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BeoGram 4000 FTGU restoration

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Menahem Yachad
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Menahem Yachad Posted: Mon, Nov 21 2016 12:17 PM

A pleasure to have one of these in my lab again - the last one was about 3 years ago.

This is a particularly nasty specimen - many issues.

 

But no worries - I'll tackle one issue at a time.

First stop - replace the main caps - these are bad 'uns. Look at the electrolyte oozing out.

And the ZD24 was shot - a new 1N5934B 3Watt is now installed.

0TR1 is just fine - an ancient RCA 2N5034 6Amp unit. I removed it, put on new heatsink paste, and reassembled.

Menahem


Menahem Yachad
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Here, I'm going to need some help.

The owner had done some creative gluework to hold the cracked SP15 cartridge. I can clean up the tonarm, and polish it like new, but I need that little black tab at the business end of the tonarm, onto which the MMC cartridge attaches.

It's the same tab on ALL that era's TT"s, including the radial TT's. So if anyone has a spare arm lying around, I'll be most appreciative.

I can remove the tab, without damaging the tab, or the tonarm, and if necessary return the arm to you, minus the tab.

 


Dillen
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Dillen replied on Mon, Nov 21 2016 12:27 PM

Rudy solved this:
http://beolover.blogspot.dk/2016/02/beogram-4000-replacing-broken-mmc-mount-with-3D-printed-replacement.html

Martin

ALF
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ALF replied on Fri, Dec 9 2016 10:10 AM

same problem I had, when you were so kind supplying a BG4002 arm assembly

changed over the arms, problem solved !!

still works fine :-))

cheers, ALF

Menahem Yachad
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I have now completed the electronic and electrical restoration. 

Apart from the standard cap, resistor, diode and transistor replacements, I was stuck on a problem that the solenoid did not operate to drop the tonarm.

I traced it to the Keyboard 3IC1 and 3IC7 (ancient FCH131), which took me a while to source, and then even longer to set aside time to install with sockets, in case I had to remove them again, which I didn't!

See attached photos.

I made a film of my electrical function test, finally - what a pleasure to reach this point!  Lets have a Party !!!

Click here to watch the film on YouTube

 

Menahem


Menahem Yachad
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Main Control PCB overhauled

Note the matched hFe marked on each of the 4 metal can transistors - this matching provides super accurate tracking, with minimal side-stress on the stylus.

The Servo motor corrects each angular deviation of the stylus, in very frequent 1/6 turn increments, which does not allow the stylus to move laterally very much, between each motor correction.


Menahem Yachad
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Here is the Tonearm, cleaned and polished.

Compare to the "before" pic at the beginning of this article.

Because the MMC cartridge tab of the tonearm had broken off, the previous owner glued the MMC20 to the tonearm with what looks like Gorilla Glue.

It worked, I guess, for him.

I desoldered the 5 wires from the chassis, and soaked the aluminium arm in boiling water with dish-soap.

After a few minutes, the glue was pliable (or brittle) enough to scrape off the outside with a credit card.

Back in the water, while I set up my vise with 2 soft wooden grips.

I then placed the tonearm between the 2 wooden grips, and from the rear, gave a couple of firm hammer knocks on a thin cylinder rod.

Out the front popped the plastic/glue mess.

I gently rubbed down the tonearm with Brasso, and then wiped clean.

Ready now for Rudy's new tab.

 


Dillen
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Dillen replied on Fri, Mar 3 2017 11:31 AM

Menahem Yachad:

Main Control PCB overhauled

Note the matched hFe marked on each of the 4 metal can transistors - this matching provides super accurate tracking, with minimal side-stress on the stylus.

The Servo motor corrects each angular deviation of the stylus, in very frequent 1/6 turn increments, which does not allow the stylus to move laterally very much, between each motor correction.

 

Just a few comments on the tech side, - not to be misunderstood or taken personally:

There's absolutely no need for Hfe matched transistors in the H-bridge.
It's a slow-moving, self-equalling analogue DC-circuit.
The motor current will never run through more than two transistors at any time (and rarely - if ever - backwards while playing) and the
laws of physics gives the current flow in a series circuit like this as exactly identical for all involved components.
Any DC offset can be ignored since the motor has no sense of the ground potential and only sees the voltage
across its leads.
Besides, there are so many other components controlling the circuit, including an adjustable mechanical lever operated
by the tonearm, a lamp and an opto component, a motor, a belt, a threaded rod, a bearing and a carriage with each
their electrical or mechanical "challenges" etc. to overcome and compensate for, so the driving transistors Hfe would be the least thing to worry about.
When correctly adjusted the angle of the tonearm will be the controlling factor.
Using Hfe matched transistors will not make the tracking any more "accurate".
It will all adjust out fine regardless.

Changing the value of the 150uF bipolar capacitor across the motor (as I noticed you suggested somewhere else) will
make the motor phase inaccurate and the motor will lose power (and possibly run warmer, particularly at the
speed for which the capacitance is now furthest off).
150uF is a bit of an odd value, but it was chosen for a reason and a "best for both"-solution for the two
main speeds and their respective drive frequencies).
Ideally the capacitance should've been changed too when changing speed, but B&O settled for this value as the best compromise.
I suggest staying with the original value.

And I would in theory also suggest going for a "real" motor capacitor.
A "conventional" bipolar electrolytic capacitor will not have suitable properties and they rarely last long when used as rough motor caps.

Martin

Menahem Yachad
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Thanks Martin for the clarifications.

Of course the motor is using only one side at a time, but as you know, I'm a bit of a fanatic when it comes to doing these restorations, so I always take an extra few minutes to choose the best components for the job.

I know it's completely anti-social in the modern era to take pride in our work, and that we should all settle for sloppy workmanship, which barely lasts 6 months. But I was never meant to fit in with this laziness.

