Rebuild of early MC275

[Highlander52]

Hello McIntosh forum,

I normally am in the Pioneer forum (lurking and trying to learn) but have had the great misfortune/fortune to inherit a nearly mint MC275/C20 combo. The 275 is s/n27510033 which I believe makes it the 33rd or 34th produced. My Grandfather purchase both new in the early 60s and they belonged to he and my Grandmother until she died at age 102 last month.
I've already talked to Terry DeWick on the phone (thanks Terry) and based on his advice am planning to rebuild keeping the circuit as per the original and that is were the rub is... the earliest schematic I could find is labeled SC 125-109C and is listed to cover my serial number but my amp does not possess C2/13 and instead appears to have a 1 Watt 973K .25% 25ppm resistor (6 band) in parallel with R11/36.

I've attached a picture of the area of interest (right side of pic) and my revised schematic(changes circled)

Am I missing anything, does anyone know why this amp is different from the schematic, has anyone run into this before?

Thank you.

 

[dewickt]

The items you are looking at are capacitors, probably 330 pf.

 

[Highlander52]

Long lost capacitors

Thanks again Terry and sorry for my bungling the spelling of your last name...the stripes are white, violet, orange, orange, blue, and finally gold in that order (see picture). I've removed one and as a resistor it tests around 15M ohms as a capacitor it tests 68pf. Guess I've learned something today!

Highlander52

 

[jjblue84]

Your amp has been worked on previously. After restoring a mint, unmolested MC275. I know first hand what the factory solder joints look like. And those aren't. Someone must have changed the circuitry around, there is suppose to be a resistor there and a ceramic capacitor.

Heres a photo of mine post restore

 

[dewickt]

The early feedback network used the tubular capacitors as did the MC60 and MC30. Here is a sheet on how to read them, a page from an Allied Radio handbook I bought in 1958 when I was first starting in electronics, well used but still usefull. Yes, I was 13 when I bought it.

 

[Highlander52]

Capacitor Confusion

Terry,

Per your book (and online sources) the stripes are white, violet, orange, orange, blue, and finally gold or maybe white, red, orange orange, blue, and gold. Either way I can't make them come out to 330pf... white is +100 or 9, red is 2 or +-2 (violet is 7 or illogical), oranges are 3, blue is 6 or illogical (violet is 7 or illogical) and gold is +100 or illogical. Sorry to be a pain but am trying to figure this out just for curiosity sake. My plan is to put a 330pf Mica in C2/13.

Thanks again

 

[Highlander52]

Progress Update

So I've finished replacing all the Diodes, the rectifier, C22 and C23, C2 and C13, the multisection caps (C24-27) and C28. R60, R61, F62, R65, and R11 and R36. Finally the thermistor was replaced with a CL-50.

I used 1N5408G for the Diodes and the 1N4007G for the rectifier, based on comments from Terry elsewhere we'll see if I need a resistor in series with the 4007.

For C-28 I used a 12uf/250V Sprague atom cap and plan to use them for the other 12uf/250V caps as well.

For the C22/C23 I used Rel Cap Audio Cap RTs.

For the multisection Caps I used Dynakit 80/40/30/20 wired for 120/50.

I've checked resistance and are within 8% for V1, V2, V3, V5 and V6.
V4 and V7 are a different story however:
pins 1 and 6 read 52 to 56.
pins 2 and 7 read 1.09M to 1.15M
pins 3 and 8 read 207.5K to 208.3K

The KT88/6550 pins are all reading way off as well.

The above lead me to believe that C10, C11, C20, C21, R28-31 and R53-56 are in need of replacement.

Any thoughts from the experts or others?

Thanks!

 

[Highlander52]

