gpapag said:
Hi George,
Finished restoring mine about six months ago, have worked on quite a few over the years. Mine required head replacement due to a manufacturing defect in the playback head, so I converted to half track stereo. It does not have very high hours, but most of these machines are now ending up in landfills, and IMO the stock machine doesn't really cut it for use as anything other than an "effect" or casual background listening. I want to use if for what the designers intended - to record music, so I felt compelled to modify it.
I'm older than this machine and find like me it benefited from some updating, I've been working on them since I was 17 and don't revere this machine, (or much else other than a Buggatti, a vintage Offy L4 OHC racing engine or early Corvette) it's a good, usable machine and IMO worth updating for everyday use. I use it with high level digital sources and needed it to be compatible with them, so I disconnected the microphone amplifier which I will never, ever use. I may record live using this machine, but will use modern mics and a home brew mic amplifier.
I have deleted the spkr amplifier which generates a lot of heat, cooking the insides of the deck. I may use the additional power available for improvements to the record and playback amplifiers.. Horrors...
Truetone said:
Hi Martin,
Is this an OEM replacement shaft? IMO you are best going to the ReVox parts dealer in TN: http://www.jmtecharts.com/ JM Technical Arts is the place to go for original parts. I haven't had to replace a shaft so unfortunately I cannot answer this question, but I'll think about it further. AFAIK there is no provision for shimming - the OEM parts were quite precise.
Thanks Kevin, Yes, I bought the capstan shaft from JMTech. I think it is original - it cost arm and leg. I guess I should swap back to the old one and see if I have lost something. Or measure them side by side. I was a bit disturbed to read that shafts are supposed to be matched to bushings, but mine fitted OK with no apparent stiffnes or side play. I wish I had saved the money actually, because the old one cleaned up OK, and I dont really mind if pitch not absolute as I mean to record and play back on same machine.
Any advice on grounding? Should one make up a chassis ground if recording from a mic only, with no output conected?
I notice my Otari MX-55N has a ground terminal on the back. I presume this is for studio installations in case of hum. The Otari has a mains earth I think.
martin
Any advice on grounding? Should one make up a chassis ground if recording from a mic only, with no output conected?
I notice my Otari MX-55N has a ground terminal on the back. I presume this is for studio installations in case of hum. The Otari has a mains earth I think.
martin
Truetone said:
Hi Martin,
When you installed the replacement shaft was it just the shaft or the entire rotating assembly? Any removable parts not existing on the new one should be transferred from the old one, unless noted otherwise. And of course check end play with the old assembly. You might have missed a spacer or two??
Safety grounding is always a good, IMO, but this deck was not really designed with an external safety ground in mind, and without sufficient care ground loops could be the result.
If you are thinking that safety grounding through an RCA cable in normal use is sufficient for a safety ground that is a very UNSAFE assumption.
RCA cables and connectors will melt in short order if mains current flows through them, and may do terrible things to the component it's connected to. (Or YOU!) The original design employs double insulation, but it is not a bad idea to measure leakage current to be safe. You can add a safety ground to this machine - try a direct connection first, next if hum is a problem the most practical approach I suspect would be a 2.2 - 4.7 ohm 5W resistor in parallel with a pair of anti-parallel >3A/600PIV silicon power diodes with one end connected to an IEC mains socket ground terminal and the other end to the chassis. This will break any loops through the AC gnd while providing some protection against faults. (Be aware that this is NOT as safe as a solid ground to chassis, and I can take no responsibility for the advice just given, use at your OWN sole risk.) When recording with microphones it is always best to make sure no potential difference exists between any component a performer can touch (or could be expected to touch, no matter how unlikely) to prevent electrocution hazards.
Plugging this deck into a ground fault interrupter would not be a bad idea when recording in public venues - adds an extra layer of safety in that fault currents will result in it tripping quickly before anyone gets hurt.
MKIII types made specifically for the US market often had a dedicated 117V transformer, and the only place mains appears is on the primary - IMHO much safer than on the European/International version where the motors and phase shift caps are connected directly to taps on the primary winding. (Mine is a 117V only unit)
Low Freq Hum?
Hi,
A short introduction, my name is Marc, make strange electronic music, originally from Singapore, now living in the Netherlands (for not much longer).
I recently aquired a G36, it unit came completely beaten to death in the mail, the whole machine was broken off from the frame... Yes painful when I opened the package...
