Monday, 20 September 2021

Sharp MZ-700 keyboard repair.

Back to the Sharp MZ-700. You may remember mine was missing a "blank" key. User psmart over on the Sharp MZ forum, very kindly provided me with a spare key and pillar/contact.

The machine is disassembled again...

The keyboard is flipped over. I want to avoid disconnecting that fragile keyboard connector, and making a load of work for myself. 27 screws are removed..

... and the PCB separated from the keys & frame.

As the key is missing, the keyboard is simply flipped over and the broken plunger falls out.

We need a spring, so time to rifle through the tub of junque fixings to see what we have that looks promising...

I'm kicking myself slightly here, as I junked a few old PC keyboards a couple of weeks ago that would have provided a perfect spring!

Hmmm ... 1 is too small and too stiff, 2 is too tall, 3 is way too stiff and 4 is suitable for closing a garden gate ...

Spring two is simply cut down to about 12mm long...

Looking good.

The replacement blank key snaps in place, and the tension is about perfect.

While the keyboard is in bits, the contact surfaces on the PCB are cleaned with a cloth and a bit of IPA. If your keyboard is not responding well, and needs a bit of force to work, you can clean the rubber contact surface on the plunger too. Mine works fine, so I'll not do that here.
The PCB can now be reassembled, taking care to ensure the power LED fits back through the hole.

And finally the machine is back together... 
A word on retrobrighting... Apparently these machines don't retrobright well, the legends fade on the keys. Now I've been watching a few YouTube videos on the chemistry and science behind retrobrighting. This one especially interests me 
What is interesting here is he uses a heat pad, rather than UV light, which is better for me, as I'm located in an overcast area of the UK. Sun is in short supply, and I don't fancy going out and getting a UV floodlight. Anyway, I won't be retrobrighting this unit, because of the tendency for the keys to fade.

What I did try was leaving it in the sun for a bit (when we were lucky enough to have some in early September (August was hopeless) .
Other than having to keep popping out, and bringing it in when it looks like rain. I think it's stating to make a difference... Look at the top and bottom section of the case here.. I'll carry on for as longs as the sun lasts! 

Sunday, 19 September 2021

Audio Innovations Series 300 repair

Dave the Quad phoned...

"Got an Audio Innovations here, it's cracking in one channel. Can you take a look?"

Yeah .... why not?

It's a Series 300 amplifier from the late 80's early 90's. Not sure I care for it's styling to be honest ... anyway...

Off with the top, and there's a nicely laid out PCB, featuring solid state rectification, an ECC83 phono pre-amp, ECC83 pre-amps, and four triode-pentode ECL86's (the two triodes in each output pair forming the phase-splitter)

Connecting a set of speakers, and waiting for everything to warm up, soon reveals the fault. 

The third ECL86's tube base is flashing over... This can be indicative of output transformer failure, a failure in the valve itself, or the amplifier having been run with the speakers disconnected. Once a tube base has started to flash over, no amount of cleaning will sort it reliably. We'll change all four to be on the safe side. Let's hope the transformer is OK. Try as I might to get a photo of it... I couldn't.

Disassembly is a bit of a pain ... 

First off, check that reservoir capacitor has discharged to a low voltage. This particular amp rapidly sinks to a safe 14V or so after a few minutes off... but check it anyway!

Remove the knobs. They're secured to the shafts of the controls by a small grub screw with a hex head.

Unsolder the wiring to the gain control. A red, white and black wire. I wonder why this network is suspended like this? Added after to remove a bit of DC from the pot perhaps?

Undo the nuts securing the controls to the front panel, and gently lift clear. 

There are three M3 nuts, and a few plastic stand offs which must be released to allow access to the print side of the PCB. I removed the valves. Keep the ECL86's in the same order that they were removed.

I gave the PCB a blanket re-solder as a good few of the joints looked suspect, and replaced the four output valve holders.

Once everything's reassembled, the amp is tested, and, thankfully, the output transformer is OK. 

