Friday, 30 April 2021

Further work on a Microbee 1248-6 mainboard

This is the definition of a long-term project. I have a bare Microbee 1248-6 mainboard. It's one of the later revisions of this board. Being blank, it's a good candidate for replication.

Some years ago I created a schematic for it in Protel, and made a PCB. The PCB wasn't particularly accurate, as I mainly just threw the autorouter at it. So (probably) functional, but not particularly exciting.

Success at getting KiCad to do beautiful curved traces reignited the spark of that particular project, so I pulled out the files and had another look.

I imaged the board at 600dpi using a scanner. Then cropped a section of that and fed it into the gimp. I played around with levels to increase the contrast between tracks and boards.

Then I put the result of that into the KiCad bmp to component converter. This tool is designed for making fancy logos, and converts a bitmap image into a bunch of vectors that can be used for gerber plotting. The file goes on the silkscreens. I have seen a board made by one of the guys at the microbee software preervation project that used this technique to create gerbers directly, but there were issues with getting board houses to accept this.

So now the next step. Trace over the image in KiCad using proper components, tracks (15 mil) and vias. It takes a while, but it's enjoyable bead therapy work. There's no need to accurately follow the original. Indeed around corners it's better to leave the round the corners plug in to do it neatly.

For the bottom layer, just process the scan the same way, then convert it to a silkscreen component, and flip it when you place it.

This makes a real board, that can be DRC checked against a netlist (when I get around to putting the schematic into KiCad), to ensure that it's correct. Finally, run the board through the round the tracks plugin to smooth things out and create absolutely stunning artwork, even better than that used to make these computers in the first place.

Edit: After a few days work, it's looking pretty cool:

And with the soldermask obscuring all the pretty curvy traces:

Saturday, 24 April 2021

Microbee SuperPAK - An EPROM Expansion Board

Quite a while ago I developed hardware for a compact flash coreboard for my Microbee. This added IDE based compact flash storage, allowing massive disk storage under CP/M. Others helped out with the BIOS software. It never got to the point where I was completely happy with it. Compact Flash cards are finicky things, and the combination of a 5V system and very slow timing conspired to make things less than 100% reliable. Being a read/write thing, every time something crashed it would corrupt the directory structure and necessitate reformatting the card. This was (and is) frustrating.

I realised that the reason I wanted all the storage was just so I could have a straightforward way of loading software on the Bee that doesn't necessitate waiting for tapes to load. I've got a couple of dozen games and suchforth that I like to play every now and again, and frustrations in getting it going mean I don't play with the gear as much as I'd like.

So back in the day I had a "ROMPAK" for my bee, which was a little expansion board that could take eight 2764 EPROMS, for a whopping 64K of storage. By writing to port 0A, you could select one of the eight ROMs, and it'd appear at C000 in the memory map. We used it to allow us to have Wordbee, EDASM and the Mytek wordprocessor in the same bee without having to swap ROMs, and it was neat. I envisaged loading Emu Joust into a couple of EPROMs and writing a short routine to copy it into RAM at the corect location and then jump to it.

So this board allows you to do that, plus maybe store some other games and bits of software. I've allowed all 256 possible PAK ROM positions, using an 8-bit latch and four 27C040 EPROMs. These are current production parts that go for the princely sum of $8 or so ea, as long as you don't mind them in a OTP flavour. You can get Flash versions too.

The idea is that the first PAK location holds software with a menu, that allows you to choose what you want to run, then copy that into RAM (or leave it bee in EPROM if it's a PAK thing), and execute it. Sort of like the PC85 but on steroids. The software to do that should be a doddle, or at least much easier than writing stuff to run CP/M. Even better, there's no FPGAs and no CPLDs to worry about. all the decode is done with bog-basic 74 series parts.

Edit: I just found the "round the corners" and "teardrop" plugins for KiCad, which make boards look like they were layed out using bishop graphics and tape. I reckon it looks so nice that it's worth leaving the solder mask off and going for an ENIG finish, ala expensive Hewlett Packard boards of the 1980s.

Sunday, 17 January 2021

Suzy's Super Rosin Paste Flux.

We're in a no-clean, water-based, lead-free world, and that sucks. Electronics manufacturing is about getting product out the door with as little effort as possible, so they leave the flux on the board rather than clean it off. This "no-clean" flux isn't just a pretty pathetic flux, it's also a right pain to clean off the board afterwards, so it should really be called "can't-clean". All well and good if your standards are low, but if you care about the quality of what you make, you'll want real rosin flux.

