Here's something I'd never thought I'd see again....
Remember the post about the rare Linn Wakonda LP12 power supply (here) ?
Well, John tracked me down (internet, eh?) It turns out he's got another one (and turns out to be a mate of a mate of a mate, twice removed...)
"Hi Andy
are you able to repair a linn lp12 Wonka pcb...its been working fine but has developed a problem where the motor stutters and jerks once its switched off
is this something your could help me with?"
Yeah ... Why not?
He's seen my blog post, and has done the cap change as I had, but still no joy.
Now this sounds very familiar to a failure on the Linn Axis (of evil) power supply where the speed selection logic goes screwy ...
It duly arrives from that bit of the UK right at the top.
Sure enough all the caps had been done (bar one), and a tidy job it was too :)
Despite the remaining cap being OK, I replaced it anyway.
Tracing the speed control logic through proved one of the counters had forgotten how to count. It's U15 on the board.
Colin dropped in to bring me an LP or two, and dropped off a mysterious black box.
"It's a Heed power supply, you plug it into your LP12, and it allows you to switch speeds without swapping the belt over, I think it's been got at. Care to take a look?"
Yeah ... why not...
It takes mains in, and spews mains out ....
The little switch on the front alters the output frequency between 50Hz and 67.5 Hz, thus changing the speed of the LP12's motor.
Mains comes in and is supplied to the larger transformer, where it is rectified into +/- 32V supplies. There is a crystal divided oscillator (with 2 crystals, one for each frequency), and this oscillator, after some signal shaping, drives a push-pull amplifier, which drives the primary of the output transformer, which turns the waveform back into 230V AC, at the frequency of the oscillator.
This unit had no output, and everything was getting hot. The heatsink, both transformers. Something was very wrong.
It didn't take long to find the negative rail was being heavily loaded, and the BD242C output transistor was short circuit.
I replaced this, and the other half of the pair, a BD241C just to be safe...
Replacement of the transistors provided the cure. I left the unit on, running into a small load to soak test it.
Now Sam, being a lady of taste and decency, decided on a piece of vintage hifi gear, and spotted a Dynatron on eBay.
Fitted with the ubiquitous Garrard SP25 (MK IV), a decent Shure M75-6S cartridge and the delightful styling to suit her grand maison, and she already had the matching speakers.
It was sold as not working ... but at least at the time it was complete...
Sadly the eBay seller slung it in the thinnest of cardboard boxes, and passed it to a courier with all the care and finesse of a rhino snowboarding down a mountain, who promptly backed over it, threw it in the back of a borderline MOT fail Ford Transit, and delivered the remnants to Sam. Thankfully not a lot of money was risked in the venture (by any party) and the eBay "seller" refunded Sam, and told her to keep it.
"Get the bits round here Sam, let's take a look"...
Shite transport services (and not at all shite) were deployed and the unit made it's way northwards.
When it arrived chez Doz, a quick inspection was made. It's cosmetically challenged, but the cabinet, whilst largely intact, is badly scratched. "Shabby shit" .... Sam says she can deal with that. I fancy a bit of pop-art decoupage.. anyway....
The turntable hadn't even had it's transit screws tightened down for the first part of the journey, but at least shite transport services had spotted that and sorted it before it got here.
As usual with 95% of all Garrard and BSR turntables of this vintage, the grease has turned to glue and stuck the thing solid. It was disassembled, cleaned and re-lubricated as required. Some levers were bent, some were in the wrong place, and the centre spindle was missing. I think someone had had a go before. Quite a job.
The top part of the motor rotor had come un-glued (as usual), so this was repaired, and the bearings cleaned and greased.
It ran like a bag of spanners ...
.. further examination shows it's got a bent spindle.
I popped down to my local, friendly spares emporium, who furnished me with a new old stock spindle and a motor which looked a bit like the one required.
Swap the mounting plates over, and we're in business !
Now the turntable was at least functional, efforts were turned to the electronics.
There was no output from the right hand channel. The right hand amplifier module had blown it's fuse (F1.6A). A replacement was fitted, and the unit switched on. There was a minor grunt from somewhere, and the fuse blew again. The module was removed and placed on the bench. It's fitted with a pair of BD130Y transistors, and gives us a nice date of 1974...
