Milli ohm meter getting warmer ...

So the aforementioned PCB arrived back, have assembled. 

I had to play with resistor values and so on,  but now I have a circuit that will pump 500mA through the device under test. There is a x100 amplifier which boosts up the voltage developed across the DUT to a suitable voltage for an Arduino 10 bit ADC to convert. 

So, a 100milli ohm resistor will develop a full scale reading. I'm going to have to add a range switch to lower the current if I want to measure bigger resistors than that. But then we have multimeters that can go down to a few hundred milli ohms in general. 

In general, quite pleased so far. I am going to assemble this into a bench power supply which happens to have one of my old boards that can read switches, write data to a flourescent display (which is only 30 years old). I am also going to float the power supply ground so we don't liberate magic smoke from things when I am using it. 

Yet more Minibrute

 So last year I bought a minibrute ... made it kind of work. I did some more work on it recently - replaced the big connector between the analogue and digital boards. That helped enormously with the stability, and functions working. There were a couple of bits left over - the pitch bend didn't work, the LFO doesn't seem to do anything and the aftertouch was flaky. 

I just spent a happy couple of hours sorting out the aftertouch and the pitch bend, and I have now worked out that the issue with the LFO is the waveform selection switch - which is completely open circuit. There is a Mouser part (Alpha?) that seems to measure up to fit, so I will be ordering one in the near future. In the meantime I just wired across the switch to give a sine wave LFO waveform - and now, things are a whole lot better. 

I've done a knob job on it - the sliders were looking very tatty, and I paid a silly amount of money for some replacement rotary knobs. So now it looks like this ... 

More Drumbrute ...

All back together and working ... 

Arturia DrumBrute ...

 So, I was attempting to repair a bit of music electronics yesterday evening. Fault in the power supply somewhere - one of those overload ones, where you turn it on, the lights flash briefly and then all quiet. I hooked it up to a power supply with a current meter, found it was sucking a few amps before dying. 

There's a resettable fuse on the input, which goes open circuit when it fails. 

Aha, I thought. I can tell what is shorting out, since whatever it is will get hot quickly, and then cool down. Having worked my way round the power supply, looking at likely culprits (is that cap a bit bulgy, shall we lever it off the board and retry?) .. I thought I would test the first voltage regulator. It's a tiny thing, (a TPS5431) brings the supply down to 5V pre-regulated before going off to a 3.3v reg, to a switcher which feeds +12 and -12 etc. 

Turned on again, with finger applied. Now that got stupidly hot, very very quickly. And behold, it is showing a resistance of about 7 ohms to ground on the output of the regulator. Trouble is, I couldn't tell if it was the voltage reg or the schottky diode (SS16) next door that suddenly got so very hot. So, I am now looking at the blister on my finger to decide if it is 8mm x 6mm or 6mm by 4mm burn.

Well, the diode is easy to lever one end off the board and have another measure. Ah, it's not the diode. So must be the voltage regulator itself which has gone deady-byes. I hope that whatever happened to the 5V during the time of failure hasn't done too much damage (the 3.3v was still regulating just fine as were the 12v ones).  

I should have been more circumspect and noticed that the voltage reg was showing signs of a little patch on its surface where it has become extremely hot. I will need to lever that off the board now, and substitute in a small external regulator to see if the beast comes up or not. If it does, I might just leave it with a replacement voltage reg on a small board. 

Fixed the Cambridge P500

Another one off the list of things to sort out .. turned out to be one of the darlington output transistors. They have integrated emitter resistors, and one had gone from 0.22 ohm to something in the hundreds of ohms, so it got replaced. While I was there I checked all the caps (no problems) and I replaced all of the heat sink compound, as there was pretty much none on the transistor I changed. Actually I changed both on that channel to make sure I had a matching pair. Applied power, and success! Sound in both.

There's quite a lot of discussion on t'Internet about the bias current, but I found in the spec for the trannys themselves that they recommend 40mA. So I put some meters on, looking for 17.6mV across the two 0.22 ohm resistors. They seemed to want to settle at 20.4mV which is close enough.

This is a budget amp, so I wasn't expecting too much, but the heatsink is a bit pissy and it feels cheap. The whole amp is not worth saving, really but it might do for a bench amp at a push. At least it is now off my bench so I can move on down the list of things to be repaired.

Reconing the Missions

And another one for the new Lab; I have removed all of the drivers from the Mission 700s. I think these speakers came from a small shop in Chandler's Ford in about 1981; I had re-coned them with some Audax drivers in the 90s, but they have lived a long, sun-drenched life in the Office on Hayling, and it was time to replace them again.

I had a good experience with the Monacor SP210 drivers in the Kef 104 I have in the lounge. They really do kick out a great, punchy but not overstated bass/midrange, so I bought another pair for the Missions.

Since the Lab is white and light Oak, with touches of chrome, the dark ?walnut of the boxes just had to go. I am not really sure how I feel about painting over veneer, but they were a little battered anyway on the corners. I took the trouble of spraying them with a white primer, and then an eggshell finish white. The new drivers went in after a couple of weeks of the enclosure drying in the garage, and they seem to have come out quite well:

Sounds? Well, having a bit of trouble with the amp at the moment. It doesn't seem to be decoding a digital input. I have the optical output from the TV, and I was hoping that the amp would just pick it up and drive the speakers, but it doesn't. I wonder if the HDMI board has left the planet when the old TV HDMI input went bang.

I did plug the Alexa into the analogue input on the amp, and I listened to Bon Jovi's Wanted Dead or Alive through the speakers. They work, and really very well.

Have a new (to me) motorcycle ..

And she is lovely .. ZZR1100 1995 (D3).

