Decca Stereo Radiogram SRG 650. Some contrarian thoughts

[paulsherwin]

I can understand the confusion. That's a very odd design decision for a relatively high-end radiogram. It's not as if it even results in a big reduction in manufacturing costs.

[AC/HL]

I seem to remember that we went through the isolation conundrum once before, and the isolation transformer was found in the depths of the cabinet behind a baffle of some description. Decca were fond of oddball designs, at least you have common valve types here. Multiple models in one manual is a definite recipe for confusion.

[Boulevardier]

I haven't analysed the convoluted earthing/chassis connections, but how are the exposed deck metalwork and pickup connections isolated from the live chassis? I suppose there must have been plenty of ac/dc record players, but none have come my way; how was their user-accessible metalwork, etc, isolated from mains voltages?
Mike

[paulsherwin]

The metalwork either isn't connected to the chassis at all, or is isolated with capacitors. The wiring to the cartridge also goes through capacitors. It made some sort of sense to do things that way for cheap portable record players as it saved both cost and weight, but in this case there needs to be a big transformer anyway because of all the E series valves, so the benefit is far from obvious.

[Boulevardier]

Originally Posted by paulsherwin

The metalwork either isn't connected to the chassis at all, or is isolated with capacitors. The wiring to the cartridge also goes through capacitors. It made some sort of sense to do things that way for cheap portable record players as it saved both cost and weight, but in this case there needs to be a big transformer anyway because of all the E series valves, so the benefit is far from obvious.

I agree, really odd-ball stuff. I think the other BB versions I've looked at have all had normal double-wound mains xformers for HT supply.

I wouldn't have thought capacitor isolation would have been much isolation at all at 240V ac - after all, the audio has to get through them, and some at least had reasonable LF response... Still, it seems to have worked satisfactorily, and I haven't heard of routine deaths from record players!

[paulsherwin]

With the high impedances of valve audio you only need very low value capacitors, but it's still a bit dodgy given the capacitor technology of the time.

It was a very common technique in cheap portables, with a UY85 and UL84 getting their heater supply from a tap on the gram motor. It's just odd to see it used in a stereogram with push pull output.

[qualityten]

Coming late to this thread, you might find mine on the SRG600 (same circuit) useful: https://www.vintage-radio.net/forum/...d.php?t=185307

[Radio Wrangler]

Sorting out differences in levels is not usually too difficult. The big problem is that all the radiogram circuitry is connected to one side of the mains. The cabinet, the knobs etc were all designed to keep the electronics fully insulated from people. Anything you connect has to follow this regime. Which is why it has no tape recorder connections.

Lack of a fully isolating mains transformer removes a lot of safety protection and saves a quid or two.

Using an autotransformer for just the heaters still costs less than a full isolating main transformer, but gives no more protection than being transformerless. But, unlike the fully transformerless doesn't make the set AC/DC. THe autotransformer does allow mainstream valve types without being tied in to the 100mA series string heater types, and avoids the heat from a dropper resistor.

If an input from a Streamer, CD or whatever, these devices were never thought of in terms on non-isolated use. There are ways to arrange isolation but none of them are al; that good, affecting the sound and leaving you legally liable should anyone who owns the thing later be harmed.

The best approach would be to leave the set as an example of what things were like, and go looking for a more suitable unit, with a proper mains transformer and an earthed chassis.

In the later fifties and through the sixties live chassis construction was very common for simple profit reasons Their era ended with the move to transistors. Transistor stuff needed lower voltages and greater currents, so mains transformers staged a comeback. Decca chose a halfway house with almost all of the disadvantages of both worlds. They just did things differently no-one's ever found out just why.

David

[Radio Wrangler]

I forgot to mention that even if you keep the neutral end of the mains connected to the chassis, neutral often runs a few volts above earth due to mains currents in houses and ohmic drops. If you connect to anything with an earthed case, you can get enough current flowing N to E to give nuisance tripping of RCDs in your consumer unit.

David

[David Man]

Originally Posted by Station X

Well at least it's not between chassis and live!

True, but there is a two pin unpolarised chassis mounted plug and socket which appears unmarked, so it's not exactly robust. I will ensure the connection is soldered.