Decent rosin paste flux is getting harder to find every time I go looking, and with the secrecy around formulations who knows what's in that stuff. The solution is to take control of the process and make our own. Before I start though here's a quick primer on what flux is, and what the various formulations for electronics work are. You have to really understand your needs to formulate a flux that's appropriate.

Flux is used to exclude air from the join while you're heating it, in order to prevent the corrosion of metals that happens when you heat them up. It also contains varying amounts and strengths of acids, which bind to the oxides present on the surface of the metal and remove them from the join, so that when you stick solder in it's able to wet the parent metal properly. You want it in either a liquid form or a paste form. Liquid flux allows you to paint it on with a brush (thick liquid) or dispense it with a pen (runny liquid) or spraycan (very runny liquid). For board assembly I like to dispense with a syringe. This allows me to use very thick paste flux that stays in place on the board. I like it to be a little tacky, so when I place surface mount components, they don't roll around. The solvents used between liquid and paste fluxes are different, but the key ingredient is still the rosin.

The various types of flux are:

  • Rosin Flux. This is the grand-daddy of fluxes. It's made from tree sap, that's had the volatiles (turpentine) boiled off. The resulting rosin is an amber crystaline solid, that breaks fairly easily. It's mildly acidic, due to the presence of abietic acid. Being a solid it needs some sort of solvent to make it useable to coat the joint with. It's mild, and perfect for board assembly where you're using good quality, reasonably fresh components. When you solder the join, most of the solvent boils off, leaving the glassy rosin behind. This is reasonably easy to remove using more solvent.
  • RA flux. Rosin Activated. This is rosin with various acids added to attack the oxides on your circuit board and component leads. It's a bit on the nasty side for most board assembly work, as we're usually working with boards and components that have been processed well (HASL or ENIG coating etc) and stored well to exclude oxides. This is used for those awful terminals that have been out in the air or on the boat for decades. It is imperative that you clean it off after soldering, as the acids will continue eating the metal and after a few years it'll stop working. The residue that's left after soldering is dark in colour, containing the oxides that have bound with the acid in the flux. It's harder to remove than the oxides from rosin flux, but still doable with a solvent and a bit of scrubbing.
  • RMA flux. Rosin Mildly Activated. Reduce the amount of acid in RA flux and you get RMA flux. It's a compromise. Good as a substitute for rosin when things aren't wetting well (old components, old boards). You need to clean it off afterwards, as it'll eventually damage the equipment otherwise. Cleaning wise, it's a mid-step between rosin and RA.
  • No-clean flux. This is a formulation made from random industrial chemicals, gelling agents, what-have you, that's supposed to mimic real flux then dry clear so the person contracting you for assembly work doesn't notice that you haven't cleaned the boards. It's motive is to skip a step, not to improve the product. Much worse though, it's often incredibly hard to clean off. The manufacturers want something that doesn't get tacky or runny in use, so they make it so that it dries hard. This equals very difficult ro remove. Don't use it. Your boards deserve better.

Here's a recipe for real rosin paste flux just like your grandma made when you were a kid, back when she worked as an industrial chemist. Nothing but the finest free-range organic ingredients, and made with love. Cook some up and gift it to that special engineer in your life, or just make a batch for your own use.

Seriously, this doesn't just resemble commercial rosin paste flux. It's the actual thing. It's not activated, which means that there are no acids added besides the naturally occurring abietic acid in the rosin. You can add additional acids if that suits your process, but I'm not particularly interested in that. Ninety percent of the time I'm happy with straight rosin, so I can use the commercial stuff for that occasional stubborn join.

This recipe makes enough to last a typical engineer a good few months. The ratio of rosin to vaseline yields a flux that's just right for syringe dispensing, with good tack.


  • 15g of gum rosin, in chunks.
  • 10g vaseline.
  • Isopropyl alcohol (varies from 0g to 5g).

I just buy rosin online. It's got a huge pile of uses, both industrially and for consumers. It's usually used for making things stick, so as a powder that you can rub on your hands when doing rock climbing, playing baseball, what-have you. It's generally advertised as gum rosin, which is made from sap collected from pine trees, but there's also wood rosin, which is made from grinding up the tree roots after harvesting the pine. I believe they're the same, but I've only ever bought gum rosin. Please be sure to only buy rosin that's certified free of angry bees.

