"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.
If you haven't seen the first bit of this ... click here.
Made quite a few boards with the laminator and toner transfer method recently, and results have generally ranged from acceptable to good.
There's the occasional fail, but I want to improve repeatability.
I've made some measurements of the temperature of the rollers, and it's temperature controlled at about 100 deg C, and sometimes, the board just doesn't get hot enough to melt the toner and cause it to transfer. We need to warm things up a bit....
I removed the PCB and reverse engineered enough of it to work out how the temperature control is done... IT'S IMPORTANT TO REMEMBER THIS CIRCUIT IS "LIVE" AT ALL TIMES, AS THERE'S NO TRANSFORMER. I used a mains isolation transformer whilst making measurements and working on this unit.
Mains comes in at JP1. Mains is dropped via a capacitive dropper C1 (R2 prevents the cap remaining charged up once the unit is unplugged, and removes a shock risk) R1 provides a bit of current limiting, D1 and D2 rectify the output, which is stabilised by ZD1, and C2 is used to smooth the supply. N3 is a 5 volt regulator, and C3 provides a reservoir for the 5v rail. The temp sensor looks like an ordinary 1N4148 diode.... it may well be. It's pressed up against the lower rubber roller. When it's at room temp, it's got about 4.2V across it. When it get up to temperature, there's about 2.4V across it. This is loaded by R32, and connected to the inverting input of N2 via R7, and back to ground (neural) via R8. A voltage reference is provided by the chain R9,R11,R12 and R13 (approx 2.6V) and applied to the non-inverting input, forming a comparator. Stability is provided by some feedback provided by R6. The output of the comparator is fed to the gate of a triac, T1... Now a note on D7... it connects to N1, which the manufacturer has very kindly filed the number off.... I imagine it's some sort of micro controller. It most likely provides some timing (the unit shuts the motor off after 30 mins) and some zero-crossing pulses to D7 so the circuit can phase modulate the triac. I've not illustrated this part of the circuit, as it's not relevant, and difficult as I don't know exactly what's going on inside the mystery N1.
OK, so we need to alter the voltage reference at pin 5 of N2. I remove R9, and place a 10K pot in it's place.
I adjust the pot until the temperature stabilises at 130 deg C.
The pot's then disconnected and measured, and it's 3.6K, which is rather convenient, as that's a preferred value! R9 is replaced with a fixed 3.6K part.
Tests prove transfer is definitely more "robust". The toner seems thicker, and more difficult to remove.
We're now operating this unit way outside of it's design spec (as if it wasn't bad enough before!), so don't leave it operating unattended!!
It's been a while. This project is taking up a lot of time (and money!) ... here's the latest instalment...
So, we have our sources selected (see here) and the signal needs to be amplified.
I'd like everything to be remote controlled. I'd like a simple and subtle bass and treble controls, and balance.
I'm going to put power supply regulation on the board, so we'll need to supply the board with +/-24V of well filtered DC.
I'm going to incorporate a "side chain" with the Korg Nutube in it, and make it switchable in or out.
I'll use NE5532 op-amps, because they're cheap and cheerful, and a damn fine performer.
So... after some thoughts about remote control, I'm left with 2 options.
Motorised pots: Damned expensive, and I'll need 4 stereo ones. Mechanical mounting issues.
Digital pots: I've used these with great effect on previous commercial projects. Very cheap compared with motorised pots, and cheap compared with normal mechanical pots, considering the spec. Enter the Microchip MCP41100. Just the job. 100K, 256 positions, simple SPI interface,linear. I'll put some resistance across the output to simulate log, and if that's not good enough we can tweak the response in the software.
OK, so our power arrives in at JP4, there's some filtration by C9,10,12 and 13 just in case any pickup has occured from the rectifier and filter unit. The +5v will be regulated externally, and has a separate ground. The +/-24V is regulated by an LM317 and LM337 respectively to +/- 17V (I know it says +/-15V on the diagram!) Each IC has it's own 0.1uF ceramic decoupling cap. There are 3 leds to show the supplies are present. Control signals arrive at JP1 from the microcontroller board. The SPI signals are taken to each digital pot, and each digital pot's CS (active low) lines for each IC are also available. There's a bit of bunkum on the web that digital pots are only capable of working within their supply voltage, of +5V and GND. This is wrong. It's digital part, yes. The resistance output, no. It's just that, and behaves like any normal pot, so will be quite happy with our audio signal.
