Revox A77 sound improvements for pre-recorded 4-track tapes ?

[david winter]

Hi,

Right now I play my 4-track pre-recorded tapes on a Revox A77 which seems to be the MK-IV version as per silver push-buttons.
I recaped almost all except the motor capacitors and two 1600µF electrolytics on the playback amplifiers (I'll change them soon anyway).
I would like to get the best sound I can from this machine, without expecting more than it can give.
As I'm using this machine in playback more only (I record on a 2-track PR99 MKIII), I removed all the plugin boards except the two playback amplifiers.
What I wanted to know is whether there are any known changes (excluding head changes) which would improve the sound quality.


Thanks,

David

 

[goldear]

If you've installed good quality caps, then you should have already improved the sound of the unit a bit. Bypassing the electrolytics with film caps might improve things a bit more though. I've not heard-of any Revox A77 specific mods designed to make them sound better. Although, if I wanted to go crazy on one, I'd probably replace the front stationary guide with a rolling guide.

 

[david winter]

I don't know what you mean by bypassing the electrolytics with film caps. Do you mean putting a small film cap in parallel ?
As my A77 is a play only machine, I removed all boards and only kept the two reproduce amplifiers. Each has one 1600µF/3V cap, two small electrolytics (replaced) and 4 small tantalum caps.
I have never seen a tantalum cap fell on these, so left them on the boards.
The only trouble is with the small 20K output level pot on the board itself which becomes faulty due to excessive corrosion on the cursor.
I had one channel sounding much under the level of the other and found that the cursor of its pot was barely touching the resistive layer. So I'll change them today and will need to know how to ajust to the correct level without test tape.
I'll probably do it by ear and pass through my PR99 to check the VU levels for saturation. Should be good enough.

 

[jan_stevns]

Bypassing are as you guessed putting another cap (small) in parallel

by design - there is a small current constantly flowing through the playback head on a A77, so they can actually benefit from degaussing

 

[tapetech]

So I'll change them today and will need to know how to ajust to the correct level without test tape.
I'll probably do it by ear and pass through my PR99 to check the VU levels for saturation. Should be good enough.

Measure the setting of the good (working) play cal pot with an ohm meter and then set the new pots for that value. I'm assuming you did not turn the pot on the good channel.

 

[stevevdb]

revox A77

The revox A77 mkIV is a good machine but the electronics is not up to japanese standards.
You could replace those 1600uF capacitors.

Very important: the wear on the heads: not above 5mm, otherwise you will need to replace to the heads.

Revox machines are alligned to use one specific kind of tape. If you want to use other tape you will need to adjust the oscillator panel.

My revox A77 mkIV 2 track is alligned on Basf/Emtec/RMGI SM911 which is a very high quality tape. The amount of background noise is also very low.

 

[KentTeffeteller]

It's not being that the deck is up to Japanese standards. The ReVox is basically a junior Studer or other high class studio machine. Which means that the specifications claimed are minimum performance limits. It is built to be reliable and easily serviced and maintained. While built to a cost, it is still in it's heart more of a broadcast machine than consumer gear. It can be aligned to far higher spec than say, an Akai toy.

 

[jan_stevns]

The revox A77 mkIV is a good machine but the electronics is not up to japanese standards..

True - a Revox is build by Swiss/German standards, which i would prefer at any time

Very important: the wear on the heads: not above 5mm, otherwise you will need to replace to the heads..

I'd beg to differ - if the mirror is absolutely perfect, and the gap is intact, there is no reason to change heads.

This head - yanked from a PR99, are still quite usable - using a mrl tape it's still within 1dB specs.

 

[stevevdb]

i agree with the picture above: this head is still ok for use.

the boards: well, you will need good quality components, a good soldering station and lots of time to find the exact components.
Try to use low noise components.

 

[LAB3]

You have to look up the serial# on the rear to see if it is a MKII,III or IV

I have a 1979 MK IV Dolby we purchased new back then. I had "Tinman"

restore it to like new again. Click on link and look at the pic's marc took when

he had it all apart. My heads could not be relapped. Click on the small pic's for a larger frame. Replace those oil filled phase motor run caps.

Mine was leaking oil on the trasnport board and I was LUCKY it did fill
the room with smoke and ruin the boards. It was still playing when I

shipped it to Marc. I sent a AGFA PER 528(Broadcasting/Radio tape) tape with it for the heads to be adjusted too. And I have noticed Maxell UD/XL, BASF LGR, LP35 and AFGA PEM 468/469 all record and play back really good. I also tried a new RMGI LPR35 and it played GREAT just expensive.

http://www.flickr.com/photos/43284454@N04/sets/72157627387811783/

 

[stevevdb]

http://www.flickr.com/photos/43284454@N04/sets/72157627387811783/[/QUOTE]

I had a look at the pictures and I must say: this guy did a very fine job!