The vaseline comes from the supermarket. Bunnings sells isopropyl alcohol.


Combine the rosin and vaseline in a 48ml Kilner hexagonal preserve jar. Break up the rosin if necessary to get it into the mouth of the jar.

Heat the mixture in the oven at 120˚C for about 20 minutes. The rosin will melt and form a thick treacle layer under the vaseline, which will go perfectly clear.

Pull it out and stir to combine layers with a paddle-pop stick.

Pop it back in the oven for another few minutes, to encourage any trapped air bubbles to rise to the surface.

Take it out of the oven and let it cool for ten minutes or so before sucking into syringes, if that's your preferred dispense method. I find if I leave it too long it thickens up and is hard to draw into the syringe.

If you find it's too thick, you can add a few percent isopropyl alcohol. Isopropyl is the solvent in liquid flux, and it's probably also the solvent you use for cleaning up your boards after assembly. Go easy on the isopropyl though. As well as thinning the flux quite well, it also boils off very rapidly on the PCB when you solder, resulting in spitting when you apply the iron if you overdo it. I find the best time to add the isoproply is as the mixture is cooling. It's easy to stir in then. Don't do it too soon after taking it out of the oven though, as the temperature initially is above the boiling point of the isopropyl, so you'll only waste it.

You can reheat the flux multiple times to get it just how you like. Warm it up to 50˚C or so, so it's runny. Add a little isopropyl. Let it cool. If you overdo the isopropyl then just warm it up over 80˚C, and the isopropyl will start to boil off.

The stuff in the jar, or in the syringe, is just a lot of rosin in suspension in the vaseline. When you heat that mixture, two things happen. Firstly the rosin melts. It flows over the joint and does the good things that the rosin does. The vaseline has a boiling point of around 300˚C, so a good amount of it will boil off. This is mostly what you get in the little wisps of smoke that come off the iron, plus of course some rosin that's caught up. It's not good to inhale, so use some fume extraction. It doesn't have to be fancy. A simple muffin fan sitting near to the work on the bench is usually plenty. If you must use RMA or RA flux rather than straight rosin, definitely use fume extraction because ingesting the acids is not at all good for your lungs.

Sunday, 20 December 2020

A new tow vehicle for Elena

My old pug that used to tow Elena became very unreliable, with crankshaft position sensor errors and other sundry issues cropping up regularly. Its age was such that we were going to have to do timing belt and particulate filter, and there was a fair bit of panel damage from an intimate encounter with a roo. So when rego came due last year we took it to the wreckers.

Since then we haven't had a car with a towbar.

I've been talking about getting a ute for ages, and recently my main car has been off the road (another roo), so we brought that forward. Top of the list was something that could comfortably tow Elena. I also wanted something that would be good for picking up piles of wood for the next boat. I decided something with a tray 2.4m long would be perfect, as so much wood comes in 2.4m lengths. Also I didn't want to spend too much, as it's a second car and will have it's arse dipped in the water occasionally.

So meet our new ute. It's a bottom of the range Mitsubishi Triton single cab, with a tray. It's a 2011 model, with 120,000km and is as simple as a vehicle can be. And it's just the ticket for towing Elena.

I've come up with a simple method of securing the mast so it doesn't scuff the floor, just by hanging it between the aft mooring cleats. I'll sew bags for the rest of the bits.

Tuesday, 8 December 2020

AEM6000 Based 50W and 100W Amps

This is a design I've had in use for rather a long time. It started life as a design exercise to see if I could do a more space-efficient board for my original AEM6000 based amps, as I wanted something that would fit on a 50mm high heatsink. Along the way it changed a little from Tilbrook's original in topology, and rather a lot in component choice. It has better performance than my original design, both through a better tighter layout and also through the use of faster transistors.

I've built a bunch of these, using both Renesas and Exicon lateral MOSFETs, and subjected them to all manner of abuse. I had one fail, due to a leaky mica compensation capacitor (see noiseUnit speaker thread), but apart from that they've been rock-solid.

Lots of component substitution is reasonable. I like to use MELF resistors, but that's mostly just bloody-mindedness. An exception is the feedback divider. No, It won't work with cheap vertical MOSFETs.

The design is free for use for non-commercial purposes.

The 50W design is available on my Google drive

The 100W design is available on my Google drive

Thursday, 19 November 2020

Tektronix TDS-340a CRO

A mansplainer on DIYAudio just told me my TDS-340a has an LCD, not a CRT.

