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Old 24th Sep 2022, 5:37 pm   #1
regenfreak
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Default Decipher Revox B285 schematic mystery

I have acquired the module for the RF section and If stages of Revox B285 FM stereo tuner. This is a high-end tuner, one is currently up for sale on ebay for $1000. One of its unique features is the initial IF stages: a 3-pole Butterworth bandpass followed by a long-tailed differential amplifier and a 7-pole Butterworth phase linear filter. The filters have asymmetrical designs with different input and output impedance.The scheme can be downloaded from:

https://www.dadaelectronics.eu/uploa...ice-Manual.pdf

On page 92 of the scheme, I am mystified by the negative polarity of the power rail at point 8 on the left; -16.5V. At first, I thought there is a mistake in the scheme. But this is also shown as -16.5V for pin 8 of the PCB on page 93.

I have attached markings with blue colour traces of the -16.6V power rails. The positive + 16.5V rails are marked red. It looks like the drains for the first pair of BF961 RF amp Q1, Q2, oscillator and collectors of some NPN BJT (Q7,Q8,Q9,Q10,Q11,Q12) are all negative. On the contrary, the differential amp CA3053 (37k resistor at the top, 2.2k resistor at the tail), Q15, Q14 seem to have the correct polarity. I cannot make sense of it. Any idea what is happening here? The polarity seems wrong as indicated in the blue traces.

The RF front end also has very peculiar double dual gate FETs. I don't know the benefits of this. Certainly I have never seen this being used in high-end Japanese tuners
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Old 24th Sep 2022, 6:26 pm   #2
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Default Re: Decipher Revox B285 schematic mystery

The circuit is just WRONG.
I think the draftsman has slid some segments of power rail to the wrong place.

If you look at the PCB drawing you can clearly see that IC1 connectes to pin 7 (+ve 16.5v supply) via R53 and L23.
I did not bother looking any further but the circuit needs modifying to match the PCB.

Good luck unscrambling that...
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Old 24th Sep 2022, 6:45 pm   #3
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Default Re: Decipher Revox B285 schematic mystery

Dual gate MOSFETs have been steadily vanishing from catalogues over the past few decades. They were always seen as low power consumption champions. Running a pair in parallel doubles the Gm and the Id, so helping large signal handling by 3dB and avoiding unobtainable devices. It also keeps the Z down and makes for easier matching into high-Q RF filtering sections. Running low L and high C (lots of varactors) keeps the signal voltages down on the varactors and so reduces intermod effects.

The whole machine is a derivative of the B261 tuner, modernised a little in some areas, simoplified and cut down in others. They've had to make space for an AM section, and for an optional amplifier. The B261 was purely an FM tuner and went with either the B251 or B252 to handle amplification. It retains the LC filtering which can be done somewhat better than off the shelf ceramic types. The linear phase characteristic lies in the choice of pole locations, otherwise it's a simple enough coupled resonator structure. They've gone to a straight-forward PLL FM discriminator, rather than the B261's pulse-count type with its length of coax cable delay line.

-16.5v has an error on the schematic. It goes to some things which need it, but the line continues on to several things which need the positive rail.

Someone went cross-eyed and extended the wrong line!

David
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Old 24th Sep 2022, 6:50 pm   #4
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Default Re: Decipher Revox B285 schematic mystery

Thanks. This confirms my doubt. I will use Photoshop to trace out the rest of the negative rail. I was going to power it up with +16.6V and -16.5V without powering the +36V oscillator and synthesizer sections. I would apply appropriate voltage to the varactor sections and inject oscillator input from signal generator to see if I can power up the RF sections. I dont want the magic smoke coming out if the schematic has errors.
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Old 24th Sep 2022, 7:01 pm   #5
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Default Re: Decipher Revox B285 schematic mystery

Quote:
Dual gate MOSFETs have been steadily vanishing from catalogues over the past few decades. They were always seen as low power consumption champions. Running a pair in parallel doubles the Gm and the Id, so helping large signal handling by 3dB and avoiding unobtainable devices. It also keeps the Z down and makes for easier matching into high-Q RF filtering sections. Running low L and high C (lots of varactors) keeps the signal voltages down on the varactors and so reduces intermod effects.

