Telling the difference between 2-track and 4-track Revox G36??

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Tubetec

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Mornin' Winst ,
Yeah I have seen some of those exact type of caps go bad in precisely the way you describe , invariably if you look closely the pour molded plastic the component was potted in has cracked and allowed in moisture long before it went kaboom .In tapedecks you might see such caps used in RC snubbers networks across the motor windings , a catastrophic fail in that position due to high voltages spikes and currents is always a possibility . I wouldnt doubt for a second going over the top and replacing everything will push the MTBF boat out a little further , but a balance needs to be struck .

Happy freedom day , I see they had to put Boris back in his box though :unsure:
 

Winston OBoogie

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I wouldnt doubt for a second going over the top and replacing everything will push the MTBF boat out a little further , but a balance needs to be struck .

Happy freedom day , I see they had to put Boris back in his box though :unsure:

Mornin' David :)
Yeah I agree, a balanced approach is probably the right way.
However, the G36 is not like your Studer C37 (I remember reading you had one once anyway... So did I but sadly had to flog it about 25years ago :( ) and getting a G36 apart for service isn't as easy as lifting the bonnet to peek at that beautiful Swiss engine while it's running.
Once you've gone through the hassle of getting a G36 apart, probably best to get all the internal stuff done before you spend hours buttoning it all up again.

Freedom day? They said that we're all now supposed to use common sense regarding masks and distancing. Trouble is, most of the folks I see round here where I'm living have no feckin' common sense... I think I'll stay in my own private little, self imposed exile.

I can't watch Boris or any of the other clowns running this show on the news anymore, reminds me of how not to organise a piss-up in a brewery.

;)
 
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Winston OBoogie

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P.S. I probably should avoid this thread now.
I spent most of last night looking at Revox A77 and G36 circuits and devising various modification schemes, then spent the wee hours scouring online for a suitable 15 IPS machine to buy.

Except I really don't need a machine. Unless it's a freebie, or I get it (or a couple) for dirt cheap like Urskov did.
 

Tubetec

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I actually have two C37's , one i've metculasly restored (around 100 hours work) and with modern tape signifigantly outperforms the original factory spec , the other is mostly complete but in need of work (and some parts, not unobtainium)before its usable again . Both these machines came from RTE(Irish national broadcaster) originally , stereo input modules but mono record and playback, full track 1/4 inch . The one I restored had much less hours on the tach but had an intermittant short on a wire suppling one of the reel motors which RTE's service dept never managed to fix , thus the lower hours usage .

I'll try and resist any further temptation to ridicule BO_JO in serious threads , he does make life easy for his detractors though :)
 

Gold

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I wasn't trying to call anyone out about nomenclature. I just figured the OP didn’t know the difference.

I’ve always wondered if there were standard EQ shapes for Hi and Lo frequency adjustments in tape decks. Is a Studer high frequency adjustment the same shape as an Ampex or Otari? I can’t read schematics well enough to figure it out.
 

abbey road d enfer

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I’ve always wondered if there were standard EQ shapes for Hi and Lo frequency adjustments in tape decks. Is a Studer high frequency adjustment the same shape as an Ampex or Otari? I can’t read schematics well enough to figure it out.
Actually most of the manufacturers use their specific version of a standard circuit. Due to differences in heads, bias the EQ points are slightly different.
A notable exception is Telefunken (M10?), who used a Baxendall-derived HF control in addition to the standard EQ.
Some boutique designers made tape recorder replacement electronics that had EQ for the LF head bump that is particularly annoying when running tape at 30 ips.
 

Gold

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Actually most of the manufacturers use their specific version of a standard circuit. Due to differences in heads, bias the EQ points are slightly different.
When you say standard circuit are you referring to the resultant response or both the response and implementation? Allowing for the variations due to head characteristics. Meaning the EQ could be active or passive as long as the shape is right?
 

abbey road d enfer

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When you say standard circuit are you referring to the resultant response or both the response and implementation?
The typical part that is adjustable is an RC series in the NFB path, resulting in a shelf that varies both in amplitbe and turnover frequency.
Meaning the EQ could be active or passive as long as the shape is right?
I've seen it more active (FB) than passive.
 

Urskov

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Here's an article from the very first issue of Glass Audio in 1988. It has a few useful tips on maintaining the G36 machine.

One caveat with the article is that I don't think it's quite correct regarding converting a machine to high speed (7.5 IPS & 15 IPS).
You do need to change the capstan's size as stated, but the HS modification also requires that the motor be repositioned by a couple+ of mm to ensure the machine has the correct tape path.

Anyway, plenty of sites online (tapeheads for one) where you'll find folks who've maintained and modified these.

