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McIntosh - Hints and Kinks

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Need help with Mac 1700 MPX problem

Hi,
I'm loosing my hair trying to restore MPX operation on this 1700. The 6EA8 tube was very weak on both sections and was replaced with a NOS. Also swapped the 12AU7. IF and RF tubes check fine. IF alignment in accordance with manual went fine, as did RF. The problem persists: there's no response from the 19khz transformer or phase coil- the manual refers to "...max negative DC at pin 7 of V8", but there is no DC present at this point; regardless. The 19khz transformer checks fine for continuity, as do voltages in the circuit. I pulled V7 and adjusted 38khz. at pin 6 of V8. I can see 19khz pilot at TP1, but not on T6-- should it not be present on pin 6 of V7?? I suspect an undetected bad ceramic cap but have not been able to get any closer- the caps check OK as do diodes and resistors.
Any help appreciated.
Thanks, Cornell
 
Common failure in the MAC1700 is C62 a 100uF at 16V capacitor, this capacitor will become leaky or short thus killing the 36K oscillator.
 
Terry, changed the cap; no difference. T6 and L5 still have no effect. With the generator in stereo, the l and R output is a modulated envelope, with one channel peaking as the other is approaching minimum. Mono looks OK.
Thanks, Cornell
 
Terry: MC2105 Temperature at 80 F Room Temp heat sinks

Terry,

With the weather change, the room the MC2105 is in is tiled and temp in the cabinet on top of the cage above the right Heatsink if you are facing the face plate is 98-100 degrees F in a 80 degree room.

The left side far left is 96 degrees F in a 80 degree room.

The heat could be from at idle mind you after 1-2 hour, the POWER Transformer being on the right front, correct?

It obviously gets hotter than the autoformers would on the left.

The whole unit seems warm, but not hot, when I put my hand on cage above the heatsinks seems like chimney is doing what it should.

Any comments or is this ok, given your last advise re: heat and bias not adjustable.

The PT is a bit hummy, if you will but probably due to the tile floors and acoustics it can be heard in a very quite room.

Thanks for any advise.

John.

PS...I did notice a slight channel imbalance on the meters but then it could be the source or preamp - tube adjustment.

right seems a bit low
 
Temperatures seem to be OK, the right side with the PT will be a bit warmer then the left, the HS and PT tend to aid each other in warmth.
 
Terry: Thanks, any comments on the Humm from PT

Any comments on the HUMM.?

I know it's not the preamp, but it could be a cheapo RCA Cable I have, I prefer cheap...will replace with some 75 Ohm HOSA Quad shielded RCAs tonight...see if it makes a diff.

Not sure.

I do hear some humm in speakers so I "think" it could be one of two things...ground loop or connection not firm.

Thanks, again, John.

Not sure about the meter issue and gain balance, seems like it's always FM and always the same channel...I am going to try a direct connect from my IPOD tonight to the amp and see if that makes a diff.

I don't want to mess with getting a 150W dummy load and trying to calibrate the meters as per service manual..the largest load I have is about 50W.

I guess I could get some large(ish) wirewound non-inductive resistors...but 20W is all I have in this area.

What is the best way to test, in your opinion, the gain controls and the meter accuracy...last night I put about a few volt 250Hz signal through it and the left meter was about 25% higher than the right.

Of course it's possible since I am running them through a phase inverting Van Alstine Bridge/Inverter and summing the inputs inverting at the amp and then correcting for phase at the speaker it's the problem...that and the humm/.
 
IGNORE HUMM ?: Found it - shorted RCA Cable

Loose ground.

replaced cable No trouble found.

Need to adjust meter calibration.

Have to take to 105W Dummy Load

Thanks,
 
Hello,
A question from a newcomer to AK McIntosh audio : I bought a 2nd-hand MC7150 in 2003, and I would like to check if everything is OK after 15 yrs.
Can anyone advice about the bias setting : which idle current, where to check and adjust ?
Is there any offset adjustment necessary with autoformers ?
When close to my speakers, I can hear a faint buzz, which does not increase when cranking the level pots ; any idea what it is, and if it can be cured ? (I don't hear it with my vintage Marantz 1200B)
Any help is welcome.
Francis
 
The service manual has the information you need, call the parts department to order one 800-538-6576. Here is the procedure from the manual.
 