A lovely sounding amp, and absolutely silent.. no hum and almost no blow. Superb. 

It's soak tested with Matt Berry - The Blue Elephant.

Another saved from landfill! 

Thursday, 16 September 2021

Martin's other Luxman R-1040

Having completed Martin's first Luxman R-1040 (and his L-80V, both documented on these pages), he wanted his second R-1040 re-capped and set up.

...why not ?

I wasn't going to do a post about this, as the job was going to be identical to the previous, but it turned out differently...

Well, after the arduous task of re-capping was complete, it was time for a test & to set the bias etc ...

This is the part where I wished I'd powered it up before I'd re-capped it ...

The bias is drifting around all over the place on the Right hand channel power amplifier. I set it up, and all appears OK, look at the meters a few minutes later and it's drifted. It's also very slightly crackling.

"That's bound to be the pot"

The bias pot, VR202b, is replaced, to no avail ... meh. VR201a (DC offset) also proves to be innocent. 

Checking the DC conditions around the amplifier, things are moving around a lot around Q204b, a 2SC945. Not having a replacement, I swap out the same transistor from the other amp. No change. 

I'm starting to think I may have swapped a cap out, and may have fitted a brand new, but faulty cap. Prime suspect being C205b and maybe C204b ... they both check out OK. 

I then decide to probe round the stage, under quiescent conditions with the oscilliscope. The voltage on the base of Q204b (and the collector of Q204b) is not steady. I scope the supply back. "Spiky" on both sides of R212b, but steady the other side of R211b ... R211b somehow noisy?? I've seen this happen with old carbon resistors in valve amps before. R211b (3.9K) is duly evicted and replaced. The supply is now steady and the amp quiet...

The bias is now set for 45mA per side, and remains rock sold. DC offset is also set, and doesn't budge.

The workshop MP3 player is hooked up, hoping to enjoy some International Teachers of Pop. No such luck ... well 50% luck. 

There's no audio on the left hand channel. Not from any source.

I put a tone into the left hand channel and trace the signal through the amp. It meanders it's way through the source switching, through the balance and volume control, then disappears across the volume control buffer PCB... 

The signal enters on P301a, via the volume pot VR301a, which, thankfully is innocent, as it's one of those 4 pin affairs (used for balance) and the signal appears at the base of Q301a. The DC conditions are, once again, off here. The -23V is fine at R309, but woefully low after R306a. The voltage on the output capacitor should be -16V, it's -2.3V. Thinking back to the amplifier, R306 is checked, and proves it's innocence. It's not going to be C302a, as that is fed by a 33K resistor, and isn't capable of pulling the rail down that far. Q301a ? Q302a?... I'm betting on Q302a, a 2SC1345 (it was easier to remove!) ... a quick diode test in circuit gives inconclusive results, so it's pulled out of circuit, and put on the Peak tester...

... eh ? Should be NPN...

... don't think so ...

... nah, it's sick as a dog ...

... and has given up having much in the way of gain!

Evil thing... 

Looking the in transistor drawer, I don't have a replacement, so a BC547 is used, making sure the legs are bent to ensure the base and collector go to the right holes!

DC conditions are restored, as is sound!

I think there was originally some foam tape against the chassis to provide some insulation between the volume control PCB and the front of the chassis. This has long since turned to dust and vanished, so a bit of capton tape is used when the PCB is refitted.

After the dial illumination is converted to LED, the amp is reassembled and soak tested with the most excellent Pop Gossip by The International Teachers of Pop 

Saturday, 28 August 2021

Sharp MZ-700 (MZ-711) repairs and renovations

About 10 or 12 years ago, I visited a car boot sale, on the look out for some vintage TV a friend of mine had seen there the previous week. I found the TV, it wasn't in great condition, and the seller wanted far too much money for it, I walked away*

This did, however, follow me home. I think it cost me £5. The seller didn't know whether it worked or not. It's missing a key on the right hand side... 

