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1970s Design Indulgence

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Post Options Post Options   Thanks (1) Thanks(1)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2022 at 8:23pm
This looks like it's going to be the sort of challenge I really enjoy. Clap  Describing the construction without the aid of photographs would be almost impossible and certainly more than a few thousand words. Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2022 at 8:43pm
... the BAK method is ... 
"for Snap-In caps, I put fresh solder on the pins and let cool before wrapping a tinned wire around and then soldering".
 It is important to minimize heat-stress on parts when soldering them... all parts.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 07 May 2022 at 5:54am
The ground stub

Nobody makes a quarter inch blade receptacle to a 4mm ring eyelet. You can get the blade to 4mm ring, but not the receptacle. Easy to punch from flat metal, but what we want is more difficult, so they don't go there. After all, who needs such a thing? We do!

The ground stub has to convert the quarter inch tag on the bridge rectifier to a 4mm eyelet, so as to nut and screw six solder tags to, in order to solder the six ground wires to.

This ground stub has to be solid and short enough to be a very low impedance, as all the grounds "mix" in it. Forget a "wire-to" or a printed circuit - don't know anybody who makes 3D printed circuits or how I design one...

But what's the fuss? On an old amp, they'd bridge between two capacitor negative terminals using some thick solid wire, and then solder each wire on in turn, and then the "wire-to" to the bridge rectifier minus terminal. But that was the problem! The problem of not understanding the asymmetrical "half-wave rectified" signal current returning to ground through badly thought out impedances.

Hardly surprising then, that on breaking new ground, that we have to create our own techniques. However, the way was shown 46 years ago in PW's Easy-build disco! Yes, A.C, Ainslie and Clive Toms (real names Pete and John) illustrated a not-so-ideal way of doing it using a screw terminal block.

Have you seen the state of a brand new terminal block from RS? Sorry, but they don't comprehend how to make them in Shenzhen - the screw holes are supposed to have a tapped thread!

Even so, we don't have the cavernous space of the inside of a twin-deck.

What we can obtain, however, is the parts from your local branch of Halfords or most car spares shops.

The photos should explain themselves. I'll will add that the crimping ends of the two parts should be tinned, then pushed one inside the other, then heated by the iron so the solder flows and they become one. You might have to squeeze the two parts together with pliers to get a tight fit. This is where you need some practical mechanical skill.

The receptacle is pushed onto the bridge rectifier minus tag so it looks like it does in earlier photos. The receptacle is then soldered to the tag such that both become one. The lower of the two holes in the tag should remain free of solder, because that's where one end of the discharge resistor is poked through for soldering (refer to earlier photos).

While you're at it, flow more solder to build the join between the receptacle and the eyelet.

The six solder tags are then mounted by screw and nut in the sequence: M4x10 machine screw, serrated washer, plain washer, 3 x solder tags, the eyelet, then another three solder tags, plain washer, serrated washer, and nut. If you don't arrange the washers in this sequence the solder tags will turn as you tighten.

Nip up to the point you feel the serrated washers bite and crunch, but don't keep tightening until the serrations are flattened, and no longer effective. Oh, and at the same time you'll need to arrange the solder tags as shown, bending them outwards. This isn't easy - it is a challenge!

Now the photos...

crimps

ground-stub

solder tag star

psu discharge resistor


Edited by Graham Slee - 07 May 2022 at 8:14am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 May 2022 at 7:27pm
Making sense of the positions of the rear panel components...

chassis layout try-out

The only sensible place for the mains inlet and voltage selector.



mains-inlet-placement

Prototypes didn't suffer from the proximity of the mains wiring. The PCB layout and input socket position allow for quite short input wires without the need for shielding. Safety is paramount, and this position harnesses mains wiring above signal and PSU wiring. Note the boots - these are a must and are used on the IEC inlet and the voltage selector switch.


rear panel

Using the arbitrary machined prototype rear panel to test positioning (ignore unused holes - they won't be on the finished panel). The soft plastic washers are used under the RCA jacks on the outside to offer some anti-rotation grip, providing the rear nut and solder tag are in tight direct contact with the metal surface inside. This ensures 360 degree grounding with the case.


rear panel positioning

Not the best place to have a mains cable, but the only sensible place to put it. However, if a properly electrically shielded interconnect is used, there will be no chance of mains noise being induced. The voltage selector will be furnished with a tamper-proof indicating cover.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lfc jon Quote  Post ReplyReply Direct Link To This Post Posted: 08 May 2022 at 7:43pm
Things seem to be looking good there Graham, what is the estimate on how far are you from putting it on the market out of interest? 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 08 May 2022 at 9:03pm
Originally posted by lfc jon lfc jon wrote:

Things seem to be looking good there Graham, what is the estimate on how far are you from putting it on the market out of interest? 

Good question. I can tell you what I've yet to do.

I have to modify the PCBs to make the fine Vq adjust pot shaft ends come flush with the front face of the front panel.

I then have to notch out a good portion of the inner front of each PCB to make room for the "mains centraliser" (DC filter as some prefer to call it). I have to draw up a PCB for it, and figure out a safe way to mount it and keep it away from inquisitive fingers - especially for those who want to DIY the amp.

The front panel positions then have to fit with the internals, which means one more bare metal piece has to be machined to try it out.

I am a bit worried about the quality of finish of the meters. A way of making them look "flashy" is to place an LED inside, to illuminate the scale. That can also be used as an "on" indicator, and so I can use a plain black rocker switch instead of the neon illuminated version (neon gas mainly comes from east of Kiev). The front panel should then appear more balanced.

There are some final mods to the PCB to correct for the circuit modifications, but that should be relatively easy. Also to add the meter LED supply pins.

And there's the anti-tamper thingy to design for the voltage selector. And also the primary tapped version of the transformer - so we have to machine some more toroidal cores before they can be wound.

All the case and heatsink machining for an initial small quantity has to be done at the Sheffield engineering works, and the three different finishing suppliers will be needed to do the paint, anodising, and screen printing.

And then I have to find suitable packaging for both parts kits and pre-assembled versions.

Oh, and before I forget, the transformer version for USA type mains supplies. Plus, the transformer mounting disc ought to be powder coated to prevent a shorted turn accident by a DIYer.

And prior release we have to flash test it for safety.

Plus I've got to write the manual. Cry Wink

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 May 2022 at 7:38am
I spent most of yesterday trying to figure out how I could shoehorn the mains "centraliser" into the case. Although the "in-circuit" voltage never exceeds four volts, the input is at live mains potential, and its output is likewise a lethal voltage.

The board has to be securely fitted and by (the now superseded) IEC60950, means a minimum of two fixings.

It also has to be difficult for anybody working on the inside of the amplifier, with it switched-on, to be able to get an electrical shock from it.

It was OK fully insulating the prototype with heat-shrink, but for safety and longevity there has to be air circulation around the rectifiers and capacitors for cooling.

The spacing between live parts and the metal casework must satisfy safety the standards, and although superseded, the creepage and clearance distances of IEC60950 are still valid. To ensure it passes the 3kV electrical breakdown test the spacing between live parts and case must be at least 5mm.

Hoping to have that part of the assembly sorted today.
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