Class B DIY

I thought I would finally get around to building the amps from the bits that I bought a long time ago.

I came across the pages at Elliott Sound Productions a long time ago, and since I am no good at designing analogue electronics beyond an emitter follower or a long-tailed pair, I bought the components. I filed them safely in a box, and forgot about them.

More recently, I thought it would be fun to exercise some layout skills and engage my friends at Itead Studios in creating a small PCB.

So I am sure that Mr Elliott won't mind me showing the results of this small bit of labour. It's his design, just I didn't buy one of his PCBs - I thought it would be fun to try my own. I am going to build five of them, after all - two for the Studio, where the reference monitor is currently running on an ageing Cambridge P60; one as a test amp for the laboratory, and two in bridge mode for a subwoofer project, to drive a Peerless 850140 10 inch driver I bought 17 years ago from Wilmslow Audio. I've been buying little bits and pieces from them for more than 40 years.

Now if only I could lay my hands on the heatsink I put aside for this project when I was young ..

  

The first prototype board. I couldn't find any BC546A in the lab, but I did find some MPSA42. They're high voltage NPN transistors so they will be fine. The pinout is opposite to BC546A so they are in backwards compared to the silk screen. 

PS: I have a handful of the blank boards left over; if anyone would like one or two, please drop me a line via Google, and we can arrange for them to be sent. 

Making progress again ..

The Dell machine arrived. After much faffing around trying to get Windows 98SE running on it (it's a pig, it can't find all of the device drivers for the network etc), I decided to go for a simpler solution. I found this FreeDOS thing, which at first sight seemed to be ideal.

So, I downloaded FreeDOS onto a bootable CD, booted the machine with that and created a 2GB (cuz that's how big a normal DOS partition could be) partition.

I then installed Linux on the beast, running dual boot so I could get the machine on the network, and copied over the files from the Fireball server on to the DOS partition. (Linux can write FAT file systems, you know).

I then ran FreeDOS up, so I could try to run LIFUTIL - and was shocked to find it works. Now all I had to do was to find a 720K diskette, format it in LIF format, and then put the right driver for the 1GSa/s card on. I went through all the files, and eventually found that the SYSTEM_0.13 file was the correct one (by a process of hex editing the files and looking for the description string), put that on the disk and moved it over to the HP 16500A front drive.

Wow. We have a working scope.

Happy New Year!

Let's hope 2017 is a good one.

Running LIFUTIL

So the laptop has died that I use to run LIFUTIL. It started with the diskette drive failing, but quickly became beyond repair when the LCD decided to go dark as well.

I have an older machine in the office, and I found an old diskette drive in the museum box. (that pile of stuff I can't bear to part with). The older machine is a Dell Optiplex 780 desktop, and it has a hole in the front for a diskette drive, so I dug around for a cable, eventually found a power supply cable for it as well, and went inside the Dell.

Couldn't find where to plug the 34-pin cable. Ah, found the site for the cable on the motherboard, but it was not populated in manufacture. Ah.

So, I remember I have yet another machine in the shed - a Lian Li case I bought off eBay with an older motherboard in it. Lian Li cases are lovely - all aluminium, all soft edges that don't cut the fingers and can carry multiple fans, huge power supplies, multiple hard disks and a huge variety of motherboards. Go get it, clean it off, vacuum it out, leave it to warm up and dry out properly before installing diskette drive and applying power.

And nothing happened.

Replaced power supply (temporarily - it doesn't fit, but it does plug in kind of), and something happens. It boots! But, it is booting Windows 7 which won't help with running LIFUTIL, so I create a supposedly bootable Win 98SE CD to put in it. (at this stage I must have done something wrong since the CD didn't turn out to be bootable later)

DVD drive won't eject. It's stuck with an old CD in it. Eventually free it without damaging the drive. The CD, not so much. However, the new CD with Win98 on it won't boot. Hmmm .. however I have the IDE drive from the laptop with Win 98SE already on it .. I could put that on one of those converter things that allow you to plug a laptop drive onto a 40-pin IDE connector. I have one in the museum box. Woo hoo.

