McIntosh - Hints and Kinks

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What are the 'got-chas' on doing all this for the C-24 and what replacement signal and other caps are advised/recommended ?
TIA!

Sorry to be replying to this several months late. FWIW, last year I sourced all electrolytic caps, including cans, for my c-24 directly from the factory. Seems like the total was around $60.
 
Having just read thru 47 pages of hints, I grabbed the information that related to tube related Amplifiers and collected it into one file. I thought this might also be useful for other:



MC30
Here is a good starter, MC30 bias upgrade for safety and reliability.
Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a

good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it

back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is

exactly what it should be with these 3 changes, and the long life of your MC30 is one step further.

440V on the plate and -42V on the grid of the 6L6, power transformer type is of no consiquence, replace the

selenium and change the value of the resistor in series with it to set the bias, about 9.1K should get you close,

a value either way may be needed to get it right on. I find anywhere from -42 to -43 is good when plate is at

440V..
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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.
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MC30 thru MC275 distortion
MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2,

12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin

7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias

voltage resulting in the phase inverter outputs being unbalanced.
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MC60 Bias update
When changing out old bias diode to a silicon (1N4007) change the 1.8K series resistor to 5.1K, check bias for -45

or slightly higher (-46) when B+ is at 435V on the KT88. If you have a fully meterd variac it should draw 1.2A at

120V.

Note: some very eary MC60 did not use a series resistor, you may have to experiment to come up with a good value

for the resistor.
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Speaker terminal screws
If your screws on the speaker barrier strips are chewed up and you want to replace them watch out - if the new

screws are too long they will short to the chassis.
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MC240 Bias update/mod
When replacing the selenium diode in the bias supply with a silicone diode (1N4007) use a 470 ohm resistor in

series with the transformer lead going to the diode, this should put the bias voltage on the button !! While you

are at it replace the bias filter, a 22uF 450V cap will stiffen it up and last forever.
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Bias Voltage MC240
The banded end of the new diode should be tied to the transformer, this will give you a - voltage, it may need to

be tweaked with a resistor in series at this point to get the proper -46V (loaded) when the B+ is 430V at the

output tubes. When all is working as it should with the AC line at 120V the AC current should run 1.2 to 1.3 amps

when you have a set of good tubes installed.
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Rebuilding Bias Supply on MC-40
I have 2 MC40's to do this to waiting on the shelf, the resistor will be in series with the diode and the lead

from the transformer. The value should be about 470 Ohms, you may have to adjust it slightly to get -52V on pin 5

of the 6L6 with the voltage on pin 3 at 470V. I use a variac to make sure the plate voltage on pin 3 is at 470

before I measure pin 5.
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First thing to check on a MC240 is the mode switch, work it a few times to get the self wiping action to clear

oxidation off of the contacts.
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" I opened up a 240 to see if the selenium rectifier had been changed and confusingly there has been a diode

soldered in but no resistor..."

A percentage of the 240s do not need the 470 ohm resistor in series with the diode, variables in the power

transformer and circuitry can change the voltage. It is best to measure to make sure 430V plate voltage and -46V

on the grid are the ideal with some variation up to -48V at the grid being acceptable.
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MC240 Thermistor
CL-50 NTC, thermal resistor available from most supply houses, or from the factory parts department 800-538-6576.
The MC240 uses a voltage doubler type power supply, the NTC is there to limit the turn on surge to the supply,

makes the power transformers surge less, and doubler capacitors last longer.
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Mcintosh Chrome

McIntosh chrome responds nicely to a good car wax, I use Turtle wax Carnuba paste, just don't rub real hard on the

lettering, and also gives the black paint a nice shine. If your chrome is getting tired with pits or hazing you

can use Wenol metal polish (www.wenol.com), it can help revive the chrome to be as good as it can be. Whatever you

do use a clean soft rag to do it, keep using a clean section as the dirt it picks up can scratch the chrome

leaving swirls and hazing.

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.
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MC225 hot 12BH7 and 22K, 1W resistors.
If your MC225 has cooked the two 22K, 1W resistors and the 4.7K, 1/2W resistor, replace the pair of .047uF bumble

bee capacitors feeding the grids of the 12BH7 at the same time you replace the resistors. When the .047uF caps get

leakage they will raise the grid voltage of the 12BH7, I have seen 96V on the grids, replaced the caps and had it

go down to 33V, close enough to get the tube running in spec. The second thing this will do is raise your output

power from a sad 22W to a happy 30W+ as it did for me on a customers 225 that was running all original McIntosh

tubes.

