C33 phono stage woes continue

[LotusFool]

Last year I sent my C33 to Audio Classics for repair of the phono stage. A loud hum could be heard on one channel when the volume on the C33 was cranked to the 2 O'clock position. I can't play the TT that loud. At 11 O'clock is really loud and is as high as I go, but I mistakenly had the volume up that high which is how I discovered it. I figure if the hum is auditable at high levels, it's there as well when played at normal listening levels and degrading the sound (although I can't hear/ detect it) at normal levels.

When I got the C33 back the hum was gone. They replaced one of the IC's in the phono stage, but some mistake was made because I discovered the TT played on all mode functions. If the TT was playing and I switched to tape 1, or aux, or any other functions the TT was still present. It was kind of amusing because I could play and hear the TT and the CD player at the same and hear both. To his credit, Ryan sent me a pre-paid shipper and I returned the C33 where that was repaired at no charge.

A few nights ago the dreaded hum returned. I reversed the wires from the TT and the hum stayed on the same channel, so it's not the TT. I reversed the IC's from the pre-amp to the amp and the hum changed sides. It's definitely the pre-amp. I need to have the volume cranked to hear it (2 O'clock). When I turn the function switch to Aux or Tape 1 or Tape 2 the hum goes silent, so I know it is the phono stage again.

I could and I might send it back to AC, or to Terry (which is becoming more appealing to me, because AC has had two shots at it, and it still hums) but I'm also thinking about just forgetting the phono stage on the C33 and getting a stand alone phono stage that works and is new. Some tech I was talking to last year said the phono stage on the C33 is very good, and would cost $1000 to better.

And so I ask you McIntosh folks what would you do? Is the phono stage on the C33 worth another repair? So far I believe I have $600 in repair's and it still hums. I'm questioning the wisdom of what could be another $400 repair bill and then it would be $1,000 into the thing. The other thing worth considering is; I listen to my system every night. Having the pre-amp out of the house for another repair for another 40 days is quite painful. Kind of like having your only car in the shop for a month.

Right now I'm completely discouraged and my gut tells me to give up on the C33. I would also be appreciative for any phono stage recommendations. Something that sound just as good or even better than the C33.

Many thanks,
Gary

 

[nbndtrain]

There are definitely ground challenges on these preamps. Is the hum present on phono1 and phono2? One thing you can do is ensure there is a star washer on one of the screws holding the phono board to the chassis. It should be on the lower one as seen here: http://img.canuckaudiomart.com/uplo...3_best_price_in_the_whole_civilized_world.jpg

This is one of the grounding points.

 

[c_dk]

Does it hum in both phono inputs? The C33 has separate phono gain stages for 1 and 2 so if another op amp has failed it will only be in one phono input.

 

[62caddy]

I would send it back to Audio Classics. They will do whatever it takes to make it right for you.

Anything electronic/mechancal can be be stubborn - just the nature of the repair business and since you already spent good money, there's no reason to spend more.

*Did they replace the phono jacks? These are well known for becoming corroded and losing ground over time.

 

[c_dk]

The C33 phono stage is in it's own isolated metal box to shield it from RF etc. The RCA jacks are securely soldered to this metal box. A cracked molex pin connection is certainly possible as is another failure of a 38 year old semiconductor.

Not knowing what AC specifically did for their $600.00 repair leaves many possibilities but I would expect for that amount of labor, even with outstate NY labor rates, a complete recap should have been done as well as the known issues with certain solder connections.

As for replacement cost.....what else will stack up but another Mac that will most likely also need restoration work unless it resells for 3k+

 

[jlovda]

I had a problem on a C34V I owned a few years ago. One of the wires going into a white plastic connector was intermittent. There is a tiny razor under each wire on the connector that pierces the insulation. In my case the ground wire was not making good contact. No hum just intermittent dead channel. Take a tiny flat bladed screwdriver and gently push down on each wire where it sits in the connector. On the phono board I believe the board is not secure along the edge where the connector are. (It hangs out like a diving board) Remove each connector first and then push on the wires.

If the hum changes when you change ICs, it would seem either the IC socket is bad or the soldering beneath the socket is cracked.

If you move your TT from phono 1 to phono 2 does the hum disappear? If you are are hearing sound from the TT when you are listening to other inputs, one of the FET switches handling the bad phono channel has shorted or has an open ground.

 

[LotusFool]

When I switch the IC from the TT, the hum stays on the same channel. When I switch the function switch to any other tape1 or tape 2 it is dead quiet. So I know it is the phono stage. I took 62caddys advise and sent Ryan at AC an email this morning and to my surprise, it is still under warranty and so it's going back to him. It was in his shop in Feb. 2017 and I would of thought it was out of warranty by now, but this just illustrates the professional level of Audio Classics and their continued commitment to excellence.

I have heard some say AC is pricy, and they may be, but when things go south, it's nice to know they have you covered!

 

[62caddy]

Just picked up my MX110 for A/C this morning for the same thing a stubborn hum that wouldn't give up. The last repair was done a good year ago but owning to the fact that I had since gotten an MX113, I never put the MX110 back into the system, and was unaware the hum issue had not been corrected until just recently. I explained to Ryan the situation and was prepared to eat the bill since I figured I had waited too long.

But the redoubtable Ryan told me to bring it up and not to fret - all would be taken care of for no charge. The best is yet to come: The cause of the hum was a new cap that failed that had been replaced in an earlier repair - not performed by Audio Classics - and still took care of it at no charge.

I cannot say enough things about Audio Classics. As far as I'm concerned, the quality of their service and dedication to customer satisfaction is second to none and the above is only one example of the many times they had gone above and beyond to help me over the years.

Incidentally - that hum occurred only when the Loudness was turned on. It was a Mallory cap that inexplicably failed within a relatively short time of being replaced.

 

[MACKIE1975]

I have heard some say AC is pricy,

Not really but they do have boat payment

 

[twiiii]

Call Ryan for a return shipping tag and they will take care of you, They have helped me for over 10 years with repairs and damages due to shipping upon return. Previous to that they were there for me and the company I worked for when we needed help either supplying parts when Mac did have them or moving older equipment that we had no market for. They also have a wealth of knowledge that money can't buy. They are truly one of the last true Mcintosh dealers where the customer and not the dollar come first!!!!!!