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Old 28th Apr 2017, 9:12 pm   #1
monaro0162
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Default Converted to REVOX

Good evening folks,

After reading so much about the Revox A77, I decided I needed to go on a Revox adventure, carrying on from my AKAI adventure.

The MK3: Bought on e-bay from a very nice gentleman who was the first owner who bought it new in 1972. It has an external pitch controller that regulates the speed by -10 +15%. He specified the speed controller when he ordered it new, and used the feature to make speed corrected recordings of 78rpm records. It hadn’t been used for the last 20 years and wasn’t working when I bought it. One of the fuses had gone and the tape sensor light was out, meaning it wouldn’t play, but after the fuse was replaced all has been fine. The only other thing I’ve done is clean the heads. A beautiful machine and sounds absolutely wonderful,

The MK4: Also from e-bay, I don’t know much about its history, but it seems to be in as good condition, although quite dusty on the bits I can see under the flap.
It needs a little work before I start using it: The volume control is a little scratchy, and there seems to be a bit of an imbalance between the VU meters on playback. I’ve only played the tape it came with so far, and the heads need cleaning.

Wondering really what would be a sensible service plan for both machines, bearing in mind their condition and history, as I’ve never been one to pull something apart unnecessarily, believing in leaving well alone. But i would like to test them properly and obvisiouly replace some of the caps.

The Revox compared to the AKAI? Leagues ahead in terms of design and performance. If the AKAI kept me interested for 6 months, then these machines will keep me interested for years!

Lots of fun ahead, happy days!
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Old 28th Apr 2017, 10:14 pm   #2
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Default Re: Converted to REVOX

Thee are some rifa type capacitors in the motor drive circuits that are prone to make smelly smoke. They go on the change before serious use list.

The trimpots on the circuit boards are known to go crumbly.

The imbalance as you said is likely to be dirt.

Are they 1/2 track or 1/4 track machines?

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Old 28th Apr 2017, 10:55 pm   #3
Ted Kendall
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Default Re: Converted to REVOX

Mmmm...nice! Welcome to the world of proper tape machines!

As you surmise, there's a large element of "if it ain't broke..." here, but there are some things which are worth keeping an eye on. Thorough cleaning of heads is a given. De-gaussing every so often is a wise precaution, although Revoxes don't generally suffer from magnetised heads unless there is a fault present. The quality of the machine justifies proper lining-up for the tape in use, for which ideally you need a test tape and test gear. It is as well to lubricate the capstan motor.

Internally, the thing is tidy and logically laid out. The audio electronics are on plug-in cards. Most of the caps are tantalum and generally don't give trouble - occasionally the motor run caps fail, but this isn't a routine replacement. What do go, of course, are the notorious Rifa 470nF capacitors - replace the lot as soon as convenient. Although failure generally doesn't cause other damage, the firework display, smoke and smell are things you can do without. The first resort on scratchy pots is Deoxit or similar and a check of any caps which may allow DC onto the control. After forty years or so, the preset controls may be in a fragile state. If they are, replacement is the cure. The pitch is 15mm, which is not common nowadays - try Nagravox of Australia if all else fails.

You have done well to buy Mk3 and later - these should have Revodour heads, which are much harder wearing than those on earlier machines. Properly adjusted, these heads go on and on - the usual replacement point is a 4mm wide flat on the head face, but I had one which went on much longer than that - it finally expired quite suddenly when the gap collapsed through wear, but was fine the day before!

You're quite right - this is quality stuff, which like as not will roll on for years yet with quite moderate attention. Some compare the plastic externals unfavourably with Japanese machines, but these are perfectly adequate and reflect the priorities applied in the design - the money is spent under the bonnet. Cast chassis, chunky motors, ball bearings, easily-serviced layout, and so on. Happy days!
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Old 28th Apr 2017, 11:26 pm   #4
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Default Re: Converted to REVOX

Quote:
Originally Posted by Radio Wrangler View Post
Are they 1/2 track or 1/4 track machines?
Both 2 track 3.75/7.5ips. I've had the MK3 for a few months, only just got the MK4.

On the MK4, just cleaned the heads and it sounds as good as the MK3. I have to turn the balance knob to the "right" a little, to centralise the sound, but the right (or ch11) VU meter seems to be reading a lot lower. Don't have this issue on the MK3, VU meters and sound seem balanced and proportional. Just playing back the tapes it came with at the moment, guess I need to try some test recordings tomorrow.