This photo is to put him in his place.

Saturday, 14 November 2020

Building the Per-Anders Sjöström QSXM2

I don't always just build my own toys. When other designers make really beautiful kit, I buy their PCBs and go to town.

Such is the case with a few of Per-Anders' designs. He and I share a common design rationalle, which can best be summed up as "components are cheap, if more of them makes it perform better, go for it!".

A perfect example is the QSXM2 phono preamp. Something like 200 resistors, 110 transistors, around 100 caps. It's really not mucking around. It's a preamp that leaves no stone unturned in the search for performance. I'm keen to see if I can improve on the basic phono pre in my NAD3240PE, and this one looks like a very good candidate. I've built some othe Per-Anders designs (mainly head-amps), and have been very impressed.

So I ordered a PCB from his website, downloaded the design doco, and started work:

I've set my work area up with a nice new A2 cutting mat, which protects it from the soldering iron and scratches. There's no need for the microscope for this guy, as it's all through-hole.

So far I've done all the resistors (I had comprehensive stocks, which will now need to be replenished), and I'm on to the caps, many of which I'll have to order in.

I'm looking forward to trying it out!

Sunday, 8 November 2020

Annual HP 3585B stinky capacitor hunt

One of the hazards of using elderly test equipment, such as my HP 3585B, is old capacitors. HP loved using tantalums, which while having great low ESR, are also teensy little time bombs. Occasionally (typically at turn-on) there's a stink, and that's the sign that another one has let go.

So this happened the other day, and I spent a couple of hours sniffing the little blighter out, quite literally, as they're quite pongy and your nose is a great diagnostic tool.

This is quite a beast of a spec-an, weighing in at around 35kg. It's really beautifully made, with gold plated circuit boards. I started by pulling boards one at a time and giving them a quick once-over.

Eventually I found it, at the back of the input module on the underside of the unit.

This one is a little 2μ2 20V Kemet axial tantalum. Luckily I have a few in a drawer. So I whipped it out and popped a new one in, and now it's good as new (until next time).

Saturday, 31 October 2020

NAD L70 receiver refresh

On the bench today is a NAD L70 receiver. It's a bit newer than the stuff I generally like to work on with NAD, but it was going really cheap, and is cosmetically pristine, so I thought I'd have a play.

There were two symptoms of bad behaviour. Firstly, a truly dreadful hum present in both channels, and secondly the CD/DVD player was really flaky, reporting "no disc" as often as not, and pausing and stuttering quite a lot while playing when I could get it to recognise a disc.

On opening it up it was really obvious what was causing the hum. There are 4 large capacitors on the top (CPU) board, that provide smoothing for all the lower voltage DC supplies. A couple of these were obviously bulging. So I whipped all four out and replaced them with 2200µf 25V 105˚ Panasonic electrolytics. While I was there I replaced a bunch of the smaller caps nearby.

That sorted the hum. Next up was the misbehaving CD/DVD player.

This is a real bear to get to, being underneath the CPU board. In order to get the transport out you have to first open the tray and unclip the oval NAD bit on the front. Then retract the tray again, open the cover, remove the CPU board, and it's pretty obvious.

The transport is an ATAPI/IDE one, made by Raymedia (RMC) part number RL-A700. This is common to a lot of CD, DVD, and SACD players of the period.

Once the transport was out, I started by popping the top cover off and giving the lens a wipe with a kimwipe and a little isopropyl. I cleaned the dried grease off the sled slides and gears, and relubed with Electrolube Special Plastics Grease (SPG), which is just for this.

Cleaning didn't help. The supplies were tested good, and this transport really has nothing to adjust, so I next ordered a replacement pickup. Part number for the pickup is SF-HD60.

It arrived after a couple of months, and I fitted it today, removing the anti-static solder blob after fitting.

This made a really big improvement, but it still stalls very occasionally and 2 or 3% of the discs I've tried today don't read. After repeating the clean, relube, reassemble, test cycle a couple of times, including pulling the spindle motor and giving that a bit of a spray of iso (did I mention that it's a pain to get to the transport under the CPU board), I've decided to bite the bullet and just order a whole new transport for it.

So that's where this one is at. It actually sounds really good, and with the exception of three useless channels will make a lovely all-in-one stereo. I think I'll set it up in my office at work once it's finished.

Tuesday, 6 October 2020

Telegraph Roads - an objective comparison of Dire Straits - Love Over Gold in various formats.