Mc225 Thermistor replacement?
In another thread I'm rebuilding the power supply of my MC 225, and a question or two has arisen. first, what does

everyone use to replace the thermistor with?

Secondly, when replacing the bias diode with a 1n4007, which way is the band oriented? The original has one wire

on one side, and two on the other.

What is the value of the bumble bee that is on the small power supply board?

I would use the G.E. CL-80, 47 ohm, 3 amp, NTC, gives a nice soft start.
The band is the + end, this is the end that goes toward the transformer R46-R47 junction.
The capacitor is a .01uF use 630V, C24.
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And here's a summary of some of the tuner comments:


Tuner alignment MR65B
Here is a useable complete alignment procedure for the MR65B, can also be use for a MR71 and MR67 with some minor

part number changes.
Alignment for MR65B

IF Alignment :
Remove V11 osc.
Inject 10.70 mHz into pin 7 V2B mixer
measure at TP1 and adjust
T2, T3, T4, T5 for maximum
T6 bottom for maximum voltage at D2
(may have to misadjust T6 top to get voltage near .2 V)
T6 top set for 0 V at D2
T7 bottom for maximum voltage at D3
(may have to misadjust T7 to to get voltage near .2 V)
T7 top set for 0 V at D3

The top adjustments of T6 and T7 set the center tune points of the discriminators for audio(T7) and AFC(T6) they

can have more than one 0 point, choose the ome that gives the most change if the 10.7 frequency is shifted (most

sensitive).

RF Alignment:
install V11 osc
FM Signal to antenna terminals
keep signal low for good reading at TP1
at 106 mHz peak C2, C5, and calibration trim C53
at 92 mHz peak L1, L2, L7, and L11 for calibration
the calibration at 92 and 106 may have to be repeated several times to make the dial correct.

Demultiplex Alignment:
FM stereo signal to antenna center tuned
T501 for maximum voltage at V14B pin 9 (19 kHz peak)
The following should be done with the stereo signal set for L or R modulated with the other channel not modulated

(L only) read output voltage at the non modulated channel output and adjust for minimum output, L501 and T502 (38

kHz Osc).
L504 is the SCA filter and is peaked for maximum seperation and SCA rejection.

If setting of T502 is in question pull V14 and set frequency at V15 pin 6 for 38 kHz by adjusting top of T502.

All adjustments in the stereo demodulator an be slightly re adjusted to maximise the seperation to a minimum of 30

dB.

A 65B in good condition can get as good as 33 to 35 dB seperation and RF sensitivity of 2 uV for 3% THD (30 dB

SNR).

For improved distortion you may want to replace C75 & C77 (10 uF) at the volume control, and C74 & C76 (1 uF) at

V9 and V10 with new quality components.
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MR65B, MR67 Increase Stereo Separation
While giving a MR65B the work over on the bench I found that I could not get better than 28dB of separation, I

remembered a modification used in the MR71. Since all three models use the same basic demultiplex and output

stages I figured that the modification would help my separation problem.

Connect a 27K 5% 1/2W resistor and a .0015mF 10% capacitor in parallel. Connect this network between pin #8 on one

6BL8 audio tube to the pin #8 on the other audio tube.

Amazing --- the MR65B now has over 40 dB of separation just like the late production MR71, and a definite increase

in sound quality…..

This modification makes up for a slight loss in the L-R portion of the composite signal caused by the SCA filter

by increasing the difference signal gain in the left and right audio amplifiers while leaving the common signal

gain unchanged.
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MR73, MR74, MR77 Bulb

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.
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12AT7 in McIntosh Tuners
Most all the tubed McIntosh tuners MX110, MR65, MR67, and, MR71, use a 12AT7 in the front end of the tuner. This

tube is used as an RF amplifier and mixer stage and can have a big effect on the overall performance.
Here is the problem, overall sensitivity is only 5uV and has a tendency to oscillate on the high end of the band,

is very tweaky when trying to align. You have put in a new tube and expect more than this !! this is a NEW stock

tube... Well take it out and use an old stock or used tube of the old design without the internal shield, all the

problems will go away. It seems that the new tubes with the internal shield works fine in preamps at audio

frequencies, but at RF they will not work!! The internal shield has plate to plate capacitance that will cause the

tube to oscillate messing up the front end of your tuner.
This is one case where new is not necessarily better.
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Thank you for the summary's on amplifiers and tuners, should save a lot of hunting, little did I know when I started this topic that it would grow to 48 pages.
Opps, just did a check and now it only has 18 pages ...
 
