Revox A77 calibration. - UK Vintage Radio Repair and Restoration Discussion Forum

NorfolkDaveUK · 5th Nov 2018, 1:13 pm

Can anyone help me with this darn thing I am driving myself up a pole .

I'm trying to calibrate the high frequency watsit via the osc card it says input 10k after setting it up so there is 1k @200mv then switch to 10k, I've done that, then then in record mode, adjust the trimmers on the osc so that it hits a peak and then (for my chosen tape) go 4db over the hump. However when I try and do it there is no hump it just goes up and up . Anyone know why this could be and how to resolve it?

I need a lay down ..haha

Station X · 5th Nov 2018, 1:28 pm

Quote:

Originally Posted by NorfolkDaveUK

I'm trying to calibrate the high frequency watsit

What is a high frequency watsit? The bias oscillator?

Quote:

Originally Posted by NorfolkDaveUK

it says input 10k after setting it up so there is 1k @200mv then switch to 10k,

What? Input 10k(Hz) then switch to 10k(Hz)?

Can you post an extract from the calibration procedure and perhaps a block diagram please.

barrymagrec · 5th Nov 2018, 1:51 pm

It`s not entirely obvious what you are trying to do but assuming you are trying to set the bias there are a number of possibilities, there is insufficient bias level available to achieve the required overbias level, your tape requireS more bias than the machine is capable of or you are recording at too high a level to get a sensible result, are three possibilities that spring to mind.

NorfolkDaveUK · 5th Nov 2018, 2:10 pm

Quote:

Originally Posted by Station X

What? Input 10k(Hz) then switch to 10k(Hz)?

Thats not what I said bud . You have to do the setting it at 10k , AFTER you have set the input level to 1k@200mv output , you THEN switch the input frequency to 10k leaving the level the same .

Barry I have been over and over and over it bud and its just doing the same thing no matter what I do . One side had nothing at all the other side it just goes up and up with no "hump" . I`m going to assume there is something is up with the osc or record cards . Man I hate A77`s

Sorry I uploaded the wrong pic , I have corrected it

ms660 · 5th Nov 2018, 2:17 pm

Quote:

Originally Posted by NorfolkDaveUK

One side had nothing at all

By that do you mean one of the channels?

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 2:27 pm

Yes if I switch from channel one to channel two on the channel selector , I get BARELY nothing from the output on channel one .

NorfolkDaveUK · 5th Nov 2018, 2:32 pm

Here`s how its set up

Inputting 10k @ -20db bridged input using crock clip leades.
Reading the output using a scope on the output .
Settings as per the pic I uploaded .

ms660 · 5th Nov 2018, 3:05 pm

He's trying to set the bias level so far as I can make out, there are four adjustments, two for each channel for the two lower speeds (3.75 ips & 7.5 ips) The 4db down after peak response comes from the figures given in table 6.5-69 in the manual which isn't shown on the page he posted, table 6.5-69 is on the next page.

Lawrence.

Michael Maurice · 5th Nov 2018, 3:11 pm

I know what Norfolk Day is trying to do, but cannot for the life of me work out how he’s trying to do it.

When you are posting things such as 10K can you tell me are they referring to frequency (Hz) or decibles (dB)

It seems I’m not the only one who cant work out what you are saying.

NorfolkDaveUK · 5th Nov 2018, 3:32 pm

Quote:

Originally Posted by Michael Maurice

I know what Norfolk Day is trying to do, but cannot for the life of me work out how he’s trying to do it.

When you are posting things such as 10K can you tell me are they referring to frequency (Hz) or decibles (dB)

It seems I’m not the only one who cant work out what you are saying.

10k decibels

oh boy I dont think inputting 10,000 dB would help it

ms660 · 5th Nov 2018, 3:46 pm

N Dave is basically saying what the manual says for the bias set up, first set the output at the output terminal of the deck to 200mV by adjusting the output level of the sig gen (set to 1kHz) If all's well in that department the output from the sig gen should be 3 to 4mV to get the required 200mV output at the output terminal, then switch to record and the bias adjustment(s) are then done with the input frequency now set to 10kHz with the output level of the sig gen remaining unchanged at 3 to 4mV.

