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EMT 927

Having partially decomposed 930, I can see some points of improvement for sure!
More advanced thurst plate for the main bearing (a piece of alu), same with motor (a nylon cap),
the motor supply and the famous speed control.
 
phixphi said:
Congratulations Patrick!

Did you also have to align the motor caps?
Click to expand...
No, I haven't touched the capacitor. The motor seems to run quite quietly, but there's some noise from the idler. I'll have to save up for a new one.

Did you get your motor sorted?

phixphi said:
Having partially decomposed 930, I can see some points of improvement for sure!
More advanced thurst plate for the main bearing (a piece of alu), same with motor (a nylon cap),
the motor supply and the famous speed control.
Click to expand...

I'm not sure there's much wrong with the thrust plates - both shafts run on ball bearings, and I imagine they are designed so that the ball remains relatively stationary and the thrust plate only needs to hold it in place under the rotating shaft.

However I agree about the motor control. It's a three phase motor so would be better run from three equally-spaced phases with variable frequency speed control. Although that's relatively easy to do these days, in the 50s and 60s when these turntables were current, it wasn't quite so simple.

Having spent some time getting my 927 running, I can now see why people say EMTs are not for tweaking. They're a well thought out system designed to do a particular job, and things like fixed arm mounts, non-SME fit headshells and built-in PSUs to drive the various internal solenoids and lamps, don't make them very compatible with components from other manufacturers. I suppose you could say that they're the 'Apple' of the turntable world.

I've learnt that my 927 came from Studio 4 (ST 4) at Granada TV, and one of the things it was used for was for groups to mime to - 'live' on air. So anyone old enough who lived in the Granada area, has probably already heard it ;-)
 
Patrick Dixon said:
No, I haven't touched the capacitor. The motor seems to run quite quietly, but there's some noise from the idler. I'll have to save up for a new one.
Click to expand...

What kind of noise do you get from the idler? "bump-bump-bump"?
When you get a new idler, don't touch it with bare hands, use gloves!

As for the cap, have you looked at the amplitude of the phases?
Hans' published method did not work for me--I could not
find a position where the motor runs but the platter not, so I aligned the cap by
looking at the phases. I was able to get +/- 1V aligment and it's ok
according to HM.

Did you get your motor sorted?
Click to expand...

Yes, HM fixed it. It was a minor thing at the end--a plastic rim holding lower
felt pads was not in place. Worse wih my cartridge...it needs a complete rebuild :(

I'm not sure there's much wrong with the thrust plates - both shafts run on ball bearings, and I imagine they are designed so that the ball remains relatively stationary and the thrust plate only needs to hold it in place under the rotating shaft.
Click to expand...

I'm trying to find a sensitive mechanic's or clockmaker's stethoscope to
actually listen to what happens there. I still believe there is a room for
improvement. The advanced materials we have on hand now where not
available in 50's-60's. Something like kokomo does for G 301?

However I agree about the motor control. It's a three phase motor so would be better run from three equally-spaced phases with variable frequency speed control. Although that's relatively easy to do these days, in the 50s and 60s when these turntables were current, it wasn't quite so simple.
Click to expand...

I don't understand how the speed controll happens. When the break slows down the platter, is it the idler that slips along the plate or(/and?) the rotor "looses" phases? I'm thinking of designing a dedicated supply. A microcontroller with a 3 channel DAC + filters + class A power amps. Anyone with a good knowledge of microcontrollers here?

Cheers,
jarek
 
I've re-capped the 139 now:-

IMGP1719.JPG



IMGP1720.JPG


and it all seems to work quite nicely. I had to move one of the connections to avoid a clash with the stereo pickup wiring - the chassis wiring in this 927 seems slightly different to all the EMT diagrams I've come across.

It sounds quite good in mono!
 
IMGP1738.JPG


You're kidding!

It's a 1950s mono phono amp - there's no RIAA, just settings labelled NARTB, IEC, BBC and DIN, and a scratch filter with an adjustable rollover point.

The DIN setting looks like it had been modified to an RIAA look-alike, but I've restored the original values. I've been simulating the circuit to see how close I can get to an RIAA setting, but I'm not sure whether to implement it or not.

The caps are nothing very special - just decent quality non-leaky new ones. MKT for most of the signal path, polystyrene 1% for the eq, and new electrolytics for the PSU.

