1970s Design Indulgence

Next jobs:

The rivet nuts in the riveter kit I bought are so hard to squeeze up that I'm sure the M3 mandrel threads will eventually strip. Therefore I've invested in some thin sheet rivet nuts that don't look as tough. All we can do is hope these days, seeing that the once trusted RS Components doesn't do such things anymore. It's all about waste these days - wasting your money to find the right thing for the job.

Hopefully, when completed, this preamp won't be a waste of money, whether pre-built by John C and Leo, or supplied as parts.

Both boards are tested and I auditioned the input stage/RIAA input and I'm happy with how it sounds.

What I now need to do is position the boards on the chassis pan in such a way that wiring is straight forward and neat, but I'll need the rivet nuts for that, as they are used for board stand-offs.

The input stage/RIAA board will sit nicely under the switch extension shafts, with the mounting holes clearing the shafts so the board can be removed without much trouble.

The tone control/output preamp board can sit in the rest of the open space and that should give short wiring distances to the controls.

So what else?

The front panel can be drawn up and punched. The shafts should centre through the holes, and the correct forward position of the chassis is where the Alps pot shafts are 12mm proud. All the other shafts can be marked the same distance and then be trimmed. The nylon extension shafts cut best with pruning shears. These shafts are made from RS part # 680-690 (£8 for 5 x 1m lengths) and are sold as being 6mm diameter. My Mitutoyo Vernier shows them to be 6.35mm diameter, so the Germans who make them must be using imperial measure

The "gantry" sub-front panel is responsible for holding the controls rigidly, but to prevent binding on the shafts, the front panel holes ought to allow for some misalignment, and the next size punch I have is 5/16ths which is so close to 8mm as to make no difference. I don't want it to be too far out of alignment or it could look silly. And here it is prudent not to encircle the control knobs with dial makings as the slightest eccentricity will be very obvious.

With the front panel out of the way, the forward distance of the chassis in the case can be determined and a block of wood machined to be used as a production spacer.

Here again, I'll need the rivet nuts fixing to the chassis pan to do the wooden base slots...

Attention then turns to the rear panel and what phono sockets to use. I'd prefer single sockets of the type fastened by a nut but I'm totally bereft of them. I do have plenty of the printed circuit type pairs, but there is no printed circuit...

However, they do have a screw hole which is between left and right sockets, but offset to one side by 9mm. The screw hole is meant for a self tapper, but is easily drilled out to 3mm (with care on a pillar drill), so could be fastened with an M3x16 set screw, load spreading washers, and nut. I think I will have to try it that way.

The only worry being that they're all shielded so I'm hoping I can avoid a hum-loop. But then again, I feel obliged to have them all shielded to the panel as that will avoid any external hum-loop (or will it? We shall see).

The power supply used in all the tests so far has been the PSU1 - long cable and all. Therefore I ought to try and replicate the PSU1 internally (in the preamp), so my thoughts will have to turn to a similar power supply pan as the power amplifier. I have a PSU1 transformer which I need to shock mount and have it in the right orientation to minimise its magnetic field. By it being on its own pan, magnetostriction eddy currents will not pass to the preamp chassis.

So, still lots to do. 

Cumulative errors creep in but aren't insurmountable. A little nudge here and a little nudge there should see to it.

I would have preferred collet knobs as it makes assembly so easy. But today, because we make now't in the UK, they're either rare as the proverbial or they charge for them like the cavalry.

They once did a 1/4" collet knob that would go down to 6mm, but that doesn't make as much money does it?

Rear panel next!