Other minor issue is that, on the MK3, when I play back the VU meters don't move (they do on record) but on the MK4, the meters work during both record and play. Intentional by design on the Mk3 or a fault?

What great machines these are. The noise levels (both electronic and mechanical) are way down compared to the AKAI, the sound is amazing. They have 4 times the amount of transistors compared to the AKAI yet background noise is way lower. How did they achieve this? Better transistors, more transistors running at lower gain?
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Old 28th Apr 2017, 11:37 pm   #5
Ted Kendall
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Quote:
Originally Posted by monaro0162 View Post
Other minor issue is that, on the MK3, when I play back the VU meters don't move (they do on record) but on the MK4, the meters work during both record and play. Intentional by design on the Mk3 or a fault?
The Mk4 is a Dolby machine, and has some signal routing changes which result in the VUs working in rep and rec. No fault.
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Old 28th Apr 2017, 11:46 pm   #6
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Quote:
Originally Posted by Ted Kendall View Post
the notorious Rifa 470nF capacitors - replace the lot as soon as convenient. Although failure generally doesn't cause other damage, the firework display, smoke and smell are things you can do without.
I've just returned to the room where I've had the MK4 running for a few hours and ... guess what ... there is a distinct smell of 'burnt coffee' in the room, so I've turned it off!

Ted thanks for your summary, I'm really looking forward to getting to know these Revoxes.
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Old 29th Apr 2017, 2:23 am   #7
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Default Re: Converted to REVOX

Quote:
Originally Posted by monaro0162 View Post
What great machines these are. The noise levels (both electronic and mechanical) are way down compared to the AKAI, the sound is amazing. They have 4 times the amount of transistors compared to the AKAI yet background noise is way lower. How did they achieve this? Better transistors, more transistors running at lower gain?
One reason for the lower noise is because the repro head on the A77 is better shielded from hum. Notice the little moveable metal flap on the repro head which swings up when play or record is selected. The flap achieves lower hum on playback even though the capstan motor is very close to that head.
Of course this only reduces the hum on playback. Even the Akais should have made good hum free recordings.

Tim
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Old 29th Apr 2017, 5:12 am   #8
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Default Re: Converted to REVOX

Check the condition of the fabric tape on the supply and take up brakes (below the reel
tables). If you hear any strange noise going from rewind or fast forward to stop, there
is a chance the fabric tape adhesive has bled out, and if this contacts the brake band at
speed it can rip it. Luckily I had a spare. (A77, B77, PR99)
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Old 30th Apr 2017, 7:43 pm   #9
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Thanks for the advice all,

I now have the cases off both machines in order to do a comparitive assesment between the two, and do the essential cap replacements to ensure stable running, before I do any proper testing. As expected, following the smell issue, the supression caps are on the way out on the Mk4, and the motor start caps don't look too good either. All caps on the MK3 look fine but will be replaced regardless. These parts are on order from Germany.

The MK4 has definitely seen more use that the MK3, as I can see more head wear. On the MK3, I'm seeing a between 2.0 - 3.0mm 'flat' across the centre section of the record - play heads with a bit more observable wear on the play head. On the MK4 the flats look wider between 3.00 - 4.00mm. Whats is considered the maximum wear, can they be lapped?

Also noticed that the reel brake tension feels different between machines, more pressure on the MK3, I did notice that the tape reels took longer to stop on the MK4.
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Old 30th Apr 2017, 8:32 pm   #10
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Default Re: Converted to REVOX

I too have a Revox A77 mk4 1/4 track machine in need of attention as I have mentioned previously here, but still not got around to working on. My problems being not stopping when the tape has emptied on rewind, and a bang and smell one day when switching on. Both problems were answered by people more knowledgeable than me may I add. Must get around to addressing them one day.

Anyway, one thing I seem to recall, the fragile trim pots on the earlier versions were replaced with much better ones on the mk4 - as I recall..

I bought a couple of 'scrap' donor machines in around '96, and was lucky enough to get one with amplifier boards, so these were subsequently fitted to my near mint Mk4 (as it was) and it was used to record and playback a friend's band one evening at a gig. The resulting quality was so good that the recording was used as a live CD of the band. I got my Mk4 for a song from the original owner who had barely used it for home 'taping' purposes and it still had the tags on it.