Well, everything still seems to work, but I realized there is a very low humming noise coming out of both carthod outputs, its louder on the right channel at around -40db. I have changed all the tubes thinking it might be that, but the noise stays. I am wondering where this is coming from and if its a ground issue, why is it higher on 1 channel?
Please all advise and direction will be greatly appreciate, I really know nothing about tape machines and electronics...
Regards.
Hi,
A short introduction, my name is Marc, make strange electronic music, originally from Singapore, now living in the Netherlands (for not much longer).
I recently aquired a G36, it unit came completely beaten to death in the mail, the whole machine was broken off from the frame... Yes painful when I opened the package...
Well, everything still seems to work, but I realized there is a very low humming noise coming out of both carthod outputs, its louder on the right channel at around -40db. I have changed all the tubes thinking it might be that, but the noise stays. I am wondering where this is coming from and if its a ground issue, why is it higher on 1 channel?
Please all advise and direction will be greatly appreciate, I really know nothing about tape machines and electronics...
Regards.
Steerpike is quite correct with regard to the hum shielding, that said I am running without the shield (it disintegrated) in a system with high efficiency speakers with bass response down to 30Hz without significant levels of hum.
You mentioned the deck was badly damaged in shipping. I am not sure I understand your description of the damage. Do you mean that the chassis separated from the case?? Or something else - if so this is a very good indication that the machine was improperly packed or dropped a great distance. (2m or more)
These machines have a few electrolytics in the PSU which do eventually need replacement and more to the point are the selenium bridge rectifiers which in most cases MUST be replaced. (Low hour machines are a risky exception.)
The deck should not be placed close to anything that generates magnetic fields - mine is more than 1.5m from the rack with the rest of the electronics..
Another possibility is a bad cathode follower tube, or the dc filament supply that feeds the first amplifier tube in the replay and recording amplifiers may have excessive ripple.
I pretty much prefer the sound of my G36 MKIII to my Otari studio mastering deck, and it gets a lot more use. Its tape handling is not too sophisticated, but I have not had any problems with tape damage even with 3600' 10" reels..
You mentioned the deck was badly damaged in shipping. I am not sure I understand your description of the damage. Do you mean that the chassis separated from the case?? Or something else - if so this is a very good indication that the machine was improperly packed or dropped a great distance. (2m or more)
These machines have a few electrolytics in the PSU which do eventually need replacement and more to the point are the selenium bridge rectifiers which in most cases MUST be replaced. (Low hour machines are a risky exception.)
The deck should not be placed close to anything that generates magnetic fields - mine is more than 1.5m from the rack with the rest of the electronics..
Another possibility is a bad cathode follower tube, or the dc filament supply that feeds the first amplifier tube in the replay and recording amplifiers may have excessive ripple.
I pretty much prefer the sound of my G36 MKIII to my Otari studio mastering deck, and it gets a lot more use. Its tape handling is not too sophisticated, but I have not had any problems with tape damage even with 3600' 10" reels..
Hi Kevin,
it definitely took a huge hit, all the styrofoam was broken, there are big styrofoam pieces inside the guts of the thing when I took it out of the box. I don't drive and don't know any techs around to check the unit. I'll see once the parts arrive if the hum goes away, but you're suggesting I need to change some other things as well? I'm really new to this, so forgive the ignorance, and you'll have to explain in greater detail.
it definitely took a huge hit, all the styrofoam was broken, there are big styrofoam pieces inside the guts of the thing when I took it out of the box. I don't drive and don't know any techs around to check the unit. I'll see once the parts arrive if the hum goes away, but you're suggesting I need to change some other things as well? I'm really new to this, so forgive the ignorance, and you'll have to explain in greater detail.
replace both 50uf x3 cans with new electrolytics
then audible hum should be tamed. my unit had similar damage and the aluminum side frames broke too. however they have nothing to do with grounding but are only intended for mounting the unit in the revox wood/plastic box.
the aluminum frames are very hard to find... managed to find one good one but never had luck to find another to make a pair. in the end i made a pair of frames out of birch ply...
then audible hum should be tamed. my unit had similar damage and the aluminum side frames broke too. however they have nothing to do with grounding but are only intended for mounting the unit in the revox wood/plastic box.
the aluminum frames are very hard to find... managed to find one good one but never had luck to find another to make a pair. in the end i made a pair of frames out of birch ply...
Hi Kevin/All
I have put the G36 back into the replacement frame that arrived, the ground hum is still evident and nothing has changed...
