Revox A77 Mk4 VU Meter - UK Vintage Radio Repair and Restoration Discussion Forum

NorfolkDaveUK · 8th Sep 2018, 7:30 pm

One of the meters is pegging out on this on power up and staying there . Any idea what might be causing it ? I know the meter is ok I just put it in out of another machine because one that was in there was broken . Its pegging out with no input connected . Playback and recording are both fine its only effecting the meter .

NorfolkDaveUK · 8th Sep 2018, 10:24 pm

OKay a bit more info .

I have 0.658v across that meter when its at a stand still which is whats causing it to peg out but I can work out where its coming from . mmmmm .

NorfolkDaveUK · 8th Sep 2018, 11:37 pm

Nevermind I got it , it was coming from the channel one record board . OK all done !

Marotick · 12th Sep 2018, 7:52 pm

The VU meter of an A77 Revox is only for input signals.

Jack Marotick

NorfolkDaveUK · 17th Sep 2018, 12:31 am

Um ...correct.

Restoration73 · 17th Sep 2018, 9:45 am

On my full track mono A77 (ex BBC) the left meter is input and right meter is playback
in case you come across one of these models.

Ted Kendall · 17th Sep 2018, 10:54 am

And the Dolby version has several changes in the signal routing, one consequence of which is that the VUs read the playback signal as well as record.

NorfolkDaveUK · 20th Sep 2018, 1:09 am

So I finally got my scope and calibration tape and I was plodding along quite happily , when I got to the RF section .

Please take a look at the pic to see the instructions and save me typing them all and making errors and confusing the issue .

Now it says "4 track - 16v on each "

This is what I have (referenced to ground)

Orange - 32vac
blue - 0.64vac

Ummmmmm..........help.....you can hear the old recorded stuff under new recordings but I put that down to the old recordings being 2 track and this being a 4 track (tape is a NOS scotch , recorded on once )

So could some one help me figure out whats causing these voltages to be so far off the mark please.

Edit : I was told when I bought this that this machine was originally a half track . The heads and osc board have been swapped out for 4 track . If that makes any difference . Also , I dont have a VTVM so I`m using a Fluke . Note : both of the OSC cards look identical . I just took the one out of the 4 track and put that in when I installed the 4 track heads . I have no knowledge of whether it is a 4 track OSC card OR indeed that the machine every was actually half track .

It seems like all the voltage is going to one side of the head

Vicboduk · 20th Sep 2018, 1:11 pm

Even a top end Fluke will not have the bandwidth to be able to measure the AC bias accurately, circa 120kHz.
A better choice would be to use your scope with a x10 probe. Note; the original VTVM would probably be average reading not RMS.
The use of the scope should make it straight forward to trace the signal path...

.

Ted Kendall · 20th Sep 2018, 1:15 pm

Have got both record buttons depressed? The volts across the selected erase head rise in mono.

The difference between the half and quarter track oscillators is in the transformer - otherwise they are identical.

NorfolkDaveUK · 20th Sep 2018, 1:44 pm

Thanks guys. .....

In the end I had to look at the osc schematic further down the manual . The reading were actually correct . I don`t know what they were going on about in the "instructions" but if you look at the schematic for the osc card , the true values are in there , so once I read that I could carry on and all values were in tolerance .

I`m now on to the RF bias and the coil isn`t doing anything .

I`m clipped onto the correct point shown in the pic ....

I should be getting < 300mV but as you can see in the pic I`m getting 700+ and the coil isnt adjusting it . Any ideas ?

NorfolkDaveUK · 20th Sep 2018, 2:02 pm

OK that problem is only existing one one of the cards by the way , the other one I`ve got down to around 98mV ish , so its working that one . This definitely has something amiss with it ..

NorfolkDaveUK · 20th Sep 2018, 2:13 pm

Its something on the actual record board . I`ll check some voltages .

NorfolkDaveUK · 20th Sep 2018, 2:26 pm

I swapped the board out to save time and they both ended up around 107mV so thats within spec . The one I replace the dodgy one with just wanted to go down and down until the core fell out so I`ve just matched it with the other one , will that be ok ?

