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Try measuring some known pure AC signal, like the voltage over the dial light bulbs (should show 6-7 volts).
Well, the curious thing is, i just tried and it's a steady 6VAC, which on the other hand should prove that it measures just fine ?
What on earth is going on here ?
Sebastian
I actually managed to find the schematic of your multimeter on the friendly interwebs. Seems to me that it simply isn't designed to measure an AC voltage riding on a DC bias, as is the case when you try to measure the ripple voltage on the supply lines. It has half-wave rectification on the AC range and probably shows the DC bias + the AC voltage, multiplied by some fuzzy factor that depends on the AC waveform. Not a big wonder, since proper AC measurement is a complex task and very few, if any, cheaper multimeters do it right.
So, just don't trust it - or if you wish to experiment, put a small capacitor (preferably non-electrolytic) in series with one of the probes when doing the ripple measurement. It should isolate the DC component out of the measurement and maybe you'll see a more sensible reading.
Hey, thanks Mika for your efforts 🙂
I actually had the manual of the meter downloaded, after having read something of the sort while googleing yesterday about AC measuring. But i wasn't able to decipher this in the manual, it said being able to read it. So you saw it directly on the schematics ? Wow.
Ok. I read about that method of a small cap in series, but what would one call a small enough cap ? I just tried with one 0.1uF Wima polyester cap, and then one tantalum 330pF, and it read 00.0 with both. It kinda jumps a few millivolts when i move the leads to verify contact, and then zeroes in immediately. So i guess in principle the trick works. But i would've preferred a "small something" reading rather than nothing. How to be sure, now ? Is there anything i could measure with it that should be sure to have ripple ? Note : it gave 00.0 with both the new cap, and the old big one that i resoldered back on to test.
Lord. If anybody can recommend the cheapest good hobby multimeter on eBay, i would've needed a new one sooner or later anyway.
Sebastian
I waited 15 years until I bought my first Fluke, and then slapped myself for waiting so long. I thought that I could get by with Japanese (now read Chinese) knock-offs.
The Fluke just removed all doubt when measuring.
My second Fluke is a model 187, now 5 years old, bought 2nd-hand on ebay, for a fraction of the new price.
It's the best investment I ever made. I wouldn't even look at anything else. And you can always get it serviced, if required.
Just check that the model you choose can do capacitance up to at least 10,000uF as well as the regular AC/DC/Ohm/Diode functions.
Keep your Mastech for doing transistor measurements.
Another excellent tool is the EVB ESR meter (made in Portugal) for checking capacitors in-circuit.
A good (used) Fluke DMM and an ESR meter is a very comprehensive toolbox for troubleshooting electronics.
Menahem
Location: Jerusalem
My B&O Icons:
I actually just opened up my 900M (please don't praise me for that effort, I was going to do it anyway 😂 ) and took a similar measurement across the main filter cap - I'm not at my "lab", so I only have an el cheapo multimeter here, not unlike the one you have.
Guess what, it shows 77.5 V on the 200V AC scale, and there's definitely nothing wrong with the power supply in this Beomaster.
When measuring through a series cap, the jump of the reading when connecting is normal as the cap is being charged to the DC voltage. I guess the zero reading is about right - as I recall, the accuracy of your multimeter on the 200V AC range was given as 1.something% of the reading, +/- 10 digits! So if there was 2 volts of real ripple, the meter might show anything between 0 and 4V and still be within specs.
I'm afraid I can't recommend any particular meter... Fluke or Meterman (currently Amprobe) should be safe bets, although quite expensive for sporadic use. Don't get rid of your current one anyway, it has the transistor test function which is sometimes very useful, and not too common in even more expensive meters.
I was away for a couple of hours and you guys solved the mystery meanwhile. 🤒
I agree on the multimeter and instrument suggestions above, I use Fluke meters myself,
but I take it you are not planning on setting up a complete and stylish repairshop
but rather deal with the things the "kitchen-table" way. Correct me if I'm wrong,
in which case, as Mika suggests, the cheaper multimeters will do fine in most cases.
That's also my experience and most will easily tell you if something is wrong or right,
no need to go full steam ahead at this point.
There's also some meters that don't like measuring 100Hz AC or AC that is
not a nice sine wave, even more expensive ones.
A scope may be the better investment if it comes to that.
But recap the poor thing and let's see what happens then.
Martin
Magnificent.
A good (used) Fluke DMM and an ESR meter is a very comprehensive toolbox for troubleshooting electronics.Menahem
I actually just opened up my 900M (please don't praise me for that effort, I was going to do it anyway 😂 ) and took a similar measurement across the main filter cap - I'm not at my "lab", so I only have an el cheapo multimeter here, not unlike the one you have.
Guess what, it shows 77.5 V on the 200V AC scale, and there's definitely nothing wrong with the power supply in this Beomaster.
