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

Michael Maurice · 17th Nov 2011, 11:14 pm

I am servicing a Revox B77 for someone and it arrived in a bit of a state.

The issues I have is with the VU meters and the speed control.

The machine is a very low speed, 15/16 IPS and 1 7/8 IPS

The VU meters are open circuit so I got some from Germany and they were open circuit! Is there a known problem with VU meters?

The speed control,is more complicated, In Low speed and Very Low Speed machines, the capstan diameter is reduced to 3mm

In my machine the speed on fast seems to be above 3¾ but not as fast as 7½

I have adjusted the speed control as per the manual ie 800Hz on TP1 but the manual only gives the info for the standard machine.

Does anyone know how to adjust the speed for these machines?

Michael Maurice · 17th Nov 2011, 11:42 pm

Further checks have revealed that someone has been changing capacitors in the speed control board and putting the wrong values in!

Hopefully Cricklewood will be able to supply.

Michael Maurice · 18th Nov 2011, 12:45 pm

Progress:

I replaced C8 and C9 with the values in the service manual. As there is no setting up procedure, I recorded 1KHz tone on a Revox A700 at 3¾ and then replayed it on the B77 at 17/8 IPS with a frequency counter connected to the output. The speed now came more or less within range with the speed preset at one end. I then added a 470pF capacitor across C8 and readjusted the preset, it now ran at the correct speed with the preset in the centre of its range.

As a further check, I connected the output to a scope and adjusted the timebase so that on input I had I cycle per division, I then replayed the tape and could check that on at 17/8 IPS I had I cycle per two divisions and at 15/16 IPS per four divisions.

Now to find that VU meter.

yestertech · 18th Nov 2011, 2:44 pm

Impressed !
I assume this was a logging machine given the speeds
What sort of rec/play response can you get on a B77 at these vastly reduced ips' ?

Andy

Michael Maurice · 18th Nov 2011, 8:34 pm

I'll let you know when I do a line up!

DOFFERY · 21st Nov 2011, 2:21 pm

Andy,
The LS gives a frequency response of 60c.s/4kcs at 15/16 ips, 60cs/8kcs at 1.875 ips plus, minus 2db.

Colin.

DOFFERY · 21st Nov 2011, 2:22 pm

Michael,
Are you still looking for meter/s?
Colin.

yestertech · 21st Nov 2011, 2:27 pm

Quote:

Originally Posted by DOFFERY

Andy,
The LS gives a frequency response of 60c.s/4kcs at 15/16 ips, 60cs/8kcs at 1.875 ips plus, minus 2db.

Colin.

Not at all bad really - better than a Ferrograph at most speeds ( ooops ! )

A.

Michael Maurice · 21st Nov 2011, 11:33 pm

Colin,

I've sourced a meter and waiting for it to arrive. Is there any info in the service manual as to how to set the capstan motor speed? i'm sure my way was accurate enough but it would be nice to know if there is a different method.

TIMTAPE · 22nd Nov 2011, 2:46 am

Michael,

If the machine's had a lot of use the capstan shaft may have worn enough to reduce the effective diameter and so the tape speed. I once came across some ex radio station logging tapes recorded nominally at 15/16ips and they were all recorded about 5% slow. Probably this was the reason.

May be best to test actual tape speed rather than rely on the Revox service manual method of 800/1600 hz (or whatever it is with the SLS motor) from the tach signal which assumes the shaft is spot on in diameter.

Tim

DOFFERY · 22nd Nov 2011, 9:52 am

The set up is the same Michael but I think Tim has the correct solution in practice, at these very low speeds & a slender capstan shaft I doubt I would use the manual set up.

Glad about the meter, just had a couple from a PR 99 in the complete switch block to hand.

Colin.

Michael Maurice · 30th Nov 2011, 5:30 pm

Well just to let you know what has been going on.

VU meters were replaced, both input level controls replaced.

There was an issue with the End Of tape sensor not working, someone had had a go at it so a new one was fitted which sorted that out.

Next a fault on the tape control board, when play was selected after rewind or fast forward, the tape would slow but just before it stopped the pinch roller would engage. Replacing C12 cured that along with C19 which decouples the 5V supply.

Now that I'd got the speed issue sorted it was time to align and check it.

As I didn't have any line up tapes at 17/8 IPS I had to improvise so I recorded 800Hz tone at 3¾ which would replay at 400Hz at 17/8 that bit was OK but the record side wasn't.

No matter what I did, I couldn't get the record side to line up, There was a 3dB difference between 400Hz and 1KHz! So out came the record card to reveal that the wrong capacitors were fitted to the equalisation circuit, they should have been 10nF and 18nF someone had put 4.7nF! There were two extra resistors which definitely didn't correspond to the circuit!

After correcting this I could set up the record level, bias and equalisation could be set up.

At 17/8 IPS I could get a reasonably good response to 8KHz after which it dropped off. At 15/16IPS you could get a good response up to 4KHZ but it started dropping off at 5KHz and was around 10d~B down at 8KHz which is to be expected.

