Mcintosh C-26 phono input problem

[ivo]

I just acquired my first Mcintosh and this is C-26. Unfortunately, i couldn't get any signal(sound) from Phono1 and Phon2 inputs. Got pretty good sound on tape1&2, AUX, Tuner but NOTHING on Phono 1&2. Is it possible to be an oxidated selector switch problem? Or this may be a deep problem in the Phono sections? Again there is nothing out of the speakers when the turntable is plugged into Phono1 or 2 input - no sound, no buzz no hum...
Thanks for any help!
P.S.The seller said he tested and ALL inputs were working good...

 

[TSmith8605]

Before you declare "deep problem", double check your set up and make sure (with another preamp) that you are getting sound from your turntable and also double check the position of tape monitor switch.

Did the seller have verifiable recent-service history. If not the controls should be cleaned per the instructions found here: https://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/

If the problem is oxidation, rotating the source selector switch several times (and operating the tape monitor switch and stereo mono switch although these would affect all sources) several times while the TT is playing should at least reveal some sound hinting that further cleaning is needed.

Not sure why both phono channels would fail unless the power supply has a problem but the line stage is apparently working.

 

[ivo]

Thanks for the help! My Technics turntable works perfectly with my Pioneer system.. As I said the Tape1, Tape2, Tuner, and AUX inputs in this Mcintosh ARE working properly with a Carver TFM-25 amp. Both channels on phono stage 1 and 2 are totally silent. Which is strange. There is no low sound with distortion or hum and buzz. Just silent.

Not sure why both phono channels would fail unless the power supply has a problem but the line stage is apparently working.

If the power supply is not good why the Tape, aux and tuner are working? Is it possible just the section for the phono to be down? I never opened a Mcintosh...
Ill check for the service manual online.

 

[TSmith8605]

And you double checked the position of the tape monitor switch while attempting to play the turntable?

 

[c_dk]

There is a schematic in the Akdatabase for the C26.

There is a 75 volt regulated supply from which 10 and 9.5 volt supplies are derived, the 75 and 10 are used in the tone circuits while the 9.5 is exclusive to the phono EQ. While it is possible to lose the 9.5 and not the 10 it would be unlikely.

A picture with the bottom cover off might be helpful to see if any high tech mods have been attempted.

 

[ivo]

If the Tape Monitor is pressed ON the tuner and aux will be silent...

 

[ivo]

And you double checked the position of the tape monitor switch while attempting to play the turntable?

if the tape monitor is pressed on the tuner and aux will be also silent...

 

[ivo]

..."A picture with the bottom cover off might be helpful to see if any high tech mods have been attempted..."
Thanks, c_dk! I didn't open yet. Will do tonight

 

[ivo]

These is the photos of the open unit. I got very, very low(almost unheard) sound on phono 1,2 inputs when the volume control on the C-26 is to the end. I have decent sound on tape1,2, AUX, tuner, but for me, this is not really powerful sound.I would say the bass is very weak... I connected the C-26 to Carver TFM-25 and i think that if the volume control on C-26 is at 2 o'clock that should be VERY laud sound! With this position on the C-26 volume control, the sound is equivalent to 9 o'clock volume control on my Pioneer amp. I don't know if this is ok for this Mcintosh or this is a sign that something general is wrong...
Thanks for any help and advice.

 

[ivo]

I did some measurements using my digital meter. Power supply board: Q302 - b 73.3v e 72.8v c 145.1v(!) R310 both sides 145.1v(connected to orange/white wire)
The service sais: b 76v, e 75v c 97v volume control full ccw but ON, loudness FLAT, selector phono1, mode Stereo, no signal at inputs.
Red/white wire points should be 10v but i have just 08.1mv(!) These points probably derived phono stage...
P.S. meter on DC, black alligator clips on the frame

 

[ivo]

R312(27k 1/2w) obviously is open. I got 75v on the side connected to C306 and 8.1mv on the other side point connected with red/white wires.

 

[clinic-audio]

Hello

I will check missing voltages first
What can affect both channel if it is not missing voltages for phono input section ?

 

[ivo]

Update: picked up one leg of R312 it's giving exact 27Kohm... so it is ok.

 

[c_dk]

So it is time four some basic audio repair detective work, that is, what part is pulling down the needed voltage?

I do not see any previous work or monkey business ever done before. Most likely first time it has been opened up in 45 years.

 

[ivo]

I do not see any previous work or monkey business ever done before. Most likely first time it has been opened up in 45 years.

My filing are the same. Ok, I'm ready!
Preamp turned ON volume full ccw
Selector switch on phono1
mod switch Stereo
loudness Flat
No input signal in any inputs
Digital voltmeter on VDC, black alligator clips on the frame.

 

[ivo]

what part is pulling down the needed voltage?

Referring to schematic Red/white wires need to be 10v on power supply board. I have only 6.8 mV...
Q302 on schematic B 76v my measurment B 73.3v E 75V me 72.9 C 97V me 148.7 V

 

[c_dk]

Are you asking us to diagnose this long distance?

Can't be done.

However if your 10 volt supply is not there how can q13 and 14 function, they are after the tone circuits and the final gain stage so would need to function for the aux, tuner and tape input paths to function.

 

[ivo]

if your 10 volt supply is not there how can q13 and 14 function

This is a question in which i can't answer... Unless I'm doing a wrong measurement. I tried to do it in the way that the service manual described, but...who knows??
What kind of capacitors are the wite cylinders? They have parameters as electrolytes but looking so different... I saw two tantalums on the phono stage and they are very suspicious(usually, right?)
Thanks anyway for the help! I really appreciated it!!

 

[ivo]

I got a very useful Mcintosh semiconductor data at Tubebook.org. So I'll start to rebuild the Power supply board. First suspect is for D301/302 I need to have there 155v but I have 176v, so I'm going to replace these two diodes.
I really can explain my self why tape1,2 and tuner are working while on the red/white wires i have just 10mv instead 10v... even i switched the selector knob to tape1 instead phono1 (as the service book required)...Anyway, I'll replace all electrolytic caps and Q302 on the supply board and then probably the big filter caps...
c_dk, I never ask for any long-distance diagnostic, ok? I just ask for help and assistance(if it is possible!) from the people with lighting years knowledge more then mine - I'm just a thobbyist in this matter...If my previous post sounded like this I apologize for it...
I will post the result of my efforts here soon.

 

[c_dk]

While restoring this series of preamps I always try to clean up the voltages of this power supply as much as reasonable, so replacing the multicaps with discrete capacitors is a given.

However if you truly have 73 volts at the emitter of q302 but nothing much a few parts down the line for 10 and 9.5 volts, well your problem is either with those parts in between the emiter and the 9.5 feed to the circuit boards; or a part on the circuit board is shortened out to ground pulling down the 10 and 9.5 down to nothing.

Disconnect those feeds to the circuit boards. Do you have correct voltages now?

If yes you have cicuit board issues if no power supply issues. Could be something in between but you need to start somewhere.