Powertran Digital Delay Line [DDL]
This page begins as some personal notes on servicing an old Powertran DDL unit - hopefully to grow and provide some useful tips if anyone else finds themselves with one..
April2010 - I saw a Powertran Digital Delay Line in a local 2nd-hand music store but they said it wasn't working - I've liked the look of them for a long time and knew that plans were probably available on the net as these were old DIY projects with kits published by E&MM magazine (design by Tim Orr). So I took a punt and took it home hoping I'd be able to get it running again..
Opening it up it seemed pretty tidy - most parts are mounted on one big PCB with two small sub-boards, one for the PSU (bipolar +/- 12 volts and unregulated c.9 volt) and one for the delay-time buttons but then I noticed some of the trimmers were gunked and that there were a load of tantalum caps used. Not that I know much about tantalums, but I've heard that they can be problematic especially in old equipment. I probed the PSU and noticed that the negative rail was running very hot - switched the unit off and noted that there appeared to be a short from ground to the -12 volt supply line - I thought it may be the tantalums and hoped it wouldn't be any of the ICs. All the +5v lines (for logic and memory ICs) seemed to be working ok.
I found a copy of the original project articles online (see link below) and began to think about which parts may be causing problems and how to replace them. The trimmers would be straight substitution, as would any electrolytic caps (mainly the large smoothing caps on the PSU). Many tantalums could be replaced easily with electrolytics - though I've still got some queries on particular areas.. (see notes below)
I began by removing all the ICs except for the RAM and tested continuity on the supply lines - still a short on the negative rail, so I figured to begin changing some parts - there are only certain areas that use the bipolar supply (the main audio chain and filtering, plus the two DACs). I changed the trimmers and then the eight 10u tants that smooth the bipolar supply replacing them with standard 10u electrolytics. I tested as I went along and, lo & behold, once I'd changed these caps I found that the negative rail was no longer short circuited. Excited by this I decided to put the chips back in and see if it worked -- it didn't.. well, until I realised I hadn't put the IC back on the delay-button board! Once this was in -- it worked!
So, there is still a lot that needs attention - I figure that ALL the tantalums and electrolytics need to be changed. IC sockets should be cleaned and some ICs have brittle legs (strange - particularly 4558s in the audio chain). The pots could do with a good clean too.
I'm now asking around for further help and info about the project and would like to compile any details here. I'll aim to get some pictures of the insides up along with notes on parts and hopefully better images of the schematic / layout etc. There is mention of a modification to expand the delay RAM, so hopefully..
Neil Johnson's Page - with magazine articles including a hi-res copy of the Powertran DDL project [ http://www.njohnson.co.uk/ & thanks to Graham Atkins]
Nick Robinson's Page - mentions modifying the machine to 6 seconds and has the magazine article scans (inc schematics) here
Loopers Delight Review
Pots / Trimmers
Capacitors (the ones to be changed)