Revox B77 no full audio

[jotape1960]

Hi!
I'm new in this forum.
I have a Revox B77 (2 tracks, 2 channels + Slide Synch option) open reel tape machine.
Even the years, almost all is fine (I need a new Counter belt, but it is just a detail).
From a week or so, I cannot to get full audio from the monitor board.
I just get -42 db audio output, if I use the L->R and R->L options (monitor switch at "Input" position).
I suspect about the muting audio relay.
If I'm right and the problem is that relay... I wonder if this relay is absolutely necessary.
According to the schematics, this electronic switch mutes the audio at the input of the monitor board. But... Why we would want to do that?
So, maybe this is a silly question but... Can I jump the relay action without to damage the electronics?
Greetings from Chile.

 

[GPS16]

Hi Jotape1960,

No!!!!! It would be a bad idea to do this. The audio circuits are AC coupled with Electrolytics. There is only a single 21V regulated supply, so all amps are approx 10V when silent. On power up these caps all need to be charged to approx 10V. Power up the machine after it has been off for 2 mins or more (to give the caps a chance to discharge). Watch the VU meters when you power it back up. If you jump the relay, the kick you see on the VU meters will be transferred to your amp and seriously damage the Bass Drivers of your speakers. The amplifier will not be too happy about it either!!! I hope you do not have a coffee in your hand at the time either as most of it will be on the floor.

If you are playing a tape the switch should be set to tape unless you are recording using the Sound On Sound facilities on the recorder. If L-R or R-L is used the level will be set by the input level controls.

Give the tape path a good clean with IPA and cotton buds and take things from there. There are 2 pots near to the output phonos. Make sure that both are fully clockwise. After the machine is calibrated, these pots can be used to match the B77 level to the other components in the system.

Cheers.

GPS16

 

[jotape1960]

Thank you, GPS16!
You're absolutely right about the start up click peak noise, yes.
But, this is something related to a lot of devices, even some computers.
It is a very, very, very good idea to set all the mixers, amplifiers, etc., input volume controls to zero when we turn on any device in the audio chain.
My question is specifically about the internal Revox B77 monitor board health if I jump that relay.
Would be some risk of damage INSIDE that monitor board without the relay? ???
Greetings from Chile!!!

 

[GPS16]

Hi Jotape1960,

The relay on the monitor PCB shorts the output from the Reproduce Amp to ground during the power up delay. It does not affect the level in any way. When the relay has clicked the contacts are O/C!! The monitor amp electronics run the VU Meters and the Headphones only. The risk to the monitor PCB itself will be zero. The VU meters would get hammered though as the kick to the meters would then also include the kick from the Reproduce amp. Why are you adding issues??!!

If your problem is low output level from the machine you need to be looking elsewhere.

Puzzled!!

GPS16

 

[jotape1960]

Hi Jotape1960,

The relay on the monitor PCB shorts the output from the Reproduce Amp to ground during the power up delay. It does not affect the level in any way. When the relay has clicked the contacts are O/C!! The monitor amp electronics run the VU Meters and the Headphones only. The risk to the monitor PCB itself will be zero. The VU meters would get hammered though as the kick to the meters would then also include the kick from the Reproduce amp. Why are you adding issues??!!

If your problem is low output level from the machine you need to be looking elsewhere.

Puzzled!!

GPS16

Hi, GPS16!!!
I tested something weird and absolutely out of the "academic" rules.
I opened the Revox B77 and I took out the monitor board. Yes, I disconnected it from the main audio connection panel.
Then, I put on the power switch of the machine and I tested the sound, and...
Voila!!!
I got full audio, again, on the Audio Line Output RCA connectors!!!
Of course, I know without the monitor board I won't have any way to get the VU meters work to show me the levels and I cannot to get sound to the headphones output, but... I get full audio inside the line to the mixer, which is the final target.
In other words, whatever it is going wrong in my Revox B77, IS SOMETHING INSIDE THE MONITOR BOARD.
So...
What do you think?
Can I be using the Revox B77 without the monitor board up I can get a new one? Is it a good idea? ???
Blessings and Greetings from Chile!!!
Juan

 

[oldvinyldude]

Why not sort the monitor board-replace electrolytics, test and replace transistors as needed, check for broken traces, etc?

You can even listen to music while you work!

 

[GPS16]

My thoughts exactly OldVinylDude!!

As mentioned in a previous post, the monitor PCB does (should) not affect the Audio Level in any way. It just shorts it out during the power up pause. Now that you have removed it and restored normality, the monitor PCB needs attention!!

Next question. Are both channels affected or just one? If only one channel, which one?

Run the machine without the monitor PCB installed at your own risk. The rest of your HiFi could be damaged. An audio system was not designed to deal with High Level DC Pulses.

Keep us posted.

GPS16