 

[david winter]

Yes, but a bit too much in my opinion:
- No real need to pass the boards to ultrasonic bath
- No real need to change tantalum caps. Electrolytics definitely need to be replaced. Same for the three 0.47 250V plastic caps. And sometimes (but not always), the three motor capacitors.

However he should have changed the miniature pots on the plugin boards. Old ones have corroded cursors which tend not to press enough against the resistive layer, causing all sorts of troubles. My A77 had that trouble with one channel much lower than the other. Until I changed both pots on the playback preamplifier board.

There is also one trick which consists in removing the mute transistor which suppresses the sound when the machine is idle, fast winding/rewinding or engaging into play mode.

 

[LAB3]

Yes, but a bit too much in my opinion:
- No real need to pass the boards to ultrasonic bath
- No real need to change tantalum caps. Electrolytics definitely need to be replaced. Same for the three 0.47 250V plastic caps. And sometimes (but not always), the three motor capacitors.

However he should have changed the miniature pots on the plugin boards. Old ones have corroded cursors which tend not to press enough against the resistive layer, causing all sorts of troubles. My A77 had that trouble with one channel much lower than the other. Until I changed both pots on the playback preamplifier board.

There is also one trick which consists in removing the mute transistor which suppresses the sound when the machine is idle, fast winding/rewinding or engaging into play mode.

One of the three oil filled phase motor run caps was leaking oil on the transport board and other borads. IMHO cleaning all the boards the way he did was a great idea, as there was oil everywhere.... And replacing ALL the caps at one time is a good idea. One and DONE. I am Not a Tech, so shipping my late dad's Revox again is not something I want to do again. He purchased it new when we got the Klipsch Cornwall speakers back then(I was 29 then) and it played great for 30 plus years. I bet it does so again as Tinman cleaned and oil the bearings in the motors, new heads etc it records and plays like it did 30 years ago. I will PM Marc and ask him about the mini pots on the plug in boards.

 

[stevevdb]

* the boards: i like clean boards as well, so I always give them an ultrasonic clean. the worst boards are the amplifier cards of a revox B780.
* the pots: yes I saw them but there is no need to replace them when you don't touch them and when they are not broken. I only replace all of them when I need to repair the oscillator panel and in case one broke of during repair. Remember well: replacing the pots means re-alligning your tape deck.
* motor capacitors: I always replace them by new ones, sometimes the motors become a bit slow or ar not at speed when rewinding. these parts are dating back from 1975 and replacement is a good thing when you want to use the A77 on a regular basis.
* doing a revision of a revox is always okay, the equipment rises in value too.

 

[tapetech]

I agree with David. Ultrasonic board cleaning is not needed. The cal/trim pots are even more failure-prone than the caps, so replacing all of them should be mandatory in any comprehensive overhaul. Also, the deck always will need a full re-calibration anyway (I always do that), so changing all the trim pots is not a big deal.

 

[LAB3]

* the boards: i like clean boards as well, so I always give them an ultrasonic clean. the worst boards are the amplifier cards of a revox B780.
* the pots: yes I saw them but there is no need to replace them when you don't touch them and when they are not broken. I only replace all of them when I need to repair the oscillator panel and in case one broke of during repair. Remember well: replacing the pots means re-alligning your tape deck.
* motor capacitors: I always replace them by new ones, sometimes the motors become a bit slow or ar not at speed when rewinding. these parts are dating back from 1975 and replacement is a good thing when you want to use the A77 on a regular basis.
* doing a revision of a revox is always okay, the equipment rises in value too.

I feel the same way and with all the oil that on them it was a Must. I guess tapetech would have left them dirty. j/k...I find it odd that all these years I have read hundreds of posts about Rifa and Franko caps going bad........ and not many on the trim pots. We purchased this MkIV Dobly new in 1979 and it's been a gem. If Marc felt it was necessary he would have replaced them as we replaced the heads so extra labor or parts was Not a issue. It's been a year now, still sounds like New and I record or play tapes on it every day. I retired last year and I am recording tapes from all LP from my late parents collection I have. Replace ALL the caps on a A-77 One time and be done with it if it's a keeper.... I will never sell Dad's Revox... LABIV, my son will get when I have passed on.:yes:
Louis

 

[Tinman]

Those who make a living doing this will always do the minimum required to get the job done. Simple economics.

Those who do this as a hobby will always do the job with pride, and how they would like someone else to treat their own gear. Otherwise they wouldn't do this.

Both approaches get the job done.

Marc