Mcintosh C-26 phono input problem

[ivo]

Disconnected two red/white two red/white wire leads and got correct voltage on this spots 10.41v
Now I'm going to connect one of these wires back to the power supply and check again the voltage to verify which channel board is bad( maybe both...)

 

[ivo]

new update: for any reason, the 10v is disappeared from the disconnected points. Now I have only 1v on the point where 10v should be...Look like a voodoo magic circle around this Mcintosh... Put back the red/white leads to the power supply board and I have phono signal! Prety laud but I would say with not so good equalization - almost no bass and with additional small popcorn noises... But so far the other input - tape1 & 2 & AUX works very well. Very powerful!! Nice treble, deep bass! Tomorrow I'll leave this voodoo Mac to works a couple of hours...
I think the next step will be to replace these tantalum caps on the phono stage...
Can't explain WHY the phono input starts to work??? I still have only 1.1v on red/white leads. And why every other selected input is normally lauded now - the volume control knob is under 3 o'clock and the preamp provides very good sound for small room(10' x 10')

 

[ivo]

I ordered caps, resistors, diodes and 2N3738(Q302) for the full restoration of the power supply board. Will post the result ones I finished the job.
P.S. I also ordered four tantalum 10uf 35v. The service book sais "electrolytic" but these caps are looking( and i bet they are) tantalum caps...

 

[ivo]

This a update regarding Mcintosh C-26 problems. I replaced a few devices in the power supply board and two capacitors in the circuit boards.
replaced parts are:
D301, D302 rectifier diodes 400v 1/2 A replaced with 1N5395.
C307, C308, C15, C16 100uf 16v electrolytic capacitors replaced with Nichicon capacitors 100uf 35v
R313 1.5K 2W replaced with 1.5k 2W metal film resistor
R314 4.7K 1/4W replaced with 4.7K 2W metal film resistor
Now I have153v on the rectifier diodes(instead 170 before) and 114v on the with/orange wire point, 9.98v at white/red wire points which feet the circuits boards and 9.22v on white/violet wire which derived power to Phono stage.
Both Phono and input lines are working really good now! Actually I have a very strong signal at Phono input, stronger than other input lines.
The only problem(or maybe not?) is, that on the phono input when the volume control is zero(0) I still have very, very low sound( if you closed ear to the speaker). I don't know if this is normal for this preamp or this is an issue. Also, I have a small hum on phono1,2 position which is independent from the volume control. This hum is appear only when the phono input is selected...
Thank you so much for any help or advice!
P.S. I'm using a Carver tfm-25 as the main amplifier.

 

[clinic-audio]

Hello

Many C26 I receive for repair need electrolytic capacitors replacement . Depending serial number these multiple capacitors are differents .
Voltages are also different . Are you sure to read the right service manual ?
One serie use :
- 2 x 066095 double electrolytic capacitor 160µF/200V and 200µF/150V
the other one use :
- 1 x 066122 double electrolytic capacitor 50µF/200V and 200µF/200V
- 1 x 066123 double electrolytic capacitor 200µF/200V and 100µF/100V

Voltage are (following schematic)
- 066122 50µF we have 115V
- 066122 200µF we have 96V
- 066123 200µF we have 76V
- 066123 100µF we have 75V
- 066095 160µF we have 155V
- 066095 200µF we have 96V

As you can see again , the right service manual is very important

There is more than 6 different service manual for C26

 

[c_dk]

Besides the service manual differences, trying to evaluate noise using speakers is fraught with variables that cloud what is going on.

Examples.......is the noise coming from the amp not preamp?
Is the noise happening at the resonant frequency of the speakers?
Are the interconnect cables between the turntable and preamp and preamp and amp picking up the noise?

There are many more!

A proper spectral analysis with a scope will allow you to study at what frequencies the noise is at and then empirically verify what remedies are applied.

Without such empirical analysis all is but a crapshoot.

 

[ivo]

clinic-audio, my C-26 with serial # 78P11 and It's equipped with 2 x 066-095 double capacitors, and I have the following voltages there:
C304 160uf 200v I have 159v orange wire, and on 200uf 150v I have 120v orh/white wire
C305 160uf 200v I have 73.3v and on 200uf 150v i have 73v
I noticed yesterday that the voltage on c304 is hi. So the problem is in the big filter capacitors? I dought I can find a replacement for these big boys...
Question: The voltage is dramatically different just on 200uf 150v white/orn wire which derivate power to the power board. Because there is no enough rooom for two separate caps, can I install a discrete cap 200uf 150v and leave the original one with only one connection(160uf 200v)?
Thanks!

 

[ivo]

c-dk, the low hum noise appears only on Phono1,2 stage. No noise on other input lines.
Very interesting(!) - the preamp is open now, no top and bottom covers and the noise increase when I approach the unit with my hand(or body) and decrease when i pull out my self from the unit! Is it normal for one opened unit?

 

[Leestereo]

clinic-audio, my C-26 with serial # 78P11 and It's equipped with 2 x 066-095 double capacitors, and I have the following voltages there:
C304 160uf 200v I have 159v orange wire, and on 200uf 150v I have 120v orh/white wire
C305 160uf 200v I have 73.3v and on 200uf 150v i have 73v
I noticed yesterday that the voltage on c304 is hi. So the problem is in the big filter capacitors? I dought I can find a replacement for these big boys...
Question: The voltage is dramatically different just on 200uf 150v white/orn wire which derivate power to the power board. Because there is no enough rooom for two separate caps, can I install a discrete cap 200uf 150v and leave the original one with only one connection(160uf 200v)?
Thanks!

