onemannation said:
I have oodles of test equipment and just basically followed the procedures as outline in the service manual. (Meters and bias adjustment)
In terms of your response error it is more likely that there is something wrong in either the record or playback amplifiers and without proper test equipment it is going to be difficult (probably impossible) to determine.
When you say it will not play in the vertical position is it that the capstan motor does not run at speed or is something else going on?
To repair this machine properly you are going to need some basic test equipment and the understanding to use it. (Good dvm, basic scope and signal generator)
I got my G36 maybe ten years ago. It was then a 4-track MkIII euro stereo machine in pretty shape. But then I also got an ugly hi-speed 2-track and made a transplant of the (new version) heads, motor and eq components. I have been updating it over the years and learning a lot in the process. The service manual has been invaluable to have.
onemannation: when the machine doesn't want to play in the upright position, it means the solenoid that controls the pinch roller is dirty or sticky or just maladjusted. Try playing with the lower plastic shield off, and press the pinch roller arm upwards manually when you press play to see if this is the problem.
Some other things I've noted during the process of restoring that may be helpful to others:
I had some trouble with channel imbalance and other intermittent stuff.
The few electrolytics in the valve section must be swapped first of all, they are small and dried out years ago out.
Changing volyme pots for modern ones made a great difference. Even cheap ones that were new sounded better than the old ones however much I cleaned them. A bit of trouble making the new audio level shafts fit the rods (drilling out like the original ones) but it worked out. I also changed the bias pots.
Some like the old philips mustard caps. After I changed those in the audio path for modern polys the sound became cleaner and the electrical circuit measured closer to specs, so some must have been leaky.
I connected one of the output RCA pairs as an input to the rec level pots to bypass the first stage. This also doubles as an output for the mic amp if I want to connect that to something else for recording through the mic amps!
I have left most of the old carbon resistors that measure ok. I am also still using the stock filter caps, together with the selenium rectifiers. A bit dangerous but as I totally removed the power amp there is less current draw in the HT section so I hope they will last until I get to put new parts in. I make sure to power the machine up every now and then to keep the caps formed.
I did make a small board in the back with a floating 12v DC supply for all the audio ECC tubes. There is'nt really any hum to be heard now even when I crank the sound up a bit.
I got lots of starting information from the great Tolonen web article, like on how to change the bias circuit. It is is around 100kHz after modification instead of 70 as it was at first. I find it plays and record well on Maxell UD and 468 tape and also some 80's BASF broadcast tapes that I have a lot of.
I have used mainly cheap valved vintage test equipment - old Tektronix scope, a HP sine wave generator, a frequency counter and an old distorsion meter all came in useful for restoring on a budget. Old gear seems good enough for valve stuff and usually fits better with the voltages and high impedances around tubes.
The only annoying things are
1) the weak erasure on used tapes and
2) the slightly insufficient startup torque when using fully loaded 10.5 reels.
Wouldn't it be possible to design some sort of improved erase circuit? Solid state would be okay.
The starting torque may in fact be okay, as mine has the improved relayed circuit that should in theory give instantaneous startup - perhaps it is just the spool motor bearings that need an overhaul. I haven't looked into this much. Any experiences here would be appreciated.
onemannation: when the machine doesn't want to play in the upright position, it means the solenoid that controls the pinch roller is dirty or sticky or just maladjusted. Try playing with the lower plastic shield off, and press the pinch roller arm upwards manually when you press play to see if this is the problem.
Some other things I've noted during the process of restoring that may be helpful to others:
I had some trouble with channel imbalance and other intermittent stuff.
The few electrolytics in the valve section must be swapped first of all, they are small and dried out years ago out.
Changing volyme pots for modern ones made a great difference. Even cheap ones that were new sounded better than the old ones however much I cleaned them. A bit of trouble making the new audio level shafts fit the rods (drilling out like the original ones) but it worked out. I also changed the bias pots.
Some like the old philips mustard caps. After I changed those in the audio path for modern polys the sound became cleaner and the electrical circuit measured closer to specs, so some must have been leaky.
I connected one of the output RCA pairs as an input to the rec level pots to bypass the first stage. This also doubles as an output for the mic amp if I want to connect that to something else for recording through the mic amps!
I have left most of the old carbon resistors that measure ok. I am also still using the stock filter caps, together with the selenium rectifiers. A bit dangerous but as I totally removed the power amp there is less current draw in the HT section so I hope they will last until I get to put new parts in. I make sure to power the machine up every now and then to keep the caps formed.
I did make a small board in the back with a floating 12v DC supply for all the audio ECC tubes. There is'nt really any hum to be heard now even when I crank the sound up a bit.
I got lots of starting information from the great Tolonen web article, like on how to change the bias circuit. It is is around 100kHz after modification instead of 70 as it was at first. I find it plays and record well on Maxell UD and 468 tape and also some 80's BASF broadcast tapes that I have a lot of.
