Revox A77 MK III Hum at Line Out

Status
Not open for further replies.
Jump to Latest
N

Nanaimo

Member
Joined 2014
#1
Old Yesterday, 06:31 PM #11
Willi Studer is offline Willi Studer Serbia
diyAudio Member

Join Date: Jan 2008
Quote:
Originally Posted by Nanaimo View Post
I have a problem with my Revox A77. There is Hum-m-m-m from the Line Out (L) CH1. The (R) CH2 is OK. I have the "Service and Maintenance Manual" The S/N is 56786.
Anyone care to venture a suggestion?? Michael (UK)
This could be a faulty or dirty potentiometer or switch. You might also have ground or decoupling problem in repro amplifier. My guess is that one of tantalum capacitors has gone bad. A77 has dual mono sound topology and is modular, so it is fairly easy to play with, provided that you have knowledge and experience in electronics.
Report Post Reply With Quote Multi-Quote This Message Quick reply to this message
Old Yesterday, 08:35 PM #12
Nanaimo is offline Nanaimo United Kingdom
diyAudio Member

Join Date: Oct 2014
Quote:
Originally Posted by cbdb View Post
You can get tape? Including alignment tapes?
I have a Reel to Reel Azimuth Alignment Test Tape from Metrosound Audio Products Ltd., Cartersfield Road Waltham Abbey, Essex U.K. Don't know if they're still around though. NAB 1/4" is still available - but where it used to be £12 is now £45. give or take. Google it. Michael (UK)
Report Post Reply With Quote Multi-Quote This Message Quick reply to this message
Old Yesterday, 08:53 PM #13
Nanaimo is offline Nanaimo United Kingdom
diyAudio Member

Join Date: Oct 2014
Default Revox A77 hum.
Quote:
Originally Posted by Willi Studer View Post
This could be a faulty or dirty potentiometer or switch. You might also have ground or decoupling problem in repro amplifier. My guess is that one of tantalum capacitors has gone bad. A77 has dual mono sound topology and is modular, so it is fairly easy to play with, provided that you have knowledge and experience in electronics.
Thanks, Willi, I have a certain amount of knowledge though maybe not enough without a helping hand (or three!) So far I have the case off, a couple of long banana plugs stuffed in the two holes, and am gazing at the amazing rigid lattice construction within - compared to my AKAI GX-635D in which I just replaced all the transistors and electrolytics of the pre-amps. The 'Service and Maintenance Manual' doesn't go into enough detail to guide me as to how to remove the Cct boards - or even which of the seven small Cct. boards is which. Is the Manual I have the manual?
I believe Tantalums are still available? Mouser? May be I should start by spraying the inside of the potentiometers and switches before embarking on Tantalums. If faulty I assume a Tantalum would present a dead short? My DMM is one of the new UNI-T UT61E's
Michael (UK)
Report Post Reply With Quote Multi-Quote This Message Quick reply to this message
Unread Yesterday, 09:45 PM #14
Willi Studer is offline Willi Studer Serbia
diyAudio Member

Join Date: Jan 2008
I hope the topic starter does not mind that we hijacked the thread, otherwise Nanaimo can start a new one. Let's start with a getting the whole picture:
1. Which variant of A77 do you have (MkI to MKIV)? If unsure, look at pictures on Google and see how to determine the model from the front fascia design. This is important because there are some differences in electronics and I can provide you with the corresponding service manual.
2. Describe better the noise you get and when do you get it: is it only in reproduction or also when you select input instead of NAB/IEC and the deck is in stop position?
3. Did you try to clean and demagnetize the heads? Are your heads in proper condition (i.e. not worn out)?

Moving on, try to eliminate/replicate the problem by playing with front panel switches and note the switch/position when there is no noise. For now, I think you should try these and report back.
I would also advise you against spraying inside because you would first need to locate the faulty part. In addition, cleaning the boards before spraying is the way to do. Yes, tantalums are available but my advice is to replace them with good electrolytics for better reliability.
Report Post Reply With Quote Multi-Quote This Message Quick reply to this message

