New B250 Owner - two questions

[MilanMilosevic]

Hello, Revox-Freunde,

A nearly perfect B250 has finally landed in my household. After listening exclusively to B251 for 25 years, I have to get used to the new sound, but I also think that B250 is a better match for my B&W Matrix 804 speakers. Of course, the B250 will need full rework, but for now I am playing around, trying to understand the logic of its settings.

The most confusing thing for me is MAX VOLUME adjustment on speakers. The factory default setting is somewhere below the half of dB scale on LCD display. Is this the setting that provides full output modulation of 120 WPC, or should I set it to Max?

Input sensitivity: if I understand well, there is only automatic source level calibration, no manual like on B251. Is this correct?

Finally, are there good sources of VFD display online, as I would like to change mine as it is unevenly dimmed?

Thank you in advance for your kind advice.


Milan

[Vapeur]

Hello
Congrats on your B250. I can’t tell you much about the settings, however, regarding the display, there is a fair chance that it will be bright again after changing the caps on the front PCB. To my knowledge the display is no longer available.

[Hamburger123]

Hi,

its from borchee a Link…

https://www.arihav.com/Item.php?Item=8432&lang=eng

45 Pieces Available.

Good Luck!

[MilanMilosevic]

Thank you all for your replies.
Regarding the power amplifier elkos: is it advisable to go down from 22000uF to 15000uF? Are there any negative sonic effects if reducing the capacity?
Thanks again.

[Niederberger Jean]

What is the reason to reduce the capacitors value? In the Service Manual they are listend as C22/23 with 22mF.

[MilanMilosevic]

According to the internet, the 22mF capacity X 4 put a lot of strain on the transformer and the rectifier and was one of the main reasons for transformer failure. So, the 15mF value would reduce PSU stress without affecting the sound and several users recommended that in their repairs. Allegedly, even Revox realised at one point that 22mF was too much and began shipping later B250 series with 15mF capacitors, but I haven't seen it documented anywhere.

[Hamburger123]

Hi,

Transformer failure often occurs when multiple speakers are (for whatever reason) operated in parallel at the same time, and there is insufficient air circulation.

The ReVox Disco Rack in particular tends to cause heat-related failure due to inadequate ventilation and heat dissipation.

This leads to the electrolytic capacitors drying out more quickly. As a result, the capacitors can develop a short circuit or become low-resistance → overloading the rectifier → overheating of the transformer → and ultimately a transformer short circuit caused by insulation breakdown of the wire.

If you want to do something good for your ReVox audio system, install four fuses between the transformer and the rectifier (one in each 33V AC line).

If you measure the power consumption at that point, you can also directly observe the difference between 15 mF and 22 mF capacitors.

The inrush current will definitely be higher, but only for a very short time. After that, only maintenance charging currents remain, which will vary depending on the volume level.

The larger the capacitance of the capacitor, the better the impulse stability in the low-frequency range. I have never measured the relationship to damping.

There are formulas you can use to calculate these causality.

Regards,
Bernd

[MilanMilosevic]

Thank you Hamburger123 for your insights. What fuse rating do you suggest here? I suppose they should be F?

I am still struggling to understand the logic of the overengineered user interface on B250, so help is kindly requested:

PWR-ON VOLUME
This one is clear, because the same function exists in B251: maximum volume on turn-on. So far, so good. But here is what I don't understand:

MAX VOLUME
This is supposed to limit the maximum output volume. The indicator on the right screen appears under the scale going from -100 to 0dB, so basically it is attenuation. If I set it to MAX (0dB) is it going to defeat the attenuation and provide access to full power?
Also, it seems to behave strangely: I will set MAX VOLUME to maximum, press store, then turn the amplifier off. But next time I turn it on and check MAX VOLUME setting, the MAX VOLUME will be equal to POWER-ON VOLUME on right display. I don't think this is a correct behaviour.

Thank you in advance for your explanations.

[Hamburger123]

Thank you Hamburger123 for your insights. What fuse rating do you suggest here? I suppose they should be F?

Hi,

you can measure the fuse rating yourself. In this case, it needs to be a slow-blow type.

That’s because, in this device, the electrolytic capacitors are not precharged; instead, when the power is switched on, the capacitors receive the full inrush current. This needs to be taken into account.

You can also just leave everything as it is. It’s worked for the past 40 years—why change it now?