I have recently obtained a Universal Loading Module (ULM60) to go with the MC60 MC card for an A&R Cambridge A60, but it comes with no instructions and online I can only find the instructions for MM settings.

Can anyone tell me how to use a Multi-meter to test impedance on this ULM module to work out what loading is set for each switch?

The blue resistor pair at the top are in the “user programmable” site. I’ve included photos of both sides, plus the MM and MC expected loadings, and the MM instruction table, in case that’s helpful.

(The MC60 card is designed to offer an input impedance of 330Ω but with ULM60 Loading Module is meant to offer additional input impedance options of 100Ω, 30Ω, 10Ω and 100nF plus one use-programmable option.)

Any suggestions gratefully received! Thank you.

Is there a Table 2. ?

I don't see loadings for 30 or 10 ohm in Table 1.

Ideally you could have a loading for 20 ohm, for the AT30E cartridge.

Your photos are nice and clear. But on the pcb side, it's hard to make out all the traces.

Can you re-do, tweak the brightness and contrast so that all the pcb traces are visible?

One should be able to reverse engineer and draw a schematic from them.

Not me right now, furiously busy, but someone else might.

Agree it’s a bit hard to see traces from photo and there is possibly the odd soldered connection between elements which needs to be confirmed. The centre output connector looks like it’s possibly the common and you might get the resistance readings from one side and capacitance from the other which and you could use a simple multimeter for the resistance ones to give you an idea of what’s going on?

1: Turns on user set resistors: currently 47 ohm soldered in
2. Turns on 100nF
3. Turns on 10 ohm
4. Turns on 33 ohm
5. Turns on 150 ohm

Afaik these are switched in parallel with the existing 330 ohm loading.

AMR-iFi R&D

Disregard my last thing. I assume the double up of resistors/capacitor is for a balanced output? Colin seems to have it.

For RDM - you are right - No table 2 - seems to be "unobtainium" anywhere. Table 1 is MM, Missing Table 2 is MC.
For RDM and Stuzzo - better photo of back of board up now - if anyone is serious about cloning this board I would be happy to loan this original - let me know.
For Stuzzo - that's exactly what I ended up doing - centre pin as "common".

And my results:
Pin 1 48
Pin 2 0
Pin 3 10
Pin 4 36
Pin 5 150

Interesting - using the Left pin at the back gave double each of those results.

But prize goes to Colin:
1: Turns on user set resistors: currently 47 ohm soldered in
2. Turns on 100nF
3. Turns on 10 ohm
4. Turns on 33 ohm
5. Turns on 150 ohm

Questions for Colin, please:
did you get that by reading the resister values?
and if so what resistor values should I use to get the 20Ω that my cart prefers?
Might my "0" on pin 2 just be because 100nF is really small?
If my cart prefers 20Ω, what effect does it have that these loading module values are "parallel with the existing 330 ohm loading".

So thanks for everyone's input - very good result!

If you want to get that close presumably you swap out the “user programmable” resistors.

The instructions for installation appear to be that the module plugs into 3 pins just behind the phono din input. Perhaps just take a leap of faith that that configuration it is right. Surely they wouldn’t offer those small resistance value options otherwise.

I don’t know enough about phono workings to advise how to test it.

I would re-flow the solder joints on those 3 main connecting pins too.

The RH one (from rear of board photo) looks particularly bad, with a semi-circular crack.

They're subject to stress from plugging in, etc. so may as well make them good.

It appears to be a parallel plug in, in which case, the new input impedance for say 30 ohm on the plug-in module with 330 ohm already will be about 30.3 ohm at the terminals as the lower value will dominate. Feel free to critique.

Jan 17, 2023 15:44:19 GMT 12 RdM said:

I would re-flow the solder joints on those 3 main connecting pins too.

The RH one (from rear of board photo) looks particularly bad, with a semi-circular crack.

They're subject to stress from plugging in, etc. so may as well make them good.