Today the 'G36 spares' I bought arrived, they are almost 2 complete set of G32s (1 Mk2, 1 mk3)! But yes, both don't seem to be working and seem to have been badly stored for many decades. I plugged one of them in (the other had the lead cut off), it lit up for a second then the fuse blew (well I hope its only the fuse!), inside, there is melted rubber and what every else you'd expect. That said though, it still had all the tubes intact, and all the electronics seem to be also untouched.
So, as for the hum, anymore ideas? And as for the parts, please advise, I wonder if its worth the time and money to get either (or both!) of the broken G36s working again? Let me know what to check for, what to look out for, and what other options are available, I heard of using the electronics as a mic pre, though I'd really hate to see this machine bite the dust...
Regards.
I have put the G36 back into the replacement frame that arrived, the ground hum is still evident and nothing has changed...
Today the 'G36 spares' I bought arrived, they are almost 2 complete set of G32s (1 Mk2, 1 mk3)! But yes, both don't seem to be working and seem to have been badly stored for many decades. I plugged one of them in (the other had the lead cut off), it lit up for a second then the fuse blew (well I hope its only the fuse!), inside, there is melted rubber and what every else you'd expect. That said though, it still had all the tubes intact, and all the electronics seem to be also untouched.
So, as for the hum, anymore ideas? And as for the parts, please advise, I wonder if its worth the time and money to get either (or both!) of the broken G36s working again? Let me know what to check for, what to look out for, and what other options are available, I heard of using the electronics as a mic pre, though I'd really hate to see this machine bite the dust...
Regards.
Hi honinbou,
thanks for chiming in. I am really really new to DIY-ing audio hardware, so please point out more specifically where I can find these parts you're referring to (I have the service manual, so use that to reference maybe?), and what value to change them to?
I won't keep your hopes up, but if I do decide not to do anything with the other frame I acquired, I could send it to you.
Regards.
thanks for chiming in. I am really really new to DIY-ing audio hardware, so please point out more specifically where I can find these parts you're referring to (I have the service manual, so use that to reference maybe?), and what value to change them to?
I won't keep your hopes up, but if I do decide not to do anything with the other frame I acquired, I could send it to you.
Regards.
Steerpike said:
There is a magnetic shield that comes into play when the capstan and pinch roller are engaged - that said some level of hum is normal, but it should be rather unobtrusive in practice.
It is also quite likely that the front end tube in the tape amplifier is bad, you might be able to pull an ECC83 from another one of your machines and give that a try.
Finally if the filter capacitors are bad you will have plenty of noise.
You mentioned that the record electronics seemed to be relatively quiet, but the only way to know this for sure is to actually record, and monitor off tape - the monitor path takes signal from relatively high level points in the circuitry and attenuates it to drive the amplifier.
Lots of parts available from JM Technical Arts in Old Hickory TN.. See earlier posts for links.
Hi Marc,
You mentioned changing the tubes but that solved nothing. Hopefully the following would help you find the problem...
With a VTVM, signal generator, and test tape find out the gain of each stage (both recording and playback sections), then compare those with values printed on the schematics. That should help you localize the problem to one or two preamp sections.
If you only have a multimeter then you could try the following (the more difficult way):
With a DMM measure the B+ coming right out of the two 50uf x 3 cans. Voltages at points A, B, C and D (see schematic) should measure something around 265, 220, 190, and 182 respectively. If you don't get these values then the resistors across each cap section could go high, or the two can capacitors (see p.52 diagram F-01) are near their end of their lifespan, or the selenium rectifier is bad.
If voltages are all good then proceed to checking all plate resistors (ECC8X pins 1 and 6; make sure the resistances are within acceptable tolerances.
Also make sure that for V1 and V10 DC filament voltages between pins 4 and 5 do indeed measure around 12VDC.
You mentioned changing the tubes but that solved nothing. Hopefully the following would help you find the problem...
With a VTVM, signal generator, and test tape find out the gain of each stage (both recording and playback sections), then compare those with values printed on the schematics. That should help you localize the problem to one or two preamp sections.
If you only have a multimeter then you could try the following (the more difficult way):
With a DMM measure the B+ coming right out of the two 50uf x 3 cans. Voltages at points A, B, C and D (see schematic) should measure something around 265, 220, 190, and 182 respectively. If you don't get these values then the resistors across each cap section could go high, or the two can capacitors (see p.52 diagram F-01) are near their end of their lifespan, or the selenium rectifier is bad.
