Friday, March 23, 2012
Playmaster Twin Twenty Five (25)
You may not instantly recognise the brand Playmaster immediately, especially if you're not from Australia. To be honest I didn't either, but the name rung a bell. Down under, we once had a magazine called "Electronics Australia" they would periodically release designs and sell them in kit form under the name Playmaster. Ranging in valve and solid state equipment, a lot of their earlier work was with Guitar Amps and PA amps, however they did do HiFi as well.
Quite beautifully built, brush aluminum faceplate, Solid machined aluminum knobs, and a nice simple design that's not cluttered. It's exactly what a nice simple amplifier should be. Simple!
Because these were usually available as a kit form, as Electronics Australia was predominantly aimed towards the hobby and enthusiast audience it makes this little amp even more special as it was a factory built by Applied Technology Pty Ltd. It even has the date of manufacture and the sign off by the technician(s) who built the unit. In this case the build date being 17th of January 1977, pushing this little beauty just past it's 35th birthday. Although by the time it's restoration is done it won't look a day over 1 :)
This particular amplifier was found, piled up with a bunch of other classics I also acquired and will (eventually) post on this blog at a local charity store. I bought it and only afterwards realised there was a sticker on it saying "not working as is". The wood sleeve was broken, veneer lifting, and was just not in a happy place.
Just looking from behind, I suspect that the speaker terminal connectors once had plastic sleeves, or at least one could hope that they did. Chances are this suffered a slow and painful death from too many speaker shorts.
It's output transistors looked the worst off, with a nice layer of corrosion on them. Overall externally the unit didn't actually look too bad. Bit of dust and 35 years of grime :)
Internally was quite a surprise however. Because it's main output transistors were externally placed there was no need for ventilation inside. As such, like a sarcophagus (ok, maybe a bit of a stretch) it was in excellent condition.
However it did not come without it's drawbacks. Unlike most of the equipment I restore, there was no underside access, so the whole board would need to be lifted in order to replace parts. A speaker protection circuit was added (bottom left corner) presumably by the manufacturer, a nice addition even if it's tapping it's power straight off the main filter capacitors. Speaking of, the main filters in this unit were 4700uf 35uF ELNA caps, respectable for it's time.
There were a few other caveats about this unit that didn't sit well, such as exposed or badly soldered joints, but nothing that could not be rectified.
Initial tests of the transistors prior to firing up showed that both output pairs were toast. As well as a couple of switching transistors. The diodes that made up the bridge rectifier were leaking current, two fuses were blown, several leaking capacitors, including the main filter caps where one was reading 900uF and the other was reading about 5500uF. There was no point in even attempting to fire this unit up, I'd probably be causing more damage.
With all that in mind, let's get started on the restoration!
As one customer of mine pointed out recently, I like to treat all of my restorations like restoring classic cars, reducing the amount of modifications, only were necessary or where a substantial improvement true to the original idea can be made. With that in mind, although having the clearance I did not want to drill a couple of additional holes in the pcb to allow the vertical placement of the main filter capacitors. Opting to use a strong but relatively easily removable adhesive to keep them placed horizontally. My star performers as always, a pair of Panasonic TS-HA capacitors, rated at 6800uF. I used 1.5mm thick solid core copper to create two 'imitation posts' where I could connect the caps.
After which the capacitors were bonded onto the PCB..
The rest of the capacitor replacement went smoothy. Tantalum capacitors were replaced with electrolytic or film where applicable. Panasonic AM series caps used for all electrolytics. To get to the bottom of the PCB, all the potentiometers had to removed from the faceplate, everything was hard wired in! But never an issue for me :)
Next up was the speaker terminals.. Pictures can say a thousand words, though maybe not so many are required to describe this...
With the speaker connector plate removed, I didn't want to fashion some kind of plastic plate job on there, at the same time, my large binding posts I had for this project were to large to fit in place. I settled on some smaller binding posts. Using the existing plate, I drilled out the holes for the binding posts and fitted them.
When installing them back on, I did solder the -ve/ground posts in place, however the +ve posts were screwed on.
A much better and safer look!
Alright next up transistors. The switching transistors were replaced on the PCB itself, but the output transistors were connected to the frame for heat dissipation. Given that they had all failed, replacements were ordered in, same type (2N3055) made by SK.
I actually bought a few too many, for starters it's nice to have spares, but secondly so I could hFe match them :)
Half way through replacement, still removing the old mica film and thermal grease:
New outputs, cleaned all the old gunk off, new thermal pads, perfect!
A couple of other small things had to be sorted out, a lot of time was spent putting heatshrink tube over exposed wires reducing the risk of anything going wrong in the future. A prime example is the power status led.
Nice and clean now..
After cleaning all the pots and switches, testing the speaker protection relay and some more cleaning it was all done.
The result? A very good sounding unit! Nice and clean, there's not that much 'warmth' to it as you would find in other brand name amps, I find it akin to the Harman/Kardon sound. But it's very clear. I opted to leave the original OP Amps in place (LN741) as they were giving no troubles and would retain it's original sound.
Overall, I like it! With a touch of tweaking to the bass and treble I could easily find a place for this little beauty in my home! However, like I've said before, the way an amp sounds is quite subjective to the user, I could rave on for hours describing the "sound" but it's my interpretation of it. Yours would be different, in fact I would be shocked if it was not!
So that's the Playmaster Twin Twenty Five. A beautiful simple 25wpc amp. As always some modeling shots below, please feel free to comment and shoot off any questions if you have any. I hope you have enjoyed reading this article, I certainly enjoyed working on it!