Decca Stereo Radiogram SRG 650. Some contrarian thoughts

[paulsherwin]

Others have said that this doesn't have a tape input. That's odd for a 1961 luxury stereogram, but not impossible - I'm not familiar with it personally.

It's also been suggested that this is a live chassis design. Assuming that's so, it does make it difficult to add a wired aux input safely. There are designs that use a 6.3V heater transformer and derive their HT directly from the mains, but that's unusual, especially for a high end model like this.

Your best approach by far is to add a BT module, which maintains safety because of physical separation. There have been many threads discussing this, e.g. https://www.vintage-radio.net/forum/...d.php?t=183586

[David Man]

Originally Posted by Michael Maurice

There are a lot of parts which need replacing in the radio section which is quite hard to do.

I was trying to obtain this radiogram but for whatever reason you beat me to it.

.

I'm sure you are right Michael, and I will be very grateful for any and all help from this forum to make sure I do the best job I can, in every sense. I am new to this, but I have wealth of other technical and practical experience to call upon and I'm ready to learn.

I didn't want to let that last statement hang in the air, I know you feel aggrieved. I am sorry you were not successful in your negotiations with the original owner. As you know, I had no part in any misunderstandings and I did offer to stand back so you could collect it within the time window being offered but you could not for whatever reason, and your loss was my gain I'm sorry to say. By the time you contacted me I had already invested plenty of time, money and trouble in it, and it was in bits in my workshop covered in french polish. I promised the owner I would do it proud and I will endeavour to. I hope you will help me, in the spirit of fellow enthusiasts.

[David Man]

Originally Posted by RojDW48

It's a good SRG. No great benefit to be had from replacing the old BSR - and 'upgrading' to an MM cartridge is the road to nowhere (unless you are an expert) plenty of good and appropriate ceramic carts to be had. One mod I think is worth doing (as I have) is the institution of a switched socket in the gram line so you can plug in a CD player or cassette player. Chances are it will match well enough - if not it's not much wasted time.

I'd be very interested in learning how you did that, and how effective it was!

[David Man]

Originally Posted by Radio Wrangler

With pairs of ECL82s in push-pull, the power available is rather modest. Modern loudspeaker drivers have a number of advantages, though at the cost of significant inefficiency. Attempts at upgrades in this area in period equipment have not proved wildly successful, as much from cabinet bass limitations as quieter results. The distance round through the back to the front creates an interference wave controlling low frequency rolloff irrespective of the driver. Deeper bass needs an infinite baffle , or a transmission line, or a tuned port.

If better sound quality had been my aim, I wouldn't have started with a stereogram. There are better starting points for no more money if you look around and can see past unfashionability of some brands.

David

Thanks David. I do understand your point, and I am very familiar with the availability of vintage hifi, my house is full of it. I will, as you say, accept the limitations and relish in the positives of this wonderful equipment!

[David Man]

Originally Posted by Radio Wrangler

You'll find that arm unsuitable for any cartridge that can work with low tracking weights. Too much friction, poor control of tracking weight. You're pretty much stuck with piezo-ceramic types with fairly low complince.

David

I have read that there are a few ceramic carts which offer excellent sound quality, which would you recommend?

[David Man]

Originally Posted by Michael Maurice

This is a live chassis, any mod like that would be dangerous.

So is that impossible, or difficult, or possible with work? What would be involved? I do have a very good tech who works on my other gear, and he is very experienced with valve equipment (40+ years in the trade). If it can be done I'm sure he could do it, though I doubt he's been asked to do it before.

[David Man]

Originally Posted by paulsherwin

Others have said that this doesn't have a tape input. That's odd for a 1961 luxury stereogram, but not impossible - I'm not familiar with it personally.

It's also been suggested that this is a live chassis design. Assuming that's so, it does make it difficult to add a wired aux input safely. There are designs that use a 6.3V heater transformer and derive their HT directly from the mains, but that's unusual, especially for a high end model like this.

Your best approach by far is to add a BT module, which maintains safety because of physical separation. There have been many threads discussing this, e.g. https://www.vintage-radio.net/forum/...d.php?t=183586

Thank you again Paul, I will read the thread and digest! Ahh, it appears I don't have access to that for some reason.

[AC/HL]

You're over 10 posts now, so modern technology should be available.

Originally Posted by David Man

I have read that there are a few ceramic carts which offer excellent sound quality, which would you recommend?

The Decca service data is a bit convoluted, but it suggests that the SRG650 has a Deram cartridge with a diamond stylus. If that is so, that was one of the best.