It's a Sharp MZ-700 personal computer. I remember these from my youth, but had never had a chance to use one. It dates from around 1983. 

I rushed it back home, plugged it into the living room TV, and eagerly switched it on. BEEP! it booted up, followed by a bang, and acrid smoke billowing out, much to the delight of the (not quite Mrs back then) Mrs Doz.

"I'll look at that later..." 

... much, much later ...

First off, let's get into the machine.

This is the rear of the machine, there are 5 screws to remove here. You can also see the composite video output, RF output, RGB output, cassette input and output jacks, the "figure-of-eight" (Telefunken) connector for the mains input, an on off rocker switch, a volume knob, a reset button, and various little covers over some ports. There's also a huge earth binding post (got to love a big binding post!)

It appears to be of modular construction. Under the centre section is a port marked "printer" , and to the left (in the photo) is the in-built cassette drive, which just lifts out, after disconnecting it's robbin connector. The right-hand most cover (in the photo) doesn't move...

There's a couple of screws which appear to attach the top to the case below.

... and finally three underneath across the front. 

Hmmm, the badge underneath states it's an MZ-711... OK it's time to consult google.

The MZ-700 is just a range of computers. The MZ-711 is the base model. It came out of the factory as a bare-bones thing. No in-built tape or printer. The tape was obviously an add on (at £50 in 1983!) for this machine.

There's seems to be quite an active forum on the web too, at I've applied to join.

So, off with the lid...

Nicely built I must say....

Z80 at 3.5 MHz, 64K of RAM.... Not much in the way of ROM.

Apparently the ROM only contains a boot loader and monitor... which means you needed to load basic from a tape... and google shows me there are lots of different basic's and many other languages too... interesting.

Zilog licensed the Z80 to sharp, in this case it's an LH0080A. It's pin for pin compatible.

... anyway , let's not get sidetracked ... 

As this machine went bang, and is utterly dead, it's out with the power supply...

It's a switched-mode affair, made by Sharp. Single output, presumably 5V.
I quickly spot the culprit....
Big bang, lots of smoke, acrid smell... it's our old friend the Rifa suppression capacitor! 

It's duly replaced with a suitably rated X-class capacitor. 47nF for future reference (or should that be rifarence 😆). One of the two 33uF 400V reservoir capacitors has a slightly domed top. I change them both out. Testing of the other caps shows excellent ESR and values within tolerance, so are left in place. The mains fuse is open, unsurprisingly. It's a T500mA. A new HRC one is fitted...

...and the supply seems to work well. 

With the supply installed back into the machine, the supply was switched on, and the machine beeped... RF was coupled into the tiny workshop TV, and ....

... well, it's a bit of a disappointment. The display is interacting with the keyboard, but the characters are corrupted. Resetting the machine changes the pattern of corruption. 

The keyboard is VERY carefully removed... go easy with the ribbon connector, I got away with mine, only "folding out" two of the contacts, and managed to repair the socket. 

All the socketed chips (the monitor and character roms, and the Z80) are removed, the sockets cleaned and refitted. No change. 

The main board is removed from the case, and there's a rusty spot on the metal screening tray beneath.. 

.. with an accompanying mark on the PCB itself...

... it's right by this IC, a 74S157, a non-inverting quad 1 to 2 line data selector chip. It's removed from the board and tested.. 

... and it's bad, which is a pity, as I don't have a '157 , but a '158 (which is the same, but inverting). A replacement is duly ordered.

.. and fitted, and makes a slight change to the display. Last time, hitting reset changed the characters on the display, now, at least it's constantly corrupt!

Now the wonderful people on the forum had suggested the CG (character generator) EPROM may be corrupted. They only had a design life of 10 or so years, and we're way outside that now. The good news is, if it is corrupted, all I need to do is erase it (with UV light), and re-programme it. I plug it into the TL866 programmer and read the contents of the ROM out. This is compared with a known good .bin file, and the bad news is the ROM is fine ...

