MM cartridge recommendations for original Mission 774 arm

[Phil Bishop]

Yes I noticed the cartridge tags looked rather like new Cardas stuff and
it generally looked nice quality to me.
I didn't want to hint at any unprofessional acts in any way,
I just stated what I would to if presented with such prob from zero..

Dedicated arm earth it seems to be then..a problem many early Rega arms share as far as I know..
Very nice arm and deck btw.

It's absolutely fine, I never took your comments to mean otherwise

I think you may have hit on the issue with the good contacts for the earth wires. I only have a bit of bare wire on hand here but the firmer I screw it on the less the hum. I think with a really firm connection from a good quality earth wire with tags (just ordered one) things could improve dramatically.

 

[Craig B]

Up and running, sounding excellent even with a humble OM10 but with a low level hum issue - always feckin' hum!

The OM looks right at home on the old 774, Phil. Very early '80s future tech.

Suggest you try connecting the tonearm earth wire to the earth terminal out back of the plinth. You could then also experiment with/without a dedicated length of ground wire between this terminal and phono stage or pre/amp chassis.

This is simply described in the instruction manual under part 3.5 as follows...

NOTE: A tone arm earthing screw is provided at the rear.​

What isn't made clear is that this terminal is common to mains earth and sub-chassis metal. On the old IIX it is also made common to the folded metal bottom plate. That deck could be placed directly upon an amp or receiver and still no hum.

P.S. If using two AS weights they are pushed together with the distance from the pivot measured to the join between the weights. You'll need consult the manual here but from memory circa 14mm out for 1.5g with both weights fitted, vs. something like 28-30mm out to the centre of a single weight for the same AS setting.

 

[Phil Bishop]

The OM looks right at home on the old 774, Phil. Very early '80s future tech.

Suggest you try connecting the tonearm earth wire to the earth terminal out back of the plinth. You could then also experiment with/without a dedicated length of ground wire between this terminal and phono stage or pre/amp chassis.

This is simply described in the instruction manual under part 3.5 as follows...

NOTE: A tone arm earthing screw is provided at the rear.​

What isn't made clear is that this terminal is common to mains earth and sub-chassis metal. On the old IIX it is also made common to the folded metal bottom plate. That deck could be placed directly upon an amp or receiver and still no hum.

P.S. If using two AS weights they are pushed together with the distance from the pivot measured to the join between the weights. You'll need consult the manual here but from memory circa 14mm out for 1.5g with both weights fitted, vs. something like 28-30mm out to the centre of a single weight for the same AS setting.

That photo was very useful Craig as it got me thinking. With this Systemdek I had replaced the motor with an Origin Live DC motor. However, I had not put an earth lead between that motor and the chassis. I have now and again the noise level has come down.

I have put a Rega arm back on for the time being but when I get the better quality earth lead and plastic screws I'll try the Mission 774 arm again. If it continues to frustrate me I may sell it on and just stick with Rega arms. First impressions are nice sounding as it is, the 774 is just a bit too delicate and fussy to set up for me.

 

[Craig B]

That photo was very useful Craig as it got me thinking. With this Systemdek I had replaced the motor with an Origin Live DC motor. However, I had not put an earth lead between that motor and the chassis. I have now and again the noise level has come down.

I have put a Rega arm back on for the time being but when I get the better quality earth lead and plastic screws I'll try the Mission 774 arm again. If it continues to frustrate me I may sell it on and just stick with Rega arms. First impressions are nice sounding as it is, the 774 is just a bit too delicate and fussy to set up for me.

Ah, I hadn't realized that you had gone ahead and changed out the motor for a DC type. This may actually explain things somewhat, with the main clue being revealed in this photo of OLs supplied mains transformer.

Basically, despite your having subsequently connected a lead between motor and chassis, this isn't an earth lead as there exists no path back to earth. Safety wise, a mains earth connection isn't required when the device is powered from an insulated AC to DC mains transformer, however, you may find that strapping the chassis to the tonearm ground pays dividends.

 

[Phil Bishop]

Ah, I hadn't realized that you had gone ahead and changed out the motor for a DC type. This may actually explain things somewhat, with the main clue being revealed in this photo of OLs supplied mains transformer.

Basically, despite your having subsequently connected a lead between motor and chassis, this isn't an earth lead as there exists no path back to earth. Safety wise, a mains earth connection isn't required when the device is powered from an insulated AC to DC mains transformer, however, you may find that strapping the chassis to the tonearm ground pays dividends.

Thanks Craig, I think there's a tonearm ground at the junction block for the Mission arm but not one for Rega arms?

Also, what's the best tool for investigating things? I was thinking some of those wires with crocodile clips at the end?

 

[Craig B]

The Rega arms are well grounded (better than most, really), only this is conjoined with Lch -ve at the external lead.

