Menahem, thank you for all the information.

Nope, no scope for measuring, it's true that i'm not really well equipped for these kinds of projects. All i have is a low-cost digital multimeter.

Dillen wrote:

Try putting your multimeter in AC-volts mode and measure across the large

filter capacitor and let us know the result. Not the best way to measure ripple but it will
give us an idea of the ripple level.

Measured in same conditions (power & FM on, volume at zero) it reads 68.5

Dillen wrote:

There's an adjustment for the 24V supply on the main board, did

you measure/adjust this after repairing the power supply ?

I did, some, but as i never got it right everywhere, i just left it at a closest acceptable across the board, not very different from the original value.

Answered to your PM, thanks.

Sebastian

68.5 - is that volts or millivolts ?
If volts, the filter capacitor is not doing anything and the rectifier not the best either.
If millivolts, no problem at all.

Martin

Erm, uh... that would be volts i'd say. Never done AC measuring before (nobody slap me please). I measure it with the multimeter at the range of 200V~ that, let me see if i get this right, measures at millivolts precision (hence the .5). So a reading of 068.5. When measured at the range of 600V~ (volts precision), it's a round 069. So yeah, must be volts. Oh dear... I don't like the sound of your answer, Martin.

I do have a bunch of new generic caps somewhere in my spares box, of which a couple of Hermei 1000uF 35V. Should i swap and test with one of them ?

Sebastian

Yes, that's bad news but mainly for the large filter cap.
You can disconnect the old large capacitor and connect a new.
Make sure to get the polarity right.
The BM900 uses positive ground. Use the old cap for polarity reference and DON'T assume red
leads being the positive.
If it does not fit into the mounting bracket, you can use two cable ties crossed
around the cap and bracket.

I strongly suggest you do not power up the radio again before replacing the filter cap.
There's a good chance that the original zener and transistors died from overvoltage
and the new components will do the same.

Martin

Nope, seems it wasn't that after all, i get the same reading with a new cap (and the polarity is good). Could it be something before the cap, even if the cap itself was good ?

Ok, so there is, what, the rectifier also that was suspected, right ? So i rechecked it once more, and this time i took notes of it all (since i already did it before, but now it's impossible for me to say how, to know if i was right or wrong, but more likely wrong, but it's always nice to have proof). Notes are good. Makes a note on making notes.

And to me, the rectifier, now, i don't know. But i'm done guessing, and will leave the analysis to those who are qualified. So ; i desoldered the four leads (so it was completely "out of circuit"), and tested it in two ways, with ohm meter, and a diode test. Between each two points the measure was taken 2 times, +/- and -/+, so a total of 8 measures per test.

With diode test i got an average of 300 in one direction, and 1 (infinity) in the other, each time.

With ohm test i was absolutely out of the simple ohm level (it read 1 immediately, both ways), should that matter. With the multimeter switched to 200k, it averaged to about 17K in one direction, and to around 50K in the other, each time.

So, i have not come this far to not make yet another mistake, and so i venture and say it could be finally the rectifier ? 👍

Sebastian

The rectifier readings sounds about right.
The huge ripple can be caused by the filter capacitor but it could also be a very heavy load on the power supply.
However, that would probably see some power supply components heat up pretty badly and as I understand it, they don't.
Still, a ripple that high would have the radio hum very much (if running at all) so I must admit to having doubts about that reading.
Try measuring AC voltage across cap 514.

Martin

Yeah, no, there's no heating in particular. To tell you, i didn't really notice anything unusual before this ripple measuring started.

Here are the latest measures :

- cap 514 : AC = 56.4V / DC = 25.8V

- power cap (1000uF) : AC = 65V / DC = 29.7V

And here's a photo of the method used to measure the value, maybe you could confirm if it's done correctly (and if i read it correctly) ?

Sebastian

Sebastian

Something is really wrong here.

Whatever you measure electrically, you can't have 56VAC with 25.8VDC - this situation would be catastrophic.

Depending on the circuit, it can be 56VAC with 26mVDC (millivolts), OR 56mVAC with 26VDC.

Can you get a better multimeter to make sure that you get the right readings?

As much as I would like to help you, and Martin is already doing a fine job, it's impossible if we can't get correct readings.

Menahem

yachadm wrote:

Can you get a better multimeter to make sure that you get the right readings?

Oh, so it could be the multimeter, yeah i hadn't really thought of that. It's true that it's an el cheapo and has had a few years of service.

This said, i couldn't really justify to get a top-of-the-line one, but i'll have a look on alternatives today (el less cheapos).

Thanks Menahem.

Sebastian

Even the cheapest multimeters are generally quite useful. Those readings are so odd that it makes me as well think your multimeter's AC range is faulty or there is something really strange going on in the measurement that makes it show potatoes.

Try measuring some known pure AC signal, like the voltage over the dial light bulbs (should show 6-7 volts).

As even the smallest AC voltage range in your multimeter is 0-200V, it will never be too accurate at measuring small voltages... I don't think it is very useful to try and measure the ripple voltages with it.