Thanks, well spotted - the module is working fine in the A60,  but next time I open it up I'll re-flow those joints.Thanks.

Jan 17, 2023 8:07:05 GMT 12 colinf said:

1: Turns on user set resistors: currently 47 ohm soldered in
2. Turns on 100nF
3. Turns on 10 ohm
4. Turns on 33 ohm
5. Turns on 150 ohm

Afaik these are switched in parallel with the existing 330 ohm loading.

Hi Colin - another question please.

What's the relationship between Ω and F? I see the MC card is 330Ω in parallel with 1000pF
and that the Loading Module has Ω and a 100nF option - how are those supposed to work together?

also
if these are in parallel, can you use e.g. switch 3 and switch 4 to get 44Ω?

Thanks -Paul 

Without doing the calculation I’m pretty sure that the 100nF capacitor that becomes parallel to the 330 ohm resistor will be the far lower impedance like with the resistors.

In summary: set the switch on the module to the 33 ohm which you should be able to test against the common or buy and install two 20 ohm resistors if you want that.(edit: 23/24 ohms is actually closest)

Plug the module into the amp just behind the phono input. You should be able to confirm with the meter that the input impedance is around 33 ohms as 33 ohms in parallel with 330 ohm is not much more than 33 ohm.

Jan 17, 2023 17:32:58 GMT 12 paulsaints said:

Jan 17, 2023 8:07:05 GMT 12 colinf said:

1: Turns on user set resistors: currently 47 ohm soldered in
2. Turns on 100nF
3. Turns on 10 ohm
4. Turns on 33 ohm
5. Turns on 150 ohm

Afaik these are switched in parallel with the existing 330 ohm loading.

[ ... ]

also
if these are in parallel, can you use e.g. switch 3 and switch 4 to get 44Ω?

10 & 33 ohm in series would get you 43Ω.

If in parallel, 7.7Ω (!)

But 30Ω & 100Ω (from your MC60 board specs - I don't know where Colin got 33 & 150 ohm from?) in parallel (if that is possible) gets you 23Ω.

If that combo is also in parallel with the 330Ω, then 22Ω.

But perhaps it will be possible to confidently replace the user programmable resistor with what you want.

20Ω & 22Ω are standard values in 1% metal film. They get you 19Ω & 21Ω in parallel with 330Ω.

I'm just guessing. Also made use of a parallel resistor calculator, plenty on the web.

[Edit: Ah, as per stuzzo, Colin is describing the actual resistor values, which in parallel with 330Ω gets close to what the MC60 description implies - 330Ω & 150Ω in parallel = 100Ω]

Just found a resistor calculator. So taking a guess to make things simple - please check me:

if we have 330Ω on the MC card, and toggle switch 4 with the 33Ω resister in parallel on the Loading Module we get the advertised option of a 30Ω final load?

Meaning:
1: Turns on 47 ohm -> 41Ω
2. Turns on 100nF -> no idea what this does
3. Turns on 10 ohm -> 9.7Ω
4. Turns on 33 ohm -> 30Ω
5. Turns on 150 ohm -> 100Ω

and soldering in a 21Ω resistor at switch 1 would give me the 20Ω I'm looking for.

or 330Ω + switch 4 and switch 5 (33Ω and 150Ω) would give me 25 Ω - which is getting close.

Which just leaves me wondering what the "F" option is for ...

F is for Farad, the unit of capacitance. It's so large, it's broken down into micro, nano, picofarads.

Maybe the n should be a p and it's a misprint?

www.digikey.co.nz/en/resources/conversion-calculators/conversion-calculator-capacitance

Yes to all!
Note that each switch on the board has 2 contacts on in, one each for left and right channels (DPST).
Close to 20 ohm would be (|| means in parallel with):
33 || 150 || 330 = 25 ohm
33 || 47 || 330 = 18.3 ohm
You could also solder in a 22 ohm resistor in place of the 47 ohm resistors, getting 20.6 ohms with switch 1 on.
F is farad. uF is F x 10 e-6. nF is F x 10 e-9. pF is F x 10 e-12. (e is exponent)
Adding capacitance, in this case 100n or 0.1uF, results in a peak in the response at ultrasonic frequencies. More info on cartridge loading here:
www.hagtech.com/loading.html

AMR-iFi R&D

Great response, Colin;- thanks!

