60hz in my tube revox?

Hello all! Here's one for the re-cappers out there:

I recently acuired a revox g36 mkIII (tubes, 1/4" 2tk, 15ips!). Initial recording tests were very reassuring, however I've noticed that it lays down a little hum (perhaps 60hz?) when recording (not heard when playing back tapes from other machines) so I was wondering after seeing pictures of someone else's mod

http://home.megapass.co.kr/~pcaudio/mod-revox%20g36.htm

is there really anything to be gained from re-capping the part of the power supply that feeds the DC rails to the audio circuits?

Also, the pics on that link show tubes pulled - like the main amps for the internal speaker (which I won't use anyway) In general terms can you just yank 'em and power up without hurting anything? A good power-saving mod there I suppose.

I have the service manual (with schematics) so we're not totally in the dark - the general idea is to shine this one up sensibly as a processing device for mixes. Any input is greatly appreciated. Thanks!

Frank

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Search for the G36-restoring article by Jukka Tolonen..

Jakob E.

> it lays down a little hum (perhaps 60hz?) when recording (not heard when playing back tapes from other machines)

It didn't do that when new.

Don't blindly "recap". Find the problem and fix it. I'd bet on one totally open power cap. A leaky tube is also possible.

Thanks for the replies!

Search for the G36-restoring article by Jukka Tolonen..

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That was a good article - I'm glad it was mentioned in this thread.

I'd bet on one totally open power cap. A leaky tube is also possible.

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Is there a way (with a voltmeter) to check a cap to see if it is "open"?

Also, I suppose I could swap around the identical tubes to see if anything changes.

For some cheap confidence building I'm going to "shotgun" replace the powersupply caps, re-route the I/O so it doesn't pass near much, and clean some contacts. If I can't find the exact value/voltage, should I err on the side of bigger? Original is a 350v 50uF, I'd be going for a 400v 56uF.

Frank

> Original is a 350v 50uF, I'd be going for a 400v 56uF.

Cool.

The hi-fi nut might even go 100 or 200uFd, just for luck. The practical techician might tack-in 10uFd: if things get a LOT better, then the old "50uFd" must really be nearly-no uFd for 10uFd to help. Of course that just proves the "50uFd" has gone lame; for a proper repair the technician would order something like the right size or a little better.

The old-timer way to proceed is to put alligator clips on a 450V 40uFd cap (used to be the most popular size) and shunt it across each suspect cap in turn until the hum changes. This is very dangerous if you are not used to getting bit by high voltage.

But wait. You have no hum on play, or in monitor, only recorded on tape? Focus on the record amplifier, after where the record monitor signal is tapped. Just for fun, check all DC voltages in there: you might find a real-wrong voltage that leads you to the problem. More likely you will have to read the AC ripple in the power rail (difficult) or simply bypass the rail caps with known-good replacements.

If it is isolated to the record amp, it is hard to test. I suppose you could put a tape-loop in and monitor the playback. Or tap right across the record head terminals into a high-impedance amp and speaker (maybe o'scope too). The head signal is weak and severely equalized, but you should be able to hear hum or lack of hum.

Ah.... don't forget the bias oscillator. Hum on it will hum-up the erase and bias and leave hum on the tape. And the bias oscillator may be the most power-hungry stage in the recorder, so a lame power supply might be "clean enough" when osc is off, but sag and ripple when the osc starts sucking power.

Aside from shunting the osc's power rail: try disconnecting the record head and then record on virgin tape. The hiss level may change from erase signal, but it should not add hum. If it does, you know it has to be in the bias/erase oscillator.

If you get bored with that G36 and want to sell it, drop me a line :wink: I'm only a few miles away from you, and although I have a couple of tube 1/4" decks, I don't have one that'll do 15ips.

If you get bored with that G36

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Not bloody likely!!! But if I come across two more I'll keep one for parts and give you the other one. I've read reviews about how the later revox solid state models "surpass" the g36 in every way. I prefer my transport buttons to need at least 5 lbs of pressure to engage. :green:

PRR - thanks for the pointers, I'll start sleuthing in the record amp or bias osc if things don't improve with the new caps. I suppose pulling the power tubes for the internal speaker might save a few joules...

You say it's difficult to measure ripple current... would that be an o-scope job or an ammeter job?

G36 schematic added to Gyraf/Resources page:

http://www.gyraf.dk/schematics/Revox_G36.gif

The Point "E" power supply - after R118 - is a good candidate for having it's 50uF capacitor checked, as this is the only powersupply part that is only active in recording (record head drive and bias oscillator)

Jakob E.

I would AC-couple a scope and connect it to point "E" and set the timebase for a slow sweep. With the machine in playback mode, adjust the vertical sensitivity and sweep rate and try to detect any ripple. There should be very little or none. Then put the machine in record mode. If the amplitude of the ripple shoots up, then examine the filter caps in the "E" supply.