I was thinking of the Mission only here.

 

[Craig B]

Also, what's the best tool for investigating things? I was thinking some of those wires with crocodile clips at the end?

I must have missed this bit.

Are you experiencing some hum with the Rega fitted, or are you referring to further experiments with the Mission?

 

[Phil Bishop]

I must have missed this bit.

Are you experiencing some hum with the Rega fitted, or are you referring to further experiments with the Mission?

Hi Craig,

More referring to further experiment with the Mission.

I have ordered some leads with crocodile clip ends, rather like one used with a multimeter, to investigate things. This should enable me to clip one end on the tonearm earth and one on the chassis as you suggest, for example.

Cheers!

 

[Craig B]

Hi Craig,

More referring to further experiment with the Mission.

I have ordered some leads with crocodile clip ends, rather like one used with a multimeter, to investigate things. This should enable me to clip one end on the tonearm earth and one on the chassis as you suggest, for example.

Cheers!

You mentioned back in post #1 that the Mission had been refurbished by The Vinyl Source. Assuming they included rewiring the wand, did they also replace the external lead, and, if so, did they retain the miniature red male connection block at wand flying leads end, as well the tiny red socket on PCB that is mounted from below via two tiny screws that thread into the arm mounting plate?

None of your photos appear to reveal this, and it all makes a difference as to how the tonearm is grounded.

 

[cobbers]

Does the Ortofon have a ground strap to one of the return pins?

 

[Phil Bishop]

You mentioned back in post #1 that the Mission had been refurbished by The Vinyl Source. Assuming they included rewiring the wand, did they also replace the external lead, and, if so, did they retain the miniature red male connection block at wand flying leads end, as well the tiny red socket on PCB that is mounted from below via two tiny screws that thread into the arm mounting plate?

None of your photos appear to reveal this, and it all makes a difference as to how the tonearm is grounded.

Yes Craig, I'll get you some photos

 

[Phil Bishop]

Does the Ortofon have a ground strap to one of the return pins?

Not sure?

 

[Phil Bishop]

Underneath and top. The blue connection is the tonearm earth lead that I take to earth on phono stage. Hope that makes sense?

 

[cobbers]

The yellow wire at the headshell connects to one of the grounds ,it's possible that may be causing a ground loop - it certainly did with my old Empire.

 

[Craig B]

Does the Ortofon have a ground strap to one of the return pins?

Yes, Ortofon connect Rch -ve to the cartridge shield.

 

[Craig B]

Underneath and top. The blue connection is the tonearm earth lead that I take to earth on phono stage. Hope that makes sense?

That looks good, Phil. Same ground connection point as original.

Next time you fit the Mission, suggest you try a second short ground lead from the opposite tiny machine screw to the metal sub-chassis. You could test this using you alligator clips lead, and if successful, make up a short one using crimp on eye connecters either end, same as the blue insulated one, with the other end connected to the same screw on the sub-chassis where your original Systemdek chassis ground wire connects.

 

[Dowser]

Umm - please note the 774 arm has a dedicated earth lead at the headshell (behind the removeable sticker), but is joined to one of the cartridge earth returns at the red 4 pin connector (at least, stock it did). When I rebuilt my 774 many, many years ago (thread here somewhere) I added a dedicated earth return - never had issue

The blue connection above only helps if the earth wire from headshell is terminated there too. See my thread for details and I think, still photos.

Richard

 

[Craig B]

The yellow wire at the headshell connects to one of the grounds ,it's possible that may be causing a ground loop - it certainly did with my old Empire.

That was because your old Empire connected shield to Lch -ve. You could have transposed the headshell wire channels at the cartridge end and then reversed the RCAs at the phono input.

 

[Craig B]

Umm - please note the 774 arm has a dedicated earth lead at the headshell (behind the removeable sticker), but is joined to one of the cartridge earth returns at the red 4 pin connector (at least, stock it did). When I rebuilt my 774 many, many years ago (thread here somewhere) I added a dedicated earth return - never had issue

The blue connection above only helps if the earth wire from headshell is terminated there too. See my thread for details and I think, still photos.

Richard

Yes, the original wiring piggybacked arm wand ground on Rch -ve (green) and this was made common to the arm base (and hence arm pillar/yoke) via the PCB/SIP socket assembly. This is why Mission were specific about connecting the external ground wire via 'hole D' in the PCB and hence into threaded 'hole E' in the metal base plate, with the arm pillar clamp tying arm pillar and bearing yoke in.

Sort of like Rega's take on Thorens' old wiring only discontinuing the ground being piggybacked on signal -ve at the external lead.

Here is the old 4-pin SIP connector wiring diagram insert from the 774 instructions.

 

[Dowser]

OPs wand was rewired? Maybe the stock arrangement was missed and that’s the hum issue?