I really do love Dire Straits. Perhaps my favourite Dire Straits album is Love Over Gold. There's a bunch of reasons I love it; it's melodically beautifully put together, there's a nice variety of different styles, and most importantly it was released in '82, when I was at the tender age of eleven, so I played it rather a lot as an impressionable teen. A whole lot of synapses in my brain are dedicated to this album.

So I've had it in various formats over the years. Originally in the '82 issue Australian pressed LP (Vertigo 6359 109/4), slightly more recently on the original Australian CD (Vertigo 800 088-2), the 1996 remaster (annoyingly also 800 088-2, but copyright '96 rather than '82) and most recently on the Japanese SHM-SACD (UIGY-9637) which is generally well regarded by audiophiles.

I'll stick to the very first track, Telegraph Road. It's 14 minutes 20 of really good music.

Before I start, all my versions sound pretty good. Telegraph road is one of the most engaging, haunting pieces of music I know of, and they're all good copies. My LP and original release CD, though old, are very well loved and looked after. My SACD version is brand-new. I cannot tell whether the original CD or SACD is playing, and I honestly can only tell the LP because of the pops and crackles. I can tell the difference with the 1996 CD though - it's louder. I've always really enjoyed the LP. It's probably simply because of the tactile nature of the medium, and the way I sit down in a nice quiet room to listen on a reasonably good stereo, but it has character that I enjoy. But these are subjective evaluations, not objective ones. They deal with my emotions, which are coloured by handling the album in the format in which I first played it. And subjective evaluations are easy to fake.

To evaluate the various versions I first import them into my mac. For that I use a variety of different things, and they are different depending on the medium.

For the LP, I play the LP on a NAD-5120 turntable (one of the later ones with a more conventional tonearm), with Goldring Epic-II moving magnet cartridge at 2.0g force. I run it through the phono amp and preamp in one of my NAD3240PE integrated amplifiers (why yes, I do have a couple of these wonderfully temperamental beasts), then to a Roland FA-77 firewire ADC, then to my mac. I run the ADC at 44.1ksps, 24 bit. The turntable, cartridge, and amplifier, while nothing to write home about, are representative of the better quality but not esoteric kit that was available in the 1980s. It's not the gear that I listened to the album on as a teenager. My parent's HiFi was a Kenwood one (with graphic equaliser!) and I actually only listened to the LP the once, while recording it to tape at 7 1/2" per second on our Akai cross-field reel deck. I was a weird kid, and I'm a weird adult.

For the CDs, I simply rip using itunes on the mac and a CD drive. I save the file as 44.1ksps 16 bit PCM, uncompressed.

I have no way of directly transferring the DSD stream from the SACD, so I go via analogue. I use a Sony UBP-X700 4K blu-ray player (which plays SACD contant), to a NAD D1050 DAC, then to my Roland FA-66 ADC, then to the mac. As with the LP, I digitise at 44.1ksps, 24 bit.

Let's work our way through the copies in chronological order. First, the LP. I started this expecting to see a gradual improvement with better equipment, but in reality it's quite a lot more subtle than that.

I've trimmed the track to 14 minutes, 22 seconds. The official length is 14 minutes 20 seconds, but I've left perhaps a second either side. When digitised, I kept the gain such that it never clipped. Then I used the Audacity amplify tool to amplify it to 0.1dB from clipping (3.6dB gain, for what it's worth).

The resulting audio reports the following dynamic range in the MAAT DR Offline Meter:

Dynamic Range: Left 12.52dB, Right 12.47dB
Peak: Left -0.70dB, Right -0.10dB
RMS: Left -17.87dB, Right -17.63dB

MAAT reports the DR as 12, which is generally accepted as very good. I've seen reports of 13 for this track on the original 1982 LP. I may be able to do better with a better cartridge and phono amp. Also I've done nothing yet to clean up the clicks and pops, which may be colouring the result a little.

So next the 1982 CD. I did nothing to the file. Just imported it into Audacity for viewing and ran it through the DR meter as-is.

This looks similar to the LP at this level of zoom, which is nice. I note there's a bit of a fade in applied to the first 20 seconds or so.

MAAT DR Offline Meter reports the following:

Dynamic Range: Left 13.40dB, Right 13.09dB
Peak: Left -0.11dB, Right over
RMS: Left -17.95dB, Right -17.06dB

This is a better result than the LP, with close to a dB better dynamic range.

