McIntosh - Hints and Kinks

[dewickt]

MX110 Power Transformer Heat

Have a MX110 on the bench, audio path and phono are updated, power supply is stock, after 14 hours I decided to check power transformer temperature.
Line voltage is 118V with a draw of .60 amp., transformer delta is 40 deg. F above room ambient, yes it is hot to the touch but is safe.

 

[dshoaf]

Thanks for that data point, Terry. I seem to remember this items comes up from time to time here and over in the Tubes forum.

Cheers,

David

 

[brichter]

Gotta give a shout out to Terry, just got my MR75 back and I have no words.

Not only do both channels now work in stereo, but the FM sensitivity and overall sound quality is much better, must be those magic caps and a good alignment. :yes:

Looks like I need to find a decent temporary amp so he can get his fingers in the guts of my MC2105, I might have a line on a Kenwood M1D.

 

[Paul K]

MC240 question re: Selenium rectifier work done on amp

Hi! 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...

I have a pic and am wondering if this is all good...any advice is needed - and there are more pics of all the work done inside if anybody can help me deduce if this is an alright restoration...
BTW I can't see the value of the cap next to the diode as the values are on the other side of it and didn't feel like doing any soldering to undo it.
The amp is working and sounds great - just wanted to know if this is the proper thing done by the tech...

 

[dewickt]

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.

 

[Paul K]

Thank you Terry for your reply. I emailed the tech and he was aware of the need for the proper voltages at those points...
Would you recommend anything else being done wrt any other resistors and caps in there being changed? This is purely for musical reasons....

 

[SoCal Sam]

Terry, fantastic resource this Hints and Kinks. I want to clear up some confusion on my part. I'm not so smart to ask with words so I took a photo of my just completed re-cap and indicated my questions.

1. I've indicated what I think are C9 and C10. Should these be .25uF or .47uF?
2. I've indicated what I think is the Bias Capacitor. I've got three 8uF in parallel to make 24 uF. IIRC, you recommended 22 uF to "stiffen up" the bias. Is this OK?
3. I just wanted to confirm the values of the other caps as well. I went off the schematic and also a later MC-60 but I couldn't figure out with certainty the values. Can you tell me the correct values?

Thanks!:yes:

 

[MX117.MC7150]

SoCal Sam - am guessing it will be a big help (even though Terry is really good) if you will tell us what amp you are asking about rather than having us try to identify from the tubes shown in the upside-down position or search back for your previous posts.

 

[dewickt]

MC60, depends on the build you have as to the capacitor values.
Bias diode current rating of 100mA or larger and 800V or more with a 1N4007 a convenient common diode to use, bias filter can be most any value above 10 uF, how you get there is optional, bias voltage is measured at pin 5 of an output tube.
The bias dropping resistor is determined empirically with 5.1K a starting point, it can vary on version.
Early versions did not have the .47uF caps to pin 8 of the output, check your schematic for the version.
The values you show are for a late production MC60 and are OK.

 

[SoCal Sam]

SoCal Sam - am guessing it will be a big help (even though Terry is really good) if you will tell us what amp you are asking about rather than having us try to identify from the tubes shown in the upside-down position or search back for your previous posts.

Oops, I need to be more clear when asking my questions. Yes, MC60.

MC60, depends on the build you have as to the capacitor values.
Bias diode current rating of 100mA or larger and 800V or more with a 1N4007 a convenient common diode to use, bias filter can be most any value above 10 uF, how you get there is optional, bias voltage is measured at pin 5 of an output tube. The bias dropping resistor is determined empirically with 5.1K a starting point, it can vary on version.

Thanks for the quick answer. I used 1N5408. I'm getting -45.5V at pin 5. and 403V at pin 3 plate.

Early versions did not have the .47uF caps to pin 8 of the output, check your schematic for the version.
The values you show are for a late production MC60 and are OK.

I'm using schematic for "SERIAL NO 3FB46 AND ABOVE" My serial number is 1402 so I'm guessing I have an early version. The original caps for C9 and C10 were .22 uF, should I install the original value or is .47 uF OK?

C2 was a 22 uF previous owner replacement so I am not sure if this value is correct. My later production MC60 and schematic show C2 as 100uF 12DC. That is what I have installed. Should I stay with the 100 uF?

I just played the amp for the first time this morning and it sounds fine.

 

[SoCal Sam]

The MC-60 re-caps are done and playing but I am having trouble getting the voltages right and concerned about some redplating in the creases. I'm running old Sovtek 6550's which I do not have any experience with but I think any redplating is bad redplating.

Here are my readings. What do you think?

The three figures are for specified, amp#1 actual and amp#2 actual. Amp#2 seems to be running hotter.