[Radio Wrangler]

The Decca Deram would be the obvious one if you could find one. A lot of domestic entertainment equipment saved a few quid by not having a proper mains transformer and rectifying mains directly. Some had transformers for their heaters just to sow confusion, but still had live chassis. Before going inside anything ALWAYS check to see if the chassis is isolated or not. Getting this wrong can kill you.

Live chassis can be handled but you need isolating transformers, experience and care.

If it isn't fully isolated it will limit what you can safely connect to it. Test equipment, tape recorders, streamers...

David

[David Man]

Originally Posted by AC/HL

You're over 10 posts now, so modern technology should be available.

The Decca service data is a bit convoluted, but it suggests that the SRG650 has a Deram cartridge with a diamond stylus. If that is so, that was one of the best.

I agree Bill, it would seem to indicate that my gram would have a Collaro C60 turntable and Decca Deram cart, however mine has a Monarch UA15 (or 16) with a Ful-fi turnover cartridge. I suppose it could have been swapped at some point. It works, in that the mechanism seems to work OK, but it is in poor cosmetic condition and I have no stylus so I don't know how it tracks. I was bouyed when I discovered a sister gram, the SRG700, often came with a Garrard 60 Mk2 deck which a Decca Deram would work on, and could track at a much more vinyl friendly 2g I believe. An altogether more attractive option, but it seems not a popular modification.

[David Man]

Originally Posted by Radio Wrangler

The Decca Deram would be the obvious one if you could find one. A lot of domestic entertainment equipment saved a few quid by not having a proper mains transformer and rectifying mains directly. Some had transformers for their heaters just to sow confusion, but still had live chassis. Before going inside anything ALWAYS check to see if the chassis is isolated or not. Getting this wrong can kill you.

Live chassis can be handled but you need isolating transformers, experience and care.

If it isn't fully isolated it will limit what you can safely connect to it. Test equipment, tape recorders, streamers...

David

Thanks, I will heed your warning I promise. I do have some experience of electronics, I did a city and guilds electronics servicing course when I was 17, but that is a very, VERY long time ago!

[PaulR]

If the radiogram is to be used at all I think that, whilst pondering how to provide another input, basics like testing smoothing caps and replacing audio coupling caps and any others subjected to high voltages should be undertaken. Without that being done there is the risk of ending up with a nice wooden box containing unusable electronics.

"E" series valves have been mentioned. Are we sure it is a live chassis set, maybe with just a heater transformer? It just seems a bit "nice" to be live chassis.

[ronbryan]

The Trader sheet shows a 1:1 mains isolating transformer T4 preceding the gram motor and the mains HT/LT transformer. It is not located on the chassis, so if fitted, must be somewhere on the cabinet. Only half wave rectified HT, performed by two EZ80s in parallel.

Ron

[PaulR]

That sounds more like Decca! Fitting an Aux connection should be fairly straightforward then.

A definitive check can be made by testing for continuity between the chassis and mains plug pins.

[Boulevardier]

Originally Posted by ronbryan

The Trader sheet shows a 1:1 mains isolating transformer T4 preceding the gram motor and the mains HT/LT transformer. It is not located on the chassis, so if fitted, must be somewhere on the cabinet. Only half wave rectified HT, performed by two EZ80s in parallel.

Ron

The RTVS schematic I have shows the anodes of the EZ80s connected to the primary of the mains transformer through a couple of 100R resistors and thus to one side of the mains input. The other end of the primary connects direct to chassis which goes directly to other side of mains input. There is no 1:1 isolating transformer shown. The "mains input" isn't marked as such, so maybe there's a separate transformer that isn't shown on the RTVS schematic.
Mike

[Station X]

Extract from the manufacturer's service info.

I think it would be best if the OP did some continuity tests, with the player switched ON but disconnected from the mains supply, to establish whether one side of the mains supply is connected to the player's chassis.

[Radio Wrangler]

Test both the live pin to chassis AND the neutral pin to chassis.

Some sockets around the country have been found wired with live and neutral reversed. Also faults occurring in street wiring, substations and other people's property can cause your neutral to suddenly go to a dangerous potential. So neutral should be treated as not safe to touch.

David

[PaulR]

And, of course some live chassis radios of the time had odd switching arrangements which lead to an apparently "dead", switched off but plugged in, set having the chassis at mains potential.

[David Man]

I can reveal... there is continuity between the chassis and neutral. Hmmph!

[Station X]

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