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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