Reply
 
#2
Hello Willi - I hope you have found this new Thread! Here are your questions:
1. Which variant of A77 do you have (MkI to MKIV)? If unsure, look at pictures on Google and see how to determine the model from the front fascia design. This is important because there are some differences in electronics and I can provide you with the corresponding service manual.
And my answers:
It is a MkIII. Although it has Speaker sockets I am not sure whether it has Speaker Amps. I've never tried it! Also the take-up guide roller is 'solid' whereas the delivery guide roller is a ball-race. That might further determine the machine.
2. Describe better the noise you get and when do you get it: is it only in reproduction or also when you select input instead of NAB/IEC and the deck is in stop position?
Answer: A mains Hum-m-m. Speed 7 1/2" or 3 3/4".
a)Settings: MONITOR = CHI. INPUT NAB or IEC = Hum-m-m
Press PLAY = Hum disappears. Release PLAY Hum returns in two stages
b)Settings: MONITOR = CHII = No Humm
Press PLAY = low Humm
c)Settings: MONITOR = CHI, INPUT at 'INPUT' = no Humm, but at full Volume: heavy 'Hissss'
3. Did you try to clean and demagnetize the heads? Are your heads in proper condition (i.e. not worn out)?
Answer: Yes, I cleaned the Heads and Tape Path with ISOCLENE and use a Ferrograph head degausser. Head wear: not noticeable.
I hoped you would negate the use of spray contact cleaner!!
Point taken about the Tantalums. I shall have to wait for your comment to find the values required.
__________________
Michael
 
Last edited:
#3
Hello Nanaimo, yes I have found the thread. You can download the service manual from the following link:
http://we.tl/ggq3GDptHJ
This is a public domain document so it is free to share.

From your description, I suspect you have a faulty playback amp. I would try to swap left and right repro amps and expect the hum to appear on CHII. In this case, replace all electrolytic and tantalum capacitors with corresponding electrolytics of good quality. Take the opportunity to inspect the PCB for cold solder joints or cracked tracks. You will find the disassembly instructions and locations of playback amps in the manual. It is fairly simple: once you free the unit from the wooden case, unscrew the bottom fascia and you will gain access to PCB modules which are plugged in the motherboard.
As for the roller on the takeup reel side, they are always fixed on A/B77. If you have one with bearing, then it was fitted by someone afterwards.
 
#4
Willi Studer said:
Hello Nanaimo, yes I have found the thread. You can download the service manual from the following link:
http://we.tl/ggq3GDptHJ
This is a public domain document so it is free to share.

From your description, I suspect you have a faulty playback amp. I would try to swap left and right repro amps and expect the hum to appear on CHII. In this case, replace all electrolytic and tantalum capacitors with corresponding electrolytics of good quality. Take the opportunity to inspect the PCB for cold solder joints or cracked tracks. You will find the disassembly instructions and locations of playback amps in the manual. It is fairly simple: once you free the unit from the wooden case, unscrew the bottom fascia and you will gain access to PCB modules which are plugged in the motherboard.
As for the roller on the takeup reel side, they are always fixed on A/B77. If you have one with bearing, then it was fitted by someone afterwards.
Click to expand...

Thank you, Willi, for the link to the Real Service Manual. I have printed it out (I work better with a book than direct from a screen) and shall put the swapping suggestion into practice and report back.
_____________
Michael (UK)
 
#5
Revox A77 hum.

Nanaimo said:
Thank you, Willi, for the link to the Real Service Manual. I have printed it out (I work better with a book than direct from a screen) and shall put the swapping suggestion into practice and report back.
_____________
Michael (UK)
Click to expand...
First: Does it matter if I remove and replace either of the Replay Amp Cards with the machine 'ON'??
I have marked both Replay Cards as (I) & (II) according to where I found them.
With Settings at: MON=CHI, NAB or IEC result: Hum-m-m
With Settings at: MON=CHII, NAB or IEC result: No Hum-m-m
Now swap the two Replay Amps over.
With Settings at: MON=CHI, NAB or IEC result: Hum-m-m
With Settings at: MON=CHII, NAB or IEC result: No Hum-m-m
When I took the cards out I applied 'contact cleaner' to card and socket terminals.
_________________
Michael (UK)
 
#7
OK, so I get that the hum remains on CHI regardless of the playback amplifier and that it occurs only in stop position, selector in NAB or IEC. This means that the hum occurs before the mentioned modules or leaks from elsewhere bypassing the circuitry and getting to the line output directly. Would have been easier if the hum were on both channels, though. This means I will have to look deeper into the schematics (that is, if you do not want the advice to break the recorder apart and overhaul every module in it). But there are some things you can do in the meantime:

