Revox B 795

[Robie]

Thanks, SaSi. While I don't give up easily, I'm thinking that this repair is now beyond both my technical skills and budget. I learned last week that no amount of staring at the thing will substitute for knowledge and equipment necessary to repair it.

In searching for available Revox parts including Hall sensors, I discovered that there happens to be a B-795 on the local Craigslist now for $250. I bet a repair of mine would end up being much more than that.

 

[JohnVF]

Thanks, SaSi. While I don't give up easily, I'm thinking that this repair is now beyond both my technical skills and budget. I learned last week that no amount of staring at the thing will substitute for knowledge and equipment necessary to repair it.

In searching for available Revox parts including Hall sensors, I discovered that there happens to be a B-795 on the local Craigslist now for $250. I bet a repair of mine would end up being much more than that.

I actually saw that listing and thought of you, thinking the same thing. If I ever see a broken one anywhere I'll grab it for parts.

 

[Sam Cogley]

I know where there MIGHT be a disabled unit available. Drop Sheltie Dave a line, he'd know more than me.

 

[markthefixer]

Thanks, SaSi. While I don't give up easily, I'm thinking that this repair is now beyond both my technical skills and budget. I learned last week that no amount of staring at the thing will substitute for knowledge and equipment necessary to repair it.

In searching for available Revox parts including Hall sensors, I discovered that there happens to be a B-795 on the local Craigslist now for $250. I bet a repair of mine would end up being much more than that.

Actually Bob, It's NOT beyond mtf's ability to advise. First we get the hall effect sensors, modern linear ones or whatever. Then we find out what the expected voltage levels out of pins 1 and 7 of IC 10 originally were, and what it is presently putting out. (you can measure that with your meter on the wires I installed, starting with the old sensor that still works, then moving to the (a) new sensor.) THEN I calculate the new 1% resistor values for the circuit, you get two groups of four at 6 cents apiece and install them like you did the capacitors.

Then you fine tune the screw heights to get the same maximum readings as you rotate the platter by hand - like some modern dj cuing up, moving the magnet's pole past the two sensors.

After that the turntable should rotate on it's own, and it comes over here for a tweaking session on the distortion analyzer OR spectrum analyzer.... (yes I have one that handles audio range - but slowly) and I think it can be pursuaded to go down to 1 hertz or so, OR we get some FFT software that analyzes WAV captures.

While that sounds "wow", the "bars" display on some audio players is exactly that on a crude scale. And some poweful downloadable software has a 21 day free trial period.

 

[SaSi]

Thanks, SaSi. While I don't give up easily, I'm thinking that this repair is now beyond both my technical skills and budget. I learned last week that no amount of staring at the thing will substitute for knowledge and equipment necessary to repair it.
In searching for available Revox parts including Hall sensors, I discovered that there happens to be a B-795 on the local Craigslist now for $250. I bet a repair of mine would end up being much more than that.

Let me tell you a story!
I once saw a B791 on German ebay that came with an Ortofon MC20 MkII cart and an Ortofon T30 step up transformer. Saved that for watching as I expected it to reach 1000 euro. In the last minutes I offered 250 euro and it was mine for a bit less than that. FWIW, that Ortofon T30 transformer sells for more than $300 in the rare occasions it becomes available.

Two weeks later the lot arrived. All was intact (or visibly intact). The TT would run but like a bat out of hell. Upon closer inspection, it appeared that the servo control was out and that it spinned up to reach about 150~200rpm. I thought it was broken, and it was.

Upon yet a closer inspection, I discovered that the servo PCB (with the servo trace on the PCB) was cracked and that was most likely caused by the platter not being removed for shipping (contrary to my instructions to the seller).

I settled the issue with the seller eventually, but the story lies elsewhere...

I hastily found a "broken down" B795 that was "TOTAL KAPUT". It went cheaply - as it was dead - but I knew that all I needed the PCB. Whatever else was a bonus. I made sure the seller shipped it with the platter removed and I received it in one piece.

The catch is; before the replacement deck even arrived, I decided to repair that PCB. A bit of sanding and extra solder was all that was needed - along with some superglue on the opposite side of the PCB. I had my B791 up and running smoothly as the replacement - parts - TT arrived.

