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
Low signal stregth on the meter to max value 1.lred light indicator for radio lid up when there is supposed to be a radio stationNo radio sound It is dedecting something.
Because stregnth value meter is fed by the If transistor stages BF194, I suppose the faulty thing is before they strength meter
I have done DC massurement on the 4 If transsitors BF194 and they seem fine, as well as the Ic circut 3012, nice values.( if 10% is okey )
powersupply give nice 15 volt. to the PCB board. Next thing will be to check DC levels in the tuner shieded input module.Anyone with some tips ? Christian
My re-capped M75 are my precious diamonds.
I have checked DC levels on all power supplies.Verified that there is a carry wave ( dont know the english word for it ) out of the Tr8( dont know how to check the wave on the earlier stages, as I dont know how to get rid of the DC offset in my stupid windows software osciloscope )
Checked that all DC values on Tr1 to Tr8, including the 3012 IC they seems fineThe Meter was just an adjustment issue, so I am now looking futher in...Back to looking at the schematics and messuring values.Tips , appreciated.
I might be on a trace.There is no 38 Khz on Tr21, and no 19 Khz on Tr 20
That's because the receiver is tuned to a station that doesn't transmit the MPX pilot, no station at all orreception is too weak.
Martin
Hello Martin, thanks for jumping in here.I have two opened BM3000 sitting side by side , tuned into same station, to compare messuerments
So, one is fully working , the opther is the one with no sound from the radioBoth are tuned into P2, with classical music,Scale put on 96 M;Hz and adjusted with the meter , so yes there is MPX signal on that station, verified by the working BM3000
The BF194 transistors are known to go weak.
Changed first TR19, as that is the last one backwards in chain, that I couldnt measue so low volt singnals on my software osciliscope. , no change.So then to Martins advice, replaced Tr 1to 4. the BF194Something happened, the strenght indcator behaved very differently, and adjusting no-signal to signal the trims ended up at exactly at the same place as the working BM3000s trims.But still no sound through radio,I have tried feeding signal directly from the output of Tr23 and Tr24 to an external amp to see if there is anything audiable, but noideas from someone ?Damn I need a better osciloscope, what are you using guys ?Tomorrow back to measuring again, after the re-transC
Can you tell (with a scope) if there's any AF signal on the tuner outputs ?
No signal on the tuner output, just noiseWith oscilocope in auto, it goes to highest resolution 50mV/scale gives just radom noise close to midpoint
Was tired last night when I wrote.BF194 is what I replced, that is not tr1-4 they are Tr5-8Didnt get so much Beoing time tonight.The collector on Tr8 that feed the filter and next module shows weird result on the oscilocope, even though I replaced that transistor.Halfcut wavesHave to do some verifications of this, tomorrow.Thanks so much for helping me a bit in this case Martin !
Tonights resultMy replacement TR8 transitor 2N4401 behaved weird on the collector .Changed back to one of the BF194, good then both machines showed same result on the osc.OkPart chain.TR8 Collector and emitor messured on the osciloscope looks now the same on both the working and the failing BM3000That means.Tr8 emittor shows clear wave.Tr8 collector shows steady DC ( must be somekind of AC there as it is the collector that feeds next stage, but I my crappy softwaare osciloscope cant meassure anything else then DC on there on both machines ) This signal on the collector feed the SF 10.7 filter and then to next shielded moduleThe output on the shielded module I can measure on the working machine, here the signal has been demodualted, which makes me believe that the demodulation is in the shielded box.So somewhere between the collector of Tr8 and the output of the the shielded module seems the problem
Just want to say that I am not an expert, I am just trying my best based on way to old knowledge, so help is appreciated.
The second shielded module, which I suspect is the demodulator seems to be the issue.I have a steady wave into itBut no signl out of that module. ( Signal that is messurable there on the working bm3000 )Now it is getting tricky. This pcb board in monted in a way that it is impossible to messure under it. And the few point I can reach from the obove side is the output of the OP703, which seems to have so high output iimpedance that my osciloscope seems to load it down.So I am in a point of guessing.....and my guess is the 703 circuit that is failing.Seem hard to get these ICs from Europe, where I dont have to pay expensive freight and import fees. from.Any comments ?
So I need to find a 703 circuit, to evaluate my error tracking in the non-working BM3000As I hadsome hours over this sunday night I wanted to verify my replacement transistor.So I replaced the 4 BF194s in the radio input stage, on the working BM3000 to the 2N4401And what happened ?It totally change the behaviour of the radio, to soo much better.The receving was much more defined, and audio quality improved .So Martins thesus about that the BF194 go bad, is something I deffinetly can verify.So now, from my experience, I truely recommend replacing themm
I was right in my suspecion, based on my messurementsThe error was in that shielded demodulation module.I replaced the entire module and radio is working now. !! YIHAAAAThis means I can actually now start refurb this BM3000 into nicer quality.If I get time later I will replace the actually 703 IC on that module to see if is actually was it.because of its mechanical contruction it was difficult to complete messurementsAaaaanyway....stubborness works, damn it took long timeEventually I will get faster with timeChristian
Christian Christensen:Aaaaanyway....stubborness works, damn it took long timeEventually I will get faster with time
Good job Christian, and dont get faster, you enjoy every moment of it, the longer the better
Collecting Vintage B&O is not a hobby, its a lifestyle.
Søren Mexico:Good job Christian, and dont get faster, you enjoy every moment of it, the longer the better
Is that what they tell you in the 'Zona Rosa' Soren?
Well done Christian. Another good save.
Dave.