MC275 Capacitor and Resistor list per SC125-109C schematic

C1 0.1 Handmade 4-TFT-R1006000
C2 330PF Mouser 598-CD15FD331GO3F
C3 100 Mouser 594-2222-118-18101
C4 0.22 Handmade 4-RT-R2206000
C5 15PF Mouser 598-CM05CD150J03
C6 0.047 Handmade 4-TFT-R0046000
C7 0.047 Handmade 4-TFT-R0046000
C8 0.22 Handmade 4-RT-R2206000
C9 0.22 Handmade 4-RT-R2206000
C10 12 250V Mouser 75-TVA1505
C11 12 250V Mouser 75-TVA1505
C12 0.1 Handmade TFT-R1006000
C13 330PF Mouser 598-CD15FD331GO3F
C14 100 Mouser 594-2222-118-18101
C15 0.22 Handmade 4-RT-R2206000
C16 0.047 Handmade 4-TFT-R0046000
C17 0.047 Handmade 4-TFT-R0046000
C18 0.22 Handmade 4-RT-R2206000
C19 0.22 Handmade 4-RT-R2206000
C20 12 250V Mouser 75-TVA1505
C21 12 250V Mouser 75-TVA1505
C22 0.01 Handmade 4-RT-R0106000
C23 0.01 Handmade 4-RT-R0106000
C24 100/30 120/50 Dynakit 80,40,30,20@525VDC
C25 100/30 120/50 Dynakit 80,40,30,20@525VDC
C26 100/30 120/50 Dynakit 80,40,30,20@525VDC
C27 100/30 120/50 Dynakit 80,40,30,20@525VDC
C28 12 250V Mouser 75-TVA1505
C29 100PF Mouser 598-CM05FD101F03
C30 100PF Mouser 598-CM05FD101F03

RESISTORS VALUE RATING MATERIAL NOTES SOURCE PART #
R4 200k 1/2W 5% Xicon Mouser 293-200K-RC
R5 200k 1/2W 5% Xicon Mouser 293-200K-RC
R6 100k 1/2W 5% PRP Handmade 8-12051003
R8 100k 1/2W 5% PRP Handmade 8-12051003
R9 1M 1/2W PRP Handmade 8-12051004
R10 22k 1/2W PRP Handmade 8-12052202
R11 2.7k 1/2W 1% PRP Handmade 8-12052701
R12 3.3k 1/2W PRP Handmade 8-12053301
R13 68 1/2W 1% PRP Handmade 8-120568R0
R14 100k 1/2W PRP Handmade 8-12051003
R15 2.2M 1/2W PRP Handmade 8-12052204
R16 18k 1w Xicon Mouser 294-18K-RC
R17 27k 1/2W PRP Handmade 8-12052702
R18 30k 1/2W PRP Handmade 8-12053002
R19 2.2M 1/2W PRP Handmade 8-12052204
R20 2.2M 1/2W PRP Handmade 8-12052204
R21 220k 1/2W PRP Handmade 8-12052203
R22 220k 1/2W PRP Handmade 8-12052203
R23 2.7k 1w Xicon Mouser 294-2.7K-RC
R24 12k 2w 1% Stackpole Handmade 8-06021202
R25 1M 1/2W PRP Handmade 8-12051004
R26 1M 1/2W PRP Handmade 8-12051004
R27 12k 2w 1% Stackpole Handmade 8-06021202
R28 18k 1/2W Xicon Mouser 273-18K-RC
R29 18k 1w Xicon Mouser 294-18K-RC
R30 18k 1w Xicon Mouser 294-18K-RC
R31 18k 1/2W Xicon Mouser 273-18K-RC
R32 220 5w Mills MRA Handmade 8-08052200
R33 220 5w Mills MRA Handmade 8-08052200
R34 1M 1/2W PRP Handmade 8-12051004
R35 22k 1/2W PRP Handmade 8-12052202
R36 2.7k 1/2W 1% PRP Handmade 8-12052701
R37 3.3k 1/2W PRP Handmade 8-12053301
R38 68 1/2W 1% PRP Handmade 8-120568R0
R39 100k 1/2W PRP Handmade 8-12051003
R40 2.2M 1/2W PRP Handmade 8-12052204
R41 18k 1w Xicon Mouser 294-18K-RC
R42 27k 1/2W PRP Handmade 8-12052702
R43 30k 1/2W PRP Handmade 8-12053002
R44 2.2M 1/2W PRP Handmade 8-12052204
R45 2.2M 1/2W PRP Handmade 8-12052204
R46 220k 1/2W PRP Handmade 8-12052203
R47 220k 1/2W PRP Handmade 8-12052203
R48 2.7k 1w Xicon Mouser 294-2.7K-RC
R49 12k 2w 1% Stackpole Handmade 8-06021202
R50 1M 1/2W PRP Handmade 8-12051004
R51 1M 1/2W PRP Handmade 8-12051004
R52 12k 2w 1% Stackpole Handmade 8-06021202
R28 18k 1/2W Xicon Mouser 273-18K-RC
R29 18k 1w Xicon Mouser 294-18K-RC
R30 18k 1w Xicon Mouser 294-18K-RC
R31 18k 1/2W Xicon Mouser 273-18K-RC
R57 220 5w Mills MRA Handmade 8-08052200
R58 220 5w Mills MRA Handmade 8-08052200
R59 16 5w Semitec Mouser 954-12D2-15LD or CL-50
R60 10k 2w Stackpole Handmade 8-06021002
R61 100k 1w Xicon Mouser 294-100K-RC
R62 100k 1w Xicon Mouser 294-100K-RC
R63 100k 1/2W PRP Handmade 8-12051003
R64 68k 1/2W PRP Handmade 8-12056802
R65 500 5w Mills MRA Handmade 8-08051001