So much fun and so much wiser - thanks everyone, and especially Colin - wisdom much appreciated 

Jan 17, 2023 19:48:58 GMT 12 colinf said:

You could also solder in a 22 ohm resistor in place of the 47 ohm resistors, getting 20.6 ohms with switch 1 on.

NO!

I have had my memory jogged, and realised that I had somewhere, stashed, an A&R Cambridge HA10 MC Preamp.

I found it, cleaned it, and found the manual and circuit diagram for it.

It's a standalone MC preamp that is identical (in function and circuitry) to the MC board with loading module for the A60.

Actually, mine is not assembled - it's just the populated pcb with the bottom half of the chassis, top missing, no plugs or sockets, has never been wired up.

At the time (decades ago) I had found no information, and didn't even know the voltage required to power it.

Now I have all the info. I will post pics later.

SW 1 (to the two 47 ohm resistors) controls GAIN! - from OUT = 15x (24dB)  to IN = 30x (30dB).

I think the "user programmable" means merely that one can switch in those other combinations of resistors (and caps).

See, for full details on loading switches, suggested switches for various cartridges - for AT30E they used 30Ω - and the circuit diagram! :

www.hifiengine.com/manual_library/arcam/ha10.shtml  (registration required)

ukhhsoc.torrens.org/makers/Cambridge_AandR/HA10_CART_PREAMP.pdf  (same info in slightly different format.)

I've also found new perhaps improved replacements for the LM394 in the HA10 & MC60 MC card but that can be another post.

Hi RdM - I'll pit up a photo of part the ULM/M instruction sheet for models with serial numbers above 13250 - I think you are right about how the external HA10 works, but the internal ULM module seems to be different.

Jan 23, 2023 17:44:41 GMT 12 paulsaints said:

Hi RdM - I'll pit up a photo of part the ULM/M instruction sheet for models with serial above 13250 - I think you are right about how the external HA10 works, but the internal ULM module seems to be different.

Oh gosh!

Maybe I was too impetuous there after all. That is different. (Sorry, Colin! ;- apologies for my hasty assumption!)

I'd quickly thought on discovery that if it was like that, changing those resistors would upset things.

Well, does it have a switchable gain setting?

You've found new data on the ULM module?

 

This is from the "Universal Loading Module ULM/M" so there remains some confusion as I have never found the ULM/C instructions. So it remains unclear what switch 1 does - is it "User available" resister loading slot like ULM/M or is it a gain switch like the HA10.

The external HA10 is for A60's with a  serial number 3981 up to 13250.

Jan 23, 2023 18:07:42 GMT 12 paulsaints said:

This is from the "Universal Loading Module ULM/M" so there remains some confusion as I have never found the ULM/C instructions. So it remains unclear what switch 1 does - is it "User available" resister loading slot like ULM/M or is it a gain switch like the HA10.

The external HA10 is for A60's with a  serial number 3981 up to 13250.

And so, I think you never wrote;- what serial number is yours?

Jan 25, 2023 21:36:36 GMT 12 RdM said:

Jan 23, 2023 18:07:42 GMT 12 paulsaints said:

The external HA10 is for A60's with a  serial number 3981 up to 13250.

And so, I think you never wrote;- what serial number is yours?

18894

From the schematic, the HA10’s 47 ohm resistor controls gain. In the ULM module, the 47 ohm resistor (in your photos) is most likely parallel with the MC input, and is a loading resistor.

AMR-iFi R&D

Yes it does look that way after all. I stand corrected!