8th Sep 2018, 7:30 pm	#1
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Revox A77 Mk4 VU Meter One of the meters is pegging out on this on power up and staying there . Any idea what might be causing it ? I know the meter is ok I just put it in out of another machine because one that was in there was broken . Its pegging out with no input connected . Playback and recording are both fine its only effecting the meter .

8th Sep 2018, 10:24 pm	#2
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter OKay a bit more info . I have 0.658v across that meter when its at a stand still which is whats causing it to peg out but I can work out where its coming from . mmmmm .

8th Sep 2018, 11:37 pm	#3
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter Nevermind I got it , it was coming from the channel one record board . OK all done !

12th Sep 2018, 7:52 pm	#4
Marotick Triode Join Date: Aug 2017 Location: Amsterdam, Netherlands. Posts: 15	Re: Revox A77 Mk4 VU Meter The VU meter of an A77 Revox is only for input signals. Jack Marotick

17th Sep 2018, 12:31 am	#5
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter Um ...correct.

17th Sep 2018, 9:45 am	#6
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Revox A77 Mk4 VU Meter On my full track mono A77 (ex BBC) the left meter is input and right meter is playback in case you come across one of these models.

17th Sep 2018, 10:54 am	#7
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Revox A77 Mk4 VU Meter And the Dolby version has several changes in the signal routing, one consequence of which is that the VUs read the playback signal as well as record.

20th Sep 2018, 1:09 am	#8
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter So I finally got my scope and calibration tape and I was plodding along quite happily , when I got to the RF section . Please take a look at the pic to see the instructions and save me typing them all and making errors and confusing the issue . Now it says "4 track - 16v on each " This is what I have (referenced to ground) Orange - 32vac blue - 0.64vac Ummmmmm..........help.....you can hear the old recorded stuff under new recordings but I put that down to the old recordings being 2 track and this being a 4 track (tape is a NOS scotch , recorded on once ) So could some one help me figure out whats causing these voltages to be so far off the mark please. Edit : I was told when I bought this that this machine was originally a half track . The heads and osc board have been swapped out for 4 track . If that makes any difference . Also , I dont have a VTVM so I`m using a Fluke . Note : both of the OSC cards look identical . I just took the one out of the 4 track and put that in when I installed the 4 track heads . I have no knowledge of whether it is a 4 track OSC card OR indeed that the machine every was actually half track . It seems like all the voltage is going to one side of the head Attached Thumbnails

20th Sep 2018, 1:11 pm	#9
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Revox A77 Mk4 VU Meter Even a top end Fluke will not have the bandwidth to be able to measure the AC bias accurately, circa 120kHz. A better choice would be to use your scope with a x10 probe. Note; the original VTVM would probably be average reading not RMS. The use of the scope should make it straight forward to trace the signal path... . __________________ It has to be here... I've just put it down!

20th Sep 2018, 1:15 pm	#10
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,670	Re: Revox A77 Mk4 VU Meter Have got both record buttons depressed? The volts across the selected erase head rise in mono. The difference between the half and quarter track oscillators is in the transformer - otherwise they are identical.

20th Sep 2018, 1:44 pm	#11
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter Thanks guys. ..... In the end I had to look at the osc schematic further down the manual . The reading were actually correct . I don`t know what they were going on about in the "instructions" but if you look at the schematic for the osc card , the true values are in there , so once I read that I could carry on and all values were in tolerance . I`m now on to the RF bias and the coil isn`t doing anything . I`m clipped onto the correct point shown in the pic .... I should be getting < 300mV but as you can see in the pic I`m getting 700+ and the coil isnt adjusting it . Any ideas ? Attached Thumbnails

20th Sep 2018, 2:02 pm	#12
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter OK that problem is only existing one one of the cards by the way , the other one I`ve got down to around 98mV ish , so its working that one . This definitely has something amiss with it ..

20th Sep 2018, 2:13 pm	#13
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter Its something on the actual record board . I`ll check some voltages .

20th Sep 2018, 2:26 pm	#14
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 Mk4 VU Meter I swapped the board out to save time and they both ended up around 107mV so thats within spec . The one I replace the dodgy one with just wanted to go down and down until the core fell out so I`ve just matched it with the other one , will that be ok ?