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McIntosh - Hints and Kinks

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C32 had cable problems, it uses a variety of ribbon cable that gives problems, both delaminating and corrosion under some humidity and temperature conditions. Check all the cables especially the ones at the volume control, if you find a bad one the best fix is to use ribbon cable (computer style) and hard solder to the board - will never have a problem again.
 
dewickt said:
I clean the optics with a cotton swab dampened with household ammonia, be careful as the optics mount is delicate.
Click to expand...
Have you ever heard of the 'lint-free' swabs used by the semiconductor industry? They utilize what looks like foam/sponge-wrapped plastic sticks for the purpose of avoiding any cotton fiber shedding, etc. However, having worn plastic lens glasses for the last 31 years, I'm also aware that wet cotton doesn't scratch plastic like most synthetic fibre can. (Besides, lint-free products are probably prohibitively expensive, so I usually get free sample packs from the vendors at trade shows.

Also, was wondering why ammonia rather than alcohol? More streak/residue-free? Would this apply to tape heads as well (cassette & vcr)?
 
Most dirt I find on the transport optics does not require alcohol type of solvents, smoke, dust and such are best cleaned with a gentile cleaner, Reel to Reel is a different matter as the tapes leave behind nasties that require stronger solvents.
 
MR-74 stereo light

I bought an MR-74 and it was in excellent condition except that the stereo light bulb was out. It was a 1835 bulb, 55 V. The wiring diagram indicates 40 V on the circuit. Apparently there are two versions of the stereo light-one with an 1828 bulb (37.5 V) with a 270 ohm 1 watt resister and one like mine which uses an 1835 bulb and the resistor was replaced by a jumper. The previous owner said all bulbs were working and, considering the condition and function, I am inclined to believe him. Even though it was very well packed with double boxes, I thought it might have suffered vibration during shipment. I replaced the 1835 bulb with a new 1835 bulb. The circuit worked as expected and I cycled it by tuning different stations numerous times. Although the light appeared dim to me, I have never owned an MR-74, so I assumed that was the way it is supposed to be. The light appeared to be steady without flickering and I had the tuner on for several hours. Later, I noticed that the bulb was out again. I am not sure if the bulb went out in that first session or if I had turned the tuner off and back on. A local tech friend of mine, with no experience with older McIntosh equipment, suggested that it was nothing more than a bad bulb.
Before I try another bulb, I thought I would ask the group if there is an issue with this stereo light, if anyone has any thoughts about why this bulb burned out so quickly, and if there is a solution other than just replacing the bulb again.
Thanks, Ed
 
Hopefully, I can get this posted before Terry shows up with the right answer. :D

I don't have the sheet for a 74 but the mpx lamp is driven by Q306 and you should have 40v at the collector when a stereo program is detected (I'm looking at the sheet for a 77). Then, going backwards there is Q305, a diode and then T301.

It should be fun doing a little detective work.

Murray
 
OK, here we go, the 73-74-77 are basically the same demux, if it has the 270 ohm resistor use a #1828 (37V), if no resistor use a #1835 (55V). Either lamp has a slim long filament that hates vibration, and looks for an excuse to break.
Have gotten several tuners with a #47 in them, is bright for a while till the 270 ohm burns open after the transistor switch shorts.
 
Thanks Murray and Terry for your prompt replies. I am relieved to learn that the bulb is the problem. I had checked the other components but was worried I had missed something. ..........Ed :yes:
 
MAC1900 - Hot heatsink

If one channel of your MAC1900 is running hotter than the other it might be a good idea to check R415/R416 on the driver board, it must not be over 22 ohms. Had a 1900 that the 22 ohm R415 measured 27 ohms, the channel ran hot and would go into shut down after an hour. Replaced with a measured 21.8 ohm resistor, and heatsink barley get warm after 2 hour idle time...
 
Assuming someone else might make the same mistake I did, I thought I would post it here: If you replace the voltage doubler caps in the MC240 (or similar) with non-twistlock caps, don't forget to ground the positive cap to the chassis...

