Sennheiser UPM 550 universal pegelmessgerät 10Hz 1MHz teardown

Sennheiser UPM 550 universal pegelmessgerät in German means Universal Level Meter in English, it is from about 1980, power on/off switch needs replacement, I was able to fix it and calibrate it, PEAK mode 10Hz to 1MHz, RMS mode 10Hz to 100kHz, powersupply uses CA723E and a couple of external NPN PNP transistors, AD536AJD is the RMS to DC converter.
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#teardown
#sennheiser
#audio
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I use auto generated subs, i am sorry there are a few spelling errors,
maybe one day they can do it better..
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(Thanks to all my Sponsors / Traders / Swappers / Sellers / Buyers, without your support and constant flow of cool items in/out, none of this would be possible)

[@K5HJ]

1 volt is not 0 dBm. It should read "10" on the meter scale. 0 dBm is 0.775 volts at 600 Ohms. It looks like the calibration was already pretty close. 1 volt into 600 Ohms should read +2.22 dBm.

[@markocebokli6565]

I think this was measuring RMS correctly in the beginning. You put 1V RMS in, and the needle was exactly at the "10" point on the linear scale. Decibels in audio are referenced to 600 ohm, so 1V = 2.2dB, not 0 dB.

[@migsvensurfing6310]

True. I remember that 0dB in audio is 770mV. I haven't measured audio since the 80s but I do remember that. One thing I find odd though is the Mohm input resistance, shoudn't it be 600 ohm ? I remember that as a standard for audio. ( Doing mostly RF stuff 😀everyday)

[@TeardownOZ2CPU]

1V = 0dB is written on the front panel.

[@markocebokli6565]

@@TeardownOZ2CPU I think they just called the 1V scale "0dB" for aesthetic reasons, "2.778dB" would look silly, and to reduce the number of different scales - if they made a "true 0dB" scale, with 0dB top scale, there would be 30 total switch positions, together with the linear ranges. 1Mohm input is probably for use as a "pure voltmeter", on live circuits. I guess when new, this box came with a 600 ohm terminator and balun transformer for dB level measurements. 600 ohm interfaces are often symmetric, I guess this standard goes back to air lines on telegraph poles.

[@TeardownOZ2CPU]

@@markocebokli6565 you could easyly be right :-) it is easy to move it back to what ever system we like, now i know what all trimmers do :-)

[@CBtronica]

​​@@TeardownOZ2CPU1V = 2.22dBm referenced to 600 ohm. The equivalence 1V = 0dB on the front panel is just for reference. I have a similar instrument, with the same functions and filters, made by Meguro Japan, model MN-446. The meter has four scales, dBm, dBV 1V F.S. and 3.16V F.S., and on the front panel attenuator it's also written 1V = 0dB.

[@yevgenkotikov]

There is built-in test oscillator. Select -70dB (test) with a range selector and push the "Test" button. The pointer must indicate "0" dB.

[@KC2QMA]

Those blue Tantalum caps almost always fail with a SHORT!

[@UlrichHarms-ci1ov]

The peak to peak filter should use the linear scale, not the DB scaling. So it looks there is still something off with the gain. There is a slight chance that someone modified the instrument from dBV to 600 ohm dBm and changed the gain somehow. That one of the filter settings gives no output is not a surprise, as the custom filter PCB has only 1 filter installed.

[@andersvandegevel8355]

Tom Evans could learn a thing or two from the way this thing is constructed

[@TeardownOZ2CPU]

the good Tom Evans surely made him self famous :-)

[@SeanBZA]

Got a few of those AD536 chips, and that 6 pin IC is most likely a matched pair of transistors used as differential pair in an amplifier. note as well the RCA chips got black topped before marking, to give a better contrast to the lettering versus the original grey epoxy.

[@UlrichHarms-ci1ov]

The 6 pin chip is an old OP-amp TA761 in the odd DIP6.

[@michaelturner4457]

Usually devices from that era would state Made in West Germany or W. Germany.

[@idleprepress]

I always wonder where do you get so much of that heaivly localized european stuff in USA?

[@migsvensurfing6310]

He doesn't. He is in Europe

[@TeardownOZ2CPU]

I live in Denmark, it is close to Germany, I think that is why i see so many products from Germany, when i am in USA it looks very different, then it is clearly US brands that is dominating the flea markeds

[@idleprepress]

@@TeardownOZ2CPU Wow, I'm subscribed for several years to your channel, but was all the time almost sure you lived in the States. Hahaha.

[@TeardownOZ2CPU]

@@idleprepress thanks a lot, i only visited 48 states so far :-)

[@dipakbhalodia7155]

मेर को आप जो review करते हो ये सब इंस्ट्रूमेंट कहा से मिले गे 🎉

[@migsvensurfing6310]

Auctions and fleamarkets

[@TeardownOZ2CPU]

do you live in Denmark ? and do you have stuff to trade with ?

[@LawpickingLocksmith]

Times when Germans built things.

[@TeardownOZ2CPU]

about dB and voltage read out scaling, it is VERY normal to have 0 dB = 1V, almost all meters I have do it that way, please look at all the other videos about AC voltmeters i released, here is one example : kzbin.info/www/bejne/i6Soenevi5iEqqc