Weird wiring in Musical Fidelity phono preamp sections

[mz543578854]

I still want to know proximity to the power transformer. A bad attempt at hum cancellation is all I can come up with.

OP, is this the correct circuit? https://www.markhennessy.co.uk/showpic.htm?mf_a1/a1_phono.jpg

The PT is on the opposite side of the amp.

Yes, the circuit is that one, AFAIK.

 

[cvanc]

Mod is right there...

It doesn't do squat. Take it out and tidy up the area. No reason not to.

 

[cvanc]

"A few minutes later" (as Spongebob would say)

Well... maybe it has one job.

Maybe the customers were told "snip this link for a 3rd gain option" which is exactly what it would do. Cutting the link gives higher gain that either switch position.

(Still needs to be pointed out both channels look different in our photos here)

 

[mz543578854]

It doesn't do squat. Take it out and tidy up the area. No reason not to.

Yeah, that is what I did on the A1X.

Maybe the customers were told "snip this link for a 3rd gain option" which is exactly what it would do. Cutting the link gives higher gain that either switch position.

At least in the A1X, the dotted R62 was not there, I think. Not bypassing the cut would also mean MC not being supported anymore, I think.

 

[caowei]

I think this is artificially increasing the "inductance", but after consulting all the information on MF A1, the PCB only cut one of the channels, so it is obvious that my idea is not valid.

 

[tiefbassuebertr]

Hi,

does anyone know what the two wires in the Musical Fidelity A1, B200, and other amps from that period are supposed to be for?
If I am not completely mistaken, the cut track is just reconnected with one of them, and the other wire is just in parallel to another (uncut) track.
This is the 2nd amp I see this "in person" on now and searching the net for pics of open A1s etc. shows this is "normal".

The tracks are both coming from the MM/MC switch.

Any reasonable explanation?

Thanks,
Mo

View attachment 1250283 View attachment 1250285

I still remember that after cutting the green wires, an additional resistor was soldered to one end of the green wire to increase the voltage gain (only for MC operation for cartridges with unusual output voltages). However, the other end of the resistor had to be soldered not directly to GND, but via an Elcap in order not to change the DC conditions.
Without the Elcap in series, it was possible to optimize the DC condition by changing the gain-determining resistor, but then the gain had to be brought to the same value on both channels with another RC series element (i.e. an additional resistor and Elcap).
The reason, why on one channel the cut track is present, and on the other channel wire is an uncut track, isn't to understand for me.
The question is, what channel show the mistake.
Uncut track means, the gain-determining resistor cannot be became a higher value after perform the above mentioned steps.

P.S.: part No R and C in the schematic in post #22 and on the image of showed PCB are different - for the voltage gain stage for moving coil cartridge the parts in the schematic are C32 (470uF) and R59 (1K) and on the PCB from image are C26 (not able to read value) and R51 (1K1).
As I recall right, more details concerning the green wires are to find in an other thread here on diyaudio, unfortunately was the chosen headline not "Musical Fidelity" and the model naming.