[vidjoman]

I think you have to accept the way it was normal to have live chassis on most radios and radiograms. They were sold as a complete unit and they were not expected to have external equipment connected to them. They were intrinsically safe in normal use. I had a Decca SRG 300 with an almost identical chassis and the only minor problem was if the 2 pin plug was put in the wrong way round then you got a tingle when touching the turntable unit. Simple answer was to reverse the mains plug. We didn’t have tape recorders to plug in in those days so it wasn’t important. Later on it became common to attach a tape unit etc to record the record charts hits of the day. Left as it was supplied doesn’t present a problem. It’s only if you want to change it’s use to do what was never intended or thought possible at the time of manufacture.

[David Man]

Originally Posted by Radio Wrangler

The best approach would be to leave the set as an example of what things were like, and go looking for a more suitable unit, with a proper mains transformer and an earthed chassis.

David

It would seem the case, and it's good advice. But, and it's a big but...what a shame! Have you ever seen such a fabulous statement piece of furniture. It has to be the best looking radiogram I've ever seen. In fact, it stands up as a piece of furniture on it's own. Since acquiring it I have scoured the net looking for other examples, and nothing can hold a candle to it for me. I so want to make this a part of my environment and use it as much as possible. I will refurbish the chassis and put it all back together. I can't wait to hear it play again. It could have been so much more.

[David Man]

Happy weekend all,
Having decided to keep the gram as is, I was pondering on the seemingly best cartridge option of a Decca Deram. From what I have read so far, although they are undoubtedly a great cart, quality styli are impossible to find for them and the standard of those that are available is poor, so what is the point? Having one retipped is financially not an option (or is it?). So are they really a viable option?

[Edward Huggins]

This is a Decca Stereogram made to exploit the virtues of the well regarded Deram cartridge. Fitting another type of cartridge would present other problems as you will find it very hard to find another ceramic cartridge that has the same output (50mV) of the unique Deram, This would be likely to create overload/distortion issues.

[RojDW48]

You are right about Deram replacement styli - they are very poor - too stiff. The real ones do come up from time to time - worth the wait.

[David Man]

Originally Posted by Edward Huggins

This is a Decca Stereogram made to exploit the virtues of the well regarded Deram cartridge. Fitting another type of cartridge would present other problems as you will find it very hard to find another ceramic cartridge that has the same output (50mV) of the unique Deram, This would be likely to create overload/distortion issues.

Thank you Edward. I will certainly look out for one, though mine was not fitted with it originally, it has a BSR Ful-fi cartridge which is U/S.

[David Man]

Hi again everyone.
After some time debating and even more time stripping and cleaning, my monarch deck is in fine condition. I managed to find a donor with a better platter and original mat in great condition, and all is good with the world. Which brings me to the next dilemma. Cartridge. The original is a BSR ful-fi TC8S which I understand tracks at around 10g. It is also likely dud from what I am reading. So what to replace it with? The Decca Deram mentioned previously seems an almost impossible dream to find. Is there anything of quality either modern or likely still working which I can use which will match the amp? Preferably with a lighter tracking force. Thanks.

[Edward Huggins]

Go for a NOS BSR SC11M or a BSR SC12H. They are an easy fit and their output levels are not too high. Or buy a "Chinese Generic Red" cartridge fitted to a standard 1/2" mount. With adjustment to the tracking weight on your tone arm, set these to c.5 grams. There should be 4 wires in your tone arm, although the old BSR TC8S only used 3. Search "Stereo Cartridge Wiring" for the correct matching of the 4 coloured leads to the 4 pins.

[David Man]

Originally Posted by Edward Huggins

Go for a NOS BSR SC11M or a BSR SC12H. They are an easy fit and their output levels are not too high. Or buy a "Chinese Generic Red" cartridge fitted to a standard 1/2" mount. With adjustment to the tracking weight on your tone arm, set these to c.5 grams. There should be 4 wires in your tone arm, although the old BSR TC8S only used 3. Search "Stereo Cartridge Wiring" for the correct matching of the 4 coloured leads to the 4 pins.

Thanks Edward. Am I correct that the TC8S is 150mV? So the carts you mention are rated at 100mV and 190mV? The SC5M is also 100mV, SC5H and SC7M are also 190mV. (Thank you Ben!)

I assume there are differences, but what are they? Is it just size or fitting?

[toshiba tony]

I thought earlier on somebody commented it wasn't a mains isolated chassis, take care!