... after a bit of poking around, it appears that one of the driver transistors, TR207, has developed a nasty short circuit...
Part of the number has worn off :( It does give us a date code of March '75 though ... not that that helps much.
Google turns a schematic up ...
and, sure enough it's a C1131 ... and this is confirmed by the part in the left hand amplifier, as the two modules are identical.
OK, so a 2SC1131? No... that's a TO-3. BC1131 (Dynatron would have used Mullard bits, perhaps? They all ended up being owned by Philips.) Nope, no such thing ... Thankfully, it's NPN best mate and complimentary chap is the M8003, TR206, which does show up as a valid part ....
A BC 559 goes in, the bias set up with R 208 for 26V on the output cap, and the thing works flawlessly.
With the amplifier re-installed, there's another issue. Hum. There's always a more than noticable 100Hz hum in the background. Here's the Dynatron's power supply..
I'd just like to say at this point, C104 is shown on the diagram as 2,200uF ... it's not. It's a more respectable 22,000 uF ... It's removed and tested, and proved innocent, reading slightly high in value, but not leaking, and has a very respectable ESR of 0.1 Ohms. C103 is likewise innocent. Whilst doubting my readings I changed C104 for a new part , still the hum remained. I also added another 22,000uF in parallel with the original, and whilst things improved, the hum was still there. Disconnecting the power supply and running from the bench supply, and the hum was gone. I think the hum was always present since the thing left the factory. What was needed was a proper filter.
A trace was cut on the PCB, and a 3.3 ohm resistor, and a 10,000uF 63V capacitor where added, making a CRC filter, as shown in the simplified diagram above....
... and lashed up for testing....
The hum is gone. The resistor barely gets warm to the touch, so that's good. The HT has dropped slightly to 48V, but we can live with that.
Why didn't the Dynatron design engineers do this in the first place? Costs probably. For a ha'porth of tar ....
The resistor is mounted up on the PCB, by drilling two holes, one either side of the cut track, and the extra capacitor is secured with a blob of silicon to the cabinet.
The next mod involves fitting a 7833 3.3 volt regulator to the output from the 16V stabiliser. It's output is connected to a 220uF capacitor. Later I added a small heatsink to the 7833, as it was a little warmer than I'd like...
... and to a BK8000L bluetooth receiver, the output of which is connected to the 5 pin tape socket on the chassis.
Powering up and selecting tape, and pairing the BK8000L module with my phone gives great audio!
All that remains is to set up the turntable.
Tracking force set to 3.5g...
... and the set down position adjusted with the little screw under the arm (anti-clockwise brings the set down point outwards)
So there it is. AM/FM/ Turntable and bluetooth... ready for the next 43 years of service!
How does it sound? I have to say, I'm surprised. Now the hum is sorted, those little BD130 amps do really provide some whack. The tone controls are sensible too. Not at all bad.
Over to you Sam, send me some pictures when it's prettied up!
Will turned up for an emergency curry. There was something ominous lurking in the boot of his car ....
It's a small Dansette Junior Deluxe record player...
"Can you take a look?"
Yeah, why not ....
It's in good cosmetic condition, and dates from around 1961.
Further inspection reveals some issues...
The valve (yes, just one!) is rattling around in the bottom, but is thankfully undamaged. The motor suspension has a few bits missing and the platter won't seat. These are really quite minor issues and are soon sorted.
Once the thing is spinning round reasonably, We try playback .... It's awful. Distorted and really low in volume...
The amplifier is simplicity itself. It has a nice mains transformer so it's reasonably safe, even has a 3 core mains lead :)
The output from the transformer is rectifier by a metal rectifier, and smoothed by the 16 and 32uF capacitor block, along with a 47K resistor. The smoothing cap is in really good condition!
The cartridge is a BSR TC8 medium output crystal cartridge, producing an output of a few hundred mV or so. This feeds the top of the volume control, and straight onto the grid of the EL84 pentode. There's no cathode bypass capacitor fitted to this unit, nor an indicator bulb, but this is the nearest schematic I had ...
The distortion is partially caused by the 0.05uF tone control capacitor (a dreaded hunts), and is changed ...
