Wurlitzer 200A electronic piano repairs.

Ah ... another YouTube thing ... a two part thing this time! 

Enjoy

Pye 169 revisited.

Remember the Pye 169 back here?

 
Well... it's been acting up. Rather than the usual write-up, here's a bit of video entertainment, also ideal as a cure for insomnia...

 

Enjoy ... ! 

Commodore 1541 Floppy Disc Drive repair.

I bought a Commodore 1541 floppy drive on eBay... not just any 1541 ... oh no... a broken one.

Apparently it was also used as a prop in a film.. http://www.computinghistory.org.uk/pages/50703/Bandersnatch/

Anyway, it doesn't work. No power.

Cursory checks to the input fuse show that's intact, so caution is thrown to the wind and mains applied... the over current trip on the bench pulls straight out (it's rated at 10A)

Checking the input fuse shows it to still be intact ..?

Ah well, in bits with it... 

Removing the main PCB allows access to the transformer etc .. 

... but what's that goop on the bottom of the PCB? 

It's waxy ... 

The mains power input couldn't be simpler. IEC Socket, fuse, switch, transformer... 

... the red seal on top of the socket looks  to have "burst" ... is that the source of our waxy goop, and excessive current draw?

Removal of the lower part of the case, and lifting the chassis out, shows more goop, right by the socket. 

The rivets securing the socket are drilled out, and it's removed.

Looks like the internal filter failed and got excessively hot at some stage, and melted the insulation. It measures a short circuit between live and neutral! A new filtered socket is obtained...

.. and duly fixed into position with some M3 fasteners. Finding a shorter socket is essential, there's just not enough room for one with tags. 

And powering up gives us the familiar sequence of LEDs, and a spinning disc. 😊

Now what's needed is a C64 or VIC20 to test it ... hmmmm ...

Someone's been here before .... The Sony TC-377.

Jason from the record shop called.

"A mate of mine has a Sony reel to reel that's not working, can you take a look?" 

Yeah ... why not.

It's a Sony TC-377, very nice. 

A quick test, and it refuses to go into play. A common issue, usually to do with sticky grease on the selector cam, worn or perished belts, or worn or perished idlers. Rewind and fast forward are in rude health, so that rules out a couple of the idlers straight away. 

Let's get it apart.

Remove all the knobs from the front panel, they should all just pull off, except the pause lever which unscrews clockwise. Remove the head cover, and unscrew the retaining pins. Now I've smelt a rat, they're lose. They're never lose. There are also five retaining screws on the front panel, or rather there should be. One is significant by it's absence. There's one that's easy to overlook... it's by the heads. 

The front panel then lifts away. 

Four screws remove the back panel. We won't need to be in here, but if you have no motor running, or it's just humming and not turning, the chances are that silver motor run capacitor as the top has thrown in the towel, and needs replacement. It's a dual value cap, one is for use on 60Hz supplies, the other, placed in parrallel for 50Hz. You only need to get one value to replace it, unless you intend to go globe-trotting with the thing... For 50Hz operation, you'll need 2.0uF , for 60Hz 1.5uF. 

OK, we need to remove the mode selector mechanism, which is a pain. First off remove the three screws circled in red, which will allow you to remove the heads in one piece, don't undo or losen any others!!!  There are three brass spacers behind, put them somewhere safe. Now remove the four screws circled in cyan. Note here than one of mine is chewed up ... hmmmm.

You can now extract the mechanism, unhooking the levels and wiggling off the take up belt from the capstan flywheel as you go...  

Now, I expect this mechanism to be covered in gold-coloured grease, that is doing the perfect impersonation of glue. Normal procedure is to clean it all of with IPA, diassembling as required, and re-lubricating with superlube or similar.. But this has already been done. Only one issue is the cam select lever has jumped behind it's slot. It's a quick fix... 

... but it's not the end of the story. There's a couple of brake pads missing in action, and the capstan drive idler lever is seized up, so the capstan never gets drive. The idler is secured to the lever with a circlip, so that's popped off and removed.

We can't disassemble the lever, as it's riveted together, so some heat is applied to the pivot with the soldering iron, and some penetrating oil gently worked into the pivot. Eventually it starts to move, and some spray grease is eased gently in to prevent re-occurrence. The lever should move freely, and spring back into position. 