I remember once recording test tones from a CD test disc on to a nice three head Technics cassette deck and also on the A77. When played back the recorded tones on the Revox were as near as damn it as pure and as stable as the CD, whereas very slightly warbly, somewhat 'dropouty' tones could be heard from the Technics cassette deck. In fairness, saying that I could not fault the Technics when playing back normal programme material including piano (the toughest test for tape). It just goes to show how in general we may not find fault with 'adequate' specs on programme material, but under test conditions a difference can be noticeable.

Is the B77 that much better than the A77?
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Old 30th Apr 2017, 9:51 pm   #11
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Interesting to hear about your live recording this is something I'd love to do as I'm a big live music fan. These A77s have a wondererfully open sound, I'm playing a Dire Straits Love Over Gold recording on the Mk3 (which came with the MK4) and and it sounds fabulous.

I Think my MK3 might have the same problem when the tape ends. It seems to stop after FF or RW, but after play it doesn't seem to stop, the take up reel keeps spinning.

I'm wondering whether the mis balance I saw on the VUs on the Mk4 was down to the Dolby calibration adjustments not being set/adjusted. Do they have an effect on the displayed VU levels during playback?

Tim, on the noise, the AKAIs did make good recordings after I'd sorted the 50hz mains hum loops, but they had quite a bit of tape hiss (reduced by the Dolby) But on switching to the Revox, I immediately noticed less background noise, and noticeably less tape hiss, even with Dolby off. Some of this must be due to the wider 1/2 track configuration compared to the 1/4 track on the Akai.

I have two Cassette machine an AKAI GXC-46D from the 70's and a Denon DRR-F100 from the late 90s. The both 'look alright' on a scope but sound quite different to each other (& neither get anywhere close to the quality of the A77!)
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Old 30th Apr 2017, 10:46 pm   #12
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Monaro wrote "I immediately noticed less background noise, and noticeably less tape hiss, even with Dolby off. Some of this must be due to the wider 1/2 track configuration compared to the 1/4 track on the Akai."
Yep, speed gives you bandwidth, track width give you increased signal to noise ratio - 3db improvement going from 1/4 track to 1/2 track. If you ever get a chance listen to 1/2" or better still 1" tape recorded as 1/2 track stereo then go home and weep I sometimes used to leave the studios and think "why do I bother" then get home and listen and think OK sounds good to me.
Currently run a Tandberg TD20A 1/2 track 9.5/19cms. Just bought a Revox B77 Mk2 1/2 track 9.5/19 cms in very good cosmetic condition, needs a bit of tlc electronically, then comes the dilemma which do I keep? Ho hum.

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Old 30th Apr 2017, 11:21 pm   #13
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Default Re: Converted to REVOX

Is the B77 that much better than the A77?

No, not really. There is a number of small improvements. The tape sequencing logic fixes a vulnerability in the A77's relay logic that allows tape to be spilled if you rush it. Mostly it's a cosmetic face lift to match a new fmily style of amplifiers and tuners.

Some people say the B77 isn't made to as good a standard as the A77. Others differ.
If you have a good working A77, you don't need a B77. And vice-versa.

If you have a B251 amplifier or a B252 preamp with the right option board fitted, it can remote control a B77 (but not an A77) from the infra-red remote. I think there may be an adaptor for the A77.

Few machines were fitted with Dolby. Most people thought they didn't need it. Their signal to noise ratio is quite good without it, and some people didn't like the side effects of Dolby processing.

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Old 1st May 2017, 11:38 am   #14
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Default Re: Converted to REVOX

For signal to noise ratio it really depends on what you're recording. Copying LP's or recording off radio is a fairly easy task as the dynamic range has usually been compressed for you. Even many CD's can have a quite small dynamic range these days because they were produced that way.

Recording live material with microphones is a different story and can quickly reveal the limitations of even a good tape machine. When we recorded audio book masters to B77's and PR99's in the 80's and 90's we used compressor/limiters upstream of the tape machines to reduce the dynamics to manageable proportions for the tapes. The compression also greatly reduced "print through" speech echoes.

Even professional 1/2" or 1" stereo tape running at 15ips or 30 ips - a big consumption of tape - will struggle to match the signal to noise ratio of the now 35 year old CD standard of "only" 16 bits.
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Old 1st May 2017, 5:37 pm   #15
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Yes, a very cracked Rifa cap in the fifth photo in post 9!
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Old 1st May 2017, 8:06 pm   #16
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Well spotted!
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Old 12th May 2017, 6:34 pm   #17
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The Supression and motor caps have been replaced on the MK4, a relatively easy job, and it now runs smell free. At the end of the month I should have some time to do some proper work on it and move it up a level in the restoration process, so I'm putting together a parts shopping list of things that I'm going to need.

