Revox A77 End of Tape fault - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 2nd Mar 2016, 1:21 am

A number of people have had problems in the past with the End of tape circuit not performing.

I have attached a circuit taken from the service manual.

Checks showed that the photo resistor was operating correctly but for what ever reason was not switching the transistors, so I decided to experiment a bit.

The problem seems to be that Q1 has too much current going through it when in the on state and cant switch Q2 off.

I first changed the opto resistors bias chain slightly by increasing R115 to 1.8K, R118 to 560R and R116 to 1.8K

This appears to have done the trick.

8 Tracker · 2nd Mar 2016, 9:31 pm

A nice bit of detective work Michael. This fault has been annoying me for ages, but I've never got round to having at look at the cause. Now you've saved me the trouble. Thanks.
Mark.

stevehertz · 3rd Mar 2016, 7:49 am

You've mentioned R116 twice?

Radio Wrangler · 3rd Mar 2016, 9:02 am

It's an emitter-coupled Schmitt trigger, by the way.

https://en.wikipedia.org/wiki/Schmitt_trigger

There is a feedback loop: Q111 collector > R117 and R120 >Q112 base > Q112 emitter and then back into Q111 via its emitter. It is a common base stage and an emitter follower in a loop, so the loop gain is non-inverting, which is positive feedback.

As Michael says, part of the game with these things is to portion the current appropriately in the two transistors in both states. Q112 has to carry enough current to keep its collector voltage low enough to keep the relays held in, and the thresholds have to be positioned right for the photoconductor to be able to flip the state in each direction when the photoconductor sees the bulb or the tape is in the way.

There's a fair amount of variance from one photoconductor cell to another, and bulbs age as well, so this circuit is a bit of a balancing act to get the tolerances to all fit and stay right over a number of machines being manufactured and over temperature.

David

Michael Maurice · 3rd Mar 2016, 10:34 am

Steve thank you pointing that out, I've edited the post so that R116 reads R118 which I changed to 560R

stevehertz · 3rd Mar 2016, 11:17 am

Since placing my post a few weeks ago on this very subject, I have not as yet investigated any further, so this latest information could be very useful when I a round tuit. Whatever, would I be advised to change the values of those resistors anyway? Not wanting to dispute Michael's good detective work and electronics knowledge, it seems strange that Revox - a company known for its technical prowess and attention to detail - would get it wrong.

vidjoman · 3rd Mar 2016, 11:29 am

Although it's many years since I last regularly worked on a Revox, I can't recall any problem except for the failure of the lamp. It may be that Michaels problem could be caused by an incorrect lamp being fitted and therefor not bright enough to activate the photocell.

Michael Maurice · 4th Mar 2016, 1:26 am

The lamp is a standard 24V lamp, the reason I chose those particular values is because if you look at the current through Q111 it is approx 60mA.

Now Q111 is just used as a simple switch to turn Q112 on and off, but if the current through Q111 is high, it may be high enough to prevent Q112 from switching off completely. By increasing R116 to 1.8K, the current through Q111 drops to around 13mA.

The reason I increased R118 was to slightly increase the voltage at the base of Q111

Anyway it worked for me!

2nd Mar 2016, 1:21 am	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Revox A77 End of Tape fault A number of people have had problems in the past with the End of tape circuit not performing. I have attached a circuit taken from the service manual. Checks showed that the photo resistor was operating correctly but for what ever reason was not switching the transistors, so I decided to experiment a bit. The problem seems to be that Q1 has too much current going through it when in the on state and cant switch Q2 off. I first changed the opto resistors bias chain slightly by increasing R115 to 1.8K, R118 to 560R and R116 to 1.8K This appears to have done the trick. Attached Thumbnails __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

2nd Mar 2016, 9:31 pm	#2
8 Tracker Hexode Join Date: Jul 2009 Location: Mablethorpe, Lincolnshire, UK. Posts: 286	Re: Revox A77 End of Tape fault A nice bit of detective work Michael. This fault has been annoying me for ages, but I've never got round to having at look at the cause. Now you've saved me the trouble. Thanks. Mark.

3rd Mar 2016, 7:49 am	#3
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Revox A77 End of Tape fault You've mentioned R116 twice? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

3rd Mar 2016, 9:02 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Revox A77 End of Tape fault It's an emitter-coupled Schmitt trigger, by the way. https://en.wikipedia.org/wiki/Schmitt_trigger There is a feedback loop: Q111 collector > R117 and R120 >Q112 base > Q112 emitter and then back into Q111 via its emitter. It is a common base stage and an emitter follower in a loop, so the loop gain is non-inverting, which is positive feedback. As Michael says, part of the game with these things is to portion the current appropriately in the two transistors in both states. Q112 has to carry enough current to keep its collector voltage low enough to keep the relays held in, and the thresholds have to be positioned right for the photoconductor to be able to flip the state in each direction when the photoconductor sees the bulb or the tape is in the way. There's a fair amount of variance from one photoconductor cell to another, and bulbs age as well, so this circuit is a bit of a balancing act to get the tolerances to all fit and stay right over a number of machines being manufactured and over temperature. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Mar 2016, 10:34 am	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 End of Tape fault Steve thank you pointing that out, I've edited the post so that R116 reads R118 which I changed to 560R __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

3rd Mar 2016, 11:17 am	#6
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Revox A77 End of Tape fault Since placing my post a few weeks ago on this very subject, I have not as yet investigated any further, so this latest information could be very useful when I a round tuit. Whatever, would I be advised to change the values of those resistors anyway? Not wanting to dispute Michael's good detective work and electronics knowledge, it seems strange that Revox - a company known for its technical prowess and attention to detail - would get it wrong. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

3rd Mar 2016, 11:29 am	#7
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,327	Re: Revox A77 End of Tape fault Although it's many years since I last regularly worked on a Revox, I can't recall any problem except for the failure of the lamp. It may be that Michaels problem could be caused by an incorrect lamp being fitted and therefor not bright enough to activate the photocell.

4th Mar 2016, 1:26 am	#8
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox A77 End of Tape fault The lamp is a standard 24V lamp, the reason I chose those particular values is because if you look at the current through Q111 it is approx 60mA. Now Q111 is just used as a simple switch to turn Q112 on and off, but if the current through Q111 is high, it may be high enough to prevent Q112 from switching off completely. By increasing R116 to 1.8K, the current through Q111 drops to around 13mA. The reason I increased R118 was to slightly increase the voltage at the base of Q111 Anyway it worked for me! __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/