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Intermittent Mc power meter

Gregory

Gregory

Soundco Kid
I've been systematically doing an MC 2205 overhaul and it's in darn good shape now after more than a year of effort. However the LEFT power meter has been cutting out (it drops to zero and stays there) at low power levels. It got so bad in the past few months that the amp would power-up with the LEFT meter non-functional.

The fix was to open the meter housing, break-loose and then re-tighten the brass connector screws/terminals inside the meter movement. Yesterday I did a VERY low power test, playing Classical FM at 0.002 Watts (-50 dB) with peaks maybe 4 or 6 dB above that, and the LEFT meter went out again after ~2 hours. It came back to life by simply increasing the LEFT GAIN control from center, up near full and back.

I think the meter terminals will need some Deoxit to finally cure the problem, not just re-tightening, as it's WAY better than before but not fully cured. I mean.. those little brass screws really cracked loose with a `snap.´ What do you think about the situation? Any suggestions?


-Gregory
 
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When I had trouble with a meter on my 2205 it was that ribbon cable that connects to one of the boards under the dust panel. Pulled it out, a little Deoxit on the bare ends, plugged it back in and no more trouble.

Murray
 
skippy_ps said:
When I had trouble with a meter on my 2205 it was that ribbon cable that connects to one of the boards under the dust panel. Pulled it out, a little Deoxit on the bare ends, plugged it back in and no more trouble.

Murray
Click to expand...

Thanks for the reply.. a new ribbon is on order from Mc Parts. Here is a photo of the ribbon in June 2009:

MC2205_Love_ribbon.jpg



and a photo of the ribbon in the past week:

MC2205_Ribbon.jpg


The ribbon has deteriorated from disconnecting and reconnecting it. There are only a couple capacitor solder joints and a few connectors in the LEFT meter circuit that I messed with while servicing the amp last year, and I'll be checking those again. There was a signal on the RED meter wire at the Lamp Board pin observed with an o-scope when the meter had cut out suggesting it was the terminals inside the meter movement.

MC2205_meter_movement.jpg



Thanks again for the helpful reply. I hadn't planned to ship the amp anywhere or take it to a service center, except to have the meters calibrated. :thmbsp:


-Gregory
 
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MX117.MC7150 said:
Most reasonable. Thanks.
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The ribbon cable specified above arrived today, but unfortunately it's the wrong size. I'd say it's about 1/2 as wide and 50% shorter that the original. Hopefully no one (besides me) placed an order for one. At least Mc Parts Department got the number of pins right. :)


-Gregory
 
Oops, that's unfortunate. Brian, in McIntosh Parts, has always been good at replying to my e-mails. Perhaps you could send him the pic of your old cable to compare to what they are offering today.
 
MX117.MC7150 said:
Oops, that's unfortunate. Brian, in McIntosh Parts, has always been good at replying to my e-mails. Perhaps you could send him the pic of your old cable to compare to what they are offering today.
Click to expand...

Yeah.. we had a few emails on the ribbon cable. I sent him four or so LARGE photos just like you've all seen in the posts here. Nailing down the correct acetate sheets was worse, but they DO try.


-Gregory
 
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MX117.MC7150 said:
What are the ribbon cables from McIntosh Parts selling for? Thanks.
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Got a call from Brian yesterday who then transferred me to Keevin (pronounced Key-vin) the Service Tech at McIntosh, who said the 2205 ribbons are NLA (no longer available.) If the ribbon is electrically intact then I'll leave it in place. Otherwise Keevin says to desolder the socket on the Power Supply PC Board and install wires to the Lamp PC board, perhaps in some kind of a sleeve or bundle.

As for diagnosing the intermittent meter, he says to measure the resistance of each meter which should be ~2.7k ohms. The meter needle should deflect a bit due to the voltage applied by the multimeter in ohms mode, or apply a SINE wave to the input, lower the amplitude and trace/measure signal levels to find where the fault is. Also one could use a small DC voltage on the meter wires directly, but not much.. just a few volts and check for proper operation.

Knowing that cables and connectors are usually the problem, especially with intermittentcies, it shouldn't be too difficult to resolve. It's just a nagging issue that stands in the way of a completely successful project!!


-Gregory
 
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Did you try Audio Classics? They stock a lot of NLA McIntosh parts and work closely with McIntosh.

I need the same cable on one of my 2205's, so tell me how it works out. Mine look like yours and have the same meter problem.

I was also told to replace the caps on the meter board as well as the diodes from Terry here on the forum. They are infamous for issues and easy to repair. I can't believe they don't stock that cable anymore. It's a frequent failure on these older amps and a common issue. I thought other amps used the same cable too like the 2125..?

Peace,

Phil

EDIT : Audio classics lists the cable as 6inch cable with 10 pins and in stock for 11 dollars. Is the one you need 6 inches and 10 pins?
 
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macman007 said:
Did you try Audio Classics? They stock a lot of NLA McIntosh parts and work closely with McIntosh. I need the same cable on one of my 2205's, so tell me how it works out. Mine look like yours and have the same meter problem.

