B251 Fan (Lüfter)

[MilanMilosevic]

Hello,

I would like to make cooling fan module for B251 and connect two silent computer fans to it. Unfortunately, there are no schematics in the service manual diagrams. Does anyone have schematics to share? I would also like to know where and how it is connected to the mP board and to the power supply. Thank you in advance for kind assistance.

[borchee]

AW: B251 Fan

Zdravo, Milane

If i understand you correctly, you're in believe that revox made/prepared/was thinking of a cooling improvement/modification with fans? As far as i know, they didn't - if you saw one, it should be DIY.

Lep pozdrav v Beograd,
Borut

[MilanMilosevic]

Re: B251 Fan

Zdravo borchee,

there actually was an original Revox aftermarket kit consisting of fan and controller. If you look at uP and stabilisation PCB schematics, you will see the connectors marked as FAN and HEAT which are not populated. I will post the picture of the fan later.

[borchee]

AW: B251 Fan

Oh, i wasn't aware of that. I don't have the slightest clue about electronics, but i'm curious...how about you taking a look into the B242 service manual.

[MilanMilosevic]

Re: B251 Fan

Here is the picture of the cooling fan and controller, as well as the rear of the amplifier, with the fan mounted. The four pins on the green PCB are supposed to be plugged in J7 on stabilization PCB, which has 11V supply and FA signal which leads to J5 connector on uP board and from there to IC4. I could never figure out how does it function, what senses the temperature and triggers the FA signal.
On the other hand. B242 adopts a simple NTC element which triggers the fan when maximum operating temperature is exceeded.

[eckibear]

AW: Re: B251 Fan

Here is the picture of the cooling fan and controller, as well as the rear of the amplifier, with the fan mounted...

I've heard about it, but actually never saw it. Now I just can't hold off. This is an awful design. No directive flow control, just a stirring wheel inside the oven. They must have been real desperate back then. I've put my B251 to rest, it's way to much pain for the moderate outcome.

[MilanMilosevic]

Re: B251 Fan

Come on, it is worth it, for the sake of its lovely sound. I have seen the fan only on pictures, but it surely doesn't look like a quiet operation design!
Eckibear, could you elaborate more on the electronics of cooling? I mentioned in my previous post the connectors on stabilization and uP PCB designed for the fan but don't know how to use them in my eventual DIY cooling fan and controller.

[eckibear]

AW: Re: B251 Fan

.. could you elaborate more on the electronics of cooling? I mentioned in my previous post the connectors on stabilization and uP PCB designed for the fan but don't know how to use them in my eventual DIY cooling fan and controller.

Unfortunately the manual seems to have various flaws in this area. There are missing pin assignments, variants of the stabilizer board... You name it.
The logic FA (Fan Activate) signal is issued by the uC and routed via IC4 (latched shift register) to the fan electronics. Probably the latter also came in variants. The one you pictured has more wires and pins than the one on the second variant of the stabilizer board in the manual.

There is a signal "H" which runs from the sensing NTC via Volume board to the uC board, J6/pin2. It is evaluated by IC9, a double comparator each with some additional hysteresis, defining two temperatures as switching points. The output H1 and H2 of these comparators is forwarded to the uC. The uC issues the thermal shutdown command at the higher temperature switching point and the FA signal on the lower switching point.

I cannot see where the other signal "HEAT" is running or ending. The stab board asssigns P1/pin9 towards Volume board. However, it dot not show up there, it is just a 8-wire connection. Probably Revox just lost control on the multiple revisions to make this oven work somewhat more reliable.

[MilanMilosevic]

Re: B251 Fan

Eckibear, thank you very much. This means that FA signal could be used in some way. I have found the schematics for the cooling unit and vent. It is the area with dashed lines. This is the execution found on PCB 1.725.810.00. It uses the FA signal (coming via R41) to open the Q7 and the Q6 which is the fan switch. If FA is logical signal, then there is no RPM regulation, only on/off function. And the motor seems to be on 50V supply.
Can you tell at which temperature would FA eventually trigger and which FA voltage should be expected? I would like to use 12V silent ventilators and for that purpose the free 11V supply from the PSU and still use FA as trigger.

[Rolf L.]

AW: B251 Fan

hi,
there is also a 12 volt external solution for the B750. Could that possibly contribute to the solution and help the B251?
8.jpg
The bottom view of the B750
nice greeting
Rolf

[MilanMilosevic]

Re: B251 Fan

Hello Rolf,

if I understand well, you mounted a fan underneath B750 which blows in or sucks out? Did you have to put extra high feet on the amplifier? Which fan is this - type, dimensions, make? This is a nice solution but not for B251 because it has a different architecture: while B750 consists mainly of vertically placed modules, B251 is all horizontal inside, so a fan mounted like this would be blocked by a large input amplifier PCB which is on the bottom of the unit.

[Rolf L.]

AW: B251 Fan

Hello Milan,

sorry if the English is not good, I use the Google translator.

The Luftsrom is sucked under the amplifier, then circulated through the cooling fins, there absorbs the heat and effect on upper cover plate off. Because hot air rises, this property is still supported. The fans are so flat that no further alterations in the level of the B750 espacially. Used a plug-in power supply for low voltage (<24V DC).
If it is now unsuitable for the B251, then it was at least worth a try and I've tried the translator again.
nice Greeting
Rolf

[eckibear]

AW: Re: B251 Fan

...Can you tell at which temperature would FA eventually trigger and which FA voltage should be expected? I would like to use 12V silent ventilators and for that purpose the free 11V supply from the PSU and still use FA as trigger.

FA logic is a CMOS output at 5V supply, so logic H is close to 5V at few mA current load.

For the temperature setting, well, your guess or preference. Lower T gives better life insurance for the B251 at the penalty of noise. Low-noise blowers may not help much, the air flow is fairly obstructed and air turbulence will give noise. You'll have to try anyhow. Perhaps you'll decide to keep the blower running all time, in case the noise is low enough.

Make sure not to overload the 11V supply, 2-3 Watts are OK though.

[xoveR redutS]

AW: B251 Fan

Hello!

FA / H are useless.
Please note that the temperature have to exceed aprox. 76°C to start the fan.

http://www.revoxforum.de/forum/showthread.php?t=6717

[eckibear]

AW: B251 Fan

Hello!

FA / H are useless.
Please note that the temperature have to exceed aprox. 76°C to start the fan.

http://www.revoxforum.de/forum/showthread.php?t=6717

Thanks for the link! Obviously the oven temperature had been discussed in detail. Although the factory settings are quite high, they do protect the amp from short-term overheating. However, this scheme does not prevent the driver transistors to fry the PCB over time.
The switching points could be adjusted fairly easy by minor adjustments to the input resistor network of IC9, i.e. the fan could be activated at much lower temperature if desired.