The rest of the distortion, and the woefully low output is being caused by a poor cartridge. It's terminals have got chronic verdigris, and this means the crystal inside will have turned to goo. It's a very common failure. (It's not all bad though, as this mono heavyweight cartridge will have chewed Will's records to pieces.)
So we dig around in the box of phono carts to find a suitable donor. The big issue here is, we have nothing to mount to, except the original "flip over" mount...
There's a couple of Acos M7 moving magnet carts that may well do ...
... and after a bit of filing, it fits !
Tracking weight is set for about 2.5g, and it manages to track the test record.... but we can't hear it ....
The output from the Acos M7 is around 1mV .... not enough to drive the EL84's grid much at all... we will need to make an amplifier. Also the output from the crystal cartridge approximately followed the RIAA equalisation curve, so no correction in the amplifier was needed. We'll have to add this, otherwise the output will sound very thin and tinny.
OK, so the above was quickly dreamed up. It's a (very) basic phono stage, and has some simple EQ in the feedback loop, to approximate the RIAA curve.
I've stolen some power from the 6.3V heater winding on the mains transformer inside the record player. You'll notice there's only a half wave rectifier here, as one side of our heater winding is connected to ground. It's imperative to get this the right was round or you'll short out the heater winding on the transformer, and it will burn out pretty quickly...
C1, R2 and C3 form a filter for this simple power supply. The cartridge is connected to JP2, and it's loaded by R1. C1 couples the audio though, which is biased to half-rail by R3 and R5. This is then fed to the op-amp, which does it's amplifying and RIAA correcting duties, and then passes the audio out via C8 to our existing volume control. C4 provides DC stability, and a little sub-sonic filtration.
A board is etched...
And duly fitted....
You'll notice the cable coming from the cartridge is screened. It was originally a twisted pair, but the hum pick-up was ghastly, as the arm is all plastic and has no shielding, lives over the mains transformer, by the mains input!
... and, because nothing in life is every easy, the mains switch failed open circuit during testing ! It was stripped and repaired....
How's it sound? ... honestly? Pretty damn awful, but at least Will's records are safe (r)
here's some more pictures, including the obligatory arty valve shot ;)
Remember my BSR UA14 record player from a while back? (No? Click here!)
Well, it deserved more than an Asda cardboard box to sit in.
My friend Julian supplied a bit of plywood to make a suitable top from, and another friend, Barry, cut the required shape out (copied from another record player fitted with the same deck, and soon to be featured on these very pages). Barry also made some MDF sides for it.
Now, I could paint it ... but I've got another couple of turntable plinths made by Carl Ellis I'd like to veneer, so this would make the ideal test piece.
Some mahogany veneer was procured from the very wonderful people at www.thewoodveneerhub.co.uk who were most helpful and rookie tolerant (even sent me some free samples)
I tried some different samples out on a small wooden box, and after some initial success, got on with the job in hand...
Trimming of the edges terrified me. What if it split? I'd have to remove it and start again! I'd read on the internet that a veneer saw was the way to go, and it needed to be razor sharp. I didn't want to go to the expense. I initially just cut the veneer to the rough size before gluing with a pair of sharp kitchen scissors! I trimmed the first part with a sharp scalpel. It went well enough ... get to within a couple of mm of the edge and sand it back with some 120 sandpaper to the edge. Don't rush any of the process. The scalpel cuts with the grain with ease. Cutting across the grain isn't so easy, and requires multiple gentle strokes.
With a couple of off-cuts, I experimented with finishes...
Whilst varnish looks good on the photo, The oil has a better sheen to it after a couple of coats. I used Liberon Finishing oil. It seems well thought of
So, now for the big piece that will cover the top.... this was difficult to get to lie flat, and, in hindsight I wish I'd done it first. Clamping it down proved problematic!! That's two dirty great power supplies, and a wooden box full of sealed-lead acid batteries holding it down whilst the glue sets!
After everything is trimmed up, and given a brief run down with some fine sandpaper, the first coat of finishing oil is applied, and left for 24 hours...
It's then given another rub down with fine wire wool, and another coat added....
This is repeated until I've built up 5 coats of finishing oil. It's important to follow the instructions on the bottle to the letter.
Once this is finished, the wood is lightly polished up with a fluffy cloth... and I must say I'm chuffed with the results!
