FYI: 78rpm modification for Revox B790

[jlbruyelle]

Okay, so I was being grossly unclear, sorry about that. There is actually a lot of PWM in the control circuit (it is PLL-based, not at all old-fashioned ), although the final stage driving the driving itself is analog, you are perfectly correct on this. However the problem does sound like a problem of Q6/Q7 staying fully open when they shouldn't. This calls for a complete check of the waveforms as a first move.

regarding the power supply I wouldn't just try changing the electrolytics, even though recapping is a common reflex in overhauling these oldies. If you want to be safe, the solution would be to change not only the caps, but also the rectifiers and the regulators, which I've seen fail much more often than the caps, and usually in strange ways that are difficult to spot. I would do this before any other action, including the waveform check above.

I have never seen a case of excessive friction on a Revox TT motor so I cannot comment, but I suspect that such a problem would be easily spotted by spinning the unpowered motor by hand. It should rotate freely and without noise, otherwise you have a friction problem.

If all this fails, there is still the possibility to send the turntable to the Revox service centre in your country, they still service them (well, they do it at least in France and in Belgium).

[enrico_rome]

Hi to all of you guys.
Before I raised C17 to a higher value, I replaced all of the old electrolitic caps in the power board and the other one in the control board. Then, once I changed the value of C17 and let the platter spin at 78, the mess started by itself!
This afternoon I've bought two 7806, one 7824 and three 2 Amps bridges which will replace the old ones mounted at the Revox factory.

@ KentT:
I got a friend who uses succesfully by many years his B790 to play 78's, he has mounted on a Stanton 500 which is supplied with a set of ten needles to play 78's, these purchased by Mr. Hodgson of Expert Stylus & Cartridge Co. in the UK.

Thanks to all of you for your kind suppport, I'll keep you updated.
Cheers,
- Enrico

[enrico_rome]

Hi jlbruyelle,
coincidentally when you posted yours, in the meanwhile I was writing my reply.
The motor spins beautifully, no friction at all. As many of you indicated, soon I'll change the bridges and the regulators.
I have no idea if a Revox service is still operating here, years by years most of the major firms closed down like Pioneers, etc.

[jlbruyelle]

@ KentT:
I got a friend who uses succesfully by many years his B790 to play 78's, he has mounted on a Stanton 500 which is supplied with a set of ten needles to play 78's, these purchased by Mr. Hodgson of Expert Stylus & Cartridge Co. in the UK.

That does not really surprise me. As a matter of fact I am aware of at least one cylinder playing machine, made specifically for the British Library, with the Revox tonearm. A parallel arm is obviously necessary and the author mentions, in a page that I am unfortunately unable to find right now, that Revox was the only company able to deliver one that showed any interest in supporting the project. Incidentally, I've seen that the BL has a 78 rpm turntable fitted with a Revox tonearm, although they don't use it a lot. Yes, this tonearm does have a record (pun intended) in 78rpm, and even with older formats.

One reason why it works at all is that, although extremely light, the arm is heavily damped, to the point of working fairly well with a DL103 which is not a small feat. The stiff needles are not what it works best with, but the problem is limited by the resonance peak being so much flattened compared to a conventional undamped design.

10 needles for 78rpm? Wow, your friend must be a true enthusiast

Good luck with the repair, and please keep us posted of your progress

[jlbruyelle]

I have no idea if a Revox service is still operating here, years by years most of the major firms closed down like Pioneers, etc.

A quick search brought these addresses. Hopefully they are still valid:

Assistenza Apparecchiature Elettroniche
Terragni Gianni Angelo
Via Vittoria Colonna, 21
I - 20149 Milano
Phone: (++39) 0243 5715
Fax: (++39) 0243 5715
E-Mail: terragniser@tiscalinet.it

New Stereolab s.n.c.
Viale Bacciglione 26
I - 20139 Milano
Phone: (++39) 02 569 4836
Fax: (++39) 02 552 12004
E-Mail: newstereolab@tiscalinet.it

Studiolab s.r.l.
Plazza di Villa Carpegna 52
I - 00165 Roma
Phone: (++39) 06 663 2763
Fax: (++39) 06 662 3990

O.T.L. di Ceolin Riccardo
Via A. Boito, 1
I - 35122 Padova
Phone: (++39) 04966 4981
Fax: (++39) 04966 4981

It all these fail, you can send a mail to info@revox.be, who service all the old Revox stuff in Benelux. They may be able to help you, or point you to the service centre in Italy. I have always had good contact with them.

[dlaloum]

One reason why it works at all is that, although extremely light, the arm is heavily damped, to the point of working fairly well with a DL103 which is not a small feat. The stiff needles are not what it works best with, but the problem is limited by the resonance peak being so much flattened compared to a conventional undamped design.

I am puzzled by your saying that it is heavily damped...

