Revox Model 40- leave it alone or dig in?

[Big John]

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.

[Eli Duttman]

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.

[wakibaki]

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

[Big John]

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.

[Eli Duttman]

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.

[kavermei]

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...

[planet10]

That amp certainly has an elegant layout, and some of the thot going into it (as Eli has already pointed out) are special.

Originally Posted by Big John

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.

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

[Big John]

Originally Posted by planet10

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?

[planet10]

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

[Franz Gysi]

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