Back to basics. Check the supply rails to the IC's... I scoped up the 5V rail, and it gets horribly noisy around the MB8264-15 memory IC's. Ugh, there's more than 500mV of hash on the rail... 

There's a couple of electrolytics there , C82 & C86 ... swap them out ... and bingo !

For the sake of it, all 9 electrolytics are changed.
For future reference, these are :
2 x 100uF 10V
2 x 10uF 16V
4 x 1uF 50v 
1 x 22uF 16V

Be careful when changing the caps, as the PCB quality isn't the greatest, and it's double sided. Were a track is soldered component side, it's best to warm that side up, and pull the cap out. There's also a couple of sold tantalum capacitors in there. I don't have any, so they can stay for a while, although they do have a reputation for failing short circuit with no warning, and going pop like little firecrackers!

Note there are several components in the top right of the picture, which are lying down flat against the board, so to clear the modulator. This includes one of our new ones. 

With the machine partially reassembled, the keyboard is tested... and much to my surprise works perfectly! I expected a disassembly and clean up of the contacts, but no. The missing key turns our to be a "blank" key on this UK machine. On the Japanese version, it has a character on it. One of the users on the MZ forum has said he has a spare key from a spares machine I can have. What a super person!

Onward with the machine.. I'm keen to get something loaded. The cassette deck is mounted, and L (for
load) followed by CR (carriage return) switches the motors to the cassette deck on, and it actually appears to work. It's cleaned up.

Now, a couple of years ago, I obtained a wav file of "Z80 Basic", which my good friend, Nathan, duly recorded it to a cassette for me. I put the cassette in a safe place.... and even remembered where that was ! 

So the tape is inserted, and, there's no take up. I quickly stop the tape before we get into a mess...

Out with the cassette deck, and remove the four screws holding the back on... the one on the bottom right is longer...

I notice there's some debris on the back panel... 

Now the belt is shot, and is replaced, but it's not that shot, because it managed to turn the capstan, and rewind is working well...  out with the deck itself... 
Undo the slotted pillar thing (there's probably a technical name for that) , there's three in total, all of
differing lengths. 

Lift the mechanism carefully from the case. More debris evident ...
... and look at the state of the supply reel table rubber, it's got chunks out of it! Despite that it's still
working (albeit a bit "lumpy" !)

The take up is reel is removed, and the reel rubber, is badly cracked, and beyond refurbishment... The Lidl box of plumbing washers is pressed into service and a suitable new tyre located!  

The unit is reassembled, and we attempt again....

Bingo! Sadly this is as far as I can get.

The deck most likely needs checking for speed and azimuth.

After an alignment of the cassette drive, and a lot of cussing, it was determined that the cassette I'd used to align the deck was of poor quality. I asked my friend Nathan to record a new one for me, which he duly did, and after aligning with that, all was well ...
The ubiquitous test program is run ... 

Except all this tedious messing around this cassettes is all very well, but ... my ageing MP3 player works really well at putting data into the Spectrum ... can this work here? 

Now the MZ-700 was also sold without a tape-drive. It's equipped with 3.5mm jacks for reading and writing data to any tape deck, so let's use those... except it doesn't work.

This is the relevant part of the circuit diagram. It can be seen that the external plugs (P-8 read and P-7 write) are not jumpered across P-12, which is where the cassette deck is connected. 

Making some jumpers up out of dupont connectors, and grounding pin5 (sense) allows the machine to load perfectly from my MP3 player via the external connector. Perfect, and easily reversible if I ever wish to use cassettes again!  
Intrigued with the "PETSCII" style characters on the keyboard, I conjured up an infinite maze programme... not as elegant as the C64's one-liner, but gives a pleasing result.  

Another saved from landfill!

*the set was there the following week, now with the tell-tale mark on the tube face that the neck had been knocked off the back, and the tube was now shot. He still wanted silly money. I still walked away.