The whole machine is a derivative of the B261 tuner, modernised a little in some areas, simoplified and cut down in others. They've had to make space for an AM section, and for an optional amplifier. The B261 was purely an FM tuner and went with either the B251 or B252 to handle amplification. It retains the LC filtering which can be done somewhat better than off the shelf ceramic types. The linear phase characteristic lies in the choice of pole locations, otherwise it's a simple enough coupled resonator structure. They've gone to a straight-forward PLL FM discriminator, rather than the B261's pulse-count type with its length of coax cable delay line.

-16.5v has an error on the schematic. It goes to some things which need it, but the line continues on to several things which need the positive rail.

Someone went cross-eyed and extended the wrong line!
Thanks. Your explanation is helpful. Yes I have been comparing the B285 with the B261 and notice the similarity. You own one from the BBC if I remember correctly.

I have been probing the values of L with De-5000 of the varactor sections and noticed unusually large L over 250nH. I guess the readings can be wrong with the inductor in parallel with the varactors and trimmers.
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Old 24th Sep 2022, 8:18 pm   #6
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Default Re: Decipher Revox B285 schematic mystery

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Originally Posted by regenfreak View Post
I guess the readings can be wrong with the inductor in parallel with the varactors and trimmers.
Oh, yes, that will get you far wrong. Think of capacitors as negative inductors cancelling some of the susceptance of the inductors, and leaving you with a higher-Z part, a larger inductance.

You have to be able to flip between admittance and impedance, resistance and conductance as well as positive and negative values once you get this comfortably mastered, then diagrams get more animated.

There are many symmetries and equivalences which can save a lot of work.

If a capacitor does something, you can make a safe bet that an inductor does the exact opposite in all respects. And vice-versa. Memorise one, and get the other for free!

David.
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Old 24th Sep 2022, 8:23 pm   #7
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Default Re: Decipher Revox B285 schematic mystery

I have marked the negative rails with blue and cyan; positive rails with red and orange.

During the manufacturing phase, the engineers would use the scruffy but correct schematic for the manufacturing of PCB. Then they passed the original schematic to a draft man who copied it wrong.
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Old 24th Sep 2022, 8:31 pm   #8
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Default Re: Decipher Revox B285 schematic mystery

Quote:
Originally Posted by Radio Wrangler View Post
Quote:
Originally Posted by regenfreak View Post
I guess the readings can be wrong with the inductor in parallel with the varactors and trimmers.
Oh, yes, that will get you far wrong. Think of capacitors as negative inductors cancelling some of the susceptance of the inductors, and leaving you with a higher-Z part, a larger inductance.

You have to be able to flip between admittance and impedance, resistance and conductance as well as positive and negative values once you get this comfortably mastered, then diagrams get more animated.

There are many symmetries and equivalences which can save a lot of work.

If a capacitor does something, you can make a safe bet that an inductor does the exact opposite in all respects. And vice-versa. Memorise one, and get the other for free!

David.

I keep bumping into admittance, susceptance and conductance when I read about the use of Smith charts for impedance matching. They are kind of ying yang in chinese, sometimes giving me headache, my brain like an aircraft in a flat spin. I have to overcome my phobia with admittance, susceptance and conductance before i can transcend to the next level.
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Old 24th Sep 2022, 9:29 pm   #9
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Default Re: Decipher Revox B285 schematic mystery

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Originally Posted by regenfreak View Post
I have marked the negative rails with blue and cyan; positive rails with red and orange.
Going from your marked-up pcb I reckon there is just one spurious join in the circuit.
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Old 24th Sep 2022, 10:57 pm   #10
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Default Re: Decipher Revox B285 schematic mystery

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Originally Posted by buggies View Post
Quote:
Originally Posted by regenfreak View Post
I have marked the negative rails with blue and cyan; positive rails with red and orange.
Going from your marked-up pcb I reckon there is just one spurious join in the circuit.
thanks
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