2205 Intermittent Power Meter and Lamp Ribbon Cable

Known issues

At least three AK'ers have an intermittent 2205 power meter, and IIRC they are all LEFT meters. Looking back through the threads and PMs, my meter cutout problem was going on BEFORE the 2205 restore/refurb activity. These meters have a known common issue. As Ron-C communicated in Dec 09, "It is a common problem in these meters which were used by McIntosh, ESS, Marantz, etc."

Here is a photo received from a user who replaced the meters in FOUR of his 2205's and apparently the end-caps as well. I'm a little worried at how tightly he's got them packed in there in terms of heat.



Photo of 2205 stack whose owner replace all eight power meters with MC7200 meters?


The MAIN SYMPTOM of the affected METER is that it's usually dead at power-up and requires a transient to `wake it up!!´


Meter voltage

According to McIntosh Service Tech Keevin the 2205 power meters are basically LARGE 1.5 Volt DC meters. A variable DC power supply could be used to test them, setting the output voltage arbitrarily low and being very careful not to over-drive of course. Here are some test data on the operation of the meters. You can see the very low voltage at which the meter will `cut out.´


Voltage across power meter vs dB reading, with cutout voltage indicated.

The drive voltage will roughly double (in this case) when the cutout occurs.


Meter resistance

If the RED wires were unsoldered from the Lamp Board pins the measured resistance across the meter movement should be ~2K ohms. That is another way to diagnose the problem.


Amplifier circuitry

Keevin at Mc also mentioned checking the ORANGE wires on the driver board to the HEADPHONES & METER DRIVER circuits but nothing unusual was found there. Going a step further, one can trace the meter signal path from the METER BOARD to the LAMP BOARD. Here is the diagram.


Sketch of MC 2205 power meter circuits through the amplifier.

There are many connections that could affect the varying DC meter voltage along the path end-to-end including the board edge connectors. One could wonder why such a route was used instead of adding tiny 2-conductors pin jacks on the meter PCB? That could be a possible Mod and would also reduce the number of conductors needed in replacing the Lamp PCB RIBBON with a wire bundle.


Diagnosis

Jumping right into the meters and popping them open can be a challenge. It's NOT too difficult to separate the clear meter covers using a FLAT bladed X-Acto knife. It's more about applying skillful prying action and not about `cutting´ through the adhesive which is very dry and brittle. The recommended approach is to diagnose first by remove the Black sheet-metal TOP COVER (four Phillips screws) then unsolder and swap the RED meter wires L for R. If the same meter still cuts out, it's likely the internal meter terminals. If the opposite meter cuts out, then the problem is likely upstream. {is that correct?}


Why the LEFT meter?

A question that keeps coming up is.. why does it always seem to be the LEFT power meter? Maybe the group can figure that out after doing more tests and thinking about the problem for a while. :scratch2:


-Greg

There is no way I would risk a customer's money or meter without ruling out all other possible reasons for a meter issue.

By swapping the ICs I could rule them out. By swapping the red wires I could cause the issue to change channels if the problem was in the circuit. I could clean and reset the pots.

But knowing the history of the 132-092 and 132-096 and the fact they are only a buck or two I would replace and see if the problem is solved.

If you are keeping track my left meter does the same thing. Often it will jump back to life if I turn the speakers off and on.

Once again, Gregory thanks for the serious look at these meters...I will be finally getting into my 2205 this week. I really appreciate you taking the time with such detail.

Also, I would like to say that those 2205s look really good without the bezels and with the addition of the new end caps - they look so modern they could have been made this year. Ron Evans originals live on for another 35 years - timeless design.

PS. Yes, he has those packed in there too tightly. The heat has no way of escaping, except though the chimney stack of 2205s, unless he has several fans.

ceynon said:

If you are keeping track my left meter does the same thing. Often it will jump back to life if I turn the speakers off and on.

Click to expand...

That's exactly what's happening here. Cycling the SPEAKERS ON/OFF will restore the meter but it's a pain. Putting it bluntly, it's a low quality situation!! We MUST find out why the failure is most prevalent in the LEFT channel. Does anyone know of a RIGHT meter (or both) becoming intermittent? Earlier today I was observing if any of the L or R component on the METER PC board were more exposed to heat from the lamp housing. Nothing is apparent.


