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
Dear All,
I am recapping a BM6000 and I have run into an issue with the radio. The amplifier plays TP1 at normal volume. When I select the radio it is very quiet, but sounds OK, tunes between stations, lights the stereo lamp and responses to the balance and tone controls. It is however picking up hum if I touch some of the wiring around the radio of phono boards.
I have double-checked I have installed the correct new caps in the correct orientations on the radio board and cannot see any connections/plugs that I have not put back.
Any suggestions what I might check next?
Thanks,
Nigel.
I have swapped in a known working radio board from another BM6000 with the same results. That would seem rule out the radio board itself and to put the problem somewhere between the radio board and the power amp. The hunt continues. N.
The pre-amp board 3 is receiving a signal from the tuner board and from TP1. I measured the signals on P9 of board 3 that control the multiplexing IC1, but these don't seem quite correct. I measured at the three pins with the different sources selected in the sequence TP1, TP2, PH and P on founds the following:
D9-1 15v, 0.5v, 15v, 15v
D9-2 0.5v, 13v, 15v, 15v
D9-3 0v, 0v, 0v, 0v
The fact that the selecting input signal to the multiplexer in the preamp is the same for both PH and P looks suspect. I checked and these are the outputs that are being generated by program selecting chip IC1 on board 2. It seems to me my problems are digital domain with the program selector on board 2 generating the correct signals to control the preamp to select the required source.
Does this sound like it makes sense?
Any suggestions for testing or how to make progress from here?
The program selecting chip seems to be a binary-to-decimal decoder. I have checked and it is getting +5v at pin 16 and 0v at pin 8. Any suggestions before I try to swap the chip?
Looking the the selecting signal values above, these would make sense (i.e. only one low at any given time) if D9-3 was 15, 15, 0, 15 instead of always zero. Checking at the board 2 end, I measure 7MOhms between pin 9 on IC1 and P6-8 (which drives D9-3 on board 3). I would expect 1kOhm. Measuring pin 10 to P6-9, I do get the expected 1kOhm.
Job for tomorrow - investigate continuity between pin 9 and P6-8.
The problem is fixed, but it wasn't what I thought it might be last night. When I checked again I couldn't measure the discontinuity between the IC and the plug that I found last night. I suspect I was measuring between the wrong points and hoping too much that I had found the problem.
I checked the binary inputs to the converter from the microcontroller and these were fine, following the expected pattern and all at 5v. I then started to think about how I could be measuring 15v outputs from the binary-to-decimal converter IC given it is a 5v device and its was receiving only a 5v power supply and 5v inputs from the microcontroller.
Looking over the schematics, it seemed something else must be feeding a 15+V signal back into the converter IC via one of its outputs. This pointed the finger at board 4 which has all the tuning pots on it and a 19V rail. Closer examination showed one of the connector strips (P15) on PCB 4 had been strained and was loose. I resoldered this and normal function was resumed, I.e. selection of all sources now nominal.
It was fun chasing down this problem and thank goodness the service manual was available as I it would have been beyond me without out it. I suspect those more experience will not be surprised at the fault, but it surprised me that a problem with the strip connector on the tuning pots board (tuning was all working fine) showed up as the inability to select a source with the preamp - weird!
You live and learn!
Well done! It was fortunate that the chip didn't get fried with that overvoltage.
--mika
Very good work and well done for tracking down the issue.
Ben
Thanks. Very good point about damaging the IC. "Chapeau" to the engineer who designed that piece.