New Beolover Carriage Position PCB for DC-Motor Beogram 4002/4004s (Types 551x and 552x)

I always found the repair and adjustment of the carriage position sensor on the PCB underneath the carriage somewhat tedious. The board and circuit are designed in a way that make a precise alignment of the LED, sensor and plexiglass ruler across the entire travel of the carriage necessary to ensure proper functioning. At the same time photosensor failures and mechanical failures of the sensor housing or the two switches on this board were sometimes difficult to fix, while broken off wires occasionally caused operational issues.

So I decided designing an improved board that would provide a reliable fix for all these issues at least in DC-motor Beogram 4002/4004s (Types 551x and 552x), where this board can be easily replaced. This shows the final design of the board:

The board features new SO and ES switches, as well as a modern monolithic IR photo-interrupter whose signal is cleaned up and shaped by an opamp in combination with a Schmitt trigger. It connects with a modern harness using a 2.54mm 8pin adapter to match the classic Molex jack on the main PCB. This connection should last much better than the original soldered wires.

The part is now available at the Beolover Store. I also gave this board a built in sensor test feature. Flip the switch to the right and an integrated LED will light up every time a black band on the ruler is properly identified by the sensor:

This shows the board installed:

My tests yielded a proper carriage position detection, while the SO and ES switch were in the correct location and performed perfectly. It seems that the modern photo interrupter due to its fixed alignment between LED and phototransistor is much more forgiving regarding the distance between ruler and photosensor. This makes the alignment between ruler and sensor across the entire translation range of the carriage much easier.

A good test for the proper functioning of the position sensor is whether it can read out the run out groove detection pattern at the end of the ruler. I measured the signal at the collector of TR17 while this section passed through the sensor:

A nice clean trace! All good in the carriage position detection department!

Beogram 4002 (5513): Adjusting the Sub-Chassis and the Platter, and RPM Calibration

Today I it was time to give the Beogram 4002 (5513) that I am currently reviving a properly adjusted platter and sub chassis. The goal is to adjust everything that the aluminum surface of the platter is flush with the aluminum panels surrounding it, while having the sub chassis centered between the upper and lower limits of the transport locks and the arms being also parallel to the aluminum surfaces. This is usually a pretty delicate process since so many things need to align. I have now a bit of experience with this and so after a while this Beogram looked very nice and up to spec. I recently made a couple videos that demonstrate this process:

Once the platter was adjusted I did a baseline RPM adjustment for 33.33 and 45 RPM using my recently developed BeoloverRPM device. Here is a picture of it in action:

This shows the readout after I already had adjusted 33.33 and then did 45:

Stabilizing
Measurement	starts
time:	0:0:9	RPM:	33.381
time:	0:0:20	RPM:	33.3782
time:	0:0:31	RPM:	33.3904
time:	0:0:42	RPM:	33.3987
time:	0:0:53	RPM:	33.4052
time:	0:1:4	RPM:	33.3937
time:	0:1:15	RPM:	33.4039
time:	0:1:26	RPM:	33.3882
time:	0:1:37	RPM:	33.3958
time:	0:1:48	RPM:	33.3817
time:	0:1:59	RPM:	33.3982
time:	0:2:10	RPM:	33.4025
time:	0:2:21	RPM:	33.4117
time:	0:2:32	RPM:	33.4088
time:	0:2:43	RPM:	33.3933
time:	0:2:54	RPM:	33.3916
time:	0:3:5	RPM:	33.3879
time:	0:3:16	RPM:	33.3815
time:	0:3:27	RPM:	33.4003
time:	0:3:38	RPM:	33.4095
time:	0:3:49	RPM:	33.4057
time:	0:3:54	RPM:	35.24
time:	0:3:55	RPM:	38.32
time:	0:3:56	RPM:	40.9
time:	0:4:0	RPM:	44.4733
time:	0:4:11	RPM:	44.8362
time:	0:4:22	RPM:	44.8325
time:	0:4:33	RPM:	44.8564
time:	0:4:44	RPM:	44.8543
time:	0:4:55	RPM:	45.0447
time:	0:5:6	RPM:	45.1634
time:	0:5:17	RPM:	45.1921
time:	0:5:28	RPM:	45.1968
time:	0:5:39	RPM:	44.8451
time:	0:5:50	RPM:	44.8174
time:	0:6:1	RPM:	45.2279
time:	0:6:12	RPM:	45.2203
time:	0:6:23	RPM:	44.7694
time:	0:6:34	RPM:	45.1452
time:	0:6:45	RPM:	45.2124
time:	0:6:56	RPM:	45.1949
time:	0:7:7	RPM:	45.1759
time:	0:7:18	RPM:	45.1953
time:	0:7:29	RPM:	45.2055
time:	0:7:40	RPM:	45.1998

It turns out that it is pretty difficult to adjust the RPM precisely with the standard single turn RPM trimmers that are in the Beogram originally. You can see at the and how I tried to get close to 45, but I always missed it by 0.2 or more by just very slightly moving the trimmer. This demonstrates that it is a great idea to put in multi-turn trimmers to get this right. On the other hand, I think it is difficult to hear such small difference...so this deck sounds now pretty good, especially with the lovely MMC20CL cartridge that came with it. I am listening to 'Song for My Father' by Horace Silver on Blue Note on it right now while writing this post! Wonderful!

I will give this Beogram some more listening to make sure there are no intermittent issues, but I think it is ready for duty again. This is Beolove!

Beogram 4002 (5513): Repair of a Broken SO ('Shut Off') Switch

This is a follow up to my last post about a SO ("Shut Off") switch that had lost its plastic actuator and hence caused trouble during shutdown of the deck. I set out to develop a 3D printed part to replace the missing plastic part. This shows the SO (left) and the ES ("End Switch", right) in comparison. The ES still has its plastic appendage, while the SO is missing it.

I took measurements and then went through a few 3D printing iterations to get a part that would come out right. Due to the small size this was pushing the limits of the 3D printer, so the outcome was significantly difficult from the 3D CAD design, and experimentation finally yielded a useful shape:

I glued it on with epoxy:

And now this Beogram can shut down again in an orderly manner:

On to the sub-chassis and platter adjustment.