So, don't make any hasty moves. $250 is an awful lot of money IMHO.

 

[Robie]

I actually saw that listing and thought of you, thinking the same thing. If I ever see a broken one anywhere I'll grab it for parts.

Thanks John! I appreciate your kind offer. If anyone can find it I know you will. :thmbsp:

I know where there MIGHT be a disabled unit available. Drop Sheltie Dave a line, he'd know more than me.

Thanks for weighing in Sam! I sent Sheltie Dave a PM and await a response. :thmbsp:

Actually Bob, It's NOT beyond mtf's ability to advise....

Mark, I appreciate the time you've spent on this thus far and didn't mean to suggest it was beyond your abilities. I know it isn't.

Wouldn't it be best to just order one or two of the new Hall sensors steerpike2 recommended (see http://www.audiokarma.org/forums/showpost.php?p=3261607&postcount=57)? They are available at Mouser for about $15/each. I presume I only need one anyway? :scratch2:

Let me tell you a story!...

....So, don't make any hasty moves. $250 is an awful lot of money IMHO.

Thanks, SaSi. I agree. I am pretty much the opposite of hasty. :yes:

 

[steerpike2]

Wouldn't it be best to just order one or two of the new Hall sensors steerpike2 recommended .... They are available at Mouser for about $15/each. I presume I only need one anyway?

I'd suggest replacing both. ReVox recommends they be matched if one needs to be replaced (they are colour coded by Studer according to their tolerance to achieve good matching). If you are using new ones that are not quite the same type as the originals, there may be enough of a mismatch that the calibration adjustments in the B795 cannot bring within spec. The result would be unacceptable wow and/or flutter.

 

[markthefixer]

I'd suggest replacing both. ReVox recommends they be matched if one needs to be replaced (they are colour coded by Studer according to their tolerance to achieve good matching). If you are using new ones that are not quite the same type as the originals, there may be enough of a mismatch that the calibration adjustments in the B795 cannot bring within spec. The result would be unacceptable wow and/or flutter.

A slight rewiring of the motor's pc board will be needed, everything else being equal, the power will have to be changed from series to parallel, one foil will need to be cut, and two jumpers wired in.

6v in series gives 3v each, the sensor wants 4 to 10v.


Are these just linear - differential out hall sensors, or are they recommended replacements, because if they are just picked out, then the Micronas HAL 401SF at $1.15 each (sot89 package but we've a better chance of getting the mountings right)

http://www.semiconductorstore.com/cart/pc/viewPrd.asp?idproduct=8453'

might be a more economical choice.

 

[steerpike2]

The one I mentioned was the only one I could find locally with similar characteristics. The Siemens SBV566 is a better match (may even be the exact one), but I can't find them at any supplier - must be discontinued. I do agree the circuit will require some mods to work with any of the modern Hall ICs that I can find in databooks.

The originals are current-diven devices, and the micronas one seems to have a lot of 'support' circuitry inside, so I'm not sure how the output would behave - will it scale the output linearly with the supply voltasge in other words? But it's cheap enough to buy one for testing.
The Honeywell one does scale the o/p according to supply.

 

[Robie]

After searching on Google for the Siemens SBV566, I called a place in California that arose as a hit on that search. I'll report what I learn on their availability and price when and if I hear anything.

As far as the Honeywell Hall Sensor suggested by Steerpike previously, they appear to be a bit cheaper here in the US: http://www.mouser.com/ProductDetail/Honeywell/634SS2/?qs=/fq2y7sSKcLXxPyD69xL2w==

Does this Honeywell appear to fill the bill?

 

[steerpike2]

The B790 uses a different motor, but the same design principle: that one's configuration of the Hall devices may be a helpful guide in getting the voltage up beyond 3V.

The Honeywell isn't a perfect fit, but it is the closest 'widely available' one I can find. If I were to be repairing my own B795, that is the chip I'd use.

Since they aren't exactly cheap, I'd first examine the original ones under a loupe, and test them out of the motor, just to be sure they were not working. Yet I can't imagine any external situation (like a cracked pcb track) that would give the results measured so far.