Boot machine. It spends an awful lot of time changing its configuration and installing drivers for motherboard chip sets, bridges, IDE controllers .. and then asks me to put the CD in to read something else it needs. Put CD in drive. Wonder why clicking on the continue ... means nothing happens. Click here and there. Nothing. Reboot machine. Nothing. The motherboard has failed.

I go to re-seat the memory DIMM and find one memory chip at a hugely high temperature. I find this with my thumb, which is still stinging. Relegate this beautiful case to the list of Things To Do for the future - buy a new power supply, disks, motherboard, memory and decent display card for a new Office Machine (Hackintosh compatible). Fireball will be relegated to be just the workhorse server she is now, but won't be tampered with by me trying to use it for desktop things any more.

Have now bought another Dell Optiplex GX620 from eBay. It's only 21 quid. Has been tested, has DVD drive, a network port and a diskette drive .. hopefully this will be the last subroutine call before we start working back up the call stack. When it arrives, I will only have to install Windows 98SE, get it on the network, download LIFUTIL and the drivers for the 1G sample/sec scope, create the diskette and off we go.

New board for the HP 16500A

I need a scope to debug the old SM4100 amplifier I have. Noisy on one channel, so it is likely a capacitor has gone a bit south. However, the scope's power supply has failed. It is a bit of a complex thing to fix without a scope. This is an attempt at recursion which I can't support. Physician, heal thyself.

So to satisfy the need for a scope to debug the power supply for the scope, I was lucky enough to find a cheap 1Gs/sec plug in for the HP16500A I have.

And, it arrived from the US! Much excitement from Wifey since she thinks I am buying stuff from the US for Christmas for her.  And I plugged it in - but it is not recognised by the mainframe. That's because the file for the board is not on the boot diskette .. so I need to get the file on to the diskette.

I have the file downloaded from Keysight Technologies (who says there is no such thing as Silicon Heaven?), and I have an old laptop with the LIFUTIL program that writes data in the strange 77-track format that HP needs .. and guess what? The diskette drive in the laptop has ceased to function.

So, I have an amp in need of repair, a dead scope, another scope that needs a software update, and a file to update it with, but now I need to find either a replacement diskette drive for the laptop, or I need to install a diskette drive into one of the desktop machines, fire up Windows 95 or 98 on it, download the files, run LIFUTIL ..

Lots of nested calls here with very few returns. 

Now to repair the SM-4100 I need the scope working..

.. and what happens? I power up the scope (HP 54502A) .. and it shuts down 10 seconds later.

It's a power supply issue. Apparently quite well known on this family of scopes. There are no like-for-like replacements around. Fortunately it does seem to work OK for 10 seconds, so I hope that the PSU going south hasn't killed the motherboard. I have a schematic for the PSU ...

There are some obvious weak points, which I have checked. What I really need to do is to break the feedback loop and fire up an external PSU to see where the issue is. Of course I would need a scope.

However, I have found in the schematic, hiding away, a crowbar circuit. It's a small bit of gizmodery on the voltage regulator board that has a thyristor in the middle; thyristors have the characteristic that once switched on, they only switch off again when the current through them drops to near zero. This one is wired so that the +5v and the +15v are shorted to ground if any of the output voltages exceed some threshold. The shutdown behaviour must be the crowbar doing its job .. so now I have the PSU disconnected from the scope, a dummy load of half a dozen 0.33 ohm resistors in series on the +5v; I am going to disconnect the thyristor, and see which of its inputs is causing it to switch (and it seems to be watching all of the rails in one way or another; +3.5v, +5.15v, +12v, -5.2v, -12v ... ) 

 

Disconnected the crowbar. It's no better, so it is not the crowbar doing it. Next theory is that it is a capacitor on the secondary side which is absorbing lots of current. Firing up the supply, and touching all of the capacitors - can't find a hot one. Not sure where to go without a scope ..

Sharp Optonica SM-4100 Schematic

I have one! Found the old paper copy from a very long time ago, and for the benefit of those out there in InternetLand, I have posted a copy of it in jpg format.

Go to this link for the big version --> SM-4100 Schematic