-Gregory

c_dk said:

There is no way I would risk a customer's money or meter without ruling out all other possible reasons for a meter issue. By swapping the ICs I could rule them out. By swapping the red wires I could cause the issue to change channels if the problem was in the circuit. I could clean and reset the pots. But knowing the history of the 132-092 and 132-096 and the fact they are only a buck or two I would replace and see if the problem is solved.

Click to expand...

All good advise, but wouldn't you need some evidence before pulling the transistors.. like a oscilloscope trace? The RED wire swap is the first logical decision block. The problem is then either upstream or downstream from the Lamp board where the METER wires connect. After all, those are the components that are in question.

Since many of the guys like to see photos, here's a view of the swapped wires. Blue dots were put on the LAMP card edge and RED wire of the LEFT channel before swapping to eliminate confusion later.


Left and Right MC2205 Power Meter RED (+) wires swapped on LAMP PC Board solder pins. Blue dots applied beforehand.

Damn that dust is everywhere!!


-Greg

The BC238/9 and BC416 were exellent low noise high HFE in the day but Mac had to cull large numbers to maximise their performance. Note paint on their tops. They were always suspect to fail and go out of tolerance. We always had bunches on hand and were cheap. The more modern replacement like the 2sc2240 is a far superior device, magnitudes better and still a buck or two. I like the odds since we might have to wait days, weeks for it to fail again.

+1 on what c_dk said. I agree that I'd swap the meter leads first while the malfunction is still present, then check general connection quality back to the driver circuitry, but if that all looked good the transistors would be outa there and new ones in.

Cold morning start-up

Up at 6am to turn on the system. Before shutting down at midnight, the gain of the system was set for a low-level that would deflect the power meters to ~10 mW (-43 dB full power.) This was after reversing the L & R meter wires on the LAMP board yesterday.

As anticipated, the LEFT meter was out of action!!


LEFT 2205 power meter is inoperative at `cold start´ after RED meter wires were swapped on LAMP board pins.


That can mean only one thing.... the fault is downstream from the LAMP board pins, where the RED and BLACK meter wires attach. Has this test shown conclusively that the power meter cut-out is inside the meter movement? I doubted there were bad solder joints the way new caps and other connections were soldered, especially after flux scrubbing and inspection.

Shall we proceed with METER surgery?


-Greg

That appears to be the smoking gun Gregory. And I was hoping it was the meter board transistors, a much easier repair than taking apart the meter!

kmanusa said:

That appears to be the smoking gun Gregory. And I was hoping it was the meter board transistors, a much easier repair than taking apart the meter!

Click to expand...

Are you sure? I mean, is there any doubt? The LEFT meter came back to life within 20 mins of power-up without `spiking´ it with the SPEAKER relay. The circuits are clearly switched, as cutting or increasing the LEFT GAIN control varies deflection of the RIGHT meter.

And why is it predominately the LEFT side? That question needs to addressed. I was looking for some heat issues on the left side but it's all pretty much even across the lamp housing. Could it be the LEFT meter wires are longer and have been pulled or `tugged on´ sometime during assembly? or during servicing?

If the meters need to be removed and opened again, then it would be good to combine that task with other upgrades like installing a NEW glass panel, and replacing the frayed LAMP ribbon which gets worse every time you handle it.


-Gregory

Well, what is left in this circuit besides the meter-- the red wires and their connectors? The only other thing I can possibly see is that the left meter is closer to the outside edge of the amp and perhaps more prone to corrosion if the amp were to be used in an environment prone to causing corrosion.

BTW, my left meter will occasionally "come back to life" after a certain amount of time and then just as inexplicably drop out again a short time later....

Gregory said:

Are you sure? I mean, is there any doubt? The LEFT meter came back to life within 20 mins of power-up without `spiking´ it with the SPEAKER relay. The circuits are clearly switched, as cutting or increasing the LEFT GAIN control varies deflection of the RIGHT meter.

And why is it predominately the LEFT side? That question needs to addressed. I was looking for some heat issues on the left side but it's all pretty much even across the lamp housing. Could it be the LEFT meter wires are longer and have been pulled or `tugged on´ sometime during assembly? or during servicing?

Click to expand...

Pure speculation here, but if my logic is flawed I'm sure someone out there can pick it apart.

My experience over the last 20+ years with replacing Mac meters (when the problem is not in the drive circuits) has been that about 9 times out of 10, the meter failure is mechanical and not electrical. Usually, the pivot points (bearings) for the coil / needle are binding or sticking. Rising ambient temps or sufficient drive level can unstick them, but this is usually temporary.