 

[markthefixer]

I see a very clear path in implementing new sensors, the op amp circuitry used will make it a lot easier, with far less guesswork.

I would expect the "slope" of magnetic field strength versus voltage (delta gauss to delta volts) to be more repeatable across different chips, especially in terms of repeatability (between devices) and stability (the sensors are internally chopper stabilized), and that the "calibration" that is spoken of in the literature reflects the twenty years of improvements and thus higher expectations of performance in modern devices.

This won't quite be shooting fish in a rain barrel, but not far from it.

ESPECIALLY with one working sensor. I figure substituting a 3v zener diode for the duff sensor will get the working one into it's normal operating region to discover the designers intent for the signal amplitudes.

I have a LOT of those zeners in various voltages here already......

Then 2 small pc boards to mount the surface mount packages and give us a connection points to the original pc board, since they are far shorter than the orginal dip devices, that height increase from the PCB will work to our advantage. There MAY even be commercially available "adapter boards".

 

[steerpike2]

I see a very clear path in implementing new sensors, the op amp circuitry used will make it a lot easier, with far less guesswork.

The circuit changes needed and the end result will be extremely interesting to note!
The project as a whole is great in proving that you can always fix things if you have the talent, and for a lot less than the cost of replacing a pre-assembled part.

I have no doubt that this will eventualy work.

(A heads-up, in case you hadn't caught it yet: from what I can see in the schematic, the 'orange wire' to energise the Hall devices is driven from a comparator, so the motor control is a variable pulse train, rather than a linearly scaled voltage. This will give a nonsense reading on some digital meters.)

I figure substituting a 3v zener diode for the duff sensor will get the working one into it's normal operating region to discover the designers intent for the signal amplitudes.

Maybe even a Zener+LED to give an indication of current flow pattern.

calibration" that is spoken of in the literature reflects the twenty years of improvements

The B795 is now THIRTY years old, can you believe it!?

 

[bikezappa]

Interesting story of people working together trying to fix the revox.

Hope you win.

Peter

 

[Robie]

Thanks, bikezappa. Not sure how this one's going to turn out but it won't be for lack of trying.

It is remarkable that so many have chipped in to help. For that I am very thankful.

 

[Robie]

I never heard from the company I contacted on the Siemens, so should I order one or two Honeywells? (I take direction very well!)

 

[steerpike2]

The Honeywells will have to go in a as a pair; They are too different from the originals to have one Honeywell and one original work together.

Even matching the two originals is a task that requires attention to detail - but I think test equipment has improved so much since the late 1970s, so it will not be as hard as the service manual makes you think. The exotic wave analyser required then can now be substituted by any old PC with a soundcard.

How did the test with the Zener diode in place of the broken Hall device work out?

 

[Robie]

How did the test with the Zener diode in place of the broken Hall device work out?

I'm headed out of town for the Holidays but will check with Mark regarding his availability next week. He'll have to assist if he's willing.

 

[guiller]

I've just won a Revox B 795 at a local auction site for cheap, with some issues though. I will start replacing the caps and see how it behaves, and most probably would have to revive this thread with further questions. It is my first tangential TT and I'm excited about it. I've got no idea about the cartridge matching without a standard pivoting tonearm. My aim is to fix the TT and then put a good new cartridge, like the Benz Micro Ace or a similar high quality LOMC cartridge. Is this a reasonable choice?

 

[Pio1980]

Bonjour!
I'd use a less "reactive" cart myself like a better moving magnet or moving iron type with a user replacable stylus, perhaps one of the Ortofons. A moving coil cart puts a lot of sonic energy into an undamped light-weight support system and the REVOX cartridge carrier isn't designed to deal with it like a rigid and somewhat massive more conventional arm. There is also the issue of having to replace the entire cart instead of a simple plug-in stylus assembly.
I use an Ortofon MC in a lo-mass Souther linear radial arm, but I have modified it and it is damped against parasidic resonances. It is also relatively simple to change it out for another identical cart, a little less so for a different model. Replacing the cart in the REVOX is an ordeal requiring the jig to get it right without a lot of trial-and-error fussing about.