Revox B790 underdamped speed regulation

[gmac]

I have been chasing this problem one step a time, so went back inside and changed the PTC thermistors with some new ones, 2.3 Ohms from EPCOS / TDK, the old one measured all over the place, 3 to 8 ohm at room temperature. Still, this doesn't seem to have made a difference regarding the speed fluctuation, but unfortunately created a new problem, at random, the carriage will raise and "go back home" as it happens when the end of a groove is detected or the microswitches are engaged. This happens at random but seems to regularly happen the first time a play a record after a day(when the turntable has been off for a while), it plays for a few minutes and then disengages.

I'm not sure this is related at all with the PTC or just something that happened during disassembly/reassembly, but I have not touched the other boards inside.

So far I have:
- checked the connectors for the arm for cold joints, reflowed some of them just in case, and cleaned it with contact cleaner
- checked +-6v and 24v and they are all within specifications
- checked the new PTC, with measure all around 2.3/2.5 ohm and the joints on the power supply board

One thing that I did not understand is that in the original board there were two big PTC (1cm diameter) and one a little smaller ( RT3 on the -6v rail) , but on the service manual they are listed all as 2.3ohm, its difficult to measure the old ones, but I suspect this one was more than 2.3ohm originally.
Still I don't think this can really be the cause, they are there just as protection and could were well be replaced by a short or testing purposes.

This is the data sheet for the PTC I have installed, is there anything inertially wrong about them? Maybe the voltage rating too low for the one on the 24v rail
B59940C0120A054 https://www.tdk-electronics.tdk.com/inf/55/db/PTC/PTC_OC_Leaded_12V_24V_C935_C995.pdf

I see there is a procedure to adjust the sensitivity of the photo sensor, but I'm not sure how to do it without the calibration disc or the alignment gauge.

Any suggestions on how to procede? This thing is such a mess to probe and I'm struggling to reproduce the fault, It seems I have stirred up hornet’s nest...

Tanks

 

[gmac]

Just an update, I have tweaked the trimpot r54 to lower the sensitivity of the runout groove detector, and this seems to have solved the issue, for now, still no idea why this started all of a sudden.

 

[Ricardito]

bought a b790 yesterday to replace my thorens td126mk3 that went to complete a tandberg set , seller thought it was smart not telling me about the noise , just 285 bucks for a nice specimen is not bad .......a couple screws were missing , i didn`t know why ....looks like they disasembled it a zillion times trying to find the cure ...they didn`t , thanks got they didn`t touch anything .....a couple ciano drops on the black square piece , and some below the main clip that helt the coil bank in place .....voila !!! now i till have to deal with a "thump i hear one time at a turn .......i think it is tonearm related ...

 

[Ricardito]

tonearm "thump" noise was caused by the abrupt move of the tonearm over a non lubed rail ....used silicon grease over it and noise went off ......cartridge sometimes and randomly gets down turning to the left , making the arm to raise and return to base ......any cure for this ?

 

[lsorrentino]

tonearm "thump" noise was caused by the abrupt move of the tonearm over a non lubed rail ....used silicon grease over it and noise went off ......cartridge sometimes and randomly gets down turning to the left , making the arm to raise and return to base ......any cure for this ?

Hi, maybe you can check if cartridge is properly aligned, an incorrect positive overhang cause centripetal force.

 

[perryinva]

You should never lubricate the rails on the Linatrak tables. All lubricants collect dust and dirt and eventually make all the issues worse, poling up on the slides and rod ends. Clean the rods and slides thoroughly and wax the rods with any synthetic car wax. They will repel contaminates and be super smooth and slick. Lasts years that way, with just a quick wipe with an eyeglass cloth, very occasionally, before needing a simple “rewax”.

Also, a drop or two of PDP-65 sintered bronze oil in the tracking motor top bearing does wonders for smoother start/stops by greatly reducing motor hysteresis.

 

[borchee]

Hi guys, this is my first post. Thanks for having me.

I see the thread is older and @Ricardito and @lsorrentino weren't here for a while but, I actually registered just to give some assistance to the ones inerested in/infected with the same problem. @lsorrentino there's also a thread on the German/Swiss forum regarding the following:

......cartridge sometimes and randomly gets down turning to the left , making the arm to raise and return to base ......any cure for this ?

The most likely cause for the cartridge wobbling at descending is:

- when the tone arm is lifted it is "locked" in position with a stabilizer to prevent it from wobbling whilst driving forwards/backwards
- the stabilizer is a plastic nipple attached to a thin brass carrier
- the carrier is clamped between the arm bearing cup and the arm carrier/slider
- if the rail is dirty and thus there's excesive friction, the thin brass stabilizer carrier rotates ever so slightly/deforms when the arm is driven forwards/backwards and the position off the stabilizer shifts, pushing the tone arm either to the left or to the right
- when the driving is stopped, the stabilizer holds the tone arm sideways/out of the gravity line as well of the magnets one
- when you lower the tone arm, it wobbles to "center" itself due to gravity and magnet pull.



The thin brass stabilizer carrier has two minute plastic sliders to glide smoothly over (better said under) the rod, so cleaning them and the road thoroughly is advised. When done, check the stabilizer to be centered in the tone arm - if not, use a toothpick, for example, and push the thin brass carrier accordingly until the stabilizer is centered.

If it doesn't help, and the stabilizer still moves whilst driving the tone arm around, it must be secured in place. You might try to glue it down somehow, I just took the whole shebbang apart and applied a very thin film of elastic glue to the ends to raise friction between it and the plastic parts it is clamped in between. Taking the arm apart is not for the fainthearted (the tiny signal wire coils require the outmost care for not to deform them), but if you must, make yourself a tool as seen in the linked thread.

Lubricating the rod on the underside might help as well.

I took a video where one can see what I mean: https://drive.google.com/file/d/1CyOKGqDYBR0zMIKY43PiwDx0rUFvgmWS/view?usp=share_link

If the tone arm drives forwards/backwards at descending, the slit in the tonearm and the emitter LED might not be centered. Check and align, if necessary.

 

[Pio1980]

Thnx for showing up and the explanation of how the cartridge carrier functions. It is not well understood.

 

[borchee]

You're most welcome.
It seems I have bigger ba**s than brain...and I'm curious, so I tend to end up doing stupid things that others, from whom I learned a lot, are reluctant to.