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.
"Any room for a little 'un - this little 'un?
It's a Grundig GMU3 mic mixer from the late '50s. I was thinking "looks pretty, but maybe doesn't sound much good..." and it was possibly in line for e-Bay. Then I plugged it in. Plenty of crackle, gain not what it should be, but the sound quality coming though from just a bog standard dynamic mic had a real presence that made me think it has real possibilities and definitely not something I should overlook.
Might this be something you could take a look at?
Hopefully see you when I'm down soon."
Sadly, as John was coming down, I was heading off to Spain. He dropped it in with a mutual friend. It's got an odd selection of knobs (the mixer, not my friend).
Back panel has 3 mic inputs , and a "channel" input, which is at line level.
Incidentally, this unit was probably intended for high output crystal mics, but is quiet enough for use with an ordinary dynamic.
Output, however, is at mic level, to feed a Grundig tape recorder of similar vintage.
Removing the case reveals some nasty electrolytics...
Which are long past their best...
and a pair of lovely Mullard ECC83's (12AX7)...
The electrolytics are evicted , and modern replacements held in place with a dab of RTV.
... and there's a lot to do on the underside, a myriad of nasty waxy's.. all replaced. The 25uF cathode bypass caps all measure very leaky except one, which was in tolerance, and had reasonable ESR too, but all are replaced anyway.
The switches, pots and connectors are all cleaned up....
... and it all works very well!
Most of these units seem to have the later, EM84 type of indicator tube.
This has the type which has two "wings" that close up, and is the brightest I've seen for ages.
The 13A fuse in the plug replaced for a more sensible 3A....
The some more suitable knobs found.
And it's pronounced fit to travel back to Scotland to John!
Despite Grundig being a German brand, this was made in the UK!
First off, read the red disclaimer to the right. I'm about to go inside a microwave oven. Don't do it.
Saturday afternoon.
There's a rather unusual heatwave in the UK. The weather's great. My wife is cooking a jacket potato for lunch in our combination microwave. I'm sat in the lounge, watching a bit of telly..
I notice a loud 50 Hz hum coming from the kitchen. I go to investigate.
It's coming from the microwave oven. Something's heavily under load, and there's a smell of warm transformer.
It stops, and appears to carry on cooking. I go and sit back in front of the telly.
So, later on the wife has gone to work, and I go to make some beans on toast (living the dream, eh?)
Beans in the microwave... and they come out cold :(
Now every microwave oven has a label on it... "Microwave energy - do not remove cover" ... so don't remove the cover. This means you.
I remove the cover.
I check to ensure there's no remaining charge in the high voltage capacitor, and proceed to check the high voltage fuse. It's open circuit.
So, remembering the smell of warm transformer earlier, and the sound of a transformer under heavy load, I check the rectifier diode....
It measures dead short circuit. Now with these high voltage diodes, you can't check them with the normal diode setting on your meter, as it requires a few more volts to start conducting. Normally a silicon junction will forward bias at about 0.7V or so, but these high voltage types are actually a stack of diodes, so the 2-3 volts your meter provides will not be enough. You can use a bench supply to check them, but a short circuit is a short circuit, and this will show up on a normal test.
A new diode is ordered and fitted and normal operation is restored. Thankfully the transformer appears to have survived it's ordeal.
It does, however, bother me that it's failed. The unit is only a few months outside of it's guarantee, and it replaced a previous oven that was also short lived.. I wonder if it can't get rid of the heat generated quick enough. You can see the big "cage" fan in the unit, and there's also another fan that forces air through a duct across the fins of the magnetron.
The oven is a built-in type, and sits above a conventional oven, and there's a vent up the back of the cupboard. Perhaps it's not enough.
Let's add some forced air cooling...
I order 5 off 80mm fans from eBay, the sort you use to cool a PC, and a small switched mode supply, capable of 12V at 2A.
The fans are mounted on some aluminium angle, so they will sit over the vent at the top of the cupboard, sucking the warm air out.
Now, we don't want the fans on all the time, and we want to keep them as quiet as possible, so a circuit is designed, based somewhere between the Simple Thermostat project and the fan controller on the Arduino Audio Wattmeter
I don't need a display, or adjustable set-point, so this simplifies the design, and the required code.
Here's the schematic...
Power comes in to JP1, and is regulated by IC1 to provide 5v for the ATMEGA328. A DSB18B20 sensor connects to JP2. The micro determines the required fan speed, and outputs a PWM signal on pin5, which is used to control the fan via the STP55N60L logic-level MOSFET. The FET should run cool, no heatsinking is required.
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!
A while ago, a friend, Alex, asked me how he could drive a synchronised clock he had procured.
It was a Westerstrand clock. The sort that you used to find in factories, offices and schools. It's an electo-mechanical driver, driving a conventional clock display. It doesn't posses any time keeping mechanism.
Now Westerstand are still alive, well and trading since 1906. Their website is here.
Anyway, according to some information gleaned from their website (they still make these things) the clock needs a pulse of 24 volts alternating in polarity to advance a minute.
I quickly drew up a small circuit, with an arduino and a GPS receiver to drive Alex's clock.... he pronounced it too complicated, and removed his electro-mechnical movement and fitted a quartz clock movement .....
.... that's cheating.
I managed to procure the same clock movement in somewhat distressed condition, and managed to put it all back together.
So what about that proper driver? Good plan. I ditched the GPS receiver, as I have the GPS master clock, and added a 433 MHz receiver to receive the signals.
The clock will "free-run" after being set, using the 490Hz interrupt driven clock, as previously seen on the Arduino analogue clock.
Pulses are sent to the clock by using a small H-bridge.
Now the electronics has no method of knowing where the clock movement is, so before the synchronising signal from the GPS master clock is received, the clock needs to be set to 12 o'clock. I've added a minute and hour button to the PCB to allow the movement to be set.
The code also automatically adjusts for British summer time.
It takes mains in, and spews mains out ....
Replacement of the transistors provided the cure. I left the unit on, running into a small load to soak test it.