1. Is the hum 50Hz or 100Hz? This could tell between ground decoupling and power supply issue.
2. Extract all plug-in modules from the motherboard and check for the hum. Alternatively, leave only playback amps in place for the same check. Avoid plugging and unplugging the modules with recorder powered on to avoid damage to electronics and potential noise spikes.
3. Check the 21VDC voltage as per service manual. Also measure the AC ripple of the regulated 21VDC.
4. With playback amplifiers out, measure the resistance of playback head coils - you should get the same or very close value
5. Please make sure that the back panel RCA terminals are soldered well and that your interconnect is OK, too.
 
Last edited:
#9
Query: Was there an 'option' for supplying a MKIII A77 without the two Power Amplifier cards (1.077.850)?? These cards are absent in my A77 yet their socket slots are there on the Power Supply board and Speaker Sockets are wired in to where? Michael (UK)
 
#11
Willi Studer said:
OK, so I get that the hum remains on CHI regardless of the playback amplifier and that it occurs only in stop position, selector in NAB or IEC. This means that the hum occurs before the mentioned modules or leaks from elsewhere bypassing the circuitry and getting to the line output directly. Would have been easier if the hum were on both channels, though. This means I will have to look deeper into the schematics (that is, if you do not want the advice to break the recorder apart and overhaul every module in it). But there are some things you can do in the meantime:

1. Is the hum 50Hz or 100Hz? This could tell between ground decoupling and power supply issue.
2. Extract all plug-in modules from the motherboard and check for the hum. Alternatively, leave only playback amps in place for the same check. Avoid plugging and unplugging the modules with recorder powered on to avoid damage to electronics and potential noise spikes.
3. Check the 21VDC voltage as per service manual. Also measure the AC ripple of the regulated 21VDC.
4. With playback amplifiers out, measure the resistance of playback head coils - you should get the same or very close value
5. Please make sure that the back panel RCA terminals are soldered well and that your interconnect is OK, too.
Click to expand...
Answers:
1. The hum is 50Hz (A1 on my Steinway)
2. Leaving the two Playback Amp cards in situ I extracted the remaining five boards and turned on. The Hum is still there.
3. I am still trying to find where to measure this. The Manual is not helpful as it refers only to colours and not to voltages that I can find . . . .
4. CHI = 94.79 Ohm. CHII = 95.25 Ohm.
5. Have applied hot iron and flux-cored solder to both RCA terminals. OK
Question 3. has puzzled me and the manual is not helping identify that 21VDC rail.
___________________
Michael
 
#12
Question 3. I have found the pinouts for the various plug-in boards and have measured at Playback Amp (pin 9) Record Relay (pin 9) Record Amp (pin 6) Input Amp (pin 6) All measure the same (and slightly rising as there are no Boards plugged in) = 22.93VDC.
Measured with my new UNI-T UT61E DMM. I am not sure whether I have a meter to measure AC ripple. It could be that the UT61E will do this - not sure.
_____________
Michael
 
#13
Good evening Michael,

your DMM can surely measure AC ripple: turn it to 20V AC range and connect to the same test point, then turn the selector to lower range if you read low values.
You can adjust 21VDC at P106 trimpot adjacent to the heatsink on the power supply PCB. Adjust the voltage with all PCBs in place.
Did you try reproducing the hum without playback amplifiers? Since it is 50Hz, it must be coming from the ground. Another possibility is that some of the motor capacitors are leaking but in that case you would have hum on both channels.
RCA terminals on A77 are rather close to each other and do not allow beefy cables/connectors, otherwise they tend to loosen and that may produce hum (seen it on two A77s). So again, check that your interconnect cable is OK, try swapping left and right wire and try fastening the connector better or moving it.
 
#14
Willi Studer said:
Good evening Michael,

your DMM can surely measure AC ripple: turn it to 20V AC range and connect to the same test point, then turn the selector to lower range if you read low values.
You can adjust 21VDC at P106 trimpot adjacent to the heatsink on the power supply PCB. Adjust the voltage with all PCBs in place.
Did you try reproducing the hum without playback amplifiers? Since it is 50Hz, it must be coming from the ground. Another possibility is that some of the motor capacitors are leaking but in that case you would have hum on both channels.
RCA terminals on A77 are rather close to each other and do not allow beefy cables/connectors, otherwise they tend to loosen and that may produce hum (seen it on two A77s). So again, check that your interconnect cable is OK, try swapping left and right wire and try fastening the connector better or moving it.
Click to expand...