I have used mainly cheap valved vintage test equipment - old Tektronix scope, a HP sine wave generator, a frequency counter and an old distorsion meter all came in useful for restoring on a budget. Old gear seems good enough for valve stuff and usually fits better with the voltages and high impedances around tubes.
The only annoying things are
1) the weak erasure on used tapes and
2) the slightly insufficient startup torque when using fully loaded 10.5 reels.
Wouldn't it be possible to design some sort of improved erase circuit? Solid state would be okay.
The starting torque may in fact be okay, as mine has the improved relayed circuit that should in theory give instantaneous startup - perhaps it is just the spool motor bearings that need an overhaul. I haven't looked into this much. Any experiences here would be appreciated.
Replaced Power Supply, Now Over Powered
Hi all,
so I went ahead and changed the 3x (3 x 50u) power supply capacitors and the 3 rectifiers. Now everything seems to be over powered, below is the read out taken from A/B/C/D on the schematic.
A - 284v (supposed to be 265v)
B- 249.5v (220v)
C- 216v (190v)
D- 205.8v (182v)
E- 231v
The machine is functioning, though I have problems with drop outs very often, I have already demagnetized the heads. I don't know what is it that needs to be fixed anymore, and are the readings I'm getting too high to run the machine?
Hi all,
so I went ahead and changed the 3x (3 x 50u) power supply capacitors and the 3 rectifiers. Now everything seems to be over powered, below is the read out taken from A/B/C/D on the schematic.
A - 284v (supposed to be 265v)
B- 249.5v (220v)
C- 216v (190v)
D- 205.8v (182v)
E- 231v
The machine is functioning, though I have problems with drop outs very often, I have already demagnetized the heads. I don't know what is it that needs to be fixed anymore, and are the readings I'm getting too high to run the machine?
hi onemannation,
I assume you are replacing selenium rectifiers with silicon replacements.
you might find this link useful
http://www.radiomuseum.org/forum/telefunken_opus_55ts_selenium_rectifier.html
I assume you are replacing selenium rectifiers with silicon replacements.
you might find this link useful
http://www.radiomuseum.org/forum/telefunken_opus_55ts_selenium_rectifier.html
Gravity.
Hi honinbou
Actually those numbers I recorded were after the initial voltage peak and when it more or less stabilized. Then again I wasn't testing it for 1 and a half minutes. Everything seems to be ok now, the dropping of frames I attribute to gravity pulling on 10.5 Nab reels, since I posted this, I have laid the machine flat on its back and the dropping out ceased. I am tempted to do the Bias cap adjustment mod but would like to know based on the previous post if Erasure will be weaken because of this?
Also more advise on modifying. How do you create a direct monitoring output to Output B, meaning monitoring via line out and not internal speaker? I also plan on removing the whole speaker configuration, where can I find more information on this? And chime in on any other mods that some of your might have done and report the results!
Regards.
Hi honinbou
Actually those numbers I recorded were after the initial voltage peak and when it more or less stabilized. Then again I wasn't testing it for 1 and a half minutes. Everything seems to be ok now, the dropping of frames I attribute to gravity pulling on 10.5 Nab reels, since I posted this, I have laid the machine flat on its back and the dropping out ceased. I am tempted to do the Bias cap adjustment mod but would like to know based on the previous post if Erasure will be weaken because of this?
Also more advise on modifying. How do you create a direct monitoring output to Output B, meaning monitoring via line out and not internal speaker? I also plan on removing the whole speaker configuration, where can I find more information on this? And chime in on any other mods that some of your might have done and report the results!
Regards.
It might have been a supply capacitor. These electrolytics need to be reformed after an idle time of decennia.
Unregular playback might be caused by the tape not running properly against the head. A simple way to find out is giving extra tension by slightly holding down the left reel. If normal playback is restored you have two options: adjustment of the tape tension or optimising the tape path.
In these machines uneven playback is often a symptom of head wear or a dirty head. There is no convenient means to adjust tape tension, however the supply reel motor does generate some back torque to provide tape tension, and with a 7" reel there should be a 100 ohm resistor (check schematic for your version of the machine to be sure) in series with the motor. (The tape tension lever is located on the head block and should be set pointing to the left) Check the take up reel motor dropping resistor for value as ReVox tended to use anything they could get in that location and mine had a dead factory installed 3W resistor in this location - you really need a minimum of 5W here and preferably 10W, mine is ultra conservative at 25W. With 10" - 11" reels the resistor is bypassed - if you have insufficient tension in this case you may have a motor issue.
Do not adjust anything in the tape path before you know what the issue is - adjustments because of the design of the head block are not easily accomplished and without an alignment tape you may actually do more harm than good.
I love my G36 III it's a great deck.
