12-22-2012, 05:57 PM
My son mentioned to me that he had a C28 that I gave him and that he has not used in several years. I asked him to bring it over so I could look at it.
It is in stock form and apart from front panel lights, seems to work.
The upper graph is the signal generator and the analyzer looped together.
The bottom is the C28.
I see two things that need to be addressed.
1 - An obviously leaky cap in the power supply. 60 hz is 70 db down which really isn't bad but it could be better.
2 - some high frequency hash or hiss.
As I progress with this project I will document it here. Someone might find it useful in the future.
I don't know what the 6kc and 10kc blip is. I suspect it is a artifact from the power supply in my computer.
The 120 Hz peak is the power supply fundamental, replacing the multicap with a modern style board and careful reference grounding improved it by 20dB++.
Replacing non shielded lines into and out of various boards (tone, Hi Low filter, main out) and rebuilding the whole ground structure, as well as slitting and soldering all shield drains, knocked down the 60 Hz noise 20++ db.
Replaced phono transistors just a few days ago to get the phono stage to show 90dB s/n using DOBs reverse RIAA just like we did back in the day (when we were lucky to get 80)
Those peaks of hum hide a lot of residual noise and low level detail while listening.
12-24-2012, 09:44 AM
I agree with your assessment.
The phono section on this one is not going to be used. Since that is the case, I doubt I will mess with it at all. Same goes for all of the rest of the low level inputs I really wish there was a easy way to convert the tape head and microphone to a high level input without padding. In other words, route the signal to the same place as the tuner or aux inputs. He will have to settle for the tape pushbuttons for additional inputs.
We touched on this in an old thread. So many new to Mac owners are finding such enjoyment in discovering vinyl and the phono stage of the C28 was such a classic, I had to delve into it.
I have found the output of my Itouch so anemic that using a padded down non eq'd mic input an improvement....the added gain stage really helps the sound.
12-24-2012, 11:49 AM
Speaking of modern devices like the Itouch, I recently bought a Behringer 302USB mixer. That little device is also a cheap USB DAC.
12-24-2012, 12:11 PM
Good to find this thread today.
Bought my C28 new 1975. Was sent to McIntosh for repair (fuzzy Channel) in 1990. It sounded great when returned. Past months the records are not as clear so tried new TT cartridge. That didn't help. So my boy gave me his spare C27 to try and that woke me up. The other inputs on my C28 are as good as the C27 but my C28 phono is not even close. Maybe it never was? Will be in the weeds to see where you go with your C28. I am thinking about changing the elec caps in the phono section for starts. Don't know what to look for in the transitors? Going to see if I can put a 1k signal into my C28 and look at it on the O-scope. Being Ham Radio I have some old test equip but don't know much about audio. Terry in North Dakota
Just restored a client's C27 a few weeks ago.
The C28 project is MrFisheys fault, an EE student who might have a chace to post now while on break. He got me thinking of all the things I had wanted to do to these early 70's vintage units.
When a late version C28 (discrete tone boards) became available, just needed to apply restoration principles other than the old time fix'in and chas'in problems approach.