Revox B77 MKII - UK Vintage Radio Repair and Restoration Discussion Forum

Redders03 · 23rd Apr 2012, 5:56 pm

Hi,

I recently got a B77 MKII and it has a problem with the tape end sensor.

The sensor has been replaced as has the control board. When the sensor is shorted out the player stops so it shows that the board is not at fault, well that is what I believe. Voltage reading is 26 volts between brown and grey.

It also has a problem with input on the left channel, for Aux it is dead, Radio is distorted and mic hi-low are fine, any ideas of where to start with this problem?

Thanks

lesmw0sec · 23rd Apr 2012, 7:04 pm

Looking at the circuit, there does not seem much that can go wrong! It may be a daft question but have you cheched that the wiring to the sensor is correct and that current is being drawn though the IR LED?

Les.

Redders03 · 23rd Apr 2012, 7:17 pm

Yes I've checked it there is the same feed for the LDR and the LED, both at 26 volts. When the contacts of the LDR are shorted, the machine acts as it should.

I have also looked at the diagram and figured the same the only thing that did not follow was that on the diagram it states the voltage should be 24v and it's 26. The only other thing is that the resistance of the LDR is not dropping enough to trip the board, unlike shorting which offers no resistance. But I have already ordered two different ones and they still don't work.

Michael Maurice · 23rd Apr 2012, 9:54 pm

What you have to do is measure the voltage on the optosensor with and without tape to see if its actually working, also you need to see if the IR LED is actually emitting IR.

There are devices on the market to establish if IR is being emitted.

If all appears well at the sensor, ie you get a different voltage with/without tape then your attention needs to turn to the board itself. The voltage change goes through a zener and a preset, they may also be faulty.

The 24V you refer to is unregulated 26v is OK.

The problem with Ch1 is probably due to poor contacts on the input selector switch. This is part of the main board and will have to be stripped to get to them

Redders03 · 23rd Apr 2012, 10:16 pm

Would you be able to see the IR light emitted on a digital camera such as an iphone, because I have done this and no light is being emitted from the sensors?

I have also ordered new contacts, hope this fixes the channel problem.

Refugee · 24th Apr 2012, 1:10 am

You will need a night vision CCTV camera to see IR light. Ordinary cameras are filtered so that they do not see it.

ppppenguin · 24th Apr 2012, 8:23 am

Quote:

Originally Posted by Refugee

You will need a night vision CCTV camera to see IR light. Ordinary cameras are filtered so that they do not see it.

Many digital cameras, inculding phone cameras, can see IR perfectly well. I've ried it with remote controls and my own phone.

Refugee · 24th Apr 2012, 9:43 am

I am pretty sure that there must be some kind of filtering otherwise the pictures would come out "wishy washy" if you use it in a place where there is hot objects. It could cause a false negative result. The sensors are cheep enough these days. Having carefully selected the wavelength of several and using a diode tester to power them find the most sensitive one and fit it in place of the original. Then the matched pair syndrome will be solved. The other way is to select the two components so that they are both the same wavelength and use them as a matched pair.
The Farnell website has a search engine that allows you to get parts from the spec and the PDFs normally have the OEM numbers.
The recorder will be fitted with a pair that was cheep at the time and as things change they will change too.

jjl · 24th Apr 2012, 9:53 am

Digital camera sensors do have IR filters, however, a relatively intense point source such as an IR LED shows up quite clearly as a white light on all digital cameras I've tried.

John

Refugee · 24th Apr 2012, 10:42 am

In this case we also need to establish that the sensor can see through the leader tape. If this is not true we will end up with a false diagnosis of a faulty sensor board.
Some tapes had metalized leaders and they will not work with this type of recorder.

ScopeMonkey · 24th Apr 2012, 3:10 pm

You could always check your phone camera or digital camera can see IR first by using any household remote control (TV, DVD player etc). As Jeffrey says, every kind of digital camera I've ever tried this with can see IR at short range, it shows up as a bright white-ish pink colour.

Ed.

Redders03 · 24th Apr 2012, 7:06 pm

With the left Channel. I have checked contacts of switches, and nothing wrong there, with the aux left input a faint signal can be heard when no ground connected but when you push the plug right in with ground the signal goes, no idea where to go next.