I was also told to replace the caps on the meter board as well as the diodes from Terry here on the forum. They are infamous for issues and easy to repair. I can't believe they don't stock that cable anymore. It's a frequent failure on these older amps and a common issue. I thought other amps used the same cable too like the 2125?

Audio classics lists the cable as 6 inch cable with 10 pins and in stock for 11 dollars. Is the one you need 6 inches and 10 pins?

Phil
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I haven't tried AC yet, and don't really know if it's the ribbon causing the meter to cut out. Right now the objective (in addition to getting the meter fully operational) is to replace an unsightly [ribbon] cable that cannot stand up to disconnects and reconnects. I think it was Murray that recommended brushing some Deoxit on the ribbon pins.

Yes I replaced the caps and diodes on the Meter board. It's been a busy week, and this weekend I'll be able to measure the width and length of BOTH the original and new (too small) one. There's another solution that I'm thinking of and will study it more, and that would be skipping all the pass throughs and connecting the meter wires direct to their source [PCB] traces. IIRC the meter circuits pass unnecessarily thru TWO PC boards where there are several connectors.

Will PM [pvt msg] you on the situation. Could you briefly describe the problem you're having with the power meter?


-Gregory
 
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IIRC, it was either dead or full deflect depending on the position of the ribbon cable as I moved it. Swapping in the other cable from my other 2205, or finding the "sweet spot" with the old one will allow the meter to work normally in all positions, Watts/Db's etc. I also recapped the Pc board for the meters and did the diodes as well, as one cap had leaked and was empty with the + lead moving around the cap end and an open diode, probably from the shorted cap.

I need the cable to complete my repairs then it will be 100%. I use it, but am hesitant because of the cable, so it's "in the system", but doesn't see much use, as I have plenty of other McIntosh amps to run everything.

I had a 2125 do the same things as the 2205 did, same meter, same repairs and cable, plus a few power guard bulbs brought it back to 100%.

the 2125 is running my Mc center channel in bridge mode as we speak. I can hear mommy listening in the other room. 250+ watts is plenty for a center channel believe me!

Peace,

Phil
 
macman007 said:
IIRC, it was either dead or full deflect depending on the position of the ribbon cable as I moved it. Swapping in the other cable from my other 2205, or finding the "sweet spot" with the old one will allow the meter to work normally in all positions, Watts/dB's etc. I also recapped the PC board for the meters and did the diodes as well, as one cap had leaked and was empty with the + lead moving around the cap end and an open diode, probably from the shorted cap.

I need the cable to complete my repairs then it will be 100%. I use it, but am hesitant because of the cable, so it's "in the system", but doesn't see much use, as I have plenty of other McIntosh amps to run everything.

I had a 2125 do the same things as the 2205 did, same meter, same repairs and cable, plus a few power guard bulbs brought it back to 100%. The 2125 is running my Mc center channel in bridge mode as we speak.

Phil
Click to expand...

Well.. I didn't get to digging into the amp today, but have been thinking what to replace the ribbon with, and surprised that no one has recommended a scheme to rewire it. There must be a suitable cable type out there. Maybe PM some of the guys?

Below is a sketch of the 2205 power meter circuits. The Meter Drive comes from the Meter PC board C113, C114 (negative side of the caps) to the edge connector, pins 2 & 7. From there the circuits are wired to the Power Supply PC board edge connector, pins 16 & 17. There are two pass-through traces that connect to the RIBBON socket, pins 27 & 28. Form there, on to the Lamp PC board via ribbon to pins 9 & 10. Then two more pass-through traces on the Lamp board to solder cups where the RED meter wires attach.

2205_meter_circuit.gif


It seems McIntosh went to a lot of trouble bringing ALL connections from the amp to the FRONT panel through a single RIBBON which then deteriorates! I'll not specify any mods but you can imagine what the options are. Probably best to just replace the ribbon with some other type of cable. :yes:


-Gregory
 
A floppy disc ribbon cable has something like 30 wires in it. Although it looks like the spacing is too close, perhaps tying two adjacent wires together and a little bit of solder might work. Or, skip every other wire as you move across.

Murray
 
Are the pins on the end of the ribbon cable soldered in place? If so, you can wick the solder away and remove. If not solder, then even better. Any good 10-conductor ribbon cable will then work. Dosen't have to be exactly the same width as you can split the end of the cable at each conductor to gain the small amount of flex needed to accommodate the difference in width. Put the pins back on the ribbon and plug back in. You can also ask McIntosh Parts who made the connector (Canon, Amphenol, AMP, etc.). Reasonable chance it is still made. Most of these style connectors started life as Mil-spec parts and they do not change very often. If available, buy the connector kit and use the new pins.

Doing a google on Belden ribbon cable brought this up:
http://www.belden.com/pdfs/03Belden_Master_Catalog/07Flat_Cable/07.7_8.pdf.
Am guessing that 28ga is fine for this application.
 
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