McIntosh - Hints and Kinks

[johndoe1]

Thanks Terry for the .JPG" Perfect

Please leave the file here so I can print it on Monday.

I appreciate it.

Any advise on the heat/ventilation issue?

The unit ran for 5-7 hours straight this weekend with no heat issues, room temp: 82 degrees and it was warm but not hot, the AC was 120-122VAC. The PT hums a bit but it's probably normal the result of the cabinet enclosure space magnifying it.

Again thank you. I think I will get the maint./serv. manual along with the feet from Mc.

John

 

[johndoe1]

Terry: Bias Success: Meter success MC-2002 Thanks

Terry,

Your scan was useful.

I appreciate the help.

The bias at 120-122VAC was about 22.6mV. I lowered it a bit to 20.4-.6 and both sides came out about right at:

L - 20.7
R - 20.7-21

The meters were very slightly sticky at ZERO...just above it and a tap in the unit showed it dropped. I realigned the meters via the adj. no issues.

Unit is less hot now, believe it or not at idle and at run.

Ran it hot and got both the power guard function to work properly and the temp never came on.

Gave it a good dusting on the meter bulbs and it's just fine.

Could not have done it without your help.

The owner got a quick electronics lesson and a free tune-up.

Thanks again,
John.

 

[cheehoi]

MC-2505 service

Hi,
I bought a MC-2505 from ebay a year ago. It arrived but one of
the channels was not working properly. Finally got off my butt and did some debugging last evening. Basically checked for shorts between transistor
junctions and found a bad power transistor (Q22). 2 of it's pins shows a short.
The surprising thing was Q22 was replaced some time ago ie. Q21,23,24 was
a Motorola 32-070 7452 and Q22 was a newer SanKen 2SC1116A.
So, I was wondering if the 2SC1116A stop working because it is an
incorrect replacement part or some other thing is causing the part to blow.
I'm going to order a replacement part from McIntosh but just want to be
make sure the new power transistor I put in will not blow again.

And since I'm at it, are there any parts I should replace since this beast is
about 30 years old ie. maybe some caps that needs to be replaced?

Thanks in advance for any advice.
Regards,
CH

 

[dewickt]

The correct replacement for the 132-070 is a MJ15003 by ON/Motorola semiconductor, odd transistor was probably blown due to mismatch.

 

[jdcheek]

Terry, I have an ms-300 that I bought in April, and have had no issues until recently. I cannot seem to get it to record a cd. I have made sure that the cd is for music recording, but there really is no trouble shooting in the manual for a machine that does not record. Thoughts?

 

[dewickt]

The MS300 is listed as 'Factory Service Only', looks like this one is a call to Chuck Hinton at the factory 800-538-6576. To be truthfull I have looked at one in a display but never had any hands on time.

 

[Victor]

Terry, I have an ms-300 that I bought in April, and have had no issues until recently. I cannot seem to get it to record a cd. I have made sure that the cd is for music recording, but there really is no trouble shooting in the manual for a machine that does not record. Thoughts?

Are you having trouble recording to the MS300 hard drive or making a CD. If you are having trouble making a CD do you get an error message?

For MS300 service and tech support there is a dedicated phone line. 866-458-6910 (I think 8am-6pm). The guys at this number are great, they have helped me out many times.

Good luck,

Victor

 

[cheehoi]

MC-2505 service

Terry, Thks for the replacement part # info. I've placed the order and will update. Rgds, CH

 

[mike5500]

MC30 biasing and filament surge

Hi Terry,
I'm wondering if you could clear something up for me. Your bias mod shows replacing SR-1 with a 1N-4007 and bumping up C-13's capacitance to 22UF@450V. I can see that however, the schematic I have only shows a 3.3K at R5 which is at the other end of the circuit on V1. Also, I'm running 5881s in my amps and they ask for a slightly beefier bias voltage. Do you think i'll be okay without throwing in the series resistor behind the diode? Also, the schematic shows -45V for a bias voltage, not -42 as you mention.

The second issue is that one of my amps seems to have a whopper of a surge at power up on the filaments. R27 and 28 measure within 1% of rated values so I'm thinking that maybe the can needs to be replaced.. any thoughts?

Thanks much,
Mike

 

[jpg123]

MC30 Makes Popping Noises at Turn On

I found this thread while looking for a fix for a turn-on popping issue, lots of great ideas - thanks! Here's the problem I've been working on.

--------------------
MC30 type A-116-B amplifier.

Problem: Makes popping noises for a few seconds after you plug it in. It sounds much like a relay closing. It would pop 5 to 10 times, then settle down.

Solution: The insulation of the yellow wires going from the P/S xformer to pins 2 & 8 of the 5U4 socket was cracked, the wires were exposed & were arcing to the chassis. The wire was stiff, brittle & discolored around those areas. I cut the yellow leads 1/4" back into fresh wire, attached new lengths of wire, covered the joints in heatshrink & connected the new leads to pins 2 & 8.

To confound things, I replaced the quad P/S capacitor at the same time as I fixed the wiring... but the popping noises are gone now. I'm guessing that the wiring was the culprit.

 

[dewickt]

MC30 biasing and filament surge

Turn on surge is common, this is due to cold filiments getting their initial hit of voltage, if this bothers you add an NTC resistor like the MC240 uses, thruthfully I would not care since it hasent caused the transformer to fail for the past 40+ years.
My schematic is for the later MC30 drawing # MC-30-4 (#15329 and above, 6-15-56), R30 is the bias droping resistor with C13 the - bias filter, the bias is called out at -42V with B+ at 440V, this is the most common version of the MC30.
Watch out on the early version that uses the 2 extra .47uf capacitors and has the square bias rectifier on the capacitor side of the board, it does not have a droping resistor and if you replace the selenium you will have to add a resistor in the transformer feed side of about 8.2K.
Remember to much bias makes the amp run cooler but also decreases output power and raises distortion.