The 4db figure N Dave was referring to is the operating point shown in Fig. 6.5-69 in the page he posted eg: 4db down at a point following the peak response.

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 3:47 pm

Correct . but in my case there is no peak , it just goes up and up until the trimmer maxes out .

NorfolkDaveUK · 5th Nov 2018, 4:06 pm

As Barry said earlier , it might be that the input it too hot . Measuring 3-4mV on my setup is difficult at best. my sig gen is a black star so it doesnt have a read of for the amplitude , I will be getting a better one before too much longer but this has to do for now . it does send out a good signal though so thats the main thing

ms660 · 5th Nov 2018, 4:22 pm

If the input is too hot the output will be too hot also, you should be able to check the input level with a scope, remember that AC voltages are always RMS unless otherwise stated.

Lawrence.

Station X · 5th Nov 2018, 4:26 pm

Thanks for that clear and concise explanation Lawrence. Including units as well as multipliers is helpful. When I see 'k' on its own I immediately think of resistors, not frequencies.

Quote:

Originally Posted by ms660

N Dave is basically saying what the manual says for the bias set up, first set the output at the output terminal of the deck to 200mV by adjusting the output level of the sig gen (set to 1kHz) If all's well in that department the output from the sig gen should be 3 to 4mV to get the required 200mV output at the output terminal, then switch to record and the bias adjustment(s) are then done with the input frequency now set to 10kHz with the output level of the sig gen remaining unchanged at 3 to 4mV.

The 4db figure N Dave was referring to is the operating point shown in Fig. 6.5-69 in the page he posted eg: 4db down at a point following the peak response.

Lawrence.

ms660 · 5th Nov 2018, 5:01 pm

Do you get the same bias adjustment problem on both channels?

Lawrence.

NorfolkDaveUK · 5th Nov 2018, 5:05 pm

Its ok I got it ,a cuppa coffee sorted it . Brain fart as usual . lol

As usual walking away swearing , drinking a coffee and coming back sorted it out . I was just measuing it wrong . My scope likes to do its own thing and you have to keep an eye one it or it will "auto" into something you dont want . Both sides now done .

On wards.

Radio Wrangler · 5th Nov 2018, 5:08 pm

As what is being looked-for is a result of the non-liner behaviour of the tape at highi-sh levels and different frequencies, then what you see will be level dependent. You really do have to run the audio levels at what is in the service manual to get the manual#s adjustment procedures to work.

Additionally, since the A77 was developed, some new formulations of tape have come along demanding ever higher pias levels. You may simply have run out of adjustment range, but you won't know for sure unless you've tried their procedure at the stipulated levels.

David

Radio Wrangler · 5th Nov 2018, 5:10 pm

Auto functions on scopes are best turned off. You need to drive it yourself to be sure of what you're seeing.

David

barrymagrec · 5th Nov 2018, 5:19 pm

What David says. Around the time the A77 was released there were some very significant developments in tape technology and most of the new types required much higher bias than the older types such as EMI H77. Many machines were found wanting both in bias and erase capability as well as the higher record and replay levels required to take advantage of the newer tapes, from memory (very fallible) I think the original A77 was marginal in at least some of these areas.

5th Nov 2018, 1:13 pm	#1
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Revox A77 calibration. Can anyone help me with this darn thing I am driving myself up a pole . I'm trying to calibrate the high frequency watsit via the osc card it says input 10k after setting it up so there is 1k @200mv then switch to 10k, I've done that, then then in record mode, adjust the trimmers on the osc so that it hits a peak and then (for my chosen tape) go 4db over the hump. However when I try and do it there is no hump it just goes up and up . Anyone know why this could be and how to resolve it? I need a lay down ..haha

5th Nov 2018, 1:51 pm	#3
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,558	Re: Revox A77 calibration. It`s not entirely obvious what you are trying to do but assuming you are trying to set the bias there are a number of possibilities, there is insufficient bias level available to achieve the required overbias level, your tape requireS more bias than the machine is capable of or you are recording at too high a level to get a sensible result, are three possibilities that spring to mind.

5th Nov 2018, 2:27 pm	#6
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Yes if I switch from channel one to channel two on the channel selector , I get BARELY nothing from the output on channel one .