V1-12AX7

pin 1 spec, measured amp#1, amp#2: 134V, 100, 101

V2-12AU7

pin 1 spec, amp#1, amp#2: 270VDC, 269, 319
pin 2 spec, amp#1, amp#2: 134, 101, 101
pin 3 spec, amp#1, amp#2: 138, 109, 111
pin 6 spec, amp#1, amp#2: 270, 269, 310
pin 7 spec, amp#1, amp#2: 110, 81, 89
pin 8 spec, amp#1, amp#2: 138, 109, 111

V3-12BH7

pin 1 spec, amp#1, amp#2: 340, 353, 369
pin 3 spec, amp#1, amp#2: 16, 16, 16
pin 6 spec, amp#1, amp#2: 340, 353, 380
pin 8 spec, amp#1, amp#2: 16, 16, 16

V4-12AX7

pin 1 spec, amp#1, amp#2: 435, 431, 469
pin 2 spec, amp#1, amp#2: -46, -46, 43
pin 3 spec, amp#1, amp#2: -46, -45, -44
pin 6 spec, amp#1, amp#2: 435, 430, 465
pin 7 spec, amp#1, amp#2: -46, -46, -44
pin 8 spec, amp#1, amp#2: -46, -46, -44

5U4, both

pin 2 spec, amp#1, amp#2: 440, 433, 475
pin 4 spec, amp#1, amp#2: 375VAC, 357, 386
pin 6 spec, amp#1, amp#2: 375VAC, 357, 386
pin 8 spec, amp#1, amp#2: 440, 433, 475

KT88/6550

pin 3 spec, amp#1, amp#2: 435, 428, 443
pin 4 spec, amp#1, amp#2: 435, 428, 443
pin 5 spec, amp#1, amp#2: -45, -46, -45.2

 

[rye667]

mc 30 with 2 speaker systems

hi there. first of all hi to everybody in here.

since i'm new to this forum and the mc world as well (using 2 mc 30 and c20) i want to ask the established old schoolers if it is possible to connect two speaker systems to a pair of mc30. i use one pair of hh scott pro100-b and a pair of kef concertos as speakers.

the scotts got 4 ohm and the kefs 8 ohm. if connecting both systems there would be about 2.8 ohms if connected in parallel. since the manual allows a minimum of 3.2 ohm maybe the answer would be no... :-(

would you state this?

the way of connecting 4 and 8 ohm speakers in series (12 ohm) would maybe be a way (connecting to the 8 ohm plug) but maybe not good according the sound convenience...

coul anybody give me some hints?

thanx

rye

 

[dewickt]

MC60 - I notice one of the amplifiers is running high B+, check the power transformer tap, it should be BK-RD for 120V, the higher B+ would occur if it was the BK-YL wire is hooked to the plug. Other than this problem your voltages look within range.

MC30, try them in parallel using the 4 ohm tap, should do no harm to the MC30, just don't know how well the levels will balance.

 

[SoCal Sam]

MC60 - I notice one of the amplifiers is running high B+, check the power transformer tap, it should be BK-RD for 120V, the higher B+ would occur if it was the BK-YL wire is hooked to the plug. Other than this problem your voltages look within range...

Amazing! Yuppers, the transformer wire was BK-YL. Changed to BK-RD. Voltages are now -40 and 411 so I'll have to change the series resistor (again).

Thanks for your help!!:yes:

EDIT ADD UPDATE: In went a 6.2k series resistor. The voltages are now -45.7 and 433.

 

[dewickt]

The bias voltage is for a B+ on the plates of the output tubes at 435V with the bias at -45V, it is designed so that if the plate voltage changes the bias voltage will track, this will keep the outputs in their operating range at all voltages.

 

[rye667]

thanx for answering Terry!

so i gonna try connecting both 4 and 8 ohm speakers to the 4 ohm connection of the mc30. i will report on the level balance...

rye

 

[swingarm]

Newbie here, I just got my MAC1900 and everything is good with it except the FM Tuning/Signal Strength bulbs are out. I found a couple of posts in here referencing it so that along with a copy of the service manual I'm a little confused on which lamp to order.

And just to be sure all I have to do to access those lamps is remove the front cover?

 

[dewickt]

The preferred lamp is a #1847, a long life lamp, you can also use the more common #47 lamp, either will work fine, same basic lamp just not rated for long life.

 

[rye667]

hi again.

one of my mc30 monos is running hot. the sound is there, but the endtubes are glowing much to much. measuring the bias voltage gave a low -1.7V.

what could be the problem? would the selenium diode replacement to 1N4007 together with 22uF cap and 9.1k resistor as reported else (http://www.investmentat.com/antique/antique-0700.html) do the job?

the other mono shows -46.8V, what i consider being allright, but i'm also thinking of doing the replacement here.

rye

 

[dewickt]

Yes this should do the job, ideal is -42V if the plates are 440V, bet the original 10uF cap has shorted with age.
Your amp with -46 is a bit high, will not hurt anything but may raise distortion slightly.