that draws 160 Watts quiescent (idle) from the service manual. I do not know where those current ratings you made last year came from but let's move forward. With one board in, the amplifier is turning on, drawing current in excess of the lamps rating. That high inrush current through the lamp heats up the filament and its resistance rises. The lamp glows brightly because all or most of the Voltage is dropped across the filament.

I really don't think those new boards have anything wrong with them. If all the wiring is correct, you should not have any issues. How do you want to proceed?
 
Get a 200W bulb and see if it works. I have seen a dim bulb starve an amp for current when the bulb picked is too small. Call around to a local electrical supply and find a 200W bulb. If it idles ok then test some voltages while it is hooked up and then try a small signal. If all is good then go direct to the power and test with more volume.
A Variac is handy to have but it is not a current limiter. I would be more comfortable starting up with a dim bulb until I saw some decent operation.
 
Refugee, since you are first, how is a Variac not a current limiter? I prefer the Variac because I can increase the Voltage gradually which helps to prevent any large initial current draw that can damage the amplifier. I can raise the Voltage slowly and increase it to a point where I can see it stable and prevent damage. This method is far better than allowing the inrush current to be great enough when the filament resistance is cold and allowing that brief, but large current flow to do damage and realize it is too late.

Dim-bulb testers are for hobbyists. You should do things correctly and spend a little money when you have a lot invested in your equipment. Take it from a retired EE who had has had his hands on designs I know you have touched, seen or heard.
 
Johnny Law, I do not come out here to post for the sake of posting something and I do have all the McIntosh information that some people on the threads here covet as if it takes the thumb over the knuckle "to see" and allocate. I truly look out for the sake of helping someone keep from making a big mistake that will hurt their pride and pocketbook. You'll be ok if you proceed cautiously and thoughtfully with the backing of people with demonstrated experience.
 
I use a variac almost exclusively. My variac has an ammeter, so I can see the current draw as I bring stuff up. You can check that things are coming up properly at partial voltage, where the current stays low enough to not cause damage. The only time I use a lamp in series with the AC, is when I am bringing an older (been out of service for years) tube unit up without tubes installed over time, to reform the electrolytic capacitors. It will grab my attention if something starts to draw current while letting the caps reform at partial voltage.
 
I feel that the DBT as it is called here, is a very helpful tool to ensure that the hobbyist, as you have pointed out, avoids plugging in / turning on equipment that has a short or other flaw that can cause catastrophic damage. There are many stories posted here where the DBT has “saved” someone who installed an EL cap backward or other mistake, sometimes schematic errors, from causing collateral damage from that mistake. Others have bought old equipment and used a DBT to plug in questionable gear to see if it is shorted. Not a bad application.

I love my variac, it is useful. I never used (nor heard of) a DBT before following this forum some years back, but have added it to my bench as a quick and easy tool.

Whatever your credentials might be, your input is very much appreciated, of that I am sure. However please note that the vast majority of members here on AK are hobbyists, not Engineers nor Technicians by training or trade, probably don’t have the advanced equipment nor deep understanding of the circuits in audio equipment, … thus the proliferation of the word “recap”, which is the standard task that the hobbyists of AK perform.

Which is the beauty of this forum, many hobbyists here partner with Technicians and other knowledgeable people, and are able to repair and restore equipment and increase their enjoyment of this hobby.
 
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Thanks all for coming in here to help - all comments / suggestions highly appreciated. Whatever your resume, I want to express my appreciation for your advice! That means you @Jose Jimenez! Thank you!

Unfortunately I don't have a variac - I tried to get one once, and got scammed by a craigslist seller in Los Angeles who sold me one that looked nice but had a melted coil inside, so it's been DBT-only testing for me.

Anyway ...

So first I wanted to check the multi-cap at @Pearly Baker 's suggestion. I measured all 4 sections without the heat sinks or cards in place, through the DBT, but saw the exact same voltage on all 4 of them (about 108V). Plugged directly into the all, I saw ~120V on all 4 sections.

Scratching my head ... In what I hope was a proper application of ohm's law (that's a joke!) I realized if the cards aren't in place, there won't be any current draw, and R303/304/305 won't be dropping any voltage. That would make sense of why I was seeing the same voltage on all 4 sections.

So I put the cards back in (still w/out the OPTs plugged in) and plugged it in via DBT again. This time, I saw voltage drop from section to section, and the bulb, while lit more brightly than before, was not shining bright, so I figured it must be normal current draw for this beast even w/out OPTs plugged in.