Have fun :)
Thanks for the article! Lots of useful information there. I'm just starting to learn the "anatomy" of reel-to-reels.
Compared to most other domestic tube era tape decks the G36 is arms and legs ahead . It even has a proper valve regulated HT supply which you normally only saw in pro level decks back in the day . If these machines have had an easy life I wouldnt go mad replacing everything and anything just because you can . First go over the machine , replace anything that has obviously gone bad . Willi Studer used the best components he could back then and the vast majority of these will still be in good condition , including Elco's (electrolytics) . Fortunately also Studer/Revox service documents are beautifully written and easy to follow . Well worth keeping the second 1/4 track machine also , it may be a relatively simple job to get the first machine up and running with new off the shelf parts so avoid using the second machine as a donor if you can and you could well end up with two fully working units in the end .
Yeah, knowing how revered Studer is among engineers and producers, I knew these bad boys were supposed to be great units, even though it's a domestic unit. My mastering engineer friend has a Studer A80 that's just beautiful.
I will definitely take your advice on just starting by replacing what's gone bad, but at this points if the caps are original... I'm more of into Winston's line of thinking, haha
 

Winston OBoogie

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. Meaning the EQ could be active or passive as long as the shape is right?

I've seen a couple of examples where guys have modified high end phono pre-amp circuits that were utilising passive eq for tape head play back use. They seemed to like what they were using.

I'm not an expert or die-hard tape guy by any means, but my feeling is that the record electronics might be a more interesting or possibly advantageous place to experiment with passive eq.
Main reason being, most amplifiers I see in machines utilise circuitry that already has diminished levels of feedback at higher frequencies due to a somewhat more primitive compensation scheme.
Once you then eat into that diminished feedback at the top end with the high end boost needed for record eq,, I could see there easily being a measurable and audible difference compared to a passive eq sandwiched between a couple of 2 pole compensated or similar amps that maintained the same levels of feedback up at 10 - 20 kHz as it did at 1 kHz.
 

Gold

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I was more curious whether the EQ curves were somewhat standard between different manufacturers.

I am batting around the idea of custom playback electronics for the A80 VU’s I have. After years of listening I think the weak spot in the A80VU’s is the cable run from heads to electronics. I’m convinced it’s low level RF/EMI I’m hearing. I want to put a preamp in the headblock and send balanced quasi line level to the rest of the electronics. I think that will help. That’s the part I proposed to Wayne Kirkwood that eventually became the Phono Transfer System. Putting the balanced front end preamp next to the tonearm makes for a scary quiet setup.
 

Winston OBoogie

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Putting the balanced front end preamp next to the tonearm makes for a scary quiet setup.

I don't have a phono setup now but, absolutely, I could see how that might be a good idea for tape head playback too.

Yes, record and playback EQ curves are standardised. Although I'm sure you experience a fair bit of difference between sources when lining up tapes you get in for vinyl mastering. Thankfully you've got tones on tape ay! :D
 

PermO

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Having all the documentation now, and the great support of this forum I think the G36 rehaul will be my project for this comming winter....

It has 100% all the original parts and is fully functioning... but it deserves some care as I want it to last.
 

abbey road d enfer

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I was more curious whether the EQ curves were somewhat standard between different manufacturers.
The curves are standardized, except there are NAB and CCIR standards. Whatever the mfgr, the head response is identical in the 100Hz-8kHz range. the differences below 100Hz are due to the head's profile and length (length along the tape travel, not the gap height); hyperbolic profile has less LF bump.
Above 8-10kHz the differences are due to gap width and stray capacitance. Since it is known, the mfgr includes the necessary compensations in the circuit.
The user adjustable PB EQ is there to compensate head wear and changes in tape wrap due to mechanical wear and temperature.
I am batting around the idea of custom playback electronics for the A80 VU’s I have. After years of listening I think the weak spot in the A80VU’s is the cable run from heads to electronics. I’m convinced it’s low level RF/EMI I’m hearing. I want to put a preamp in the headblock and send balanced quasi line level to the rest of the electronics.
A few mfgrs have done that. It shouldn't be necessary because the heads are floating, but the wiring may be subject to interference.
I think that will help. That’s the part I proposed to Wayne Kirkwood that eventually became the Phono Transfer System. Putting the balanced front end preamp next to the tonearm makes for a scary quiet setup.
That shouldn't be too difficult since the heads are center-tapped.
 

Gold

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A few mfgrs have done that. It shouldn't be necessary because the heads are floating, but the wiring may be subject to interference.
Including Studer. I also have an A80RC. It sounds better and cleaner. The electronics are directly below the head block. They are the B67 electronics but I think the difference I'm focused on isn't the topology of the electronics but the absence of low level EMI/RF. The A820 put a preamp in the head block. The Telefunken M15 I have has the electronics directly below the head block and sounds fabulous with no hint of "tizziness".
 

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