 

[mike5500]

okay... thanks very much...by the way... since I don't have a variac to get the B+ right at 440, what if I used a 1/2W 10K trim pot instead of the drop resistor? how do ya interpret the serial numbers on these things? I'm sure mine aren't from the same run, or same year for that matter but i am curious.:yes:

Thanks again,
Mike

 

[audio1970]

question on headphone output on mc502 power amp

hi,

I'm running a couple of mc502's in mono, the other day I plugged my headphones into them and noticed that on one of them only the righthand side has any signal. the other one plays as expected a mono signal through both sides of the headphones. I switched the amp back to stereo and plugged in the left input and then both channels had the stereo signal through the headphones. Swtiched back to mono and only the right channel again. Would this be a fault with the mono/stereo switch or something further down the line ?

thanks

A

 

[Face]

At low volumes on my MX115, I hear a constant static noise. Once I go past 1/3rd, it goes away. I used some Rat Shack contact cleaner and it helped a little, but it's still there. Is the static noise a problem with the switch itself, do I need to use Deoxit, or is something else the problem? A bottle of Deoxit arrived today, but I don't want to take the unit apart again unless I have to.
Thanks!

 

[HalM]

MC-501 Will not Remote Start

Factory-Recon'd '501 came last week and even though the knob on the right would power up the amp, using the remote start would not. The red LED for Remote would light up, but no matter what combination of remote cables and sockets tries would not fire her up.

Even though the shipping container is in perfect shape, sending the '501 for a trip from Tampa to upstate NY is out of the question. Two different carriers already cracked glass on a C-28 and an MC-352, all within the last month!

So the amp came out of the box and on to the bench. Lotsa terrycloth towels is the way to go. As the amp is rolling over on its back to get the bottom plate yanked there is the unmistakeable sound of bits and pieces tumbling inside the amp chassis.

Disassembly starts with the remote board and a magnifier inspection looking for cold solder joints, then moves on to yanking the entire front panel, then the amp modules and moving on to the autoformers.

Removing the transformers revealed that one of the rivets holding the threaded transformer case securing screws was snapped. The nut just turned and turned inside the chassis. Best idea was to remove the outer part of the rivet and use a 4-40 stainless allen screw into the hole of the securement stud.

The debris was found in the undersides of the output autoformer: Little pieces of transformer potting compound had cracked off the edges of the dried compound right near the wires as they emerge from the tar-pit and were flopping around. Whew! It coulda been much worse. Putting the chassis back together gave rise to a warnig for everyone who wants to work on their stuff: The edges of the stainless chassis metal are like little razor blades. Especially when you need to jam your hands into tight spots to start hardware. First blood-Hmmmm.

Using a continuity setting on a DMM, the tip of a remote cable was used as the signal injector and the circuit was traced through the J-5 connector on the input board, throught he connectors on the right channel amp module and into the power supply board. There the problem ended. The mddle connectors on the power supply board are J-14 and J-15. The plugs are not lettered and look exactly the same. You got it: The plugs were jacked in backwards, meaning P-14 went to J-15 and vice versa. The amp would still play in manual mode but the +5 VDC start signal would never reach the optoisolator's LED pin 1 as the P-15/J-15 is not redundant.

A bit of Windex to wipe off the blood, a bit of Wax'n'Feed to nourish the walnut and she's back in shape, operation normal.

Hal

 

[gdillingham]

Transformer issue??

I've just moved up from an Eico 2080 (which I have enjoyed for years) to an MC240/MX113 which is blowing my socks off! (Playing through E-V Aristocrats). I've had the amp for about a week: an untouched find from a used furniture store. I haven't done anything to it yet. A concern: one of the output transformers gets warm to the touch; the other stays totally cool. I've switched the 7027A's from channel to channel with no change in the effect. I'm not sure that there is any component beyond the power tubes and speaker impedance that could affect this. Any hints or suggestions?

Regards,

Giles D.
Cincinnati

 

[aball]

The power transformer is the one on the right. The two on the left are the output transformers. The middle one will get much warmer than the left one because it is cooling the hot power transformer. The heat from the power transformer is conducting into the middle transformer through the chassis. So if you have the far left one cool, the middle one warm, and the right one hot, then it is all normal. You are simply feeling conducted heat on the middle one - and not heat induced by the circuit. Hope that is clear.

You will want to have the capacitors, power supply diodes and main bias resistor pairs replaced. This will guarantee another 40 years of trouble-free life and have it sounding better than ever. I have done this to 2 MC240s now and it makes a world of difference. Enjoy your find! I wish I could be that lucky.

Arthur

 

[gdillingham]

Arthur:

I made the erroneous assumption that the center transformer was the power transformer! What a relief!

Do you advocate replacing the signal caps, or are you referring to only the power supply caps? I replaced all in my Eico to very good effect, but there appears to be two schools of thought on doing the same for the McIntosh instruments.

Giles

 

[dewickt]

Replace the power supply capacitors, 2 doubler and 2 filters, possibly the bumblebes (striped oned), and leave the Black beauty's alone, the 160P black beauty is a mylar-foil capacitor and it's not likley you can beat it's sound and quality at any price you can afford.

 

[mrz80]

Huh. I thought that Black Beauties were oil-impregnated paper dielectric, unless more than one variety of cap got that nickname. With old Collins ham radio gear, first thing anyone tells you to do with it is replace every Black Beauty cap in the radio.