Beogram 4002/4004: New Belts and RPM Adjustment

A few months ago I received new Beogram 4002/4004 belts from 'Dillen' (Beoworld.org). Now was finally the time to install them into the Beogram 4002 that I am currently restoring. It seemed to need a new drive belt, since I had to considerably increase the speed trimmer to achieve 33 RPM. The Servo Belt also showed cracks. It seems it may have still been original. It had the prescribed triangular crossection, matching the groove in the pulley that drives the carriage spindle.

As in the case of the Beocord 5000, I thought it useful to take some measurements of the old and new belts for future reference (like when diving archeologists bring up this turntable in the year 2358 from the ocean floor where Florida used to be before global warming...;-):

Belt	Old ID	New ID	Old Crossection	New Crossection
Drive Belt (flat belt)	17” length when pressed flat	17” length when pressed flat	Width: 5 mm /0.2” Thickn.: 0.5 mm 0.020”	Width: 5 mm /0.2” Thickn.: 0.85mm/ 0.033”
Servo Belt	31 mm /1.2”	31.5 mm /1.23”	Triangular Height: 1.5 mm /0.059” Base: 2 mm /0.078”	Square 0.75 mm/0.030”

I really like the much tauter feel of the new, thicker, flat belt...and indeed, after I put the new drive belt in, the speed was considerably higher, indicating slippage with the old belt. Now the speed trimmer was much closer to the '0' mark for a precise 33 RPM, i.e. the slippage was much less. I decided to make this perfect, and adjusted the primary speed trimmers on the circuit board (R14 for 45 RMP and R15 for 33 RPM) for a correct RPM setting when the user accessible trimmers are set to '0'. I did this with a small all-metal watch maker screwdriver for 33 RPM while holding down the '33' key (as one does for sweeping the platter before starting to play a record). There I noticed a remarkable behavior: The speed of the platter would immediately increase, seemingly beyond 45 RPM, whenever I stuck the screwdriver into the slot of the trimmer. After extracting it the speed would normalize...it appears that my body's resistance path to GND via the key pad affected the time constant formed with C3, which then throws off the difference amplifier in IC3, resulting in a change of speed of the motor.

Lesson learned: Finally get rid of the all-metal screwdrivers...they got me in trouble before when I fried one of the outputs of my Beomaster 6000 4-Channel by a screwdriver-caused short when adjusting the quiescent current...;-).

Beocord 5000 (4716/4715): Removing the Drive Mechanism

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Note: This post is 'historic'. There is a much easier way to extract the drive from the enclosure without lifting the Pre-Amplifier board (PCB #1) and removing the solenoid linkage as is described here. Check out my subsequent post about this process, which discusses the updated procedure.
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Per advice by 'chartz' and 'sonavor' of Beoworld.org (here is a link to the thread) I started working on replacing the belts of my Beocord 5000 (4716), which are all out of spec...they expanded over time and have too much slack. 

The first step is to take the drive mechanism out. In order to do that one needs to first flip up the board on the left to be able to get to the solenoid linkage. Chartz suggested to bolt the operating panel in reverse orientation back to the bottom of the enclosure with three screws. A great idea! This helps preventing breaking leads off when manipulating the expanded mess. Here is a pic:

The next step is flipping up the board on the left. This board is held in place by one screw in the back right corner, as well as a screw into the bottom of the enclosure next to the red DIN socket in the front part of the board. After removing the board one can move it a bit to the right to pull it out of the slits in the two brackets on the left that hold the board in place. 

A great thing is that there are two more slits that point upwards where one can secure the board in upward orientation. At this point it is a great idea to protect the plexiglass cover with an old T-shirt or the like from scratches. In my pic this was not implemented yet, as I was still ramping my learning curve...;-):

So far so good. Now it was time to get the drive out. 

The first step is to remove the solenoid linkage:

This requires removing the retaining ring that holds it on the shaft it pivots on. This requires a retaining ring removal tool. since the ring is seated flush with the linkage, this is a bit painful: I was able to get it off unscathed by carefully prying it open with the retaining ring tool, while jamming a small screwdriver in-between the ring and the linkage, thereby pushing the ring up. This enabled me to get the tool further in, which then allowed me to remove the ring just by prying it open and moving it further upwards.

Once the ring is gone, the linkage can be rotated 90 degrees, and removed:

the next steep is to remove the plug that connects the heads (make sure that the deck is unplugged when the plug is pulled to prevent magnetization of the heads - not sure why, but that is what it says on the half-obscured small print warning label in the operating panel - obscured and small print: that is how warning labels should be..;-):

After this I unsoldered the solenoid connections:

The drive is held at the front by two screws that go in from the bottom of the enclosure into the solenoid bracket. Here is a picture from below the enclosure:

These screws needed to be removed or just loosened that the drive can slide forward.

In the back there is no screw that holds the mechanism, only a bracket that fits into a slit in the mechanism. By sliding it forward the drive can be lifted out.

Almost done. The last step was to remove the large wire to board connector from the back of the drive:

This connector sat fairly snugly, and I had to carefully pry it loose with a small screwdriver from the side, and then pull it downwards. At that point I wished I had taken out the plexiglass cover for better access, but finally it came loose, and the drive was liberated.

Here is a pic of the drive after I took it out:

And the 'cavity' left behind in the enclosure:

That is it for today...next step is to partly dismantle the drive mechanism to get at the belts.