IMGP1728.JPG
 
Oooops!
I guess if you listen to modern records
best possible RIAA is an absolute must...
Have you also replaced the scratch filter pot or
it's not necessary? I've been wondering
if the old pot negatively affects the signal.
 
The scratch filter in the 139 uses a (ganged) variable capacitor. The only pots are presets for the heater balance, and for monitoring and output levels - and they are nice quality 50s ones. I just cleaned them with Servisol.
 
NAB or NARTB is fairly close to RIAA. A little more rolled off (-16db v -13.7db at 10Khz) but the bass turnover's the same. It's close enough to be pretty usable.
 
Just to say hello, after reading with interest Patricks experiences with the EMT - it is nice to see it going to a good home. I was in fact the source of these, two of which spent 25 years in my garage and one in my living room behind the grand piano! As an ex Granada employee, I have in fact used them many times on air.
At my previous house, one of them lived in the hall , next to the telephone. One day someone rang up to say " Have you heard of an (spelt carefully!) E. M. T. Turntable?"
My reply "yes I'm leaning on one!"
By the way, it sticks in my mind that we used IEC equalisation to simulate RIAA.
 
I've bought a set of SG Brown headphones from eBay so that I can correctly cue my music.

Should be fun, and all for £4.95.

Just need a band to mime for me now ...
 
Patrick Dixon said:
It's a three phase motor so would be better run from three equally-spaced phases with variable frequency speed control. Although that's relatively easy to do these days, in the 50s and 60s when these turntables were current, it wasn't quite so simple.
Click to expand...

Very easy, if you've already got the gear lying around. Audio oscillator. Digital delay line. Three power amplifiers:

4967736656_4b5bb134a4_z.jpg


100V/phase, 120º between phases. Much quieter audibly, significant reduction in vibration. (Better than a Lenco, now!)

Added bonus: Variable speed without needing to source and re-instate the felt brake components, missing from mine. (I've gathered the felt brake can be noisy; acceptable in the environment for which it was designed, less so in a domestic environment. Hence I'm not concerned about re-instating it.)

Later, I'll design/build a somewhat smaller self-contained unit. That lot is a bit impractical (and also doubles as a room-heater.)

Kat
 
Great Kat!
I've been thinking about synthesizing the voltage with a microcontroller
with a tripple 16bit DAC on board, then filtering, then 3x class A power amp
 
An easier (and just as good) way is:

12 bit shift register driven by a simple NAND clock

3x low pass 2nd order op amp filters (TL074?)

Either 3x 100v line PA amplifiers (say 50w) or
Crude power amp feeding 100v transformers

The inductance of the transformers and motor will smooth out the rubbish...

kjt
 
Agreed. You don't need to synthesise sine waves; square waves and some filtering will suffice. The voltage is fixed, the frequency is 50Hz ± not much; so you can reduce the requirements down to 1-bit DACs and a LPF. Then remove the DACs and microcontroller completely in favour of something far less complicated which doesn't need programming.

Class A is overkill, I can't say I'm worried about a bit of crossover distortion when I'm driving a motor. (Have you seen what the mains waveform usually looks like?!)

The three amps I used are class-B 100V-line public address amplifiers, two 60W and one 30W. The o/p transformers in these are marked "80Hz - 18kHz" (but that lash-up worked well enough at 50Hz to prove the point. The 30W amp wasn't too happy but survived half an hour of abuse anyway.)

Class-B amplifiers driving mains transformers 'backwards' would suffice for the output stage IMO. They're designed for 50Hz; split-primary types with the primaries in parallel would provide a 115V 'secondary' in this application. Probably better than the o/p transformers in those PA amps.

Keep it simple! (And I've done more software and digital/microprocessor hardware design than anything; I'm used to throwing microprocessors and software at problems. Some solutions don't need them. This is one of them. IMHO.)

With a bit of luck (and toroidal transformers) the whole lot will go in a 2U case and won't do astonishing things to my electricity bill... ;)

For efficiency, I've been thinking about something involving PWM and MOSFETs; it'd remove the need for bulky transformers and it wouldn't need much heatsinking. That might result in something small enough to fit within the frame of the turntable. Last time I tried anything like that I ended up with a pile of dead MOSFETs and a lingering burning smell, though... :rolleyes:

Kat
 
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