"New" Mc2300

Terry,
I was wondering if you would be so kind to give me some advice? I recently purchased Mcintosh Mc2300.that is in very good cosmetic condition. There is some rust on the power transformer (and maybe the Autoformers) that concerns me. Should I leave it alone, or try to clean it up?
I was also wondering what components typically need to be replaced in a 42(?) year old first gen. Mc2300? (1Y983) I'd like to use the best possible caps, and whatever else might be necessary to bring this amp back up to spec.
Any advice would be greatly appreciated!
 
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Rust is common and just cosmetic, never have seen it cause operational problems.
Sensitivity may be low from bad capacitors on the preamp board, I use Nichicon Muse both the EH and KZ where appropriate.
If in doubt on condition of the 2300 get it to a test bench with proper load resistors and a good distortion analyzer, then test to verify it meets specs, too capable an amplifier to test by ear.
 
Rust is common and just cosmetic, never have seen it cause operational problems.
Sensitivity may be low from bad capacitors on the preamp board, I use Nichicon Muse both the EH and KZ where appropriate.
If in doubt on condition of the 2300 get it to a test bench with proper load resistors and a good distortion analyzer, then test to verify it meets specs, too capable an amplifier to test by ear.

Thank you for your time, and trouble, sir. Your advice is very much appreciated!
 
Being new to Mcintosh I am not familiar with the type of wire used in the older amps. I recently acquired a MC225. Does anyone know what the wire was made of? Replacing it would be a awful mess so I would not want to if not needed but I do have some oxidation concerns if it is not made of silver.

Thanks
 
Pre-tined OFC copper I think with PVC insulation, used in all McIntosh, changing it out for silver wire would be a waste of time, the gains would be minute. The worst part would be the change in a known good layout and the possibility of fatal errors, change out the known bad bumble bee capacitors, and out of tolerance resistors. The brown dog bone capacitors are what gives the 225 it's fine sound, leave them alone.
 
Thanks Terry. I will stick with that approach. I combed through this thread and documented those items along with the power supply. Sounds like a sensible approach.
 
new thread? anyhow, a newly acquired mc30

it's rough, i'm wondering how to test the tranny.. one looks questionable. clearly, the output transformer. wires look like they got hot. one coupled cap in the board had a bad solder. i resoldered. was planning on using a lighter gauged fuse, and bringing it up on a variac. This particular one came from a military base. i can provide pics, specifics.. before stepping up to the plate and recapping it, etc. i at least want to know if it's a go, or just a ebay boat anchor.. thanks, rich
 
C34V turn-on noise

I have a C34V that I bought on ebay several years ago with numerous problems. I thought I had them all corrected but now something else popped up. When I turn it on, one channel has an ~2 sec loud buzz followed by a snap/pop after which it is fine. Turn off is quiet. The other channel has no problem. It seems to be the monitor amp since the noise is not present when I send the line level outputs to another amp. I notice there are two FETs that shunt the audio at the input of the amps for two seconds. However, that would not silence any noise created downstream. It's very hard to get any probes onto the amplifier components. Would anyone have suggestions where to start looking? I have found a source for the J174 P-FETs used as the turn-on shunts so I will probably try changing them. That is however just a shot in the dark.
Thanks
 
I'm about to bring an old tube favorite back to life ( MC30) but before I start digging in anyone with testing procedures for the trannies? Regards..Paul
 
Terry,
I have an MX 114 tuner/preamp and it sounds dull and tired. What can be done to improve the performance?
Thanks Terry,
Paul
 
Replace the electrolytic capacitors in the preamp and MPX sections, the originals are tired !! May as well do the power supply while you are into it.
 
Thanks for the quick response Terry. Any recommendations for best capacitors to replace the electrolytics with.
Thanks,
Paul
 
Terry,
In the MX 114 service manual, the preamp printed circuit board diagrams show C305 and C307 in the left channel and C306 and C308 in the right. The caps list show these to be electrolytic caps # 066-048. In my MX114, they're actually Tantalums in those slots. Should I replace these or leave them be?
Thanks,
Paul
 
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Tantalums were a good cap in their day, as they aged they develop problems like shorts and bad ESR, replace any you find !!
 
MA6200 question

I would like to replace the electrolytic caps on both driver boards - does the amp need to be recalibrated after that or is it a 'simple' swap job?

Any feedback will be appreciated.

Wolfram
 
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