The voltages you report for C304A and C304B are very similar to the values I measured on a C26 that was on the bench. The 2W 1.5kohm carbon comp. resistor (R313) that feeds C304B had drifted to 850 ohms and was replaced with 3W 1.5kohm metal film resistor (advantage of low TCR); see here for details: https://audiokarma.org/forums/index...-and-upgrade-of-a-late-production-c26.782810/ . After R313 was replaced the voltage on C304B was ~95V.

 

[Leestereo]

...the low hum noise appears only on Phono1,2 stage. No noise on other input lines.
Very interesting(!) - the preamp is open now, no top and bottom covers and the noise increase when I approach the unit with my hand(or body) and decrease when i pull out my self from the unit! Is it normal for one opened unit?

Is the low hum heard on Phono with a turntable connected to the input? If no, a hum noise is expected/normal.

 

[c_dk]

c-dk, the low hum noise appears only on Phono1,2 stage. No noise on other input lines.
Very interesting(!) - the preamp is open now, no top and bottom covers and the noise increase when I approach the unit with my hand(or body) and decrease when i pull out my self from the unit! Is it normal for one opened unit?

The casework is the final shield for the circuits so a person's electo-magnetic halo certainly should effect the circuit.

 

[ivo]

Thanks for the information, Leestreo! I already replaced R313 with a metal film resistor but with the same 2W 1.5K and still have 120v on the point after the resistor. Do I need to go 3W? I didn't know the number of watts is related to the voltage drop... Also, I replaced C16 but with the same parameters 100uf 35v...
Do i need go to220uf?

 

[Leestereo]

...I already replaced R313 with a metal film resistor but with the same 2W 1.5K and still have 120v on the point after the resistor. Do I need to go 3W? I didn't know the number of watts is related to the voltage drop...

The power rating of the dropping resistor should not affect its function. I don't know why your 2W 1.5kohm resistor isn't dropping the voltage sufficiently; I would examine the connections on C304 for any anomalies (e.g., solder bridges, component leads making contact with conductive surfaces...)

Also, I replaced C16 but with the same parameters 100uf 35v...Do i need go to220uf?

The C307 capacitor was increased to give a little more reserve/headroom, this is desirable but not obligatory.

 

[ivo]

Resoldered all connections before and after C304 and also the derivate points on the power board to circuit boards(red/white, white/violet, white/orn and the voltage after R313 drop to 102v. Regarding the low-frequency hum: after the top and bottom metal covers were installed and the unit moved to the music rack away from the big subwoofer(who is just below my working bench) this noise disappears
THANK YOU for all help and advice which allow me to restore this beautiful unit!!
Ivo.

 

[Andre Garcia]

hi to all,
from where you can find the c304 and c305 in case i need them?
thank you in advance
Andre.

 

[Refugee]

hi to all,
from where you can find the c304 and c305 in case i need them?
thank you in advance
Andre.

I have a later model unit. The schematic calls for Two 160/200@200VDC

In my unit this is achieved (original from the factory) by using Two 150/80/80/50 250VDC. The two 80 sections make 160 and the 150/50 sections make 200.
CE manufacturing is currently making these using the old machinery from the Mallory factory. It can be purchased here:

Can Capacitor - CE Manufacturing, 250 V, 150/80/80/50 μF | Amplified Parts

150/80/80/50 uF, 250VDC USA-made electrolytic multi-section can capacitor by CE Manufacturing. Reproduced to exact Mallory specifications using original Mallory machinery and equipment.

www.tubesandmore.com

In the later units like mine the cap is mounted using standoffs - you may need to do this if the cap is too tall to mount directly to the chassis. (see double arrow below). The sections can be tied together using a trimmed lead from a resistor or other component (see single arrow).


I know this is a pretty old post but I hope this helps someone. Better late then never.
Alternative methods include:
*mounting discrete caps under the chassis
*or even re-stuff the cans with them

 

[Andre Garcia]

I have a later model unit. The schematic calls for Two 160/200@200VDC
View attachment 3037259
In my unit this is achieved (original from the factory) by using Two 150/80/80/50 250VDC. The two 80 sections make 160 and the 150/50 sections make 200.
CE manufacturing is currently making these using the old machinery from the Mallory factory. It can be purchased here:

Can Capacitor - CE Manufacturing, 250 V, 150/80/80/50 μF | Amplified Parts

150/80/80/50 uF, 250VDC USA-made electrolytic multi-section can capacitor by CE Manufacturing. Reproduced to exact Mallory specifications using original Mallory machinery and equipment.

www.tubesandmore.com

In the later units like mine the cap is mounted using standoffs - you may need to do this if the cap is too tall to mount directly to the chassis. (see double arrow below). The sections can be tied together using a trimmed lead from a resistor or other component (see single arrow).

View attachment 3037271
I know this is a pretty old post but I hope this helps someone. Better late then never.
Alternative methods include:
*mounting discrete caps under the chassis
*or even re-stuff the cans with them

thanks a lot. always useful information here.
AG.