Beocenter 4000 (T1603) Cassette Problem - UK Vintage Radio Repair and Restoration Discussion Forum

Krolroger · 23rd Jan 2021, 2:47 pm

Afternoon all,

I'm repairing a Beocenter 4000 cassette tape section (the one from 1979 not the later twin deck version). The rest of the unit (amp, radio section) is functional but the cassette section has suffered a major failure at some time in its life.

The symptoms (dim bulb tester showing a short on FF or Rewind) pointed to the pull-in switch (PC27) which controls the two solenoids that pull in the tape bridge and the brake bands.

Sure enough, TR1 (2N6101) was shorted and a couple of associated traces vapourised.

I think there are probably some other problems lurking elsewhere awaiting discovery but clearly a first step is to replace TR1 and repair the board and eliminate the short.

My question is whether I should replace any other components on the board, all of which test ok though I have changed a couple of slightly burnt looking resistors? This was clearly a fairly high energy failure with 60v across the shorted device until the fuse went.

Thanks,

SiriusHardware · 23rd Jan 2021, 2:52 pm

Disconnect one end of and check D3, the back-EMF suppression diode across RL1, in case that has gone short-circuit. If so, turning T1 on would effectively short the +Ve supply to 0V through TR1, which would get deeply unhappy about that. Does the relay coil have the resistance you would expect? It looks as though someone has written the 'usual' resistance of the coil on the diagram.

Krolroger · 23rd Jan 2021, 7:17 pm

Hi Sirius,

Thanks for your quick reply.

Flyback diode D3 checks out ok and the resistance across the solenoid coil (RL1) is 150R. (The written figure you refer to is an earlier inaccurate scribbling of mine). This would seem to give plausible results given that the initial 60v across RL1 falls immediately back to 30v as a holding voltage.

I guess what you are driving at is whether an upstream problem might have caused TR1 to short or whether it was a case of spontaneous failure.

Either way, is there anything in the immediate vicinity I should change on spec?

Thanks,

G6ONEDave · 24th Jan 2021, 12:39 pm

Is the 2N6101 the original component, as it's voltage rating is rather close to the 60v mentioned. The transistor has a max of 70v collector to emitter according to Towers.
Dave

Krolroger · 24th Jan 2021, 5:02 pm

Hi Dave,

Thanks for that.

Yes, 2N6101 is the original component. B&O didn't offer any alternatives in their parts list.

23rd Jan 2021, 2:47 pm	#1
Krolroger Pentode Join Date: Nov 2017 Location: London, UK. Posts: 108	Beocenter 4000 (T1603) Cassette Problem Afternoon all, I'm repairing a Beocenter 4000 cassette tape section (the one from 1979 not the later twin deck version). The rest of the unit (amp, radio section) is functional but the cassette section has suffered a major failure at some time in its life. The symptoms (dim bulb tester showing a short on FF or Rewind) pointed to the pull-in switch (PC27) which controls the two solenoids that pull in the tape bridge and the brake bands. Sure enough, TR1 (2N6101) was shorted and a couple of associated traces vapourised. I think there are probably some other problems lurking elsewhere awaiting discovery but clearly a first step is to replace TR1 and repair the board and eliminate the short. My question is whether I should replace any other components on the board, all of which test ok though I have changed a couple of slightly burnt looking resistors? This was clearly a fairly high energy failure with 60v across the shorted device until the fuse went. Thanks, Attached Thumbnails

23rd Jan 2021, 2:52 pm	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,548	Re: Beocenter 4000 (T1603) Cassette Problem Disconnect one end of and check D3, the back-EMF suppression diode across RL1, in case that has gone short-circuit. If so, turning T1 on would effectively short the +Ve supply to 0V through TR1, which would get deeply unhappy about that. Does the relay coil have the resistance you would expect? It looks as though someone has written the 'usual' resistance of the coil on the diagram.

23rd Jan 2021, 7:17 pm	#3
Krolroger Pentode Join Date: Nov 2017 Location: London, UK. Posts: 108	Re: Beocenter 4000 (T1603) Cassette Problem Hi Sirius, Thanks for your quick reply. Flyback diode D3 checks out ok and the resistance across the solenoid coil (RL1) is 150R. (The written figure you refer to is an earlier inaccurate scribbling of mine). This would seem to give plausible results given that the initial 60v across RL1 falls immediately back to 30v as a holding voltage. I guess what you are driving at is whether an upstream problem might have caused TR1 to short or whether it was a case of spontaneous failure. Either way, is there anything in the immediate vicinity I should change on spec? Thanks,

24th Jan 2021, 12:39 pm	#4
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,700	Re: Beocenter 4000 (T1603) Cassette Problem Is the 2N6101 the original component, as it's voltage rating is rather close to the 60v mentioned. The transistor has a max of 70v collector to emitter according to Towers. Dave

24th Jan 2021, 5:02 pm	#5
Krolroger Pentode Join Date: Nov 2017 Location: London, UK. Posts: 108	Re: Beocenter 4000 (T1603) Cassette Problem Hi Dave, Thanks for that. Yes, 2N6101 is the original component. B&O didn't offer any alternatives in their parts list.