Fg810 - A Working Title

[Mike H]

You approve? That's made my day.

I was going to do some more work on it, but you know how it is.

Anyhoo, power supply board(s) arrived today so there's that. However I was intending to start chassis assembly next.

Watch this space (as they say) ...

[Mike H]

Just to mention, this style of construction, i.e. a heatsink + bracket, seemed to be the 'thing' back in the day, the first one I made heatsink was made from folded alu sheet, not angle (done in the best tradition of a combination of naivety, hammer, and vice, complete with hammer dings and vice jaws imprints), it must have come from somewhere, I'm sure I didn't dream it up on my own. A little later, I also ordered a ready-made board from an advert in back of Practical Electroncs (what else), same construction. Heatsink was a flat plate, but black!

[Mike H]

Mocking up the chassis - wondering if the small brackets will be enough to support the front and rear panels - going to sleep on it - or else, to use my favourite phrase, 'Only one way to find out' ...
-

Fg810 chassis-1.jpg (109.05 KiB) Viewed 3798 times

-

Fg810 chassis-2.jpg (125.88 KiB) Viewed 3798 times

-

Overall, this is way more sophisticated than anything I could have done back in the mid '70's.

[Dave the bass]

You approve? That's made my day.

Well, The Dammed do!

IMO though.............

Shoddy!

[Mike H]

High praise indeed.

Still not got around to drilling any more holes and getting on with it. Diversions.....

[andrew Ivimey]

Drilling holes - that's the worst.
What's also the worst is that having drilled the holes I find they are not 'lined up' - quite dissipates the enthusiasm.

[pre65]

Drilling holes - that's the worst.
What's also the worst is that having drilled the holes I find they are not 'lined up' - quite dissipates the enthusiasm.

What about the bleeding ! Don't forget the bleeding.

[Mike H]

Drilling holes - that's the worst.
What's also the worst is that having drilled the holes I find they are not 'lined up' - quite dissipates the enthusiasm.

I'm so glad it's not just me - yes my inabliity to drill a hole accurately is only matched by my inability to cut a straight line in anything.

Well I broke the back of the remaining drilling today, hardly any (apparent) cock-ups, so win win.

I then made and ate a late lunch, as usual put on YouTube while I ate, then that was me sat in front of YouTube nodding off. So that's another day over with.

Transformer is ensconced, PCB's mounting pillars ready, just needs screwing together. Oh and wiring up of course, minor detail. (Arf.)

No bleeding yet Phil. Maybe that's a ritual sacrifice that's needed?

[Mike H]

Just managed to avoid bleeding today. Thumb - why it didn't is still a mystery.

A 'dry run' of screwing things together (meaning, not final), I don't think any more drilling is expected.

Bu first, while I was on a roll I also threw the PSU board together...
-

Fg810 chassis-05-THIS.jpg (111.39 KiB) Viewed 3276 times

-

The latest assembly... vol pot is on, + knob, and mains switch...
-

Fg810 chassis-06.jpg (136.47 KiB) Viewed 3276 times

-

Now this is all fine and dandy, but will it fit in the wood case..... Yes! It does! A triumph of planning and forethought. (I saw that wrote in a book once and been looking for an excuse to use it. )
-

Fg810 chassis-07.jpg (81.01 KiB) Viewed 3276 times

[Mike H]

The arse end.....
-

Fg810 chassis-08.jpg (84.2 KiB) Viewed 3274 times

-

Fg810 chassis-09.jpg (133.66 KiB) Viewed 3274 times

-

Awethome! (© DTB)

I think that's probably it for next couple of days, but one job I haven't done yet is set the amplifiers' idle currents. More later.....

[Mike H]

Wanted to set up the amplifiers' bias (quiescent current) but wanted a current limiting power supply to do it with (keeping in mind past catastrophes - as in, increase bias from zero, bit more, not there yet, not there yet, still not there yet, glowing red smoking emitter resistors. Which means the o/p transistors are scrap). To be fair the presets I was using back in the day weren't the best, with dirty tracks or even loose end connections, so not surprising.

Anyway, amplifiers arev supposed to run on 45V so I went and unearthed the very ancient Marconi (at least I think it's Marconi) DS 50/2 bench PSU, which does 50V and current limiting. I also have a 2A fuse as 'backup'.
-

Marconi DS 50_2b.jpg (78.12 KiB) Viewed 2919 times

-

Doing something else with it beforehand, testing 555 timer boards, suddenly it caught fire - smoke billowing out everywhere!

Took the top cover off, and found two very dodgy looking 'lytics on the board ... the bulging one (on the right in photo) was actually hot ...
-

Marconi DS 50_2 dodgy caps.jpg (55.13 KiB) Viewed 2919 times

-

Replaced them thinking that was it, but no. Turned it on again, no Volts, front panel virtually dead. Then more smoke.

Took it apart a bit more to find where the smoke is actually coming from (bearing in mind I have to turn it on each time to see any).

Looks like it's coming from the mains transformer - smells like transformer smoke as well, so seems like it's toast. Poo! Snivel snivel.

I might pull it part later to see if it could be replaced...

In the meantime, I got out another spare...
-

Apelex PS 120 TR.jpg (125.44 KiB) Viewed 2919 times

-

That's as far as I've got today.....

[andrew Ivimey]

Should stuck to valves. All this solid state stuff....bah!

[Mike H]

Valve transformers can burn up too! Guess how I know


In the photo below - amplifier #2 on soak test (#1 has already been 'done').

I am monitoring the Voltage across R13, which is the emitter resistor for o/p transistor Q6. It's 1Ω, so display in mV translates directly to mA.

The MO seems to be get it to 30 mA while cold, then watch it. The heatsink is quite warm at 50 mA, while it's more stable (less drift) a around 40 mA - 35 to 40 seems to be it.

No magic smoke released yet.
-

Fg810-006B-PCB set-up-1.jpg (148.55 KiB) Viewed 2878 times

-

Fg810-006B-PCB set-up-2.jpg (68.12 KiB) Viewed 2878 times

-

Ignore the '0.08' - I forgot, this PSU won't allow its output to switch on without a load already present (haven't used it for a while!) - so I have a 1.5k resistor in parallel. The total current is then more like 50 mA.
-

Fg810-006B-PCB set-up-3b.jpg (78.37 KiB) Viewed 2878 times

-

And the Fluke needs a new battery.

More later.....

[Mike H]

Back onto the main event -

I wasn't overly convinced by the double-diode temperature compensation for the bias, (read as, 'not good enough'), for example it would have helped if the diodes were closer to the heatsink end maybe?

So I changed the design slightly, and opted for the adjustable temperature sensing transistor configuration.

This is where we were before, when the power supply caught fire. But this time with an extra BD139 added to the heatsink ...
-

Amp board set-up2-01.jpg (103.42 KiB) Viewed 1437 times

-

Board is slightly rewired to replace the diodes.
-

Amp board set-up2-03.jpg (98.19 KiB) Viewed 1437 times

-


The modus operandi seems to be, set for 50 mA when cold, let it warm up then it reduces and ends up hovering between 30 & 32 mA.
-

Amp board set-up2-02.jpg (84.11 KiB) Viewed 1437 times

-
It's been on for a while and is stable at that, so we'll go with that for the time being.

Next, wiring up .....

[Mike H]

Revised schematic .....

-

Fg810 amplifier version 7.png (91.99 KiB) Viewed 1435 times