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DC Offset Mc-2105 L: 2mV R: 12mV

Terry,

As b4 the offset I am seeing is as in title.

Is this something to be concerned about or is it alright, within spec.

This is the MC2105 I asked you about when discussing bias/temp on heat sinks, etc.

No output problems or other issues, just wanted to make sure this is not a problem on the outputs - idle no input about 30-40min on or at first start up, same as above.

No offset adjustment.

THanks,
John
 
No problem, the heatsinks are cool this says the bias is OK, if the bias is OK the offset you are seeing is nothing to worry about, more than likely some noise that is showing up as offset to your meter.
 
Offset to meter

terry,

Thanks, could be. All voltages spec out inside the unit per the schematic +/- 10%.

Hot or cold no issues.

Meter is a Simpson VTVM

Other meter is a Fluke.

Thanks again.
John
 
MC240 temperature rise

Been running a MC240 for burn in after rebuilding the complete power supply, 1/2" feet, on the workbench.
After 5 hours here are the readings.
.
Room = 75F
Front = output transformer = 88F
Middle = output transformer = 92F
Rear = power transformer = 110F
.
Can still hold a hand on the hottest transformer but yes it is a bit warm to touch though entirely in the safe zone...
 
Temp

I am guessing as always that the close proximity of the Output and Power trans, in my case to the heat sinks on the rt side, looking from the front are driving the slight elevation in Rt v left temp with a room temp as stated.

Unit of course was in the cabinet with cage, a few inches from back wall and clear on both sides, elevated about 3 inches on a wood block.

Seems normal...

Room at about 80 degrees F, PT runs the hot part it seems varies when incoming AC is 115 or over 120VAC.

This is the MC2015 I spoke to you in email about, within this forum.

I've seen hotter tube PT's...but generally not "smokin': hot.

Good to know your thoughts though.

I have had one Cap can on a MX110 that is always hot due to proximity to rectifiers underneath and PT...even in open space but the one next to it is less hot, further away by inches....these never fail, run fine and leak not one bit.

i rebuilt the PS some years back and it runs and runs.

Not a problem anywhere except a 6CS tube failure.

For what it's worth. MC stuff runs and runs./

John/.
 
dewickt said:
Been running a MC240 for burn in after rebuilding the complete power supply, 1/2" feet, on the workbench.
After 5 hours here are the readings.
.
Room = 75F
Front = output transformer = 88F
Middle = output transformer = 92F
Rear = power transformer = 110F
.
Can still hold a hand on the hottest transformer but yes it is a bit warm to touch though entirely in the safe zone...
Click to expand...
Adding to the database; with roughly the same room temperature, I get maximum 117F transformer temp from my MR-71 in a McIntosh slant-foot case. I took readings several times over roughly a week.

Murray
 
How to replace speaker barrier blocks on MC2120?

Another question regarding the amp mentioned above. The outside barrier block has four dividers broken off. I will have to call Mcintosh shortly to see if replacements are available (I'm crossing my fingers. They already said no regarding the the front panel.) What is the procedure to replace them? Do I unscrew the old one and pull it out to expose the solder terminals or do I have to work from the inside? The inside is very tight between the back wall and the headsink. Does the heatsink have to be unscrewed and raised out of the way? This amp must have been used by roadies. The accessory outlet is also cracked and the two volume controls were rewired so they are bypassed. The output level has to be controlled by the source.

Thanks,
John
 
Attaching feet to bottom of MC2120

I purchased an MC2120 that was rack mounted with no feet attached. I have a set of four feet with threaded screws that I got a few years ago, probably for a C34V. I cannot figure out how to attach them to the bottom plate of the amplifier. There are four holes that look to be in about the right position but they are relatively small and not threaded. Which holes do I use and what are the correct fasteners?
Thanks,
John
 
Not sure if this should be a separate post (I'm new here but I already love it). Recently got a 2505, but one channel does not work. The meter does not move either. Any suggestions?
 
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