Fg810 - A Working Title

[Mike H]

Have today ordered Schaeffer panels, so am now financially committed (or, even more financially committed), so may as well start a topic.

So far I have this, and a box of bits.....
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Recap -

from 'Fig. 8.10', out of a book.
Bipolar push-pull power amp, old design, 1960's - '70's
Pretty sure I made this in germanium, but there was only ever one board, I balls'd it up by not understanding how NFB works, left out a couple of parts, so it had no Voltage gain. To be fair tho it was one of the very few SS amp boards I made in those early days that didn't immediately catch fire when first turned on, and even more impressively, kept on working. Haven't got it anymore, of course. So it's kind of 'unfinished business' ...

A 'scan' of the page from my school exercise book, I copied it from a library book... around 1972?
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I had also made a PCB for the first germanium version, etching it with ferric chloride, was quite a lot of work in those days. As I say there was only ever one, and somewhere along the line it got junked, which is a bit of a shame because it probably would still have novelty value if nothing else. A note about these library books - schematics had been donated for inclusion by amplifier manufacturers, but since they don't want anyone trying to do exactly what I was trying to do, i.e., make my own (they'd prefer you to buy their ready-made products) some information was omitted, on purpose. Most often, it was transistor (and diode) type numbers, or in one case, esoteric in-house device number codes that you couldn't match to anything you could buy from anywhere. Also, no indication as to the power rating of resistors and so on. But most of all, power supply circuits were never shown or even mentioned. This made it quite awkward for a beginner who's not sure what he's doing.


Modern take on the circuit, courtesy of LTspice -
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There will be a fuse in the supply line (for each amp) and I also have a speaker protection board to use, as the supply is single-sided and we don't want any of those horrid and alarming thumping noises when we switch on and switch off, do we?

To be continued.....

Laters!

[Paul Barker]

It’ll end in tiers.are you mad?

[Max N]

I think I know where Mike is coming from on this. I have several (too many if I’m honest) “unfinished business” projects. It’s great to see someone cracking on with one!

Mike, I can’t believe you kept that school book all these years - the project obviously means a lot to you. Good luck!

[Mike H]

It's one of two that had anything really worth keeping in them as a future reference. Some other pages are about different types of valve oscillators circuits for radio, hastily copied from Foundations Of Wireless by M.G. Scroggie. I've since got a later edition of that book, but by which time sadly the valves had been ousted from the pages by transistors.

It’ll end in tiers.are you mad?

Probably.

[pre65]

I've still got (somewhere) an exercise book from my days at Mid Essex Technical college, when I was a Marconi apprentice.

I marvel at entries I made regarding the biasing of transistors, did I understand ? Probably not.

And I still have a copy of "Radio and Line Transmission" .

[Mike H]

Bit of a funny day today ...

Here is a picture of two new speaker protection boards I bought a couple of months ago -
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And here is a picture of two kits I bought 4 years ago, but couldn't find. So I buy 2 new ones, and shortly after, these magically reappear. Of course. Naturally. (Been filed in the 'wrong box'. Found them while looking for something completely different. The time I spend looking for stuff is nobody's business. )
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The boards and components are identical, but somewhat interestingly, the new ones came fully assembled. Was there a problem with buyers not building the kits correctly? Who knows.

And then, I 'lost' a packet of 10n ceramic disc caps I'd only earlier this same day had got out of their storage box and 'parent' packet - they were there on the table, and then they weren't. I'd been rummaging through another (bigger) storage box looking for other stuff, and they'd got scooped up and 'put away' there by mistake. Got them back eventually, after I'd had to rethink what did I just do the last half hour.

Spent the rest of the afternoon and evening wrestling with the LTspice simulation for the UPC1237 chip (Speaker Protection IC). I had created the model 4 years ago, but it still had some problems I wasn't then aware of. Not understanding, or maybe not reading, the datasheet properly, I seemed to have overlooked, or ignored, the part implying that pin 8 (the supply pin) is shunt regulated, basically (range, 3.0 - 3.8V). After going around in circles for a few hours tryingt to find out why the circuit falls over if I just change the value of the resistor between it and the supply Voltage, or even the supply Voltage (anything greatly differing from 12V DC). Taking better notice of what the datasheet says, I modified the model to include a 3.5V Zener between the supply and ground pins. And lo, the timing interval doesn't go all peculiar if the supply Voltage or the resistor changes, the timing period stays the same. Better yet, the Schmitt triggers keep working (which are native LTspice 'generic idealized behavioral [logic] gates', not real world devices). Coincidentally, the default logic '1' high for these is 3V, which is handy because that's around what the internal supply of the UPC1237 turns out to be. So no need to change the settings.

More about this later!

[Mike H]

Still progressing ... chassis panels are on their way, so I'm told - also, PCB's have been designed and accepted by JLCPCB.

Think I've licked the speaker protection module requirements, it may seem I'm fussing about this but it has to be right. The eBay sellers say bung in 12VAC and off you go. No. For one thing the supply ripple is horrendous. It's much happier with 12VDC. For other Voltages, need to read the datasheet. (Example therein shows running it from 45V, the relay is 24V and has a ballast resistor in series. The relay on the board is 12V and the series resistor (yes there is one) is 2.2 Ohms, which will not do for the claimed operating Voltage range of 12 - 24V, and cannot be AC either).

Power supply is designed, transformer obtained and waiting, so it's coming along.

More later!

[Mike H]

I decide to go for having PCB's made. While at Owston, Tony Moore reccommended using JLCPCB. I opened an account, then drew up a board layout for the amplifiers with FreePCB, submitted online, accepted OK (Gerber files are compatible). Stupidly cheap. I think it was £3.78 or something, but I got a discount, so £1.38! Or something like. For 5 boards 100 x 70mm. Crazy. Single-sided, solder resist, top silk screen. 'Course there's shipping and taxes which was most of the cost, but still it's all under £20. Definitely doing this again!

Well boards arrived today by DHL, and looking very good. Photos later!

In the meantime, when you upload the zip to the site, as soon as it's scanned you get these really neat and realistic looking pictures...
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This is perfect 'cause it's recreating, pretty much, the complete process I went through with the germanium version years ago, for which I made a PCB layout with a 'PCB pen', then etched in ferric chloride.


Am now just waiting on aluminium angle to make the heatsinks/brackets.

Also there's a power supply board to do, might as well!

Schaeffer order arriving tomorrow, they reckon. ('They' being FedEx.)


Laters...

[simon]

If you can wait 8 or 9 days Mike the shipping is around £7 if you use "ordinary" rather than DHL!

[Mike H]

Can't remember if I saw that, but OK - still cheap as chips, considering!

[simon]

It's when you check out - it defaults to DHL IIRC, but you can select other means. Can't remember what they call it but it's essentially ordinary airfreight rather than courier for about a third if the cost. And I'm tight.

[Ray P]

It's when you check out - it defaults to DHL IIRC, but you can select other means. Can't remember what they call it but it's essentially ordinary airfreight rather than courier for about a third if the cost. And I'm tight.

Yep, it's what I use when ordering from JLC and it never takes anywhere near as long for delivery as it quotes.

[Mike H]

Ta for the info chasps. Will try that next time.



Meanwhile - the boards are populated - without heatsink brackets yet (TIP41's just screwed in at the mo, not soldered yet) .....
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The alu angle has arrived so the heatsinks can be drilled and cut.

Also, the Schaeffer stuff arrived, so I can start mocking up the chassis.

More laterer...

[Mike H]

Now with heatsinks on.....
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[Dave the bass]