Also in my MC240, I used the IXYS DSEI 8-06A HexFRED as my replacement bias diode and that got the voltages close but still a little high at -55V (instead of -48) and -155 (instead of -150). Any idea what resistance value I need in series with this diode to get them in check? Also, is 1/2W resistor sufficient? I couldn't figure out the current draw from the schematic and I don't really want to do trial-and-error or use an underrated pot. Great thread, thanks for all the help!

Arthur
 
Well I just did some tube datasheet reading and some rechecking of my bias voltages. Maybe this info will help others using 5751s in place of 12AX7s in a MC240.

It appears that the 5751 can handle more grid voltage than the 12AX7s - specifically, -55V max, -50V design, whereas most of the 12AX7s I checked are -50V max. I measured my pin 2 voltages on my GE 5751WAs and have -50V on the dot so I don't think I need a bias resistor afterall since last time I tested the tubes, they were as new. The IXYS diode is close enough for these tubes. All the other tube voltages were at or slightly lower than spec.

If someone else is using this IXYS diode with 12AX7s, a "correcting" bias resistor value would still be helpful to post here. Maybe I will go back to 12AX7s down the road myself. Thanks

Arthur
 
Cleaning Older Mac Gear

Any advice on cleaning older 60's and 70's Mac gear would be appreciated. Specifically the cleaner to use for cleaning the brown transformer covers and such like so that I don't remove the silk screened lettering. Any advice on what cleaner to use for cleaning light rust and oxidation from the chrome.
Thanks!!
 
For chrome I use Wenol (http://www.wenol.com/), it gets the rust and oxidation. For the glass I use Turtle Wax Carnuba, can use it on painted surfaces just don't rub it a lot or you will end up with a beautiful gloss finish. For general dirt I use clear Ammonia, is very gentile on lettering, both the Wenol and Turtle Wax can eat at painted lettering if you rub to hard, the metal ID/information plates are safe.. Do not use Windex, or 409 type cleaners as they attack lettering and painted surfaces.
 
I've followed Terry's suggestion about using auto wax on the chrome, glass and painted surfaces. It looks great and a side benefit is that it seems to repel fingerprints a little better and makes the surfaces easier to clean.

Wenol is indeed great for getting the corrosion off of chrome. It'll (as will most cleaners) get your stamped ink serial number off too. :tears: Does a nice job on zinc plated hardware.

Murray
 
MAC 4300 question for you Terry: The unit did not come with jumpers for pre-amp to amp when I bought it new back in 1988. Should I get some anyway? Why would some units need jumpers and others not?

Thanks.
 
The MAC4100 and up do not need external jumpers for their preamp to amplifier connection, they have internal contacts in the RCA connectors that if an RCA is plugged into them the connection between the amp and preamp is automatically opened. This is a nice feature that helps keep the clutter on the rear panel a bit less, but if a unit has been in a damp area and has some corrosion problems these switching contacts have been known to fail necessitating the old style external jumpers.
 
Are the phono RCA shorting plugs on the back of a preamp/MX really helpful? Should I bother with replacing missing ones?
 
MR 74 Stereo Indicator

After having all the bad bulbs replaced, including the stereo indicator, the stereo indicator now ALWAYS glows red, but it does get brighter when tuning in the station.

Why doesn't it stay dark and glow only when tuning?
 
MC2205 help

I just picked up a MC2205 this weekend at a garage sale (along with a C28 and a MR71 that is in rough shape) and noticed a couple of things that weren't working:

1. The 2 right channel lights in the Output Mode box do not light (Limit, Normal).

2. The right channel big blue analog meter doesn't appear to be in synch with the left channel when the Meter Range knob is set to Watts mode. When set to any of the Decibel modes it funtions normal.

One last question on what you use to treat the Walnut cases with. I have used Formsby Lemon Oil on them and they have definitely improved, but they are somewhat soaking up the oil in a hurry.

Thanks,
Mike
 
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