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Revox B 795

Frako electrolytic capacitors are a ticking time bomb. Exactly why technicians call them "Flako" caps. Those tantalums are also in need of replacement. Most issues with these machines are cured by this measure. Especially intermittent issues! I have replacement caps on order covering those bases. Will install them on arrival. I now have the owners and service manuals for my B 795. Will let you folks know how it runs!
 
Just an update. After a lot of head scratching I found the diagram in the Service Manual that illustrates the conversion from 110 to 120 volts. Very easy to do and it is now done. Now that it's done, I'm hoping it is just a simple conversion doesn't necessitate any follow-up readjustment. Could someone please confirm this.

I ordered the replacement caps for the Frakos, the power supply filter and the rectifier. Should have those this week and will proceed as soon as they arrive. Once all this is done, I plan to reassemble and see if the problem is solved before tackling the Tantalums.

Thanks again for everyone's help!
 
Hi Robie -

The rewiring of the Revox B795 Power Transformer to 120vac DOES NOT change any settings, etc. The Power Supply PCB has 3 Voltage Regulators on it for the 3 principal voltages that the table uses: +/-6v, +/-12v, and +24vdc. All that changing the power transformer's input voltage does is reduce its output voltage and hence the stress from overload on those regulators.

Dale
 
K.A.B said:
When you clean the rails, you can lubricate it with WD-40 this is similar to that Revox used. Anders
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One correction needs to be made. While you can clean the rails with WD-40, you cannot lubricate them with it. WD-40 is not a lubricant. It is primarily kerosene. Has a few non-volatile oils in it, but is not designed nor sold as a lubricant. Once you have cleaned (with whatever), then you might want to lubricate with a light machine oil. I find the 5-30 weight Mobil 1 to be an excellent oil for most all things TT.
 
MX117.MC7150 said:
One correction needs to be made. While you can clean the rails with WD-40, you cannot lubricate them with it. WD-40 is not a lubricant. It is primarily kerosene. Has a few non-volatile oils in it, but is not designed nor sold as a lubricant. Once you have cleaned (with whatever), then you might want to lubricate with a light machine oil. I find the 5-30 weight Mobil 1 to be an excellent oil for most all things TT.
Click to expand...

Yes, that's correct. I wouldn't use WD40 even for cleaning. Can't expect any linatrack rails to be that dirty (or rusty) requiring WD40 or similar. I've used pure alcohol with a piece of cotton and then lubricated with a single drop of light machine oil per rod.
 
I told myself not to "touch" the WD-40 replies, but what the hey!!

"While you can clean the rails with WD-40, you cannot lubricate them with it. WD-40 is not a lubricant."

If you look on the WD-40 can, THEY STATE IT IS A LUBRICANT (not yelling). OK, it isn't suitable for the crankshaft bearings of a Formula 1 race car, but it still has lubrication properties and you have to evaluate what they are and what you are trying to do with this lubricant.

The chrome rails on the Revox Linatrak tonearm are very smooth when new. The plastic guides of the tonearm sled are smooth also, but plastic is not a self-lubricating material (at least not what Revox uses!). While it is possible to clean the rails really well (talking Acetone here, being VERY careful) and also to clean/recondition the drive cord with Acetone, there will always be a very slight drag of the plastic tonearm guides on the chrome rails. I have used WD-40 for years now on the chrome guide rails without any problem, and the addition of the WD-40 reduces any plastic sled drag that exists. Notice that I did not say I LUBRICATED the rails. I put a small amount of WD-40 on a cotton swab and then wipe the rails with the swab - that's it! The addition of the WD-40 acts as an "anti-seize" agent as much as or more than a "lubricant" - it allows the tonearm sled to traverse the rails without any minute binding.

I would NEVER use any oil on the chrome railings unless it was of the very thinnest watch bearing type. Any normal weight oil on those chrome rails is going to be a "dirt catcher" and further impede the sled's movement over time.

