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Sunday, 8 July 2018

Sam and the Queen Anne (Dynatron 1275A series)

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!

2 comments:

  1. Hi thank you for this wonderful article. I have had a similar problem with the Dynatron. One channel overheating. I note TR207 is BC1131, this is really helpful. Would I be right in thinking that TR203 is a BC1115? Any suggestions as to what is likely to cause the overheating in one channel? Your help would be much appreciated.Thank you.

    Kenny
    kdhun@hotmail.com

    ReplyDelete
    Replies
    1. Hi Kenny. How's the bias looking (voltage-wise) on the hot end of the output cap?

      Delete