Hi- This is my first post after digging around for a few weeks, what a great forum!
Here's the story: I was given a dead Revox Model 40 integrated amp a while back. Cute, simple, 10 watts per channel. The friend who gave it to me has the same model and it is his only amp since downsizing from a huge Conrad Johnson (house to Swiss apartment 🙂), and he is in love with his.
This got me back on a path I started years ago- learning more about electronics and trying to soak up as much info as possible before hitting the on switch on the soldering station.
In the meantime I've come up with another working Model 40, which is working quite well.
Where I'm confused is in some of the info regarding capacitors. I read or been told to replace all the caps, period. Also to replace only electrolytics. Also to keep it as original as possible and only replace components if and when they fail.
So what does one do? I feel like this amp could give a little more, and considering it's 45 years old I'm sure that the electrolytics at least are in poor shape. But it also has loads of 'mustard' caps and I'm not too keen on arbitrarily mucking up Mr. Studer's work in there.
I suppose I will start with the dead 40 and get it going.
Here's a pic of the topside
The underside is bare except for 3 big filter cap cans and the solid state bridge rectifiers.
Here's the story: I was given a dead Revox Model 40 integrated amp a while back. Cute, simple, 10 watts per channel. The friend who gave it to me has the same model and it is his only amp since downsizing from a huge Conrad Johnson (house to Swiss apartment 🙂), and he is in love with his.
This got me back on a path I started years ago- learning more about electronics and trying to soak up as much info as possible before hitting the on switch on the soldering station.
In the meantime I've come up with another working Model 40, which is working quite well.
Where I'm confused is in some of the info regarding capacitors. I read or been told to replace all the caps, period. Also to replace only electrolytics. Also to keep it as original as possible and only replace components if and when they fail.
So what does one do? I feel like this amp could give a little more, and considering it's 45 years old I'm sure that the electrolytics at least are in poor shape. But it also has loads of 'mustard' caps and I'm not too keen on arbitrarily mucking up Mr. Studer's work in there.
I suppose I will start with the dead 40 and get it going.
Here's a pic of the topside
The underside is bare except for 3 big filter cap cans and the solid state bridge rectifiers.
John,
Electrolytic caps. literally dry out over time. So, replacing all 'lytics is a given.
The "mustard" caps. may or may not need replacing. If they are leaking (electrically), replacement is in order.
Do you have a scan of the schematic? If so, please post it.
Electrolytic caps. literally dry out over time. So, replacing all 'lytics is a given.
The "mustard" caps. may or may not need replacing. If they are leaking (electrically), replacement is in order.
Do you have a scan of the schematic? If so, please post it.
This looks just great.
If you can get someone who is sensitive to look at it and do the minimum to bring it back into spec. based on some test and measurement and judicious replacement of electrolytics, that would be my recommendation. If you are out of practice, you could easily leave what is a minor work of art looking a bit sorry. This is for you to judge tho', I've seen first-time builders produce really nice work.
w
If you can get someone who is sensitive to look at it and do the minimum to bring it back into spec. based on some test and measurement and judicious replacement of electrolytics, that would be my recommendation. If you are out of practice, you could easily leave what is a minor work of art looking a bit sorry. This is for you to judge tho', I've seen first-time builders produce really nice work.
w
It is beautifully laid out. I've attached the schematic, hopefully it will be legible.
There's one lonely electrolytic in one of the tube "turrets", otherwise it's just the filter cans. I think I will replace those as they must be way off, but will certainly do it in a sympathetic and reversible manner.
There's one lonely electrolytic in one of the tube "turrets", otherwise it's just the filter cans. I think I will replace those as they must be way off, but will certainly do it in a sympathetic and reversible manner.
Attachments
Considerable "skull sweat" went into this design. Rotary switches in the tone controls. A separate B+ rectifier bridge for each channel. Yowzah!
Disagreeing with some of the choices made is something we "always" do here. For instance, active RIAA EQ in the phono section. All in all, a very fine effort. FWIW, the thing I'd pick on most is that the value of the coupling caps. between splitter and finals is too small. That is common in units of this time frame. The net pentode grid to ground resistance seems to be 352 KOhms. Combining that with 47 nF. coupling caps. creates a high pass pole with F3 = 9.6 Hz. That pole should be <= 5 Hz. Increasing those coupling caps. to 100 nF. would accomplish that. 400 WVDC 716P series Orange Drops would be my choice. BTW, the same sort of argument applies even more forcefully to the cap. coupling the voltage amplifier to the "concertina" phase splitter. I suggest a 22 nF. part there.
Disagreeing with some of the choices made is something we "always" do here. For instance, active RIAA EQ in the phono section. All in all, a very fine effort. FWIW, the thing I'd pick on most is that the value of the coupling caps. between splitter and finals is too small. That is common in units of this time frame. The net pentode grid to ground resistance seems to be 352 KOhms. Combining that with 47 nF. coupling caps. creates a high pass pole with F3 = 9.6 Hz. That pole should be <= 5 Hz. Increasing those coupling caps. to 100 nF. would accomplish that. 400 WVDC 716P series Orange Drops would be my choice. BTW, the same sort of argument applies even more forcefully to the cap. coupling the voltage amplifier to the "concertina" phase splitter. I suggest a 22 nF. part there.
