ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello all. I repair Dual turntables, but I'm new to B&O. I have a Beogram 8000 that has an issue on startup.
When initially plugged in, I don't get the usual single dot in the LCD. Sometimes two two or four dots, sometimes nothing, sometimes the platter just starts spinning fast.
After replugging the power cord a few times, I get the single dot and everything works as it should.
I thought perhaps it was C20, the capacitor that connects to the processor reset line. Subsequently, I recapped the entire TT including the main board, processor can, inside the transformer and chassis regulators. All is well, but I still have exactly the same LCD dots and reset issue on startup.
I'm wondering now if I should be looking at the rest of the reset circuit, TR17, etc., or if it's something else.
Has anyone experienced this issue? (I've perused the forum looking for similar posts, but alas, I got overwhealmed with it all.)
Thank you kindly in advance.
Duncan McAllister
Toronto, Canada
I have never seen that condition. When you recapped did you get C1, the 47uf power supply cap in the fuse box?
Hi Mark and thanks for your reply.
Whatever this is, it's related to the power on sequence. I don't think it's capacitors anymore.
Not sure which C1 you meant (there's three in that TT). I replaced the C1 cap in the power transformer black box. On mine, it was a 27uF/55V bipolar electrolytic. (It's not really the motor capacitor as some have surmised. Looks to give a kick to the motor brake.)
I didn't replace C1 in the processor enclosure, a 4.7nF/63V non-electrolytic. But I did replace C28, that tricky 47uF/10V electrolytic to +5V VRR on processor pin 1.
On the Beolover Blogspot, the guy posted a similar issue with his BG-8000 restore. Turned out it was an intermittent 5V supply from the black jumper wire from the main board rectifier.
I've resoldered that jumper as well as reflowed all the header pins. Cleaned and reseated all the ribbon cables.
I've got it on the bench now, and I'll just poke around with the scope.
Today I'll check the regulator voltages, the reset pulse to the processor on pin 39, the power on/off pulse to pin 38, and test the crystal oscillator. Not sure what else to think of.
Incidentally, does anyone know what that processor is? Is it like an old Z80 or 6502, or maybe something custom? Haven't seen a 40-pin DIP for some years.
Duncan McAllister: Hi Mark and thanks for your reply. Whatever this is, it's related to the power on sequence. I don't think it's capacitors anymore. Not sure which C1 you meant (there's three in that TT). I replaced the C1 cap in the power transformer black box. On mine, it was a 27uF/55V bipolar electrolytic. (It's not really the motor capacitor as some have surmised. Looks to give a kick to the motor brake.) I didn't replace C1 in the processor enclosure, a 4.7nF/63V non-electrolytic. But I did replace C28, that tricky 47uF/10V electrolytic to +5V VRR on processor pin 1. On the Beolover Blogspot, the guy posted a similar issue with his BG-8000 restore. Turned out it was an intermittent 5V supply from the black jumper wire from the main board rectifier. I've resoldered that jumper as well as reflowed all the header pins. Cleaned and reseated all the ribbon cables. I've got it on the bench now, and I'll just poke around with the scope. Today I'll check the regulator voltages, the reset pulse to the processor on pin 39, the power on/off pulse to pin 38, and test the crystal oscillator. Not sure what else to think of. Incidentally, does anyone know what that processor is? Is it like an old Z80 or 6502, or maybe something custom? Haven't seen a 40-pin DIP for some years.
Check solder joints everywhere. Particularly at connectors and sockets. And replace the CPU socket.The CPU is a mask-programmed custom job.
Martin
I did some more testing today. It turns out I'm not getting 15V.
The input to the TIP32 regulator transistor is 23V, but there's 0V output.
TIP32 tests OK out-of-circuit. But I'm also getting 23V on the base.
I thought then, perhaps R77 is shorted, but it's bang-on 6K8.
So I'm back to testing the main board again.
At least I know what's wrong. I'm thinking a lot of things won't work without the 15V supply.
After more testing, I believe it's related to the on/off transistor. It's not providing the voltage to turn on the + and - 15V transistors.
In any case, the unit is going back in the box and put on the shelf for a while. I've no more time to spend on it.