5th Nov 2018, 2:32 pm	#7
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Here`s how its set up Inputting 10k @ -20db bridged input using crock clip leades. Reading the output using a scope on the output . Settings as per the pic I uploaded .

5th Nov 2018, 3:05 pm	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. He's trying to set the bias level so far as I can make out, there are four adjustments, two for each channel for the two lower speeds (3.75 ips & 7.5 ips) The 4db down after peak response comes from the figures given in table 6.5-69 in the manual which isn't shown on the page he posted, table 6.5-69 is on the next page. Lawrence. Last edited by ms660; 5th Nov 2018 at 3:11 pm. Reason: clarifying

5th Nov 2018, 3:11 pm	#9
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Revox A77 calibration. I know what Norfolk Day is trying to do, but cannot for the life of me work out how he’s trying to do it. When you are posting things such as 10K can you tell me are they referring to frequency (Hz) or decibles (dB) It seems I’m not the only one who cant work out what you are saying. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

5th Nov 2018, 3:46 pm	#11
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. N Dave is basically saying what the manual says for the bias set up, first set the output at the output terminal of the deck to 200mV by adjusting the output level of the sig gen (set to 1kHz) If all's well in that department the output from the sig gen should be 3 to 4mV to get the required 200mV output at the output terminal, then switch to record and the bias adjustment(s) are then done with the input frequency now set to 10kHz with the output level of the sig gen remaining unchanged at 3 to 4mV. The 4db figure N Dave was referring to is the operating point shown in Fig. 6.5-69 in the page he posted eg: 4db down at a point following the peak response. Lawrence. Last edited by ms660; 5th Nov 2018 at 3:54 pm.

5th Nov 2018, 3:47 pm	#12
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Correct . but in my case there is no peak , it just goes up and up until the trimmer maxes out .

5th Nov 2018, 4:06 pm	#13
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. As Barry said earlier , it might be that the input it too hot . Measuring 3-4mV on my setup is difficult at best. my sig gen is a black star so it doesnt have a read of for the amplitude , I will be getting a better one before too much longer but this has to do for now . it does send out a good signal though so thats the main thing Last edited by NorfolkDaveUK; 5th Nov 2018 at 4:26 pm.

5th Nov 2018, 4:22 pm	#14
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. If the input is too hot the output will be too hot also, you should be able to check the input level with a scope, remember that AC voltages are always RMS unless otherwise stated. Lawrence.

5th Nov 2018, 5:01 pm	#16
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 calibration. Do you get the same bias adjustment problem on both channels? Lawrence.

5th Nov 2018, 5:05 pm	#17
NorfolkDaveUK Banned Join Date: May 2018 Location: Norwich, Norfolk, UK. Posts: 605	Re: Revox A77 calibration. Its ok I got it ,a cuppa coffee sorted it . Brain fart as usual . lol As usual walking away swearing , drinking a coffee and coming back sorted it out . I was just measuing it wrong . My scope likes to do its own thing and you have to keep an eye one it or it will "auto" into something you dont want . Both sides now done . On wards.

5th Nov 2018, 5:08 pm	#18
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,877	Re: Revox A77 calibration. As what is being looked-for is a result of the non-liner behaviour of the tape at highi-sh levels and different frequencies, then what you see will be level dependent. You really do have to run the audio levels at what is in the service manual to get the manual#s adjustment procedures to work. Additionally, since the A77 was developed, some new formulations of tape have come along demanding ever higher pias levels. You may simply have run out of adjustment range, but you won't know for sure unless you've tried their procedure at the stipulated levels. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Nov 2018, 5:10 pm	#19
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,877	Re: Revox A77 calibration. Auto functions on scopes are best turned off. You need to drive it yourself to be sure of what you're seeing. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Nov 2018, 5:19 pm	#20
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,558	Re: Revox A77 calibration. What David says. Around the time the A77 was released there were some very significant developments in tape technology and most of the new types required much higher bias than the older types such as EMI H77. Many machines were found wanting both in bias and erase capability as well as the higher record and replay levels required to take advantage of the newer tapes, from memory (very fallible) I think the original A77 was marginal in at least some of these areas.