Frag Jan zuerst --- Ask Jan First have some new production filter caps with old-style looks, though I'm not sure they have 50+50+50...
That amp certainly has an elegant layout, and some of the thot going into it (as Eli has already pointed out) are special.
Depends on what your goals are. A restoration to maintain collectors value, has you doing the later, trying to get the most out of the iron and tubes that are there, means probably replacing most of the passive parts.
Modern capacitors, to be specific, are much better than vintage. So caps are an obvious target. Also the old rectifiers, even if for only safety. Those old resistors tend to drift in value, so they are candidates for change.
If you want to go crazy on the amplifier stage, you might want to search out Yves ECL86 amp that was the precursor to Baby Huey.
Can you post pictures of the front and other side (ie all the iron)
dave
Depends on what your goals are. A restoration to maintain collectors value, has you doing the later, trying to get the most out of the iron and tubes that are there, means probably replacing most of the passive parts.
Modern capacitors, to be specific, are much better than vintage. So caps are an obvious target. Also the old rectifiers, even if for only safety. Those old resistors tend to drift in value, so they are candidates for change.
If you want to go crazy on the amplifier stage, you might want to search out Yves ECL86 amp that was the precursor to Baby Huey.
Can you post pictures of the front and other side (ie all the iron)
dave
Attached are more photos. I was already leaning toward replacing the rectifiers, as one of the large ones in the dead amp looks a bit puffy... The dead amp, which is the older one, has Siemens rectifiers and the running amp has Semikron, both with the same reference B300 C120. I have found replacements searching that number, but I don't know that the C120 means?
Attachments
Lovely amplifier.
I have dozens & dozens of those rectifier blocks. I won't use them in anything. I'd use UF4007 or maybe one of the swoopier soft recovery diodes. You want to go all out Schotkies. Eli knows more about those than i do.
If you intend max performance, I'd be looking at replacing the RCAs as well... kinda tight together thou. The usual suspects are probably too big.
dave
I have dozens & dozens of those rectifier blocks. I won't use them in anything. I'd use UF4007 or maybe one of the swoopier soft recovery diodes. You want to go all out Schotkies. Eli knows more about those than i do.
If you intend max performance, I'd be looking at replacing the RCAs as well... kinda tight together thou. The usual suspects are probably too big.
dave
Hello Big John
I know the M40 pretty good and restaured some of it.
How I proceed:
- replace the Selen-rectifiers (Selen gleich-riecht-er) by silicon rectifiers
- replace the electrolytic caps in the power supply chain. Jan is a good source, you can use 100+100uF instead of 50+50. Choose 400V, better 450V types (idle voltage with silicon rectifiers)
- now, you have higher voltage because of the silicon rectfiers. I normally insert another 100uF cap and a resistor between the rectifier and the existing PSU
- attach a dummy load to the speaker outputs and measure all voltages (Ub, Ua, Uk, Ug). This is showing you possibly faulty caps or resistors. The mustard caps in my opinion are generally still reliable.
When you want help in my workshop nearby Bern, you can send me a PM.
Regards
Franz
I know the M40 pretty good and restaured some of it.
How I proceed:
- replace the Selen-rectifiers (Selen gleich-riecht-er) by silicon rectifiers
- replace the electrolytic caps in the power supply chain. Jan is a good source, you can use 100+100uF instead of 50+50. Choose 400V, better 450V types (idle voltage with silicon rectifiers)
- now, you have higher voltage because of the silicon rectfiers. I normally insert another 100uF cap and a resistor between the rectifier and the existing PSU
- attach a dummy load to the speaker outputs and measure all voltages (Ub, Ua, Uk, Ug). This is showing you possibly faulty caps or resistors. The mustard caps in my opinion are generally still reliable.
When you want help in my workshop nearby Bern, you can send me a PM.
Regards
Franz
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... here you can download the complete manual as a pdf:
ftp://ftp.studer.ch/Public/Products/Revox/Revox_Model_40/
Franz
ftp://ftp.studer.ch/Public/Products/Revox/Revox_Model_40/
Franz
Thanks for all the advice- it's nice to know that I was sort of on the right track.
I will order some components and see if the dead amp breathes again. The story is, though, that it has a bad transformer. I am surely revealing my ignorance here, but what is the recommended procedure for checking one? My gut tells me this is not the problem.
Franz, do you buy from Distrelec ever, or is it easier to source parts from outside the country? Also- as you have one of these, I'm a little perplexed by the output. On my working amp, there's almost no sound until the volume control reaches ~10/11 oclock. For a decent listening level it is usually at around 2 oclock. I also have to boost the bass and treble one or two notches or it's very murky. This is running a pair of little Mirage SM.5's, 4 ohm speakers with the trannies wired correctly for 4 ohms. Just seems like I really have to push it to get a normal listening level... sounds good though! Perhaps freshening up the power supply/recifiers will make a change here.
