Adapted MC Phono amp circuitry query - UK Vintage Radio Repair and Restoration Discussion Forum

Bookman · 20th Apr 2019, 2:07 pm

A lack of response from AT concerning the hum associated with their AT632 MC step up txfr prompted a need for what I hope is a reasoned approach to alternate solutions.
In the first instance I need an amp to drive the output from an AT32 cart I have. It works very well through my old treasured Harman Kardon PM650 amp in our TV lounge. This has a MC input with a fascia mounted cap trim pot.
My main amp is a MF M3Si (in another room) which has an MM input but will play the AT32 albeit at around 80% full volume which attracts amp noise at that level.
I have therefore been looking for a used PM650 with a view to extracting the appropriate circuitry and took delivery of one this a.m. It is relatively straight forward in terms of I/P and O/P to preamp connectivity. I also have the diagram and can therefore trace the Voltage rails. Furthermore, I am of a mind to use the amp power supply and smoothing circuitry all packaged within a different box.
As a power engineer I am looking for guidance in terms of any potential pitfalls I may encounter that may detract from my efforts in this regard. I have attached pics from which one can see how compact the phono input pc board is. The black module affixed to it has no reference other than “MC amplifier module”
Incidentally for those wondering why I don’t buy a new step up transformer, would they please note I am a true Tyke.
Thanks in advance
Joe

Bookman · 26th Apr 2019, 4:08 pm

I can now report that the adapted circuit alluded to previously has proven to be a worthy endeavour. I finally tested the item today via the line input on my trusty and fully operative Sony radio cassette; see pic.
I used the AT32 via the MC input on the device and could not believe the sound that it produced; absolutely fabulous.
There were only two drawbacks for consideration, the first being that I had cut up the PC boards accordingly as pictured and then undertook full Voltage tests prior to configuring the circuits. The net result of this was not realising that the 4700 µF smoothing caps had not discharged which led to a few scary moments.
The second being, that in the interim I had decided to strip down the humming AT630 step up transformer that gave rise to the endeavour in the first place. Here, I removed all of the perished bracing foam and stuff and repacked it which then worked fine when tested. Probably more luck than skill what?

20th Apr 2019, 2:07 pm	#1
Bookman Hexode Join Date: Aug 2017 Location: Taunton, Somerset, UK. Posts: 318	Adapted MC Phono amp circuitry query A lack of response from AT concerning the hum associated with their AT632 MC step up txfr prompted a need for what I hope is a reasoned approach to alternate solutions. In the first instance I need an amp to drive the output from an AT32 cart I have. It works very well through my old treasured Harman Kardon PM650 amp in our TV lounge. This has a MC input with a fascia mounted cap trim pot. My main amp is a MF M3Si (in another room) which has an MM input but will play the AT32 albeit at around 80% full volume which attracts amp noise at that level. I have therefore been looking for a used PM650 with a view to extracting the appropriate circuitry and took delivery of one this a.m. It is relatively straight forward in terms of I/P and O/P to preamp connectivity. I also have the diagram and can therefore trace the Voltage rails. Furthermore, I am of a mind to use the amp power supply and smoothing circuitry all packaged within a different box. As a power engineer I am looking for guidance in terms of any potential pitfalls I may encounter that may detract from my efforts in this regard. I have attached pics from which one can see how compact the phono input pc board is. The black module affixed to it has no reference other than “MC amplifier module” Incidentally for those wondering why I don’t buy a new step up transformer, would they please note I am a true Tyke. Thanks in advance Joe Attached Thumbnails

26th Apr 2019, 4:08 pm	#2
Bookman Hexode Join Date: Aug 2017 Location: Taunton, Somerset, UK. Posts: 318	Re: Adapted MC Phono amp circuitry query I can now report that the adapted circuit alluded to previously has proven to be a worthy endeavour. I finally tested the item today via the line input on my trusty and fully operative Sony radio cassette; see pic. I used the AT32 via the MC input on the device and could not believe the sound that it produced; absolutely fabulous. There were only two drawbacks for consideration, the first being that I had cut up the PC boards accordingly as pictured and then undertook full Voltage tests prior to configuring the circuits. The net result of this was not realising that the 4700 µF smoothing caps had not discharged which led to a few scary moments. The second being, that in the interim I had decided to strip down the humming AT630 step up transformer that gave rise to the endeavour in the first place. Here, I removed all of the perished bracing foam and stuff and repacked it which then worked fine when tested. Probably more luck than skill what? Attached Thumbnails