Good afternoon, Willi.

Before I go on I have noticed on the Power Supply Board 1.077.540 that two of the connecting cables are reversed. I am looking at the 11 contacts by the two Fuses where ED1 is Red and AD1 is Yellow. They should, according to the photo 'POWER SUPPLY ALIMENTATION' be the other way round and ED1 should be Yellow, AD1 should be Red. This may, of course, be a trivial item - but then it may be important.
There, of course, I found the trimpot P106
I shall wait your comment before going further.
____________________
Michael
 
#16
Nanaimo said:
Good afternoon, Willi.

Before I go on I have noticed on the Power Supply Board 1.077.540 that two of the connecting cables are reversed. I am looking at the 11 contacts by the two Fuses where ED1 is Red and AD1 is Yellow. They should, according to the photo 'POWER SUPPLY ALIMENTATION' be the other way round and ED1 should be Yellow, AD1 should be Red. This may, of course, be a trivial item - but then it may be important.
There, of course, I found the trimpot P106
I shall wait your comment before going further.
____________________
Michael
Click to expand...
However, reading the Cct. Diag.2 both AD1 and ED1 are the same potential so I have corrected their position. In doing so I found the Yellow connector extremely loose and have applied the long-nosed pliers.
There, of course, I found the trimpot P106. I shall therefore go ahead and adjust for 21VDC with all Cct.Bds in place and read the voltage at my newly discovered ED1 and AD1 Test Points! Then measure AC Ripple as you suggest.
As to the RCA terminals . . aren't they awful? And they're so close to the Power socket. Why they were used I can't fathom - but then DIN plocketry is awful as well and my UHER Report 4200 has them. But both those RCA sockets have loose middles - yet the connections are secure and I use small light-weight RCA screened coloured cables.
I will Post my findings.
__________
Michael
 
#17
Nanaimo said:
However, reading the Cct. Diag.2 both AD1 and ED1 are the same potential so I have corrected their position. In doing so I found the Yellow connector extremely loose and have applied the long-nosed pliers.
There, of course, I found the trimpot P106. I shall therefore go ahead and adjust for 21VDC with all Cct.Bds in place and read the voltage at my newly discovered ED1 and AD1 Test Points! Then measure AC Ripple as you suggest.
As to the RCA terminals . . aren't they awful? And they're so close to the Power socket. Why they were used I can't fathom - but then DIN plocketry is awful as well and my UHER Report 4200 has them. But both those RCA sockets have loose middles - yet the connections are secure and I use small light-weight RCA screened coloured cables.
I will Post my findings.
__________
Michael
Click to expand...

Later: Measuring VDC at ED1 or AD1 to GND I have adjusted it from 22.91VDC to 21.00VDC at P106. Thank you for this suggestion.
However the hum-m-m is unchanged. I removed both Playback Amplifiers and the 50cps Hum is on both CHI and CHII and making any changes to vol,NAB etc. makes NO difference. The Hum remains. Reinstalled both Playback Amplifier cards and the Hum is back on CHI/NAB and not on CHII/NAB. Is this where we came in?
Ripple. With UT61E on mV AC measuring at AD1 or ED1 = 19.74mVAC.
______________
Michael
 
#18
Good morning, Willi.
I seem to have come to a dead end here on the Revox A77. I have adjusted the P106 and now have 21VDC. On removing both Play Amplifier cards the Hum is still there on both channels but much lower in volume and changing NAB, VOL or CHI/CHII makes no change in the hum at all. Reinstalling the Play Amplifier cards the hum is back on CHI/NAB/IEC and the VOL controls it. No hum on CHII.
_____________
Michael
 
#20
Test for Hum at Line Out:
Result: Settings:NAB/CHI = Hum. Settings NAB/CHII = No Hum
Change over the two Playback Amplifier Cards.
Result: Settings:NAB/CHI = Hum. Settings NAB/CHII = No Hum
Restore Playback Amplifier Cards to proper places.
Both Tests required VOL Adjust to be Full Vol.
End of Test.
_____________
Michael
 
Last edited:
Status
Not open for further replies.
Top Bottom