Oh- and thanks for the link Franz! I have a copy of a copy of a scan of the manual but the link is much clearer.
I will order some components and see if the dead amp breathes again. The story is, though, that it has a bad transformer. I am surely revealing my ignorance here, but what is the recommended procedure for checking one? My gut tells me this is not the problem.
Franz, do you buy from Distrelec ever, or is it easier to source parts from outside the country? Also- as you have one of these, I'm a little perplexed by the output. On my working amp, there's almost no sound until the volume control reaches ~10/11 oclock. For a decent listening level it is usually at around 2 oclock. I also have to boost the bass and treble one or two notches or it's very murky. This is running a pair of little Mirage SM.5's, 4 ohm speakers with the trannies wired correctly for 4 ohms. Just seems like I really have to push it to get a normal listening level... sounds good though! Perhaps freshening up the power supply/recifiers will make a change here.
Oh- and thanks for the link Franz! I have a copy of a copy of a scan of the manual but the link is much clearer.
Fantastic guys- Jan has 50+50+50+50 cans and a 2000 which if I'm not mistaken should be ok to replace the 3x 600 (or no?), all drop-ins for the originals.
Dave, are you suggesting diodes rather than a block? What would you recommend for the little B30?
Sure, will work fine, if the working voltage is at least the same as the original. Be careful, the negative of that capacitor may not be connected to the chassis or ground in any way!
Kenneth
Hello
I would recommend, to localize the fault(s), before ordering parts.
What transformer is defect? Output transformer?
Then better forget a renovation.
Mains Transformer? Just measure the AC voltages.
Distrelec is O.K., ordering by Jan or other sources outside CH also no problem.
But not for heavy goods like transformers, you have to pay often custom tax.
Franz
/Edit
I dont have one of this amps: I restaured some and sold to Hongkong, Taiwan or China. There are better tube amps than the M40.
I would recommend, to localize the fault(s), before ordering parts.
What transformer is defect? Output transformer?
Then better forget a renovation.
Mains Transformer? Just measure the AC voltages.
Distrelec is O.K., ordering by Jan or other sources outside CH also no problem.
But not for heavy goods like transformers, you have to pay often custom tax.
Franz
/Edit
I dont have one of this amps: I restaured some and sold to Hongkong, Taiwan or China. There are better tube amps than the M40.
The OP's photo shows plenty of room where the B+ rectifier bridges go. This is, fortunately, a relatively low PIV situation. 8X 600 PIV Schottkys get my nod. It's hard to better zero switching noise. Cree CSD01060As or Infineon IDT02S60Cs should do quite nicely.
Use Schottkys in the negative supply bridge too. 50 PIV/1 A. parts, with axial leads, make sense to me. "Battle" TO220 case parts only when necessary.
Big John,
That is a beautiful amplifier. I love the industrial design. You Swiss sure make nice things. Take good care of her! When replacing the electrolytics caps, I'd personally check the dimensions to make sure everything fits well and you don't have to hack the chassis at all. This from a guy predisposed to wire ties and glue.
Paul
That is a beautiful amplifier. I love the industrial design. You Swiss sure make nice things. Take good care of her! When replacing the electrolytics caps, I'd personally check the dimensions to make sure everything fits well and you don't have to hack the chassis at all. This from a guy predisposed to wire ties and glue.
Paul
Paul
I correct:
We Swiss sure made nice things (O.K., you still can have some nice things out of our country).
Today, Revox is a German company and the original roots, Studer Professional Audio Equipment, was recently reduced to the R&D part.
I also lost my job.
And: it is owned by an US company: Harman.
Back to the M40: even when you like the layout (it is really nice) you will realize it was not very service friendly.
Some of the parts below the tube sockets are not easily accessible until you dismount parts of the chassis.
Kind regards
Franz
I correct:
We Swiss sure made nice things (O.K., you still can have some nice things out of our country).
Today, Revox is a German company and the original roots, Studer Professional Audio Equipment, was recently reduced to the R&D part.
I also lost my job.
And: it is owned by an US company: Harman.
Back to the M40: even when you like the layout (it is really nice) you will realize it was not very service friendly.
Some of the parts below the tube sockets are not easily accessible until you dismount parts of the chassis.
Kind regards
Franz
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I'm not Swiss, but I am a watchmaker 🙂. I'm especially fond of Swiss machine tools, but alas there too much of it's gone and what's left generally is simply not as good as the old stuff.
Franz, I agree reaching some of those components on the tube turrets looks next to impossible without some disassembly. For this amp I'm going to do the large filter caps and the rectifiers and leave it at that for now. The dead one will be more of a challenge and I have quite a bit of reading to do there from the sticky post at the top for newbies.
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