If voltages are all good then proceed to checking all plate resistors (ECC8X pins 1 and 6; make sure the resistances are within acceptable tolerances.
Also make sure that for V1 and V10 DC filament voltages between pins 4 and 5 do indeed measure around 12VDC.
Thank you everyone for chiming in, so strangely today, when I powered up the G36 and connected everything after not having used it in a couple of days, the hum disappeared! I don't know why, what happened, but now the ground hum, though still there, registers at -60 db, a pretty workable level. That said, a new problem has arised!
When placed in an upright position, reels facing front, the machine won't play, the transport seems to be underpowered or something, then when I loosen both reels and the tape tension mechanism kicks in, then it starts to play. Seems pretty strange no this behavior?
Also, I ordered a wrong tube, I didn't realize that tube 12 is an ECC 82, I am using an ECC83 instead. What exactly does V12 do? What difference does using an 82 in place an 83 make in the global picture? Everything seems to be working ok minus the transport and there seems to be a constant noise of something rotating internally.
Again, please forgive me for these ignorant questions, I really don't have any background in tape machines nor electronics...
Thanks again for previous replies, and thanks in advance for whats to come
Regards.
When placed in an upright position, reels facing front, the machine won't play, the transport seems to be underpowered or something, then when I loosen both reels and the tape tension mechanism kicks in, then it starts to play. Seems pretty strange no this behavior?
Also, I ordered a wrong tube, I didn't realize that tube 12 is an ECC 82, I am using an ECC83 instead. What exactly does V12 do? What difference does using an 82 in place an 83 make in the global picture? Everything seems to be working ok minus the transport and there seems to be a constant noise of something rotating internally.
Again, please forgive me for these ignorant questions, I really don't have any background in tape machines nor electronics...
Thanks again for previous replies, and thanks in advance for whats to come
Regards.
V12 is the recording bias oscillator and must be an ECC82- an ECC83 will not work properly due to the very different rp, etc.
No your deck will never be completely silent, but it can be extremely quiet, mine as I have indicated has audible hum which is pretty unobtrusive even without music playing. You still have plenty of work ahead of you..
The mechanical noise you hear is the capstan motor which runs continuously when the power is turned on.
Do you have the schematics and service manual for the deck?
The information is all available here:
http://www.johnmcculloch.net/documents.aspx
No your deck will never be completely silent, but it can be extremely quiet, mine as I have indicated has audible hum which is pretty unobtrusive even without music playing. You still have plenty of work ahead of you..
The mechanical noise you hear is the capstan motor which runs continuously when the power is turned on.
Do you have the schematics and service manual for the deck?
The information is all available here:
http://www.johnmcculloch.net/documents.aspx
Ok, it seems to be pretty constant, the hum has stayed at -60db, pretty workable now. Thanks for those links Kevin, I've already seen them before, but its still very technical for me. Today I tried adjusting the VU meters and the Bias.
I was using Pure Data on the computer as a signal generator sending -6db of different frequencies. How do you all normally adjust your VU ballistics? In the service manual, everything is again cryptic to me, I basically set digital -6 to the VU 0.
I then record and played back immediately the sine tones and adjusted the bias until it also gave me the same levels as what was going in. Probably the wrong way of doing it, but without any test equipment, I thought this might be the way forth. Then I started sending all different frequencies from 100 - 20000 hz, everything was pretty even on both channels until I reached about 10k, the right channel is suddenly 5 dbs lower than the left, and only at 15 did both the channels level out again. What is the reason for this behavior? Do I need to physically adjust the heads as maybe its not aligned right?
Also, the transport is still not working properly when I stand the machine up, it only works properly (well almost properly) when laid on its back.
Regards.
I was using Pure Data on the computer as a signal generator sending -6db of different frequencies. How do you all normally adjust your VU ballistics? In the service manual, everything is again cryptic to me, I basically set digital -6 to the VU 0.
I then record and played back immediately the sine tones and adjusted the bias until it also gave me the same levels as what was going in. Probably the wrong way of doing it, but without any test equipment, I thought this might be the way forth. Then I started sending all different frequencies from 100 - 20000 hz, everything was pretty even on both channels until I reached about 10k, the right channel is suddenly 5 dbs lower than the left, and only at 15 did both the channels level out again. What is the reason for this behavior? Do I need to physically adjust the heads as maybe its not aligned right?
Also, the transport is still not working properly when I stand the machine up, it only works properly (well almost properly) when laid on its back.
Regards.