I've purchased a new Hantek DSO5102P digital storage scope, and a signal generator, from ebay admittedly they are budget products but perform way better that old CRT scope which wouldn't trigger very well that I was using before.

I would like to achieve baseline 'as is' figures for Capstan motor speed/frequency, Frequency Response, noise, amp levels and the amount of power laid to tape on recordings, before making any adjustments. Then work on them for improvement. Intending to use the MK3 as a reference machine if needed, as its performing great, and is from the first owner and untouched as far as I know.

Calibration tape has been ordered. Looking at the position of the trim pots on the various boards it seems that most of them are in more or less their initial setting position judging by the dust marks. With the exception of the 3 & 3/4 oscillator pots but I can't see how that could be affecting the major L/R difference observed on the VU meters during rec and playback. Both channels sound fine on playback with just a little adjustment needed on the balance/record levels to hear balanced playback.

I Can feel some end float in the capstan motor, how much is allowed? Interested in the brake adjustment, the resistance on the MK3 feels stronger.

I might order the right hand tape guide roller bearing upgrade, initially I thought it was stuck until I read that it was intended that way.
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Old 12th May 2017, 7:56 pm   #18
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Default Re: Converted to REVOX

If the bias levels are seriously out, this can affect recording levels as well as frequency response.

An A77 is best left with the RH guide solid - the friction of tape against guide filters out the 50Hz pulsing of the take-up motor which would otherwise impair the flutter performance. This function is performed by the RH swing arm pin on the PR99. Just take it apart and clean it - reassemble with a fresh face presented to the tape if worn.
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Old 21st May 2017, 4:15 pm   #19
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The trim pot set and capacitors have been ordered along with some other bits and I now have a calibration tape, so I’m looking forward to making a proper start on the MK4 week after next once the last parts arrive.

My test setup so far comprises: A FY2200SP Function generator running a 300mv sine signal sweep from 30hz to 30khz over 5 seconds.
The Signal is fed into Aux in on Revox via BNC cables and t-pieces with a Hantek DSO5102P oscilloscope connected in parallel, to allow signal monitoring. The Revox Record levels are set to about the 5 position which gives a reading of between -10 to -5db over the duration of the sweep.
On the output side the line out is fed to a PC 24bit/192khz HD audio card running audacity software, I am effectively using audacity as a spectrum analyser.
Also got a Racal dana 9910 frequency counter.

Tests carried out so far:

1: A frequency sweep direct into the PC card, to verify that the generated response curve of my test setup is flat. It is bar some drop off at the bass end for some reason.

2: A frequency sweep into the Revox, with the machine recording at 7.5ips/nab, with the monitored output being fed into the PC card for analysis. In a perfect machine the response curve should be flat between ~30 to 20khz. The right channel (chII) looks good with a flat response up to about 18khz, the left (ch1) is not so good and down a little at the top end. So some work required on the MK3 but I don’t want to touch this yet it’s the MK4 that I want to set up first.

3: Frequency sweep into the Revox (not recording) with the output being fed into the PC card for analysis. The selector switch set to input so just checking the frequency response of the Revox internal amps. I wasn’t expecting the response curve that I got but maybe it makes sense as it seems to match the frequency response of the record amps as show in the service manual (to deal with the physical limitations of magnetic tape?)

The lowest voltage resolution on the signal generator that I’m using is 100mv which will be too high for signal injection purposes (particularly into the dolby circuits) so an analogue generator with 1mv resolution is on its way.

One thing I haven’t got is a VTVM and I can find none available in the UK, nearly all on e-bay are in the states. Presumably I could use the (high impedance) scope to take measurements, but it won’t be very easy to measure step changes in db. Does anybody have a spare VTVM for sale or trade?

I found this great site with a lot of very useful information on setting up these Revox machines. Co-incidentally the machine featured is a MK3 with Dolby.

http://www.hccc.org.uk/revox/a77servicingman.html

Is the author anybody on here?
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Old 21st May 2017, 6:20 pm   #20
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If you're using software and a soundcard, there is the possibility that if the measurement is tracking, frequency selective the tape delay can be messing your results up. By the time the siganl at some frequency reaches the output of the recorder, the measurement frequency has moved on. Different parts of the scan will have different measurement bandwidths.

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