I've used WD-40 for years on regular tungsten light bulb (speaking 60-100watts) threads to prevent "seizing" when they are removed after "burning out". The light bulb's thread has never been "gummed up" or in fact exhibited any residue. You have to remember that lubricants (anti-seize agents) vary and to make a global statement that a lubricant is no good is to ignore what you are asking from them. The Linatrak's chrome rails demand a very fine "anti-seize" coating to keep the sled free - remember, we are not talking of a bearing under great load or stress which is what most critics of using WD-40 cite as the reason it shouldn't be used (also the "gumming up" tendency).

Anyway, I've used it (sparingly as described) on the Linatrak's chrome rails for years (bought my first table in 1978 - a B790 and it's still working flawlessly), and I'll stand by my experience to refute the last 2 "anti WD-40 posts.

Dale
 
I have found that liquid lubricants - oils, etc, on plastic cause 'stiction'. On plastic where grease is not approprate I always would only use graphite.

I think WD40 is the most vile stuff - suitable for garden swings, removing chewing gum & nought else. It tends to creep and infuse itself everywhere, doing more harm than good.
 
Thanks for the input all. Sounds like there is a bit of personal preference on the WD-40.

Update: Still waiting for parts. Old parts have been removed. I'm now armed with a solder sucker, solder wick and some liquid rosin.

If the parts don't arrive today, I'll clean up the old solder on the boards this weekend so I'm ready to go when the parts get here.
 
First and foremost, thanks for the input thus far.

Due to a USPS screw up, the last cap arrived today. I installed it and, unfortunately I have the same warbled sound/speed problem as before. On the bright side, I believe I have learned how to be a competent solderer and de-solderer.

To date, I have replaced the 3 Frako caps on the power supply board with Vishays. I also replaced the small bridge rectifier on the same board. On the small board behind the transformer, I installed a new "X" cap to suppress noise and resoldered the leads on the transformer to convert it from 110V to 120V.

Any suggestions as to what I try next? I'm hoping it isn't the Tantalums as there appear to quite a few of them.
 
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Hi Robie -

Got your PM.

Hate to be the bearer of bad news, but the Tantalums and the other Electrolytics on the Motor PCB are now the logical suspects. I would work the Tantalums first, getting Panasonic FCs from Digikey as their replacement. You might as well get the replacements for the regular electrolytics on that PCB as well when you place the Tantalum order in order to save future postage, there's always a chance the Tantalums are "innocent".

I had an instance on my B790 where it wouldn't always start up to speed - it would start to "rev up", but couldn't lock in and would "waver" around the quartz lock value. Turns out to have been an 0.47uF TANTALUM capacitor on the Motor PCB. The B790 SM listed that cap as being a standard nonpolar poly - how it got to be a polarized Tantalum is anybody's guess!! There was also another cap nearby that was also a 0.47uF Tantalum that was spec'd in the SM as nonpolar poly! My guess is that since my TT was in the SN 5k range (very early), I got a PCB that may have been stuffed with early components that were later changed when they realized problems would develop. The really odd thing is that the table had worked for 25 years before the problem developed - although to be honest I didn't use it that much during that time, archiving most vinyl to the Nakamichi 1000II and then play music through that.

Anyway, time to start learning how to solder the Studer "double-sided" PCBs. One hint, keep the heat to a minimum and don't leave the iron on the traces very long - Studer PCBs lift traces VERY EASILY.

I really think that replacing the Tantalums may do it for you,

Dale
 
Thanks again, Dale.

As I see it there is only one electrolytic (220uF/16V so I'll order a 4019PHCT-ND Vishay) and 7 Tantalums on the Motor Control PCB.

As for the replacements for the Tantalums, I do not see Panasonic FCs on the Digikey site in the Tantalum capacitor listing. Since they do list Panasonic ECGs, I presume those are suitable replacements?

Thanks!
 
Thanks, Sasi.

