C-26 Phono Section left channel issue (wooshing sound and very low output)

[Refugee]

I don't know how else to describe it. It's only one channel of the phono section. It sounds like a microphone in the wind. Is this what a bad transistor sounds like? Or is it like everything else in my life, "it needs to be recapped"

There is a schematic here http://www.hifiengine.com/library/mcintosh/c26.shtml

Above is from an old post. His description accurately describes my problem. Looks like the OP did not get much action - LOL

I have the same problem. It was working fine last week. I tried to play an album last night and the left channel was shot. I only had time to test a few things. I switched the left and right RCA inputs from the turntable and the problem remained the same. I moved the selector switch to 'phono 2' and the same sound was coming from the left channel (phono 2 has grounding plugs in the RCA jacks). I changed the mode switch to reverse and the problem moved to the right speaker.
This quick testing makes me think I have a problem in the phono section on the left board as all other functions of the pre are working great (tuner, aux). Before I take the unit out of service I was wondering if anyone else has had this problem or knows what usual suspects I should look for. Also, am I doing any further damage by continuing to use the pre in aux and tuner?
The unit has never been recapped that I know of and probably needs a full refurb but as always, I would like to find and fix the problem before I start a refurb project.
I was hoping that there was a separate board for the phono section but it seems that the phono sections are on the left and right boards.
Thanks for any ideas.

Attached is the service manual.

 

[w3jn]

Could be anything. Check the voltages on the three transistors associated with the phono amp (Q1/3/5 or Q2/4/6), mode switch in phono1 and your turntable connected. That stage is supposed to have 42 dB of gain, so a bad bypass cap could have it oscillating. If you have a scope, take a peek at pin 10 of the preamp card, or, lacking a scope, you can look at it with a DVOM in AC mode. Should be zero VAC there.

Report back with the base/collector/emitter voltages and we can go from there. Since the other channel is working we can presume the power supply is OK, but take a look at pins 9, 15, 19, 21, 24 for correct supply voltages.

 

[Refugee]

Thanks for the quick reply @w3jn -
I will get those numbers. I have not opened it up yet. I requested the proper service manual from McIntosh via email and am awaiting a response. I could only find one online and I think mine may be a later serial#. I have read that there were several SM's - we'll see.

 

[Refugee]

Update:
I ran the unit one more time to see if the problem was with cords or switches that I overlooked before opening up the unit. It worked for about two hours and then I started getting the low output and some low frequency intermittent burps etc...
I pulled the unit and opened it up and planned on getting the DVOM readings. I also planned on getting a recap list together since it is surely due - Surprise - the unit had all the small electrolytic caps already changed, so that's good. Nichicon ES in the signal path and Nichicon KZ in the power sections which is what I was going to replace with. Since the problem seamed to be intermittent and heat activated I tried some easy stuff first.
Some of the soldering on the newer caps looked a little suspect so I reflowed the five new caps and a signal wire that looked funny and cleaned the flux off the board. Next I cleaned the input select switch with some deoxit and a brush and some Qtips. I ran the unit for about 30 hrs. and it sounded great! I don't know if it was the reflow or just the switch cleaning - or both - but it works!
It might be my imagination but the unit sounds better on all input sources.

 

[w3jn]

Great news! I'm betting a crummy solder joint. Hazards of recapping.....

 

[Refugee]

I am looking to do a little upgrading to the phono stage of this preamp.
Specifically, I want to change some of the caps in the phono section of my preamp. These are part of the RIAA circuit. The caps are ceramic and I want to upgrade to PP film since they are in the signal path - or at least the feedback path. Namely C9,10 and C11,12.
I was able to find a replacement for C9,10 - .0012uf. I chose:
FKP2J011201D00HSSD
FKP 2 1200 pF 630 VDC 4.5x6x7.2
WIMA

My problem is the C11,12 replacement. It is a 430pf at 5%. I cannot find a replacement PP film, or PP film and foil, cap for this location. I can find a 470 but in the RIAA circuit it seems that I need to keep the value firm. Does anyone have any suggestions? What would you replace this part with?

Thanks!

 

[w3jn]

Polystyrene. Justradios has 430 pf and 1200 pF, both in 1%. Neevo replaced the ceramics in his long-running C20 restoration thread with polysyrenes and I think he was happy with them.

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[Refugee]

Polystyrene. Justradios has 430 pf and 1200 pF, both in 1%. Neevo replaced the ceramics in his long-running C20 restoration thread with polysyrenes and I think he was happy with them.

I'll try these. They had the 430pf in 5% only and I got the 1200pf in 1%.
Neevo's C20 thread is a great read! He was happy with these caps. I hope they work as well for me.
Thanks!

 

[Refugee]

What do you guys think of adding inrush current protection to the C-26?
I was considering adding an Amphenol CL-160 thermistor or two like the MC-2105 has. I know the filter caps are not as large and it is not a must to add these, but it can only protect the components and lower the wall voltage a little closer to the 117V that the unit was designed for? Right??
Do you guys think this should be added?? if so where would you put them?
Two in line between the red wires of the transformer and D301 and D302??? or One right after those diodes before the first filter cap??? Before the transformer??

 

[w3jn]

It would go after the fuse, on the primary side.

The thing only draws about 15 watts, I'm not sure a CL80 would get hot enough to fully reduce its resistance. I'd leave it alone.