At the slower speed you could get a 12 Hours continuous recording.

I'll give it a quick listening check before returning it to its owner.

17th Nov 2011, 11:14 pm	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Revox B77 Woes I am servicing a Revox B77 for someone and it arrived in a bit of a state. The issues I have is with the VU meters and the speed control. The machine is a very low speed, 15/16 IPS and 1 7/8 IPS The VU meters are open circuit so I got some from Germany and they were open circuit! Is there a known problem with VU meters? The speed control,is more complicated, In Low speed and Very Low Speed machines, the capstan diameter is reduced to 3mm In my machine the speed on fast seems to be above 3¾ but not as fast as 7½ I have adjusted the speed control as per the manual ie 800Hz on TP1 but the manual only gives the info for the standard machine. Does anyone know how to adjust the speed for these machines?

17th Nov 2011, 11:42 pm	#2
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 Woes Further checks have revealed that someone has been changing capacitors in the speed control board and putting the wrong values in! Hopefully Cricklewood will be able to supply.

18th Nov 2011, 12:45 pm	#3
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 Woes Progress: I replaced C8 and C9 with the values in the service manual. As there is no setting up procedure, I recorded 1KHz tone on a Revox A700 at 3¾ and then replayed it on the B77 at 17/8 IPS with a frequency counter connected to the output. The speed now came more or less within range with the speed preset at one end. I then added a 470pF capacitor across C8 and readjusted the preset, it now ran at the correct speed with the preset in the centre of its range. As a further check, I connected the output to a scope and adjusted the timebase so that on input I had I cycle per division, I then replayed the tape and could check that on at 17/8 IPS I had I cycle per two divisions and at 15/16 IPS per four divisions. Now to find that VU meter.

18th Nov 2011, 2:44 pm	#4
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,135	Re: Revox B77 Woes Impressed ! I assume this was a logging machine given the speeds What sort of rec/play response can you get on a B77 at these vastly reduced ips' ? Andy

18th Nov 2011, 8:34 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 Woes I'll let you know when I do a line up!

21st Nov 2011, 2:21 pm	#6
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Revox B77 Woes Andy, The LS gives a frequency response of 60c.s/4kcs at 15/16 ips, 60cs/8kcs at 1.875 ips plus, minus 2db. Colin.

21st Nov 2011, 2:22 pm	#7
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Revox B77 Woes Michael, Are you still looking for meter/s? Colin.

21st Nov 2011, 11:33 pm	#9
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 Woes Colin, I've sourced a meter and waiting for it to arrive. Is there any info in the service manual as to how to set the capstan motor speed? i'm sure my way was accurate enough but it would be nice to know if there is a different method.

22nd Nov 2011, 2:46 am	#10
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox B77 Woes Michael, If the machine's had a lot of use the capstan shaft may have worn enough to reduce the effective diameter and so the tape speed. I once came across some ex radio station logging tapes recorded nominally at 15/16ips and they were all recorded about 5% slow. Probably this was the reason. May be best to test actual tape speed rather than rely on the Revox service manual method of 800/1600 hz (or whatever it is with the SLS motor) from the tach signal which assumes the shaft is spot on in diameter. Tim

22nd Nov 2011, 9:52 am	#11
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Revox B77 Woes The set up is the same Michael but I think Tim has the correct solution in practice, at these very low speeds & a slender capstan shaft I doubt I would use the manual set up. Glad about the meter, just had a couple from a PR 99 in the complete switch block to hand. Colin.

30th Nov 2011, 5:30 pm	#12
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,225	Re: Revox B77 Woes Well just to let you know what has been going on. VU meters were replaced, both input level controls replaced. There was an issue with the End Of tape sensor not working, someone had had a go at it so a new one was fitted which sorted that out. Next a fault on the tape control board, when play was selected after rewind or fast forward, the tape would slow but just before it stopped the pinch roller would engage. Replacing C12 cured that along with C19 which decouples the 5V supply. Now that I'd got the speed issue sorted it was time to align and check it. As I didn't have any line up tapes at 17/8 IPS I had to improvise so I recorded 800Hz tone at 3¾ which would replay at 400Hz at 17/8 that bit was OK but the record side wasn't. No matter what I did, I couldn't get the record side to line up, There was a 3dB difference between 400Hz and 1KHz! So out came the record card to reveal that the wrong capacitors were fitted to the equalisation circuit, they should have been 10nF and 18nF someone had put 4.7nF! There were two extra resistors which definitely didn't correspond to the circuit! After correcting this I could set up the record level, bias and equalisation could be set up. At 17/8 IPS I could get a reasonably good response to 8KHz after which it dropped off. At 15/16IPS you could get a good response up to 4KHZ but it started dropping off at 5KHz and was around 10d~B down at 8KHz which is to be expected. At the slower speed you could get a 12 Hours continuous recording. I'll give it a quick listening check before returning it to its owner.