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Revox B 795

Definately the heatsink for the 4 motor power transistors is missing. General purpose TO-126 heatsinks should fit there with no problem.

If you do decide to add a single heatsink as originally used, be sure to use mica insulators between the heatsink and the transistors, and plastic bushes for the screws, or things will blow up. For the same reason, if using 4 individual heatsinks, ensure they don't touch each other.

The messily replaced IC is the monostable that does the timing for the tacho frequecy-to-voltage conversion, a CD4538. I bought a few of these not a month ago - very easy to get and cost me exactly 25 cents (US) each, should you want to replace it neatly (think IC socket!) That soldering around it is a mess, and there may be shorts, broken tracks there, and breaks in the thorough-hole plating.

Since the IC is so cheap, I wouldn't bother trying to unsolder it: that will just do more damage to the board. Rather cut off every pin of the IC with fine nippers, and then unsolder each pin stub individually. I like Solderwick for that sort of tidy-up.
(on close inspection, it looks like somebody maybe did that already, and just stuck the replacement IC on the remaining leg stumps of the old IC)

The 'empty board space' back right was never used.
 
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Thanks for the help! I'll address those issues in the Motor PCB and will show pictures of the progress. All the best
 
There may well be board feedthru issues, looks like the previous person did the work from the top of the board without pulling it.
 
There may well be board feedthru issues, looks like the previous person did some of the work from the top of the board without pulling it.
 
Pio1980 said:
Thnx for the wonderfully useful images!
Lotta work done there, one wonders what for. Shoulda changed out the yellow Frako e'lytics first, then the rest. Looks like it's mostly there, I think those motor board drive transistors are supposed to be on a heatsink. It looks worse than it probably is, the work was somewhat clumsy but purposeful. Nothing that can't be tidied up or redone.
Here's a photo of the interior of another B-795. Apologies to the unknown owner/poster.
Click to expand...

I believe that is my B795 in the photo in Pio's post quoted above (my home built record cleaner to the right of the TT is visible in the photo) and I can provide any other pics you need. Mine is still basically intact save for replacement of the 3 Frakos, all the motor control board Tantalums, the rectifier on the power supply board and the X2 cap on the small board to the left of the power supply board. Also, all of the ICs on mine are now socketed thanks to markthefixer's efforts. Lastly, I increased the voltage on the transformers from 110 to 120V per AK member boyerd001's recommendations but that change is not visible.
 
guiller said:
<snip>

Motor Control PCB



This board has been modified. There is a IC on top left that has been replaced. Some transistors look bad and seem to have been replaced too.

<snip>
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Looking at that motor board in the picture makes me want to wash my eyes out with soap.

Unfortunately I cannot UN-remember it too.

The top of board solder pads of that IC are just plain GONE. ALL he had to do was cut each pin from the IC, heat the pin and when the solder was molten, lift UP, surface tension would have held it to the iron. Then use braid or a sucker to clear the hole... it's clear he was in over his head. Melted parts on components etc - he couldn't even watch his solder iron clearances...

Bob - I'm keeping that table up in the front room so we have space no matter what's open on the bench or waiting for pickup....

As for the matched sensors, back then their performance was pretty wide tolerance, modern sensors and processes hold things to tolerances they didn't even dream of back then.
 
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markthefixer said:
Looking at that motor board in the picture makes me want to wash my eyes out with soap.

Unfortunately I cannot UN-remember it too.

The top of board solder pads of that IC are just plain GONE. .
Click to expand...

Do you mean it is not recoverable? I'll post pictures of the rear part of the board later today or tomorrow. If this is the case, there is no much I could do unless I get a donor machine for this board or try asking Revox if there could be a replacement.
 
guiller said:
Do you mean it is not recoverable? I'll post pictures of the rear part of the board later today or tomorrow.
Click to expand...

Everything is repairable if you have the will & patience :-)
I think a complete board from ReVox will be very expensive; and its not like this is a huge multipin IC with a lot of top-side traces going to it.
The apparently melted components are probaly fine; just their cases are rather mutilated.
 
steerpike2 said:
Everything is repairable if you have the will & patience :-)
I think a complete board from ReVox will be very expensive; and its not like this is a huge multipin IC with a lot of top-side traces going to it.
The apparently melted components are probaly fine; just their cases are rather mutilated.
Click to expand...

Thanks for the hopeful words! What would be your suggestion? Following your previous suggestion, I was planning first to add the heatskins to the power transistors and also to replace frako caps in the power supply board. At that point should I venture in giving tension to the circuits and see how the TT behaves? According to the seller, the platter rotates but the speed is not right. Anyhow, it seems that somebody did a mess with the motor PCB, and there was not mentioning of that from the seller.
 
I was trying to be nice, it is a royal botch.
I used to spend a lot of time on "sloppy seconds" just getting things back to the point where I could determine the original fault and then fix it. This is going to be one of those projects without having another cleaner motorboard on hand to replace this one.
It is do-able, you have deep and broad resources right here.
 
markthefixer said:
Bob - I'm keeping that table up in the front room so we have space no matter what's open on the bench or waiting for pickup....

As for the matched sensors, back then their performance was pretty wide tolerance, modern sensors and processes hold things to tolerances they didn't even dream of back then.
Click to expand...

I'll order them today along with your IR part and call you when they arrive.
 
guiller said:
Thanks for the hopeful words! What would be your suggestion?
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Remove the offending IC with fine wire cutters, then clean up the mess with acetone on cottonwool to remove flux, and narrow Solder-Wick to remove excess solder. I'd do the same with the power transistors.

Then you can see if the top-of-board tracks are broken or pulled up anywhere.
Get an IC socket of the 'turned pin' variety, since you may need to solder the pins on the top of the board and you can't do that with stamped-pin sockets.
Anywhere that there are top-and-bottom pads for the IC, solder the socket pin both top-and-bottom. Any lost or torn tracks can be replaced with fine insulated wire - not soldered to the broken track, but to the nearest available component pin on that conductor.

You need a fine soldering iron bit and caution/care, the PC board tracks lift easily, and you dont want to do any more damage if possible.

Mount the transistors on the heatsink first, then see that they all drop into the PC board holes neatly without bending or stressing the transistor body. Only when they slip in freely should they be soldered. (NB again: 4x mica TO126 insulating washers). I think not much heat is disspated here, so zinc oxide heatsink grease probably isnt essential - but if you have it, why not use it.
 
Thanks guys. Although I am not a professional tech, I hope to be able to go through this proyect following your great suggestions. Many thanks for your support and patience!
 
Your excellent photographic skills will serve you well collaborating on this project. It'll make assisting and tutoring you much easier than otherwise.
 
boyerd001 said:
You might also check the integrity of the connections to the tachometer in the Motor itself - SaSi had the problem of a "run away" motor due to no tack feedback.
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Slightly off topic but I have a B790 with a runaway motor and investigation shows pin 15, one of the tachometer connections, has become detached from the PCB inside the motor.

Any tips for getting the motor apart, please?
 
QuadMusic said:
Slightly off topic but I have a B790 with a runaway motor and investigation shows pin 15, one of the tachometer connections, has become detached from the PCB inside the motor.

Any tips for getting the motor apart, please?
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First, Welcome to AK!

Be patient, someone will weigh in soon. Lots of folks warned me about fiddling with the motor so try to wait for instructions.
 
Robie

I have lost track of your Revox table issues. Have you fixed it?

Are you going to install an IR sensor and control your table with the Revox remote?

Peter
 
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