Two other things to check are the cathode resistors on the bias oscillator tube, and the tube itself. If the two side of the push-pull oscillator are severely out of balance, they won't cancel out whatever residual ripple is riding on the supply.

You'd do all this after pinpointing the problem to the bias oscillator as per PRR's suggestions.

I would AC-couple a scope

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New York Dave - I remember you mentioning that you futzed around with scopes before you knew what you were doing - I'm at that futzing stage... I've looked at signal, observed clipping, etc... could you please explain how one AC couples a scope, and if you don't mind also include some do's and dont's for the beginning o-scope user?

You'd do all this after pinpointing the problem to the bias oscillator as per PRR's suggestions.

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Right on - I ordered the parts for the cap replacement today. From there I will proceed scientifically and will post results as they come.

As always, thanks for the help.

Frank

I've been doing a lot of work on my G36 (MKIII, 2 trk 19 and 38cm/s).
A few things I've found out (sorry, it's a bit long):
The large electrolytics hold up well, at least mine still do their job. If you want to change them in a nice way, you could try a guitar amp supplier for new caps - they still make large multisection caps for old Fenders that will fit in the chassis and also look original. But modern 105 deg C low-resistance (ESR) caps made for switching supplies are probably the real upgrade. The 2 selenium rectifiers are an accident waiting to happen, replace with solid state bridges (the voltage will rise slightly, but see below).
There are almost no electrolytics in the signal path, just 2x 125uF at V10, and 2x 40uF in the meter driver circuit. These do go bad, small electrolytics always dry out quicker. Replace these with 105C of closest value.

All the yellowish caps are plastic (polystyrene? polyester?) and are stable, and they don't sound any worse than modern polypropylenes so you could keep those. Tubular ceramic caps are not too good in the audio path, replace those with polypropylenes at least in the bias traps (2 on osc board, and 2 on small board next to output pots).

I removed the speaker and the power output section in mine. If you pull the outputs, the overall supply voltage rises by 20% or so. This could make for better headroom, but also stress various parts. Raise the value of the 470 ohms after the first filter cap - I put 680 in mine, it needs to be over 2W or it gets too hot during recording (the bias oscillator draws as much current as all the preamp stages together, but only when "rec" is pressed). Also play with different values for R118 that feeds the bias osc, +215v is the recommendation in the manual for the 2 track version but a little higher makes erasure work better.

Like the Japanese modder, you can wire a "clean" direct input to where the 0.22uF's meet the input 100k pots, and route that to "radio" input sockets if you like, or an extra pair of sockets somewhere. You can keep the preamp tubes in circuit, to get extra gain or distorsion from the diode or mic inputs when you need it. You'll get the added bonus of the new input acting as output from the first preamp stages (say, if you want to pass something through a tube stage before recording on the computer
Oh, and a big improvement is replacing the volume pots! A bit of a pain, because the shafts need to be drilled hollow.

Don't expect perfect results. The signal-to-noise (and hum) ratio will at best be 50dB, while a cheap Soundblaster Live has what, like, 90dB? Also changing individual tubes can make a lot of difference. The old Philips Miniwatt ones are wonderfully good sounding, they last forever, but are not too quiet. Newer Chinese tend to be quiet but sound a bit more sterile. Try to fit real low-noise 7025's or similar in the 2 shielded positions.

I couldn't have done a thing without the service manual, try to get one if you haven't already got it. If you find a late one, all the different versions of the G36 are outlined. Good luck!

Such good information! Thanks for the post! :thumb:

I did notice that after replacing the power filter caps that my voltages increased, which I thought was due to my increasing their values slightly, but I also had yanked the power amp tubes, which futher explains the higher voltages. I found that it sounded "stiffer" like this.

Can you describe what your selenium rectifiers looked like? The rectifiers in my unit look just like the ones in the Japanese modder's. Are these selenium? The wires from the main power transformer are pretty scary.

I actually have half a mind to make the entire power supply an exernal affair, since that power transformer is bolted to the metal plate that sits directly behind the tubes. I'm sure they rotated that one around a few times for the least hum, maybe it woudn't make a difference but it would make it lighter!

I had done the power supply mod I mentioned before and it worked out, I even found a cheap, big 6.3v dc supply for the heaters and wired that in (more complicated than it sounds). I did have a breakdown related to the wires connecting the power trafo to the rectifiers being frail.

The sound was cleaned up, the afformentioned hum during recording went away, and I was excited to push it farther - which is where marzipan's post will take me. I had talked to a revox parts supplier on this side of the pond (jm tech arts?) who recommended replacing the heads if they were the plastic backed ones.

That service manual is NECESSARY! The wiring inside the deck is hard to follow and some parts are hard to ID.

Frank