Unfortunately the former owner forgot to tell me about the issue with starting from anything other than cold. It happened to me once on the way back from Southampton on my first outing, but it was really bad the first time over to the Isle of Wight. I bump-started the dang thing five times in one day!  It just clicks when I press the starter .. so I changed the battery (no improvement), changed the starter solenoid (no improvement) so it's on to the starter motor. Since I really need the bike to get to work - especially on the Isle of Wight, I thought I would buy a new one rather than try to refurbish the older one. So I went to a reputable bike bits supplier, and they duly sent one for a rather large lump of money. Part duly arrives, set aside some time this Bank Holiday weekend to do the swap.

I eventually got down to the bit in the bike - removed petrol tank, disconnected battery, removed lower side panel, removed alternator (eventually! had to go and buy a long allen key to get to the bolts required) and then removed the old starter motor. This at least avoided the guidance in the service manual which says Remove Fuel Tank. Remove Carbs. Remove Engine Oil. Remove Engine.

So, out comes the old starter motor and I go to unbox the shiny new one. And, guess what? It is completely the wrong one. The picture in the advert was the correct one, much like the one closer to the camera below. Unfortunately the one further away in the picture is the one they sent. It has the wrong size, wrong fittings, wrong oil seal size and the wrong pinion. *sigh*. 

So I may end up trying to repair old one instead. I have cleaned it all out (it was a big mess inside), checked the armature which seems to be connected in all the right places, and insulated in all of the (different) right places, and I have a kit of bits coming from the very capable Wemoto to replace the brushes and springy things, and I hope for a good result.

Meanwhile, catching the bus and stealing the car from wifey, who is very understanding.

Fixed amp!

So with a scope available, I had a good look around the Optonica power amp to find out what was going on. It seems that one channel is spitting out so much noise that the protection circuit is tripping, so the speakers are disconnected. (Stock picture, posting from work lol)

I searched everything around the main power amp IC (tsk tsk .. fancy using an integrated amp in a quality product), and eventually came to the conclusion it was faulty. I'd replaced a few caps around the place, but nothing would explain why one channel was all over the place while the other was quiet.

So, I bought a new one from eBay. I had no confidence at all that it would do the job, but you have to accept the improbable, once you have eliminated all of the other possibilities. And it did.

(Picture from the eBay advert I bid on). 

Fixed Scope!

The power supply issue on the HP 54502A eventually succumbed to my repair type ministrations.

I had found a bunch of capacitors in the ageing power supply that needed replacement. 12 in all, all 2200uf 35v that I bought a bulk of Panasonic replacements for. That, and a test load brought the power supply back to something that worked a bit, for a while.

The thing with SMPS is that they are a nest of feedback loops. Especially early ones like this, using discrete componentry. After having reminded myself of what a UJT oscillator looks like, I found that the feedback for the +5v from secondary to primary was way out of whack.

That took me to a circuit in the secondary that compared the +5v and the -5v rail in a long tailed pair. -5V being generated by a linear regulator from the -12v, itself a linear regulator off a separate secondary.

Trouble is, with all this feedback going on, the secondary outputs all look wrong, so how to find out what was working? The trick in the end turned out to be switch on, run down all of the outputs to see if they start off ok or not. And guess what? The -12v that fed the -5v that was compared to the +5v to drive the feedback to the oscillator never got bigger than -6v.

So I drove the -12v regulator from an external 15v power supply, to see if it worked by itself. Still faulty! Getting warmer .. but it is only a few components. Swapped all the capacitors. No change. Measured resistors (by probing them near the body from the top) no problems. Hmm ...

Then noticed some burning or black stuff on one leg of two resistors - near the big capacitors on the 5v I had replaced. I hadn't noticed that one of the caps had leaked. I removed the resistors, used a Dremel to remove the black stuff and clean up the PCB as much as possible, replaced the resistors ..

Nothing happened. But all of the rails look like they are at the correct voltages?
And then noticed I had removed the connection to the CRT while working on it, and hadn't put it back. Tried again ..

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

Repaired my biiig Plasma TV

I was gutted when my lovely plasma TV decided to die. It was only the video inputs that went - I could still get some things on the screen like the volume control. So, on the offchance, I went on that auction site again and looked for a new digital board for it.

Well, I was surprised to find a TH42PZ70BA digital board for 13 quid.

After removing what seemed like a million screws, I got down to the point where I could actually see the board I was trying to replace ..

 

I was even more shocked when the replacement worked - all back to normal again! 

This is probably the last time in my life I will ever repair a TV, I just thought. I grew up with mono TVs and new colour TVs with their tube output push-pull output stages (Philips G6), LOPTs and triplers and CRTs with rimbands, convergence with 22 pots to adjust and a ton of X-rays shooting out. I used to help repair TVs (and amps, receivers, tuners etc) for pin money as a student. TVs were most challenging since the focus voltage was a few thousand volts, and the EHT was 25KV. Quite scary to work on the things, and the dust would collect on the high voltage components. I used that as an indication on the neck hardware which pins to avoid touching (all of them).

Interestingly enough I had to apply a vacuum cleaner to this Plasma before I could repair it, as it was just coated in dust. 

These days, I do repair the odd LCD but they are very uninteresting. Occasionally it's a power supply capacitor that has let go. Sometimes that takes a switching transistor with it. Most other faults though, including the mechanical stuff - broken screens, broken connectors - I am sorry, but it's easier, faster and cheaper to go buy a new one. It rankles a bit, and offends my sense of wanting to repair rather than replace, and my care for the environment. When did manufacturers give up on designing things for repair? Well, if the plasma is anything to go by, about ten years ago.