I've never really evaluated a left vs. right meter failing more than the other, but on the model(s) you are discussing, I will note that during any transportaion of the amplifier, whether during the original shipping or handling by the owner, it is not hard to damage the meter with an externally applied shock, such as by dropping the unit even a few inches.

These guys are heavy. They're easy to set down roughly. The box and packing helps, but still the meters are subjected to mechanical shocks which inevitably target the bearings.

Now, if the unit is set down roughly, how likely is it that it would be dropped exactly evenly on the bottom? Not too likely, I would say. One side or the other will likely get the shock first. The side that gets the shock first will be statistically most likely to damage a meter movement.

Now-- which meter is closest to the side of the amp, and therefore gets shocked the most during an uneven drop on that side?

In this case-- that would be the LEFT meter.

Reasonable? Possible?

Classic_CJ said:

Pure speculation here, but if my logic is flawed I'm sure someone out there can pick it apart.

My experience over the last 20+ years with replacing Mac meters (when the problem is not in the drive circuits) has been that about 9 times out of 10, the meter failure is mechanical and not electrical. Usually, the pivot points (bearings) for the coil / needle are binding or sticking. Rising ambient temps or sufficient drive level can unstick them, but this is usually temporary.

I've never really evaluated a left vs. right meter failing more than the other, but on the model(s) you are discussing, I will note that during any transportation of the amplifier, whether during the original shipping or handling by the owner, it is not hard to damage the meter with an externally applied shock, such as by dropping the unit even a few inches.

These guys are heavy. They're easy to set down roughly. The box and packing helps, but still the meters are subjected to mechanical shocks which inevitably target the bearings.

Now, if the unit is set down roughly, how likely is it that it would be dropped exactly evenly on the bottom? Not too likely, I would say. One side or the other will likely get the shock first. The side that gets the shock first will be statistically most likely to damage a meter movement.

Now-- which meter is closest to the side of the amp, and therefore gets shocked the most during an uneven drop on that side? In this case-- that would be the LEFT meter. Reasonable? Possible?

Click to expand...

Yes.. you have some good points. The thought of a mechanically damaged meter is depressing to the max!!

Let's debate this some

Notice the VOLTAGE vs Power level chart in the OP. The voltage roughly doubles when the meter cuts out (although it's a tiny voltage) which says there's no longer current flowing through the 2K ohm meter coil? If the movement was `stuck´ the same voltage would still be observed? The meter doesn't really get `stuck,´ it de-energizes and falls to zero.

Also if you analyze the physical position of each meter (L & R) when the amp is INSIDE the CARTON, the difference in position from the edges become LESS significant and therefore a shock to the LEFT corner of the box shocks both meters more of less the same. It's not like 5:1 or even 2:1, is it?

Procedures

I believe the problem is electrical because both Ron-C and Keevin of McIntosh have communicated [to me] procedures for repairing a faulty meter involving the brass ring lugs/crimps and the possibility of brass-to-brass surfaces becoming ohmic.

Let's have a close-up look at one of the meter movements below. You'll see a RED and BLACK wire coming in from behind and a black jumper with ring lug extending from the RED wire screw to a tiny solder pin on the movement.


Close-up view of a MC2205 Power Meter with clear cover removed. Four brass ring lugs are used in its assembly.

Ron says to operate on the brass-to-brass contact issue, and Keevin says to REMOVE the crimp terminals and SOLDER them back on the wires. That would mean FOUR solder operations for each meter.

When the meters were opened Feb 2010, someone needed to be around who could hold onto the meter housing when loosening/tightening the brass screws. You need to work a nut-driver with one hand in the front, and a screw driver with the other hand in back. IOW someone, or some fixture must hold the meter housing steady during this process. It should probably be in a fixture while working on the internal wires.


-Greg

Gregory said:

Known issues

Photo of 2205 stack who's owner replace all eight power meters. -Gregory

Click to expand...

That is a very nice setup and the fact that the plastic meter frames are removed really makes these old amps look very modern.

I have two 2205's powering my bi-amped speakers, I bought them both used about 25 years ago, on one amp one of the meters, I believe its the left meters always shows half the power compared to the one on right side, but I never experienced the issue that you describe here, at any rate the sound is not affected and that is what counts for the moment until I can figure out a safe way to get them to Terry for a tune up job.

Vahe

Ron says to operate on the brass-to-brass contact issue, and Keevin says to REMOVE the crimp terminals and SOLDER them back on the wires. That would mean THREE (possibly four) solder operations for each meter.