Audio arrives at JP2, and is given some gain by IC2. this is fed to a buffer amp, IC1, which then returns the audio to the REC OUT socket, for recording purposes. Audio is also fed from IC2 to the tone control, formed by two digital pots (per channel) and IC10. Bass is controlled by IC3 (and IC4) and the treble by IC7 (and IC8). Maximum boost and cut is limited to about +/- 3dB, centred on 20Hz and 20KHz. Audio leaves IC10 and passes to IC11 (and IC12) which are the volume controls. I did, originally add a balance control in, and then realise this was utterly pointless, as I could have independent control over both the left and right volume controls, and do the balance in software, and not run the risk of compromising channel separation. Audio leaves the volume controls, passes K1 into the final buffer. IC13 , and out via JP5. If we want more (or less gain) we can alter the value of R59 (R60) to suit. K1 is used to switch in the Nutube valve. The Nutube cannot operate differentially, so the audio is DC isolated by C29 (and C40), and bias is applied from the 3.3V regulator (IC5) via R30 (R48). Some experimentation will be required to set the correct bias. Anode is supplied from our 17V rail, with a bit of filtering courtesy of R17 and C11/15. The audio is developed across the anode load resistor R32 (R45), and passed to a simple FET buffer Q2 (Q1). Now. I think I've dropped a whatsit here, I've passed the audio via a 100K pot R58 (R61) which is going to present a rather high impedance to the output. I'll take some measurements, and see how it performs. Sadly I didn't notice this schoolboy error before I sent the board for manufacture.. The required 1.7V for the filament is supplied from the 3.3V reg via R22/R23. As the tube is directly heated, the filament is also the cathode, so it's AC coupled to ground via C19/22 (C21/23).
Audio is also passed from the output to the meter drive circuit, buffered by IC14 (IC15), and rectified by two germainium diodes D5/6 (D7/8). This output is used to drive the output transistor T2 (T3) which drives the meter connected at JP6 (JP7). R68 (R69) is used to control the calibration of the meter. It's responce should be more PPM-like than VU (which, as every recording engineer knows, stands for "virtually useless"). It's not going to be that accurate, but will give us something to look at ;) You may need to alter the values of C57 (C58) and R82 (R83) to suit your meter. If you can't find germainium diodes, try a schottky diode. The relay for the Nutube is driven from the microcontroller board via T1. The 5v ground and audio ground are kept separate, to avoid clicks and pops when the relays switch, and to avoid noise being picked up from the uP.
Board layout....
Single sided board is going to be a problem. There's quite a lot going on, and I want to keep the form factor reasonably small. Double sided it is, and whilst I can do this at home using UV film resist, it's not easy to get a decent etch. I could purchase board readily prepared with UV resist, and these usually etch very well, but they are damned expensive here in the UK now. It's a shame that the spray on UV etch resist is no longer available. So, I'll get the board professionally made. I used to use a company called Olimex in Hungary, but they are at capacity with their own work now, so are not currently taking on small batches, which is a pity as pricing was good, and quality excellent. My friend Ben had some small boards made in China by www.smart-prototyping.com. The quality was great, so after a brief and helpful conversation on-line with Minnie Yu, I sent over the gerbers. Within 10 days, 5 boards arrived. Quality is at least as good as Olimex.
When I laid the board out, I tied to keep the digital signals away or perpendicular to the analogue stuff. A friend of mine always used to say "Keep the ones and noughts out of the rig" !
There's an analogue ground plane each side of the board.
A most exciting delivery from China...
Parts were ordered, and the board assembled...
One thing worth noting is the output coupling capacitors (C1,C2,C51 & C52)... I left plenty of room on the board, as I originally intended to use some poly's.. but Mr Self has written about non-linearities in some, so I though I'd try some multi-layer ceramic's... and they are TINY! This is a 2.2uF 50V example!....
I was going to carry on in this post, with control electronics and firmware, but, quite frankly, it's taking some time, and it's bloated into a project in it's own right... So far it consists of a Teensy 3.2 microcontroller, and a few logic gates to expand the IO, I'll show more in the next exciting episode!
Which are long past their best...