I then plugged it straight into the wall - reminder, at this point the cards are in place but the OPTs / heat sinks are not plugged in.

Success! I measured 90.5V at the C305D / the blu/wht wire. So the multi-cap appears to be OK.

BUT THEN! SMOKE!

I shut it off immediately - pulled the cards, and found that one of them had a blackened R135!! (Worth noting - both new cards are marked as if Left channel cards - so both have R135, neither have R136.)

I put in a spare 100ohm, 1/4 watt resistor from my parts pile ...

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So I'm scratching my head again ... what could cause R135 to start smoking if the cards are brand new (and if the OPTs / heat sinks are not plugged in)?

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Because there's plenty of evidence someone had fun sticking a hot soldering gun in the guts of this thing in the past - maybe some melted insulation somewhere has led to a hidden short? Maybe between the green or green/white wires to chassis?

But I measured with my DMM and can't find any shorts.

I took a closer look at all the internal wiring using a flashlight to help, and can't see any obvious re-wiring or melted insulation anywhere.

Hmm.... any more ideas as to what to look for?
 
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Hello, again Johnny Law. I am looking at the circuit and see that the number 9 connection on those pc boards connects to the output and part of the feedback circuit, bias and output limiting. It is swinging in the breeze, hanging open. That may be the reason the Voltage balance on the board is screwed up. (I have work around the home after the storm, so I have to get my thoughs back in line again otherwise I would have responded earlier.)

The other end is the autoformer that the amplifier works into. I would think the white wire would measure a couple of Ohms to ground. It is an amplifier and what many people do not realize you could hook it up to speakers without the autoformer and listen. I'm not saying to do this, but I have in the past with MC2205s and others.

But with the output transistors in and the bias in adjustment, you are at or very close to 0 Volts, meaning the offset should be minimal. This is #11 on the pc board. That resistor you had heat up was because the emitter Voltage on Q114 was not close to -39.4 Volts with the negative rail on the other end connected to the resistor at -40.

If the amplifier was at my home and if someone was curious, I would put a 4.7 Ohm resistor on pin 11 to ground and bring up the supply Voltage to the amplifier slowly observing all the Voltages on Q112 and Q114. That is the beauty of having you know what. Let some of the other chime in but read what I said, look at the schematic and ask if you need help. That is what we are here for.

McIntosh went away from being circuits with adjustable bias in the early transistor models after producing a few and then it returned in the mid 70s on models like the MC2200/05. The bias is fixed and needs a complete circuit to operate properly and be able to check it.
 
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I have brought up MC2300's with a Variac (while monitoring current draw), with the output transistors installed, and the driver boards out, to see if there is an obvious short in the outputs, but I have never brought one up with the driver boards installed but the outputs disconnected. I'm not sure that is appropriate.
 
Well - I figured it out. (The smoking resistor issue.)

Anyone care to guess? Or is that too mean? :whip:
 
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Turns out, the R135 smoking was caused by a bad Q113 (a 2N5679) in the brand new output board. :eek: That's the second faulty new TO-39 device I've seen lately for my stereo repair bingo card. (See here for the first one.)

I diagnosed it by at first trying to replicate the failure condition, to take some measurements. If R135 was smoking there must have been more than the correct amount of voltage across it (back to ohm's law!). But if you look at the circuit, without the OPTs plugged in and pin #8 "swinging in the breeze", there shouldn't be any voltage developed across R135 at all. So I put my DMM leads across the new R135 and turned it on via DBT with the 53W bulb. But I saw 0V - then again, last time it didn't smoke until it was in the wall. So - kind of like turning the variac up - I put in my 100W bulb and turned it on again. This time after a ~5 second delay, all of a sudden the DMM started reading positive voltage that kept increasing! I shut the unit off when it crossed 2V as it kept climbing.

I swapped the card into the other channel, same result.

Then I took that card out, and put the other card into the L channel ... 0V across R135. Swapped it into the R channel ... same. Totally fine.

So: looking back at the circuit ... again, with pin #8 hanging open, my first guess was a leaky Q113.

To test this theory, I swapped Q113 from the good card into the bad ... bingo. 0V across R135.

I installed the suspect device into the good card ... bingo again. Crazy voltage across R135.

So - I'll need to add a 2N5679 to my next mouser order.

At least, it seems, I have one good pre-amp card to use to get this thing going one channel at a time.
 
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