I don't disagree with this, in fact if the problem is in the meter and isn't a mechanical failure, this would be exactly what I would do, except that as a tech, doing this for a customer, it would be a last ditch attempt that would only be done if the meter was NLA.

(The reason being is that the labor to disassemble the meter, repair the electrical connections, get it back together, all without damaging something, is risky plus would exceed the cost of just buying the new meter. If you're a DIY-er, that's different.)

Mechanical failures aren't hard to determine once the meter has been removed and you can tilt it about and observe the pointer movement. Also, lightly tapping on the face of the meter while it's acting up with signal applied will show this also.

If the problem is electrical, that doesn't explain the tendency for the left meter to fail more than the right one. My theory applies only to mechanical damage of the meter movement.

Question for anyone reading to consider-- if you are a right handed person, as is the majority of the population, and you are carrying something very heavy in a horizontal orientation (like an MC2255), when you set it down, do you set the left side down first, or the right side down first?

(Never thought about that? Well, neither did I until this thread came up. But we do a lot of things unconsciously and never notice a pattern. However, I can state with certainty that the carpeting on my steps between the first and second floors is worn in a pattern that indicates I always step off the top step in the same way-- the pattern has a distinct sequence.)

(Note. I think too much. Sorry about that!)

I am sure that with over 12,000 of these amps out there 3 or 4 with a bad LH meter is not a conspiracy.

What amount of time do you think a meter rebuild would take? I would guess about 3 hours each maybe 2 1/2 bills or so. Still would scare me to death to possibly destroy one without absolutely KNOWING FOR POSITIVE it was bad and then would want to make all the watchmaker type repairs while inside.

We had problems with the smaller meters on the MC2125 overheating and discoloring and melting the case. Standard procedure was to melt holes in plastic to vent. Drilling could crack the brittle plastic as would removing the lightbulb circuit board on all models if you are not very carefull. I usually file off some of the tab to minimize how much clearance is needed.

A simple bulb change on these 35 year old amps can become a major headache if you are not carefull.

CountD said:

Once again, Gregory thanks for the serious look at these meters...I will be finally getting into my 2205 this week. I really appreciate you taking the time with such detail. Also, I would like to say that those 2205s look really good without the bezels and with the addition of the new end caps - they look so modern they could have been made this year. Ron Evans originals live on for another 35 years - timeless design. PS. Yes, he has those packed in there too tightly. The heat has no way of escaping, except though the chimney stack of 2205s, unless he has several fans.

Click to expand...

Kool.. There's a four day weekend coming up which would be a great time to have another go at the meters. A new glass panel hasn't been ordered yet. The amp will get a new face once it's working ok.

Yes, attention to heat dissipation is important IMHO. If you notice in the Cold Start photo of this thread, there's a ~1 1/4" opening (just barely visible) across the back panel of the amp compartment where heat can escape. Few will acknowledge the heat that builds up inside the top black meter cover. Being inside the walnut case makes the situation worse as the three vents in the cover are mostly blocked. There's always the PANLOC detent position which exposes the vents.


-Greg

c_dk said:

I am sure that with over 12,000 of these amps out there 3 or 4 with a bad LH meter is not a conspiracy.

Click to expand...

In the description, the seller mentions the LEFT meter problem. Just saying...:scratch2:

http://cgi.ebay.com/McIntosh-MC-2205-Amp-/220700511593?pt=Vintage_Electronics_R2&hash=item3362c68d69

-Mike

Found the guy's emails from June 2009 on the 4x, 2205 amplifier stack (with replaced power meters) and will ask him which of the meters were faulty. He says that one of the meters "wasn't working properly" but we'd like to know which one, and what the symptoms were.

Another point to make is: When opening up the meters you'll probably need a can of compressed air (such as for a PC) to blow out any dust or specks that fall inside the clear cover when apart. It's surprising how easily stuff gets in there, and how noticeable pieces of lint and dust are if not removed. Be sure to inspect for this when reassembling, that is if you dare take them apart!!

It will be a good time while the front is off to measure and write down the brass screw, wire and ring lug sizes in case NEW terminals are needed. What do you think about using brass hardware? how prone to oxidation is it? Will also be measuring the LAMP ribbon length, width and pin spacing, etc. If all goes well with the power meters, a new glass will be ordered and the ribbon will be replaced next time it's apart.

Imagine that some of the Pro techs could do all this restoration at one time, which took an amateur 1-1/2 or 2 years to complete.


-Greg