Could it be something to do with a resistor on the main board, parts on the input amplifier or a component in the connection box itself? I noticed there were some resistors in there as well.

With the IR sensors I have plugged them in and no IR light can be seen, voltage is there so it points to the light gates, I am replacing the zeners and preset just in case.

Refugee · 24th Apr 2012, 9:59 pm

I have tried it and you have to point the remote right at the lens to get the effect.

Redders03 · 24th Apr 2012, 11:28 pm

Yes I have and nothing can be seen, this leads me towards the idea of the switches at fault, anybody got any ideas when I can source the components brand new so that I can rebuild the unit, available in Maplin?

Darren-UK · 24th Apr 2012, 11:36 pm

Have a look and see... http://www.maplin.co.uk/

Redders03 · 24th Apr 2012, 11:38 pm

Looked and maplin only offer LDR not opto sensor of transmissive.
Found this though

http://www.ebay.co.uk/itm/OPB866T55-...b#ht_500wt_689

Would this be suitable ?

Michael Maurice · 24th Apr 2012, 11:49 pm

No that wouldn't be suitable but this would

Redders03 · 24th Apr 2012, 11:53 pm

I have already sourced 2 from this seller and both have failed to work I cannot see the IR when put in front of a camera but the voltage is there !!

Michael Maurice · 25th Apr 2012, 10:19 pm

Quote:

Originally Posted by Redders03

I have already sourced 2 from this seller and both have failed to work I cannot see the IR when put in front of a camera but the voltage is there !!

Wouldn't that suggest that the problem is not with the sensor?

Can you measure the voltage across the IR LED? It should be about 1 - 1½ Volts. If its 0V it means that its not drawing any current.

With the machine switched off, measure the resistance between the cathode of the LED and ground, it should be around 3.3K. if its a lot higher, then either R2 is open circuit or there is an Open Circuit between the LED cathode and pin 6 of the transport board.

Refugee · 25th Apr 2012, 10:55 pm

I would be tempted to say that with all the activity around this part that the track between the resistor and the connector has worked loose. I think the photo-transistor has already been linked out and it stopped. With the tape playing there should be about 2 volts less than the 24v unregulated on the resistor and that end of the IR led. If the resistor itself has zero on it then it is an open circuit wire or track. To test the sensor on the bench apply 12 volts to the LED via a 1k resistor and measure the sensor with a diode tester. It should close when the beam is not blocked and open when the beam is blocked.

23rd Apr 2012, 5:56 pm	#1
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Revox B77 MKII Hi, I recently got a B77 MKII and it has a problem with the tape end sensor. The sensor has been replaced as has the control board. When the sensor is shorted out the player stops so it shows that the board is not at fault, well that is what I believe. Voltage reading is 26 volts between brown and grey. It also has a problem with input on the left channel, for Aux it is dead, Radio is distorted and mic hi-low are fine, any ideas of where to start with this problem? Thanks

23rd Apr 2012, 7:04 pm	#2
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,509	Re: Revox B77 MKII Looking at the circuit, there does not seem much that can go wrong! It may be a daft question but have you cheched that the wiring to the sensor is correct and that current is being drawn though the IR LED? Les.

23rd Apr 2012, 7:17 pm	#3
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII Yes I've checked it there is the same feed for the LDR and the LED, both at 26 volts. When the contacts of the LDR are shorted, the machine acts as it should. I have also looked at the diagram and figured the same the only thing that did not follow was that on the diagram it states the voltage should be 24v and it's 26. The only other thing is that the resistance of the LDR is not dropping enough to trip the board, unlike shorting which offers no resistance. But I have already ordered two different ones and they still don't work.

23rd Apr 2012, 9:54 pm	#4
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 MKII What you have to do is measure the voltage on the optosensor with and without tape to see if its actually working, also you need to see if the IR LED is actually emitting IR. There are devices on the market to establish if IR is being emitted. If all appears well at the sensor, ie you get a different voltage with/without tape then your attention needs to turn to the board itself. The voltage change goes through a zener and a preset, they may also be faulty. The 24V you refer to is unregulated 26v is OK. The problem with Ch1 is probably due to poor contacts on the input selector switch. This is part of the main board and will have to be stripped to get to them __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

23rd Apr 2012, 10:16 pm	#5
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII Would you be able to see the IR light emitted on a digital camera such as an iphone, because I have done this and no light is being emitted from the sensors? I have also ordered new contacts, hope this fixes the channel problem.