 

[dshoaf]

Next step is to bring the amp up on a variac and check the power supply voltages. They will need to be in the ballpark before proceeding. Also check the bias voltage while you are at it. See the procedure in Hints & Kinks to do the adjustment after the diode replacement.

Once you are ok with them, then proceed to test the operating voltages on each stage. Record the to compare with the specs in the schematic. If they are in spec, the amp should be in good shape to run.

Cheers,

David

So I've finished replacing all the Diodes, the rectifier, C22 and C23, C2 and C13, the multisection caps (C24-27) and C28. R60, R61, F62, R65, and R11 and R36. Finally the thermistor was replaced with a CL-50.

I used 1N5408G for the Diodes and the 1N4007G for the rectifier, based on comments from Terry elsewhere we'll see if I need a resistor in series with the 4007.

For C-28 I used a 12uf/250V Sprague atom cap and plan to use them for the other 12uf/250V caps as well.

For the C22/C23 I used Rel Cap Audio Cap RTs.

For the multisection Caps I used Dynakit 80/40/30/20 wired for 120/50.

I've checked resistance and are within 8% for V1, V2, V3, V5 and V6.
V4 and V7 are a different story however:
pins 1 and 6 read 52 to 56.
pins 2 and 7 read 1.09M to 1.15M
pins 3 and 8 read 207.5K to 208.3K

The KT88/6550 pins are all reading way off as well.

The above lead me to believe that C10, C11, C20, C21, R28-31 and R53-56 are in need of replacement.

Any thoughts from the experts or others?

Thanks!

 

[Highlander52]

Next steps

dshoaf,

So I shouldn't be worried about the high resistance readings on V4 and V7? Some are almost 50% higher than spec.

The paper electrolytic caps were interesting...the external case would spin freely around the body.

I still need to replace the .047 bumblebees before I'll try it on the variac.

Thank you for the inputs!

 

[jjblue84]

Replace all the caps, everyone but the ones with red lettering. All the small value electrolytic. Check the fuse in the bias supply, mine looked fine but had no continuity. Also looks like your missing a resistor.

 

[Highlander52]

jjblue84,

I'm using a Mills for the 500ohm resistor you have pictured, it's smaller and doesn't show up well in the picture but it is there...thanks for checking work, I need it!

 

[Highlander52]

Finally finished, almost!

Had a 3 week vacation and have now pretty much finished the amp refurb.

Installed the last of the capacitors and checked the resistance which are all really good and even except for the 12AZ7 pin 1 and 6 which are 51-55 ohms instead of the specified 38 volts, pins 3 and 8 are 201.6-202.2 instead of the specified 170Kohms.

I brought it up on the variac and no smoke!!!...pins 3 and 4 on the KT88s read 435-437volts with no load and bias is -56.7 volts bumping the variac to produce 450 volts on 3 and 4 results in -59.2volts at pin 5.

Do I need to mess with anything?

Other pin voltages are all within 6% of specified values, amp sounds good but am going to hurt anything with these voltages? I can provide specific voltages if desired/needed.

Thanks for any input and for the great help that this forum always provides as I would not have gotten this far without it!

 

[Highlander52]

The 12az7 s are running much more "lit up" than the other small tubes and the getter appears to be damaged as well... Amp has only run about 45 minutes and I'm not sure if something is wrong or if this is normal for this amp/tube?
Any help would be much appreciated!

 

[dshoaf]

As long as all the operating voltages are in spec around the tube, then it should be fine. Note where the illumination is coming from. It could be just a bit more of the filament is showing.

Cheers,

David