I must have missed that in the previous posts.

As you can tell, I'm definitely in over my head but not afraid to stumble ahead.
 
Breaking news (with an emphasis on still broken):

The Motor Control PCB has been updated with new electrolytics in place of the old electrolytic and tantalums.
  • (1) 220uF/16V electrolytic replaced with new electroytic (Digikey P/N: 4019PHCT-ND)
  • (3) 10uF/16V tantalums replaced with 10uF/25V electrolytics (Digikey P/N: P11212-ND)
  • (2) 100uF/3V tantalums replaced with 100uF/10V electrolytics (Digikey P/N: P11181-ND)
  • (2) 4.7uF/10V tantalums replaced with 4.7uF/50V electroyltics (Digikey P/N: P10315-ND)
Unfortunately, my warbled sound continues. I'd like to say it's better but don't think I can. The platter still does not spin up when the arm is moved tangent to the record and requires a hand assist to start spinning.

On the bright side, my efforts have not made the situation any worse, my soldering/desoldering skills are vastly improved and I'm not afraid to make repairs to PCBs, provided someone directs those changes.

I did not replace the tantalums and electrolytics on the Arm Control Board but wonder if I should have. Reaching for straws here but wonder if it's possible the arm is causing the slow warble.

Thanks for "all y'all's" help again and wonder if there are any other suggestions?
 
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The ReVox uses a two phase motor. If it doesn't start, it sounds much like one phase is dead. There are several components that could acount for that, (any of the ones in the diagram below) including the Hall sensors within the motor itself.
(DIY Hall sensor replacement is a no-no; DO NOT remove them to test them, it requires a special alignment jig to get them back in and correctly set.)

One of the drive balance trimpots R57 and R58 may be broken. The trimmers used by revox sometimes become brittle & just break. This may account for wow, but should not cause starting failures.
You could try 'wiggle' them each from side to side, but be sure to leave their positions unchanged after you've wiggled them.

Might alternatively be one of the 4 power transistors that's died.

Do you have measuring equipment? (An oscilloscope would be ideal.)
 

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I think I may have partly the same problem as you do on my B791. I posted about it a few months ago. The problem appeared after storing the TT on a shelf for 3 months, as I moved into a new house.

Initially, my B791 had some trouble starting with visible "warble" - let alone what the sound of that was.

One it started, the Quartz Lock LED wouldn't light up. Speed was a bit off to the slow and I also had audible wow.

I did lubricate the spindle and then let the TT rotate for a couple of hours. That apparently fixed the issue as I would get a QL Light and also Wow was eliminated (at least audibly).

After leaving the TT inactive for a day or so, the QL led again fails to light up. After rotating for half an hour or less that is cured.

In the beginning I was inclined to believe it was a dry spindle well but the fit of spindle there is perfect and the spindle is still lubricated, so I now am more inclined to consider an electrical issue. I am guessing that leaving the TT operating for an hour or so warms things up and brings them closer to their normal operating range. I have seen dirty (and rusty) trimpots cause havoc on equipment so I share Steerpike's idea on checking them as well. But if the TT reverts to normal operation after some time left to "warm up", then I would suspect something else. There's a pair of TO-220 transistors that drive the motor windings (IIRC a pair of BD139/BD140 or two). There may be a bad solder there or a transistor in terminal condition.

I also have a project B795 that shows the same symptom (no QL light) but it came with an AKG cart and a broken stylus. As I have no replacement stylus for that I can't listen to it. And you all know that replacing the cart on a Linatrak is a project by itself.

Perhaps it's now the time to work on these two TTs and see if we can nail it down.

For the moment I would like to ask you to warm up the TT for an hour and see how it performs then.

BTW, Like Steerpike asked, what kind of measurment instruments do you have? I know an Osciloscope might be too much to ask, but do you have a multimeter?
 