Now for the 1996 "remastered" CD:

MAAT DR Offline Meter reports the following:

Dynamic Range: Left 10.34dB, Right 9.89dB
Peak: Left -0.00dB, Right over
RMS: Left -14.31dB, Right -13.96dB

It's pretty obvious to see why this isn't a well-regarded mix. It's certainly subjectively louder than the originals, but that's because of judicious use of compression. Alas the '90's were awash with this sort of awful treatment of audio.

Finally we come to the "definitive" 2014 SACD:

This is interesting when compared to the original LP and CD. Note the gradual increase in gain during the final instrumental passage, up to around 1.5dB for the finale.

MAAT DR Offline Meter reports the following:

Dynamic Range: Left 13.91dB, Right 13.68dB
Peak: Left -0.38dB, Right -0.00dB
RMS: Left -19.27dB, Right -18.50dB

I keep looking at the fiddling to the end of the instrumental passage. Could this simply be a cynical attempt to fudge the DR readings? Could the record industry really be that blatant?

What happens if I undo this? On a whim I applied a fade-out of 1.1dB from 11 minutes onwards (I actually added silence to 22 minutes, then selected the audio from 11 minutes to 22, and used the "studio fade out" effect in Audacity, then amplified back to -0.1dB peak:

So now it looks... Pletty much exactly like the 1982 CD. MAAT DR Offline Meter reports the following for my "un-tweaked" version:

Dynamic Range: Left 13.37dB, Right 13.18dB
Peak: Left -0.73dB, Right over
RMS: Left -18.55dB, Right -17.81dB

Which is very close (we're talking within 0.1dB) to the 1982 CD. Enough so that I'm wondering if perhaps this isn't the extent of the "remastering"?

So, what to say? In raw numbers, the SACD wins. But only, I feel, because they cheated. Discounting the cheat the 1982 CD is the winner.

So that's a "whole of track" view showing the really loud bits. One of the really engaging things about this track is the very quiet bits between passages. These don't affect the overall dynamic range figures terribly much, as they're derived by simply taking the difference of the whole-of-track RMS vs peak. If a quiet passage is a few seconds long, it won't have much impact on a 14 minute track. Sections of silence, or near silence, stretch the medium, and are a good indicator of a really good recording. CD should have an overall signal to noise ratio of 96dB, so we should be limited entirely by what the original (analogue) tapes could manage.

The bit I'm interested in comes immediately after the line "There's six lanes of traffic, three lanes moving slow" at about 5:10 into the track. It moves around a little on the various recordings due to small changes in speed, plus slightly varying start times. In any case, there's a very quiet passage, then some piano. Let's examine the second before the first piano note.

Again, we'll start with the LP:

I've simply used the contrast analyser (a power meter) within audacity to show the power in the second before the piano note. -49.36dB is pretty good. Remember this is a media that's full of pops and crackles, and I haven't removed these.

The 1982 CD:

Interestingly, significantly noisier than the LP version, at -46.45dB. Nearly 3dB. This is incredibly strange, as one assumes that CD is much quieter than LP. There must be something else at work here. Perhaps the original master tape has degraded somewhat between the LP pressing and the CD mastering?

The 1996 CD:

This result of -43.77dB is pretty much what you'd expect from something that's been compressed. The process boosts the quiet bits. Nice for listening to in the car, not really what you want otherwise.

Finally the SACD. I've gone with the version as recorded, with the slowly increasing gain at the end to fudge a better DR value:

Now this is interesting. It's -48.01dB, which is good (though not as good as the LP) on the face of it. However when we recognise that this bit of audio is essentially attenuated by something like 1.5dB, in reality it's about the same as the original 1982 CD. This lends some weight to the hypothesis that the SACD is simply the original CD with a slow increase in gain for the last few minutes.

Examining spectrograms of the quiet bit really throws the difference in noise levels among the different versions into sharp relief. First the LP:

The 1982 CD:

The 1996 CD:

And the 2014 SACD:

This is maddening. The LP version is obviously quieter than everything else, in between the pops and clicks! The 1982 CD version and SACD version are really the same thing, and the 1996 CD is garbage. Interestingly the liner notes for the '96 CD version go on about "super bit mapping", a technique using HF dithering to reduce the noise level for the lower frequencies. I'd expect to see an increase in noise towards the high frequencies, but I see no such thing. I'm guessing any dithering improvement is completely masked by the noise level of the source, which simply isn't as quiet as the source used for the LP master.