Beocord 5000 Type 4923: A couple of service manual retests and wrapping up the project

Recording and playback testing is going really well with the Beocord 5000 (Type 4923) unit.

Whenever a project reaches this state in the restoration I am always reminded how good the component is.
The Beocord 5000 is a really good cassette deck. Although it doesn't have auto-reverse and a monitoring tape head (it's only a two head tape deck) it performs great and some people prefer not having those extra niceties as they also add complexity to a tape machine.

In the previous post I performed the Beocord 5000 electrical adjustments called out in the service manual.
The adjustment procedures were not terribly difficult to follow but do require some investment in proper calibration tapes (and bench testing hardware).

A couple of the service manual adjustments left me thinking about them afterwards.

One of those was the Advance adjustment.
That is the procedure where a test input is applied to the Beocord 5000 record inputs and the record potentiometers adjusted where the first LED of the PPM begins to glow.
Then the test signal is lowered by 30 dB and the frequency set to 2.5 kHz.
The adjustment procedure is to perform test recordings with this input setting and adjust the 1R182 "Advance" trimmer so the measured DC coupled oscilloscope measurement (made at 1TP12) alternates between 12 volts and 0 volts.

The purpose of this test is to adjust the Advance tape functionality of the Beocord 5000 Type 4923 cassette deck.

The Advance function is a cool feature Bang & Olufsen designed into the Beocord 5000.

It allows the owner to advance a cassette with recorded audio forward by one to eight audio tracks depending on how many times the Advance button is pressed.

Pressing the Advance button results in the Beocord fast forwarding the cassette while monitoring for blank spaces between audio tracks.

The feature attempts to count the number of blank spaces between tracks and match it with the number of times the Advance button was pressed.

I was surprised that the Advance function works on audio cassettes that were recorded on other cassette decks.  

So the objective of the Advance adjustment must be to tune the sensitivity of the detection of the tracks.
I wasn't able to fully verify that as I did not notice a measurable difference in the Beocord's ability to detect tracks using various positions of the 1R182 trimmer.

I wish the service manual specified a minimum and maximum range for the detected pulse but it doesn't.  It only says to adjust 1R182 until the level at 1TP12 is alternating between 0 and 12 volts.
Does that mean to adjust the trimmer to the minimum detection point of a space between tracks?
Perhaps.  However, I was not able to adjust 1R182 where that condition could clearly be marked.
So I left the position of the 1R182 trimmer pretty much in the center location.

Here is what I typically measure at 1TP12 with an oscilloscope when playing an audio cassette.
The oscilloscope shows the detection of the gap between audio tracks.

That pulse width is 70 milliseconds but I don't know if that is the correct target measurement value for the test.

Another service manual adjustment procedure that puzzled me was the Dolby Skewing test.

The following pictures show the test setup.

A blank, chrome type tape.
Measurement probes on 2TP5 (R-CH) and 2TP6 (L-CH)

A tape recording source signal of 19.9 KHz, 300 millivolts.

The recording potentiometers are adjusted so the 2TP5 and 2TP6 test points measure 300 millivolts.
Note that the Dolby C switch is also turned on.

The adjustment procedure says to adjust the Dolby Skewing (variable) inductors (2L102 & 2L202) to achieve minimum deflection in 2TP5 and 2TP6.

This measurement confused me a bit. Per the setup instructions I had already adjusted the potentiometers to get to 300 millivolts at 2TP5 and 2TP6 so those measured values should be right on.
How would adjusting the variable inductors make the measured 300 millivolts better?

I decided it makes more sense if the measurement goal was for minimum deflection between the two test points 2TP5 (R-CH) and 2TP6 (L-CH).

I performed that check by using two channels on my oscilloscope  to measure 2TP5 and 2TP6 while also displaying the oscilloscope math function of 2TP5 minus 2TP6.

Here are a couple of photos of that measurement.

I could measure the left and right channels while adjusting their respective Skewing inductors to get the flattest Math function result I could for the Right channel minus the Left channel.  I could also check that the left and right channels are perfectly in phase.

After that last electrical adjustment re-test I moved to a service manual mechanical adjustment I had been wanting to look at.

The Take-up Moment adjustment.
I have had a Sony TW-2231 Torque Meter cassette for this type of adjustment for a while now but hadn't really tried it.

In this test a specially produced cassette tape is used to measure the moment of the Beocord take-up reel.

The Sony TW-2231 Torque Meter cassette measures the moment as g-cm.
Here is the Take-up Reel during the test.

The Take-up Reel measured right at 60 g-cm. The Beocord 5000 Type 4923 service manual says the Take-up Reel moment should be with 30 - 80 p-cm.  From what I could find in comparing units I believe that is the same as the Sony meter's stated g-cm.  There is still more to learn about this adjustment procedure for sure.

To adjust that take-up moment value the Beocord 5000 22061 Clutch Assembly has a tension spring that can be adjusted.  It isn't easy to get to however.
Disassembly of the some of the tape transport is necessary to make this adjustment.

Spring tension of the clutch is increased or decreased using the brass tabs shown in the photo.
The tabs are lifted up and moved left or right depending on whether increasing or decreasing the force.

That check completed my service manual adjustment re-testing and I resumed testing the Beocord recording and playing of audio cassettes.

This Beocord 5000 performs wonderfully and should soon be on its way back to its owner.