So, permit me to continue being the "fanatic" - It's a great place to be!

Lets have a Party !!!

Regarding the capacitor, I don't have direct (vibration) experience with the B&O Sonceboz motor, but on the Thorens 145 and 160 (on which I have substantial experience, but not for this forum), changing the capacitor to something more than the standard 0.15uF cap does wonders for the motor's stability.

This is done by connecting a magnetic Vibration Sensor to the motor, and noting the vibrations.

Generally, the Thorens Sonceboz motor has the least vibration with a cap in the range of 170-280nF, which in any case is more than the original 150nF, which Thorens originally specified. Even though they all came off the identical production line, each motor is different, and one size cap definitely does not fit all.

Is this relevant to the BG's? I think so.

 

On my first BG4002 restoration, about 20 years ago, IIRC, I decided to be a fanatic, and connected 100uF and 47uF BP caps in parallel, and went through the entire calibration procedure. I noted the scope and DMM results for best performance.

I then swapped those 2 caps out, and installed a single (and cheaper) 220uF BP, and for the life of me, I could not determine any better or worse results than I had previously obtained an hour earlier with the 147uF.

So, I took the approach (which you mentioned regarding the motor transistors above), that the other components in the circuit compensate for whatever change there may be (due to the 220uF instead of the 147uF), and it's all adjustable anyway.

Since then, tens of BG4002's have passed under my knife, and every customer is ecstatic.

If you have any empiric evidence which would negate the use of a 220uF, you know I'm always ready to consider that.

 

Many times, on vintage equipment, the values installed had more to do with physical size limitations, than actual optimum values.

Example - the earliest BG4000's had 3000uF main capacitors, and the later ones had 4000uF, just like the 4002's. What happened? Simply, the higher-value capacitors which suited the circuit better, became available in a physical-size which would fit inside the BG4000.

Actually, nothing's changed - on cheap equipment today, the capacitors are woefully underrated for the specs they need to deal with in the circuit.

 

I have thought long and hard about a "real" motor capacitor, but have not found any which would fit. Have you?

 

Menahem

Dillen
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Dillen replied on Fri, Mar 3 2017 3:19 PM

Permanent-split capacitor motors are all about frequency, impedance and phase and
perhaps a bit out of scope for this forum, but, well...

This is easy reading, the section about "Run capacitors" is of particular interest.
https://en.wikipedia.org/wiki/Motor_capacitor

This is the slightly more involved reading:
http://what-when-how.com/electric-motors/general-procedure-for-calculating-the-performance-of-permanent-split-capacitor-psc-motors-electric-motors/

The most interesting bit, I suppose, is the background for calculating the run capacitors capacitance (and working voltage).
Note that surprisingly many factors are involved in this fairly simple setup.

Note also at the bottom of the last page, the multispeed motor example has
several windings to select between speeds. Ours doesn't.

The "motor phase" trimmer provided will allow for a slight adjustment (and still it has quite an effect!) and to some extent allow to
compensate for component and motor tolerances, but it will not change the basic factors of the motor or its characteristics.

 

"I have thought long and hard about a "real" motor capacitor, but have not found any which would fit. Have you?"
What B&O fitted seems to be just fine.
Actually, until a few years ago, I recommended owners to keep that capacitor in the deck, - for the above reasons and because most
of them were still doing absolutely fine.
Now, however, they have started to go in larger numbers (a rough guess is apprx 10-15% of the decks I get in), so I would advice
to replace them if the deck is acting up.

This is by no means some kind of heavy industrial equipment, not even a washing machine,
so there's no reason to "gun it up there".
A good quality bipolar capacitor with a not too high working voltage rating - I don't remember the original rating but
perhaps 16V or so. It's probably not much higher.
Don't go much higher than the original or it won't be excited (remember it's an electrolytic).
Two solid electrolytics back to back would also be fine if a suitable capacitance value can be found.

Martin

Menahem Yachad
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Martin

That's excellent info.

 

Did you do the calculations?

I won't shoot you, if you said "Whoa!!!" and backed away Big Smile

 

So, it states very clearly that the Run capacitor should NOT be electrolytic, but polymer, or polypropylene film.

Take look at this Vishay MKT1820715165 , the $48 price not withstanding.

Would this be a good choice - it says nothing about a Motor capacitor?

Menahem Yachad
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The machine is running "fine", but I noticed something which I didn't like.

With the tonearm down (solenoid activated), when the tonearm is moved to the left (as if playing a record), it takes too long for the servo motor to start rotating.

However, if I moved the tonearm to the right (as if playing a record in reverse), immediate response occurs.

I had already done a complete alignment and calibration of everything required to normally take care of the problem, including adjusting the sensor window tab on the vertical tonearm drum.

Being that this machine was incredibly filthy when I received it, I had an idea that perhaps there was a thin film of dirt on the actual ORP61 photo-sensor.

After such a nice assembly, to disassemble this machine again, was not a decision I took lightly!

Like Nike - Just do it!

I disassembled the tonearm sensor assembly, removed the 2 ORP61 photo-sensors, cleaned the black case inside and out, wiped the 2 ORP61's with alcohol.

I straightened the leads - the left one has a bend at the base, which would influence the angle at which light would strike it.

I then reassembled, tested, and got the results I wanted - immediate response on both left and right tonearm movement.

Just FYI, this being a resistive photo-sensor, I measured the resistances across the 2 leads (out-of-circuit):

1. Background light - 56KOhms

2. Intense white LED light - 3.8KOhms

This would probably be "drastic measures", not normally required on most restorations, but it's nice to know that a clean ORP61 definitely influences the sensitivity of the tonearm movement.

Click the picture to open full-size.


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