IMO A regulated supply is probably not going to do much to improve noise performance. The ripple on the supplies provided the caps are good is already pretty low. Magnetic fields in the machine probably account for most of the audible hum (not much) anyway.
Do not adjust anything in the tape path before you know what the issue is - adjustments because of the design of the head block are not easily accomplished and without an alignment tape you may actually do more harm than good.
I love my G36 III it's a great deck.
IMO A regulated supply is probably not going to do much to improve noise performance. The ripple on the supplies provided the caps are good is already pretty low. Magnetic fields in the machine probably account for most of the audible hum (not much) anyway.
Last edited:
You need to add a little bit of resistance in series with the new bridge rectifier as it is much more efficient voltage wise than the selenium it replaced.
Drop outs could be tape or head related. Have you checked that the supply reel motor dropping resistor is ok. (Note that it is only used with smaller reels 7" and below.)
Last edited:
Umm, lower oscillator frequency and lowish erase current. These decks have 70kHz oscillators, but may be converted to 100kHz and the power increased. I boosted the erase current in mine when I converted to half track, but have not changed the frequency of the oscillator.
Look here: http://koti.mbnet.fi/siliconf/JukkaTolonen/ga/revox/revox.html for a lot of good information on restoring and modifying the G36 and also look at some of my early posts. (I'm the OP)
Agreed they are lovely old equipment -I own three stereo 36, including G36iii- noise is the second major problem after bandwith. Even though the PS caps may be ok the noise originates for a good part from the PS. Unfortunately time is lacking to look deeper into it, maybe in a couple of months.
I quite like mine, and have a line on another machine. I'm hoping eventually to score one with the rare 15ips option with half track heads.. Probably faint hope of that.. 
Some models unlike the North American only version appear to be easier to get parts for for this particular conversion..
Let me know what you find wrt to the psu, in mine it seems to be mostly 60Hz residual which would be magnetic in nature, hence my prior comment.
Some models unlike the North American only version appear to be easier to get parts for for this particular conversion..Let me know what you find wrt to the psu, in mine it seems to be mostly 60Hz residual which would be magnetic in nature, hence my prior comment.
Perhaps a hum-bucking coil situated somewhere strategic on the chassis could null this?
Older and tube tape recorders often had such coil located in the vicinity of the playback head, but some machines suffer of motor generated magnetic field on the recording side, modulating the field of the heads. One such recorder I know of is the early example (ca 1970) of the Uher Royal de Luxe, later ones had a heavy metal shield around the record and the erase heads in the head block. Recording made with an early Royal de Luxe would record the hum on the tape.
Yes, if you raise the frquency ,than you have to pay attention to re-establish the adequate erase current. Can be a pain on some machines. While adjusting the bias on the four series Nagra machines one constanly have to monitor and readjust the changing oscilator frequency.
Hello folks,
Does anyone tell me what should be a problem in my old G36...
When I press play button or FF/RW buttons - apropriate function work ok...until I hold button.
Once button released - all stops.
Interesting - when machine is not powered, all buttons work properly.
Thank you for your time..
Does anyone tell me what should be a problem in my old G36...
When I press play button or FF/RW buttons - apropriate function work ok...until I hold button.
Once button released - all stops.
Interesting - when machine is not powered, all buttons work properly.
Thank you for your time..
Tommy,
Assuming you have loaded and threaded tape properly through the tape path It sounds like your tape tension sensor is not working properly. Which version of the G36 do you have?
The MKIII has an optical sensor, the earlier models have a mechanical switch which senses the tape tension with a little formed wire thing the tape rides across.. (Not sure how to describe it better.)
Assuming you have loaded and threaded tape properly through the tape path It sounds like your tape tension sensor is not working properly. Which version of the G36 do you have?
The MKIII has an optical sensor, the earlier models have a mechanical switch which senses the tape tension with a little formed wire thing the tape rides across.. (Not sure how to describe it better.)
Revox G36 Head Resistance?
Hi Folks,
Just picked up a pair of working G36s for cheap and I'd like to get them up to spec. Specifically, I'm curious about getting new heads for them. There are a few for sale labeled "For G36 / A77 / B77" but I understand this is inaccurate.
I saw an earlier post on this thread that suggested a head resistance of +/- 400 ohms. Does this sound like it's in the ballpark? (Not sure if that's playback, record, erase.) Any thoughts or advice?
Many thanks,
Seth
Hi Folks,
Just picked up a pair of working G36s for cheap and I'd like to get them up to spec. Specifically, I'm curious about getting new heads for them. There are a few for sale labeled "For G36 / A77 / B77" but I understand this is inaccurate.
I saw an earlier post on this thread that suggested a head resistance of +/- 400 ohms. Does this sound like it's in the ballpark? (Not sure if that's playback, record, erase.) Any thoughts or advice?
Many thanks,
Seth