24th Apr 2012, 1:10 am	#6
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Revox B77 MKII You will need a night vision CCTV camera to see IR light. Ordinary cameras are filtered so that they do not see it.

24th Apr 2012, 9:43 am	#8
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Revox B77 MKII I am pretty sure that there must be some kind of filtering otherwise the pictures would come out "wishy washy" if you use it in a place where there is hot objects. It could cause a false negative result. The sensors are cheep enough these days. Having carefully selected the wavelength of several and using a diode tester to power them find the most sensitive one and fit it in place of the original. Then the matched pair syndrome will be solved. The other way is to select the two components so that they are both the same wavelength and use them as a matched pair. The Farnell website has a search engine that allows you to get parts from the spec and the PDFs normally have the OEM numbers. The recorder will be fitted with a pair that was cheep at the time and as things change they will change too.

24th Apr 2012, 9:53 am	#9
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: Revox B77 MKII Digital camera sensors do have IR filters, however, a relatively intense point source such as an IR LED shows up quite clearly as a white light on all digital cameras I've tried. John

24th Apr 2012, 10:42 am	#10
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Revox B77 MKII In this case we also need to establish that the sensor can see through the leader tape. If this is not true we will end up with a false diagnosis of a faulty sensor board. Some tapes had metalized leaders and they will not work with this type of recorder.

24th Apr 2012, 3:10 pm	#11
ScopeMonkey Pentode Join Date: May 2008 Location: Leeds, West Yorkshire, UK. Posts: 236	Re: Revox B77 MKII You could always check your phone camera or digital camera can see IR first by using any household remote control (TV, DVD player etc). As Jeffrey says, every kind of digital camera I've ever tried this with can see IR at short range, it shows up as a bright white-ish pink colour. Ed.

24th Apr 2012, 7:06 pm	#12
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII With the left Channel. I have checked contacts of switches, and nothing wrong there, with the aux left input a faint signal can be heard when no ground connected but when you push the plug right in with ground the signal goes, no idea where to go next. Could it be something to do with a resistor on the main board, parts on the input amplifier or a component in the connection box itself? I noticed there were some resistors in there as well. With the IR sensors I have plugged them in and no IR light can be seen, voltage is there so it points to the light gates, I am replacing the zeners and preset just in case.

24th Apr 2012, 9:59 pm	#13
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Revox B77 MKII I have tried it and you have to point the remote right at the lens to get the effect.

24th Apr 2012, 11:28 pm	#14
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII Yes I have and nothing can be seen, this leads me towards the idea of the switches at fault, anybody got any ideas when I can source the components brand new so that I can rebuild the unit, available in Maplin?

24th Apr 2012, 11:36 pm	#15
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Revox B77 MKII Have a look and see... http://www.maplin.co.uk/

24th Apr 2012, 11:38 pm	#16
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII Looked and maplin only offer LDR not opto sensor of transmissive. Found this though http://www.ebay.co.uk/itm/OPB866T55-...b#ht_500wt_689 Would this be suitable ?

24th Apr 2012, 11:49 pm	#17
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 MKII No that wouldn't be suitable but this would __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

24th Apr 2012, 11:53 pm	#18
Redders03 Pentode Join Date: Apr 2012 Location: Bromley UK. Posts: 161	Re: Revox B77 MKII I have already sourced 2 from this seller and both have failed to work I cannot see the IR when put in front of a camera but the voltage is there !!

25th Apr 2012, 10:55 pm	#20
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Revox B77 MKII I would be tempted to say that with all the activity around this part that the track between the resistor and the connector has worked loose. I think the photo-transistor has already been linked out and it stopped. With the tape playing there should be about 2 volts less than the 24v unregulated on the resistor and that end of the IR led. If the resistor itself has zero on it then it is an open circuit wire or track. To test the sensor on the bench apply 12 volts to the LED via a 1k resistor and measure the sensor with a diode tester. It should close when the beam is not blocked and open when the beam is blocked.