Hi Robie -

I'm so sorry the original suggestions didn't fix the problem - it's a wonderful table that's just going to take some time and work to fix what's taken 30+ years to break.

First, I'd second Steerpike2's suggestion to "tweak" the 4 pots on the Motor PCB - mark them and then turn the wipers CW and CCW a couple/three times before returning them to their original positions - you can't adjust them to a new value, the tools to do so are beyond this thread! Replacing the 4 Motor Driver Transistors probably won't help - I tried that when I had my "speed" problem described earlier that turned out to be a bad Tantalum. Those transistors are very durable and I would really be surprised if they were failing.

Second, you really need a scope and DVM - if you don't have access/knowledge to use these, then just think about what I'm going to say. In the Service Manual in Section 3.1.2, there is a good description of what happens with the Motor - it sounds like the Motor can't "lock" onto the reference quartz clock frequency. The ICs 1-4 on the Motor PCB (Section 4/15 thru 4/16) have control over this and the expected voltage waveforms along this circuit are displayed in Section 4/14. At this stage you need to trace through that circuit with a scope to see if there is a "screw up" occurring. Without a scope to trace this circuit, your only recourse would be to replace ICs 1-4 completely in a "brute force" repair, VERY difficult without a professional de-soldering gun to get those DIP package ICs out.

Third, there is always a small (really small) chance that the quartz crystal oscillator is failing, but that's really small. As before, you need a scope to measure its frequency output.

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. Measure the resistance through that circuitry to make sure it's intact and not intermittent. A scope here would allow you to see its output when the table was running to see if that was truly the problem.

You've got 2 Hall Sensors in the Motor, measure them in the DVM's diode mode to see if they are both the same - if not, one may be bad (did this once long ago, if your results are not conclusive try the measurement in the standard resistive mode of your DVM).

Lastly check the PCB under bright light for any solder problems - in all Studer PCBs I've seen, that's never been a problem.

One last point that you could check would be the micro-switch at the tonearm's base that gives the signal that the arm is in position to "play". It is conceivable that if the contacts are corroded that the arm might not be sending a steady start command to the motor - knowing how this works I don't think this is likely though, but measure across the switch and note its resistances between opening/closing (infinite in "play" and zero in "standby" position).

I'm afraid at this point that this is as far as I can go with you. We're at the hands on troubleshooting phase that only someone who's had EXACTLY your problem would be able to give an exact/on the nose answer to.

In the long run, buying a "broken" table with intact PCBs to use as spares is the way many of us survive these problems - by swapping out the boards you can zero in on the troubled area quickly and then take your time to figure out what went wrong.

Keep the thread alive, if you work through some of this maybe some of us will have some more ideas to offer.

Dale
 
A key issue here is that the motor does not start. This rules out a problem with the quartz crystal and the tacho signal.

Any faults in the arm in/out switches would cause the cue mechanism to lift the arm.
 
Thanks for the input, steerpike2, SaSi and Dale. At the risk of overstatement, this is a great example of international cooperation.

Unfortunately I do not have an oscilloscope. I only have a fairly rudimentary DVM and I'll have to learn how to use its features. This provides me some incentive to do so.

I'm doing the "warm up" test that SaSi suggested for 3-4 hours and will try to "exercise/wiggle" the trimpots next. As I understand it, this B795 was in its box for ~10 years so our problems sound similar.

As for the "no platter start problem," with a small hand assist the platter rotates up to speed quickly and the QL light now stays constant so perhaps our efforts have helped a bit. One other observation that may be related: When the on/off switch is depressed, the platter "surges" (starts to rotate (I thought it was CW but it just surged CCW) but then stops). Is this normal? It only stays rotating with the arm in "Play" position.

I'm going to have to ponder the input above regarding next steps. I can also check with a local tech/AK member to assist on the required measurements. I'll PM him and see if he's willing to participate.
 
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After letting the platter spin for almost 2 hours, the wow (slow warble) is still present.

More, later.
 
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