Dave and his blog

  • Two interesting repairs

    Its always interesting to get equipment that has been around for 20 – 30 years but has suddenly developed a fault. I have just had a couple of bits which could have been scrap, one of which was a relatively easy fix the other confused me! Read on!

    The first was a Studiomaster Vision 12 mixer amplifier which, when you get a good cared for one really do work well. They have decent power amplifiers, the mixer stages are all modular and the reverb and effects work really well. These were quite special in their day. This one was made in 1996 so its been around a while. All the cables internally plug in and there are no conflicts as to where they go or the lead lengths. A dedicated output stage buffer come power regulator come send return loop come protection circuit board does all those little bits that either end up forgotten or messy or jammed onto other boards. So much better than most of the subsequent models from other manufacturers! It could do with a bigger set of main PSU capacitors from new but otherwise it a good piece of kit. This one arrived with a dead low level PSU or at least one that was dead short somewhere as the boot up didn’t seem to work properly there being no display and certainly no welcome message! I disconnected each channel of the mixer and started it up after each one until I came to channel 12, the last one obviously and then it started properly. So channel 12 was the culprit. Taking the channel out the obvious thing is a damaged or blown op-amp pulling the supply lines down this usually being caused by an incorrectly sourced input where someone plugs in an amplifier output straight into a line level input. However after lifting a couple of jumpers to isolate things I spotted a ceramic decoupling cap with a tiny burn at one end. About the size of a half mm dot. Removing it cleared the short and all was well, or at least the thing started properly. I then discovered there was no 48volt phantom supply so while the PSU board was out I swapped the phantom regulator, its capacitors, its fuse and the two smoothers on the 15 volt supplies. So it was all back up again.

    The second was a Tascam 144 portastudio. No headphone on the right channel was a failure of the LM386 output chip and again whilst in bits I changed the capacitors around it to save call backs. Confident I had fixed it I plugged in a pair of headphones but I now had no left channel. Head scratching resumed. After about an hour I suddenly realised, one of those horrid 6.25 – 3.5 headphone jack converters on the end of the phones wires was faulty! If I had picked up the same phones as those I started with I wouldn’t have wasted my time! The mechanism was something else though. Even with a hammer, heat and all sorts of liquid lubrication I could not break open the cassette mechanism to change the belts. This is a sandwich construction that, if you dent one half it will stop the thing working as it will be out of alignment. Much as I would love to get into it I had to think of costs and the effect that drilling out all those screws would have on the remains. Drilling, re-tapping and finding new screws in 9 places is okay if the customer wants to pay the bill. Belts seemed to be just the start as it sounded like there was an idler wheel in there that was very miss-shapen during play and in the take up reel as it made thumping noises as they went round. It still worked but obviously could have been improved. With £100 of work to do on it I think a 30 year old machine like this is of little use nowadays so it has to stay as it is. Shame, but unless the customer says otherwise I have to limit the things I do to what is a reasonable job. This one wasn’t!

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  • With colour TV turning its 50 year celibration I thought I should remind people what it was like before the arrival of the flat screen and what it was like when, as a early teenager I got my hands on my first TV!

    My parents had a Pye V210a TV, manufactured on or about the year of my birth in 1958. There is one in the Science and Media museum I understand from a browse of the internet. Now this was a single standard 405 line set with what was then a facinating tuner mechanism as I found out much later. A push button next to the on off switch started a squirrel cage motor that revolved a drum containing individual selected tuning arrangements. These were slug tuned coils as part of the oscillator and preamp stages. I can still remember the noise the motor made when you pushed the button. We only ever had BBC1 on this TV and it wasn’t until it was in its final death throws that I figured out that the knob on the back could and had to be rotated 30 or 40 times to tune in a particular channel and that the drum, if it didn’t have anything selected from the back would just cycle around back to the BBC!

    Of course in my early teens I also got my own TV to play with around 1970. Parents wouldn’t dream of letting their kids play in the back of a TV now but I had an old Philips 21gt100 unit (1959) with lots of extra speakers connected up You can have no idea how dangerous a TV was then. Forget about the mains! Very early console style sets derived the extra high tension (EHT) electricity direct from the mains with anything up to 8kv. Once the likes of the Philips came along the EHT was derived from a winding on a transformer driven at the line rate so we are talking a harmonically tuned transformer producing up to 14kv and using the capacitance of the inside of a CRT to act as the reservior. All of 2000pf of it was charged and of course it didn’t go away when you turned the set off. Then there were the valves. If I remember correctly my Philips had about 16 of them. Many of these valves you can only find in museum’s or second hand now. PCL805 frame output valves or the miniature PCC types in tuners, or the EF85 and the like in the intermediate frequency stages and PCL82 speaker output stages.

    Next came the dual standard black and white sets which were basically a bit of a stop gap until the 625 standard took over from 405 line to give us all BBC2 of course. There were lots of these made, with two tuners, lots of RF trickery, modifications as the video was clamped properly in 625 and had never been in 405, different line rates so switching all around the line output transformer and all supported by switches running along several printed circuit boards. You cannot imagine how much more complicated the sets became when dual standard and colour TV arrived.

    With colour the EHT rose too. 25 -28kv wasn’t uncommon. We then also saw the rise of the switched mode power supply (SMPSU) its generic term which was a new concept to most people as it also formed what was to be one of the most influential technology in electronics even to this day. Think of all those little power units to charge your phone, your laptop your life! One of the early ones was fitted to the Thorn 3000 chassis. As it got older its reliability was somewhat suspect. The output voltage could easily double and since everything else including the EHT was derived from it turning a faulty one on in the customers house could be dramatic! My first encounter was with a set with a blown CRT or the screen. My customer purchased a new one with high expectations. I carefully stripped the chassis, swapped the tube all in the customers living room. Reassembled the chassis and with no sign of anything untoward in the electronics turned it on. The blue flash that came out of the tube neck some 45 seconds later was startling! Having previously checked the HT at around the correct voltage it had then risen with the new tube to almost double and hence the EHT hit probably 50kv tracking through the CRT guns and down to earth via the focus spark gap! Ouch!

    We have become very disconnected from what our technology does for us. Post war there were a whole generation of people who knew radio and radar from their wartime experience and were likely to be the very people repairing your TV or radio in the corner shop. Now we generally have no idea about the design, the manufacturing, the development, the fault finding process or much else of our technology. Everything is a box that does things. I also regret the loss of the old school development process with things breadboarded on the bench as so much is computer designed, tested in software and assembled without the hands on touch. Isn’t that a shame? Electronic dangers now are likely to be just the mains or if you love valved Hi-Fi 350v HT. Kids are probably safe!

    CRT? Cathode ray tube!

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  • Hello World!!

    This is the first post in your new blog. To add another or delete this one click the Blog option on the toolbar above.

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