When measuring through a series cap, the jump of the reading when connecting is normal as the cap is being charged to the DC voltage. I guess the zero reading is about right - as I recall, the accuracy of your multimeter on the 200V AC range was given as 1.something% of the reading, +/- 10 digits! So if there was 2 volts of real ripple, the meter might show anything between 0 and 4V and still be within specs.
Yeah the Mastech is rated at 1.2% for AC. This is great news, and really the only confirmation needed then. It was the multimeter. Whew. Thanks so much for the trouble, even if you meant to service your 900M anyway. That was very nice of you.
I was away for a couple of hours and you guys solved the mystery meanwhile. 🤒
I agree on the multimeter and instrument suggestions above, I use Fluke meters myself,
but I take it you are not planning on setting up a complete and stylish repairshop
but rather deal with the things the "kitchen-table" way.But recap the poor thing and let's see what happens then.
Martin
So i guess it's back on track, then.
Martin has, in his infinite kindness, proposed to send me a recap kit free of charge, in exchange of some detailed informations about my specific subversion of the 900K, which apparently isn't the most common kind (well, uncommon enough for him to not have encountered one).
So i'd like to take profit of this unusually calm moment on my "restoration" project, where everything seems once again going well (before something else will come up), to repeat my sincere thanks to everybody around here, and in particular those who have helped me directly. And of course in first place Martin, who really has shown a patience out of this world.
This said, i won't say more, because it's still not finished 😎
Cheers guys.
Sebastian
Martin has, in his infinite kindness, proposed to send me a recap kit free of charge, in exchange of some detailed informations about my specific subversion of the 900K, which apparently isn't the most common kind (well, uncommon enough for him to not have encountered one).
🤒 Both of you!
To tell the truth, I didn't know about this kind of problem in AC measurements myself before this, as I've always used a scope to check for ripple... So this has been a highly educational thread all over 🙂
Yes Mika, I'll second that - I also learnt something new here. Actually on the Fluke 187, it's so stable, that if I see fluctuations on the AC or DC scale (decimal points as appropriate), I'll suspect ripple, and connect my scope to confirm it. If the Fluke is rock stable, I know there's no ripple.
Those Uni-trend UT models are no longer suitable for our type of audio work. They changed their models, and I couldn't find anything over 100uF, let alone the minimum 10,000uF I'd want.
Perhaps check new Amprobe, as Mika suggested. But I can tell you that plenty used Fluke 8x or 11x go for well under $100 on ebay. You don't need a 187 like mine.
Menahem
Location: Jerusalem
My B&O Icons:
The next chapter ;
i thought to report on how it went, so that maybe one day it can help someone else, in any way, as my good Karma payback.
So, got the recap kit (thanks Martin !), nicely documented, and attacked the job.
Luckily this is old circuitry we're talking about, because my soldering equipment isn't what you would call the most advanced kind (nor in the best condition, to top it all). But it went well. Well, i'd say so, and i won't show you any too-close-up photos so that you could tell me the contrary 😂
Turns out that this was indeed a version of the 900K that was still partly undocumented in Martin's information vault (just how unlikely is that ?!). So photos were taken. Notes were written. PM's exchanged. And here's a résumé of all that. A recapping of a Beomaster 900K, subversion S 290, export model with a six input voltages selector, and whose wooden cabinet has a stamped date of 8 JAN 1965.
This is an extremely image-heavy post, so my apologies to those who it might derange, but personally i would have loved to find one like it when i first encountered my radio, so here goes. The first few photos are for identification, of the untouched unit before the recapping. You'll notice (well, i just did) that the volume and radio dial buttons are reversed (the volume button has the black point) for the moment, during the project. It goes pretty much in the following order ; the original unit, the recap kit, and then the recapped unit.
Front
Back
Power unit ; 1 cap to change
Tuner ; 1 cap to change
Bottom side ; in my model version there are 12 caps to change (+ the two 800uF output caps that are fixed to the back wall)
Top side (on topmost there are the amp board on the left, and the stereo decoder on the right)
Amp board top ; in my model version there are 17 caps to change
Stereo decoder top ; in my model version there are 6 caps to change
Stereo decoder bottom
The recap kit from Martin
Bottom after recapping
Amp board after recapping
Power unit after recapping
Here are all the original caps from the main board, with their respective designations on the schematics that i have. After discussion with Martin, i've come to learn that these designations may well vary between model versions and/or schematics, so it just might not be the same ones for you. But this is the complete set on my specific model/version.
And here's the same thing for my amp board, to be considered with the same grain of salt.
And here's something i discovered only while taking the photos, even though i had manipulated the board frequently during days before (i've bent the cap slightly to make a point, i hadn't noticed it at all before). A severed + leg on a cap on the stereo decoder board. Just for the curiosity factor. It'll be fixed by the recapping of the stereo decoder board, coming in the next chapter.
So there, for now. What remains is the recapping of the stereo decoder, and the single one on top of the tuner box, both to be done in the coming days. I've also completely redone the wooden cabinet and given new luster on the gray speaker grilles with some polishing. It's starting to look good
Sebastian
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