There are a few other levers on this side of the mechanism which look like they are in need of some attention, so they're sorted out, and new brake pads fabricated and fitted. 

Finally the mechanism is reassembled and tested. 

The tape has some Christmas carols on, how appropriate.

And the case is reassembled. 

Another saved from landfill! 

Trevor's Tandberg TP41 and the mystery of the "Stabistor"

 Trevor called..

"Do you remember my Tanberg radio you repaired for me a while back?"

As it happens, I do... I fitted a new telescopic aerial, and cleaned up the controls a bit.

"It's howling, can you look?"

Yeah, why not...

Well, Trevor made a couple of attempts to get it round to me, each time thwarted by the radio, obviously feeling threatened by a trip to see Doz, putting itself right, and behaving again for days.

Eventually the fault was more or less ever-present. 

It's a Tandberg Portable 41, dating from around 1970, and just oozes Nordic quality.

I got the thing on the bench , and sure enough it's sat there ... making a sort of mooing noise, that varied in frequency with the volume control. 

Let's get the thing in bits... 

First off remove the two screws to the left and right of the handle.. 

Gently bend the handle out and remove it..

Unhook the aluminium side pieces from the bottom of the case, one each side... 

You can now remove the wooden "clamshell" from the chassis..

Remove the knobs.. 

You can now remove the top. You'll notice I removed the telescopic aerial too, there's a screw in the base, and, once removed, it just slides out of the top. I'm not sure it was necessary to do this. 

Remove the two handle retaining plates before they fall off.. 

(It's worth noting how they sit the the grooves in the top panel to aid reassembly later!) 

We can now get at the PCB, but access to the component side is difficult... 

So remove the speaker , and place it carefully to one side, you won't find another... 

Of course, once it was disassembled, the fault vanished....

Some googling occurred, and a schematic obtained. 

This website, had a very detailed description of the set, and suggested the "Stabistor" was at fault...

https://www.electronics-notes.com/articles/history/radio-receivers/tandberg-tp41-circuit-block-diagram-maintenance.php

The what? Stabistor?

So, the schematic is consulted.... 

It's a weird looking thing, and, as suggested in the website, is very corroded.. It's shown on the schematic as an ST1,5, given the designator D501, but it's not some sort of diode. It's drawn as a battery, and that's sort of what it is... it's acting as a voltage stabiliser, feeding a stabilised 1.5V supply to the RF & IF stages. 

It doesn't look corroded here, as the green corrosion just fell away. It had sort of made it's way down the wire end, and had even corroded the PCB, which took quite a bit of cleaning up before it could be persuaded to take solder again.

"That's bound to be the fault" ... a big bag of red LED's is opened, and each one tested, until I find one with a sensible forward voltage drop, that I can use in place of the stabistor. It's tacked into the circuit, along with a 220uF capacitor to decouple it, and the radio is powered up .... no mooing! Excellent.

I put the radio back together again , and listen to it for the rest of the evening in the workshop.

The following morning , back in the workshop ... and the cow is back ! Damnit. 

If the quiescent current adjustment (R518 on the above schematic) is just barely touched, the fault can be *almost* cleared. The instablility is once again permanent. 

(As an aside, you may notice the audio output stage is a mix of silicon and germanium transistors) 

The audio input to the output stage is disconnected by lifting C506. The fault still persists, so it's in the output stage somewhere.... 

There's a few electrolytics in there ... C512 (1000uF) is the output coupling capacitor, C513 (1000uF) is some local supply smoothing. These appear to be in reasonable condition, but changing them clears the fault temporarily. It can be made to become unstable again by adjusting the quiescent current again. Voltages on the output pairs base's are unstable... There are only a few electrolytics in the whole set, so they are all swapped out. The fault vanishes once more. The quiescent current control can be rotated and no instability re-occurs .... The quiescent current is reset by measuring the current flowing into the collector of Q503, set for 5mA.

Here's our stabistor replacement LED glowing away. It provides about 1.45V, slightly less than the 1.5V of the original component, but it shouldn't ever fail, and performance doesn't seem affected.

The guilty parties ... and another saved from landfill !

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...

(details here) 

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 https://www.youtube.com/watch?v=YPl356YKcVs&t=1557s 

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! 

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!