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Late Beogram 4002 and the 4004 (Types 551x and 552x), which have DC platter motors instead of the earlier synchronous AC motors usually suff...

Saturday, June 24, 2017

Beogram 8000: Trouble-Shooting Test Phase 1

I made the choice to try switching the Beogram 8000 control panel board first in the trouble-shooting of the intermittent record play interruption. This is the problem where occasionally the Beogram tonearm lifts to the Pause position while playing a record.

This is the easiest thing to check first and I kind of suspect the photo resistor circuit in the control panel on this Beogram anyway. When I made the service manual adjustments earlier in this restoration I noticed the adjustment for the adjustment screws were quite a bit different between the forward (<<) and reverse (>>). The adjustment screws control how much light shines on the photo resistor. The service manual says the normal voltage across those resistors (when forward and reverse are not engaged) is 620mV.

The following picture shows the control panel board removed. The position of the adjustment screws are where they needed to be to get the 620mV measurement. The difference in position of the two screws make me think one of the photo resistors is weaker than the other. Perhaps that is the reason for the fault I am trying to fix.

On the spare control panel board in the background you can see what I expect their positions to be like in relationship to each other.

One annoying thing about this adjustment is that B&O didn't provide good test points to check and adjust these controls without opening up the Beogram. It is easy to open just the control panel so if there were a nice test connector there I could check and make this adjustment after the Beogram is closed up.

The P5 connector pins 3, 4 and 6 are the nodes necessary to measure for this adjustment.
So I decided to add a test connector that I can access with just the control panel open. I also decided to do this test with one of my spare Beogram 8000 control panel boards.

I used a female connector for the end so I could leave the test connector attached. The picture above shows the test connector with the male test plug attached.

The test connector works great!  The Beogram 8000 is all closed up except for the control panel and I can easily use the test connector to make the adjustments.

I discovered that I will have to remove the test connector in this first attempt as the 24 gauge wires are too big and interfere with closing up the control panel all the way. If I want to have a "leave in" test connector I will have to go to a 30 gauge wire.

For now though it is time to return to the record play testing of this Beogram to see if the control panel was the problem. I am using the spare control panel board with the test connector and adjusted to the voltages specified by the service manual. On this spare board both adjustment screws are pretty symmetrical so that is a good sign that both photo-resistors are a pretty good match.

I am now on the last side of the second record play test. Only about eighteen more uninterrupted record plays (36 sides) to go for me to say the problem is fixed.

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