Revox A700 pinch pressure ??? - UK Vintage Radio Repair and Restoration Discussion Forum

StevanP · 6th Mar 2018, 4:18 pm

Hi, I'm trying to ressurect an old A700. I did recap all tape control PCBs, except motor cap. So I got it work, but I find now another problem. Pinch pressure is too low, so if a tape was not at the start , I got flutter and at the 3/4 of the tape, tape stops. Revox service manual presure suggestion is 1000 to 1300grams, I got only 200 grams. I cleaned solenoid, check voltage, working 20V. I even supply voltage from bench PS, same situation. Does anyone have any idea what is wrong?

Thanks.

Ted Kendall · 6th Mar 2018, 10:56 pm

Perhaps the solenoid is in the wrong position. The spring in the linkage should be extended slightly when the pinch wheel is against the capstan, and this ensures the correct pressure. The mounting holes of the solenoid are slotted to enable this to be adjusted.

StevanP · 7th Mar 2018, 9:49 am

I try to adjust soledoid moving it up and down , at least 10 position. I even replace solenoid with tape solenoid ( same product number as I saw in Studer service manual). But the results are the same. The linkage spring doesn't extend a bit, what ever position of the solenoid is. The force of the solenoid is too low, I can very easily with finger move the roller arm, and it can't go back. Is there any possibility that the solenoid plunger lose magnetic force or being magnetize ?

StevanP · 7th Mar 2018, 10:24 am

I tried several solenoid position, even I switched it with tape solenoid, and the result is the same. Still I can easy, remove pinch from the motor shaft with my little finger. Linkage spring doesen't extend at all. Is there any possibility that the solenoid plunger gets magnetised or lose magnetic properties?

Ted Kendall · 7th Mar 2018, 11:58 am

Is the diode across the solenoid leaky?

barrymagrec · 7th Mar 2018, 12:16 pm

If the pinch solenoid is set too far out the core won`t go fully home into the solenoid and pinch pressure will be minimal, as here.

I don't know the Revox circuit, is it possible that the solenoid should be fed with an initial surge to pull it in and then the voltage is dropped?

TIMTAPE · 7th Mar 2018, 1:00 pm

Agree with Barry. Check that the solenoid core is going all the way in. Try it with pinch roller removed. Mark that position on the core. When you try it with the pinch roller back on the shaft the core should go to the same position. All the way in.

The solenoid adjustment is critical. Too much to the left and the pressure will be weak. Too much to the right and the pressure will also be weak. It's the same generally with solenoid operated pinch rollers.

As Ted says when it's set properly the spring should extend a little.

StevanP · 7th Mar 2018, 1:00 pm

Quote:

Originally Posted by barrymagrec

If the pinch solenoid is set too far out the core won`t go fully home into the solenoid and pinch pressure will be minimal, as here.

I don`t know the Revox circuit, is it possible that the solenoid should be fed with an initial surge to pull it in and then the voltage is dropped?

Yes! That was the problem, since this is very old, unused for decades, R2R, roller arm was set bad, so I couldn't set the solenoid right. As soon I reset the arm angle on the shaft, the force came back. I must right now only set the proper force.

Thanks for a tip, I stumbled across another problem, too much noise in signal while the tape is in Stop.

TIMTAPE · 7th Mar 2018, 10:27 pm

Quote:

Originally Posted by StevanP

... i stumbled across another problem, too much noise in signal while the tape is in Stop.

Is the noise increase hum? In stop the small metal hum shield over the play head swings down. This increases the hum and is normal.

StevanP · 9th Mar 2018, 10:57 pm

Hi, Timtape.

This is very old recorder, I had problem with bearings, so Ichanged them. I didn't have time to look further. I will check what is going on with headshield. Thanks.

6th Mar 2018, 4:18 pm	#1
StevanP Retired Dormant Member Join Date: Mar 2018 Location: Belgrade, Serbia. Posts: 5	Revox A700 pinch pressure ??? Hi, I'm trying to ressurect an old A700. I did recap all tape control PCBs, except motor cap. So I got it work, but I find now another problem. Pinch pressure is too low, so if a tape was not at the start , I got flutter and at the 3/4 of the tape, tape stops. Revox service manual presure suggestion is 1000 to 1300grams, I got only 200 grams. I cleaned solenoid, check voltage, working 20V. I even supply voltage from bench PS, same situation. Does anyone have any idea what is wrong? Thanks.

6th Mar 2018, 10:56 pm	#2
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Revox A700 pinch pressure ??? Perhaps the solenoid is in the wrong position. The spring in the linkage should be extended slightly when the pinch wheel is against the capstan, and this ensures the correct pressure. The mounting holes of the solenoid are slotted to enable this to be adjusted.

7th Mar 2018, 9:49 am	#3
StevanP Retired Dormant Member Join Date: Mar 2018 Location: Belgrade, Serbia. Posts: 5	Re: Revox A700 pinch pressure ??? I try to adjust soledoid moving it up and down , at least 10 position. I even replace solenoid with tape solenoid ( same product number as I saw in Studer service manual). But the results are the same. The linkage spring doesn't extend a bit, what ever position of the solenoid is. The force of the solenoid is too low, I can very easily with finger move the roller arm, and it can't go back. Is there any possibility that the solenoid plunger lose magnetic force or being magnetize ?

7th Mar 2018, 10:24 am	#4
StevanP Retired Dormant Member Join Date: Mar 2018 Location: Belgrade, Serbia. Posts: 5	Re: Revox A700 pinch pressure ??? I tried several solenoid position, even I switched it with tape solenoid, and the result is the same. Still I can easy, remove pinch from the motor shaft with my little finger. Linkage spring doesen't extend at all. Is there any possibility that the solenoid plunger gets magnetised or lose magnetic properties?

7th Mar 2018, 11:58 am	#5
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Revox A700 pinch pressure ??? Is the diode across the solenoid leaky?

7th Mar 2018, 12:16 pm	#6
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Revox A700 pinch pressure ??? If the pinch solenoid is set too far out the core won`t go fully home into the solenoid and pinch pressure will be minimal, as here. I don't know the Revox circuit, is it possible that the solenoid should be fed with an initial surge to pull it in and then the voltage is dropped?

7th Mar 2018, 1:00 pm	#7
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox A700 pinch pressure ??? Agree with Barry. Check that the solenoid core is going all the way in. Try it with pinch roller removed. Mark that position on the core. When you try it with the pinch roller back on the shaft the core should go to the same position. All the way in. The solenoid adjustment is critical. Too much to the left and the pressure will be weak. Too much to the right and the pressure will also be weak. It's the same generally with solenoid operated pinch rollers. As Ted says when it's set properly the spring should extend a little.

9th Mar 2018, 10:57 pm	#10
StevanP Retired Dormant Member Join Date: Mar 2018 Location: Belgrade, Serbia. Posts: 5	Re: Revox A700 pinch pressure ??? Hi, Timtape. This is very old recorder, I had problem with bearings, so Ichanged them. I didn't have time to look further. I will check what is going on with headshield. Thanks.