The two magnets appear to work primarily in the horizontal plane - damping effect in the vertical plane would be expected to be minimal...

My impression is that the level of damping in the horizontal plane is also relatively light - Certainly far lighter than what is achievable given a fluid damping trough...

Mind you, I have not measured the arm damping (something for another day?)

[enrico_rome]

Good afternoon to you all guys and thank you jlbruyelle for taking your time to forwarding me the contacts of the Revox laboratories in Italy.
Power board has today new 2 Amp bridges, new 7824/7806 and fresh elec. caps.
I've measured the output voltages and they're sharp at 6+6+24. I put then a digital ammeter in series at RT 1 (the 2,3 ohm PTC) and turned on the platter, everything works fine, no noise from the motor, 45+VAR brings the speed to 78 rpm. Max current measured on the 24V line is 400mA. Good.
But after three/four minutes of working condition... the system stops by itself.
On the control board, Q6/Q7/R17 and a couple of resistors around are as hot as hell. Same for RT 1. The bridge of the 24V line is very warm too, same thing as for Q8/Q9/Q10/Q11.
When RT 1 reaches a high temperature it interrupts the current flowing from the secondary into the bridge, protecting so the system. Once RT 1 is cooled off, the 'table works again. But why the above cited components reach over 50 C°?

Bestest,
- Enrico

[jlbruyelle]

You are welcome Enrico. I tend to think the power drain is too high at 78rpm, and the components are not sized to handle it. If I am correct the overheating will not occur at 45rpm.

[jlbruyelle]

I am puzzled by your saying that it is heavily damped...

The two magnets appear to work primarily in the horizontal plane - damping effect in the vertical plane would be expected to be minimal...

My impression is that the level of damping in the horizontal plane is also relatively light - Certainly far lighter than what is achievable given a fluid damping trough...

Mind you, I have not measured the arm damping (something for another day?)

I spoke according to my own experience with the DL103 on a Revox. I tried to locate the patent of the damping system, but I can't seem to find it (if you have a link, I'd be very grateful). For "ordinary" 78rpm, the vertical damping is not so important since those records are horizontal mono, but if memory serves my measurements did not show a difference between in-phase and out-of-phase resonances, so I tend to think the damping is high in both axes. I'd need to check the measurement files I made back then in order to confirm.

[enrico_rome]

Hi guys,
this is a message I received by a British guy who was working at Revox in the 80's:

[...] When you test the turntable, do you have the heavy platter fitted? The B790 drive electronics would always overheat (at any RPM speed) if run without the platter.
It expects the inertia of the heavy platter to smooth-out small speed fluctuations. Without the platter, the control circuitry is continually speeding and slowing the motor, so the drive transistors overheat.

I hope that's useful.

Regards
Alan

[jlbruyelle]

Hi Enrico, it is indeed the normal behaviour of all DD turntables that without the platter they won't run at all, or very poorly, more a random series of jolts than actual rotation. In any case you would notice this long before any overheating

[enrico_rome]

Today I've realized for the first time that my paper service manual is slightly different from the pdf one. In fact the power board is 1.179.160 while on the pdf is 1.179.150. Some component value is different too, like the PTC RT3 is 5,6 instead of 2,3.
I've seen that the value of some capacitor is different in the switch board/speed as well (1.179.145).

I took the two boards out of the base to inspect the parts and the tracks with a lens and I've found that a tantalum cap, C28 100uF, is holed from side to side, see attachment. I'm going to replace all the tantalums, one never knows what's wrong with these too.
Also, I'm replacing all the BC337 with more robust 2SC2655 (max Collector current is 2 Amps against 1 Amp of the 337) and the BD135 with the BD681 (1,5 Amps against 4). I've found in a local shop only a handful of caps, the rest of the parts I'm going to buy it on ebay soon.

Cheers,
- Enrico

[jlbruyelle]

Hi Enrico, I see you've been working a lot

Good finding, the damaged tantalum. If you replace them all you will need to check the proper operation before going further, and maybe change a few adjustments, but it may well be worth the trouble.

Re. the differences between your unit and the service manual, it is not at all surprising and it is not an issue either. Revox used to make incremental improvements to their design, and many late B790 have boards designed for the B795, which are perfectly compatible. The changes only made the turntable a little more reliable, and cannot be the explanation to your issue.

Incidentally, it is not a good idea to change the transistors: if Revox chose a type that can withstand 1.5A (BD135) or 1A (BC337), it is because the design did not need more and you can bet that the choice was made with a substantial safety margin. Trying to make this stage more robust will only displace the problem to another part, probably much more difficult to replace if it fails: the motor coils and the transformer may be in danger here. Unless you have determined that the design is flawed on the transistor side and safe otherwise, it is a much better approach to start by determining why the current draw is excessive.

Keep up the good work