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Old 23rd Oct 2020, 2:25 pm   #21
vidjoman
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Default Re: Adding a selectable line input to valve radiogram

This is the power supply from a Decca SRG300. The later red book says the 400 & 600 are the same except for 2 extra 100 ohm resistors fitted to each EZ80 anodes and 2 capacitors that are fitted in the stereo pickup headshell are deleted.
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Old 23rd Oct 2020, 8:17 pm   #22
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Default Re: Adding a selectable line input to valve radiogram

Hi vidjoman
Thanks for the drawing It seems that the chassis is live as you say. I have finished my chores for the day and dug out it’s service manual which agrees with your image (attached)
Richard
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Old 24th Oct 2020, 6:24 am   #23
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Hi again, I have managed to sit down and concentrate a little more on the electrical side of this project and my original intention to add a selectable modern stereo line input to this needs to take a back seat until I have achieved an electrical safety level I am comfortable with. My post is lengthy so thanks in advance to anyone who takes the trouble to read through it and potentially help with this.

I will explain further.

My background is one of industrial electrical engineering and electronics, and as such the idea of discovering the technicalities of a late 50's radiogram was very appealing especially as the end goal of a nice piece of furniture that plays my 78's and 1950's 45's, which I could also integrate with a modern audio source.
I have obtained an original service manual but need to familiarise myself with elements that are particular to this such as live chassis and valves, hopefully with sympathetic replies from this site. Some symbols and elements of these drawings are currently confusing to me. Thank you so far for the constructive help I have received.

Bearing this in mind I have decided firstly to ensure this appliance is electrically safe.

I have only run this SRG600 via an earth trip (a common plug in DIY device with no specified trip curves) and as I look deeper I am glad I have.

This is probably no surprise to many on here, but I must admit the safety element of this device has taken me back. I find the following a bit of an eye opener.

1. It is connected to the mains via a three pin plug, but only has a two core cable.(This cable appears to match the others inside this unit so I would assume it is original)
2. As there is no earth, this would render the metal bodied Garrard deck live in the event of a phase wiring fault to the metalwork.
3. The live amplifier/tuner chassis is accessible via a rear cover without the use of tools (retained by pivot tabs) and no signage of live exposed conductors.
4. The deck illumination lamp is chassis mounted and only covered by a small plastic blister which removes with a tap.
5. There is no isolation transformer.

My intention now is to provide mains AC via a three core supply cable directly to an on board rear panel mounted 6A RCBO with a 'C' curve to cater for the capacitor inrush. This would sit electrically where the previous phantom 'T4' isolation transformer was expected and I intend to provide earth terminations to all the exposed metalwork (Garrard deck etc) upstream of the chassis.

As a residual current device is not going to provide any protection from an earth fault of the secondary side of the power supply transformer (T3) I would ideally like to supply the chassis of the SRG600 via an isolation transformer. I am not sure where I would obtain this from but I would prefer a 'period' one if possible if anyone has any suggestions where i could find one it would be much appreciated. Someone has suggested other models mount this in the speaker compartment which sounds perfect.
Little points like signage for live conductors and tool removed fasteners for access panels will be made.

Again if i have missed something here constructive corrections will be welcomed.

I expect the purists would consider these modifications to detract from the originality of the radiogram, but I wish to have an appliance I can trust in my home which has a level of electrical safety acceptable with modern standards. I hope this principle is acceptable (to most)...

Once this is done I need to consider the line input mod further, and am appreciative of the kind help and suggestions I have had here.

It appears that the Ronette BF40 stereo cartridge will be providing a similar signal to a modern line input and i intend to break into this connection (to the amp) via a rotary channel selector (any suggestions where i can get one of these is appreciated) tying all the audio grounds together to the new earth connection. I need to understand the service drawing further as it is not clear to me how this connection to the chassis is shown grounded considering the live chassis design.
The new proposed line input will be via panel mount RCA's and I intend to include a 2 ch variable attenuator to trim the line input to match the deck signal to ensure volume compatibility on source selection.

The safety of this external connection one one side is made via grounding the audio grounds to the new earth terminations. Regarding the stereo signal side, i need to come up with something, and suggestions are welcomed here, perhaps via a opto isolator circuit.

I have started the refurb of the exterior of the SRG600 (french polish) and then refurb of the Garrard 121 deck, which I intend to complete before tackling the electrical issues I have discussed here, so hopefully i can check back for any any suggestions/constructive comments regarding my proposals in the meantime.

Thanks in advance

Richard
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Old 26th Oct 2020, 2:45 pm   #24
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Default Re: Adding a selectable line input to valve radiogram

Also could someone recommend the best place for replacement stylus (GC8-78 and 33/45) and Ronette 33/45 stereo.
Many thanks
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Old 26th Oct 2020, 2:58 pm   #25
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Default Re: Adding a selectable line input to valve radiogram

No externally accessible metalwork should become live in the event of L and N being reversed. It should be insulated, either by physical separation or by isolation capacitors. Live chassis construction techniques were very widely used in the valve era, though less commonly for radiograms. Appliances had to remain safe with L and N switched because non polarised 2 pin mains plugs were still very common.

You should think carefully before modifying the original safety arrangements in any way, even if you think you are making things safer, as there are legal and liability implications.

The easiest and safest way to add a line level input to a live chassis receiver is to use a Bluetooth module.
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Old 26th Oct 2020, 3:31 pm   #26
Edward Huggins
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Default Re: Adding a selectable line input to valve radiogram

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Originally Posted by Rich400 View Post
Also could someone recommend the best place for replacement stylus (GC8-78 and 33/45) and Ronette 33/45 stereo.
Many thanks
You can get replacement styli for the Ronette BF40 and the Garrard GC8. But why would you use the GC8 in a Stereo unit? Maybe just for 78s?
Be aware then that the tracking weight of these 2 cartridges will be different. You can buy these (if they have stock) from Mantra Audio, The Stylus Lady, Musonic and others.
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Old 26th Oct 2020, 3:44 pm   #27
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Default Re: Adding a selectable line input to valve radiogram

Beware references to the SRG300. It is the same basic chassis, and it is not isolated as noted.
The SRG600 should have the additional isolation transformer, not illustrated on the trader notes because it is in the cabinet.
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Old 26th Oct 2020, 5:48 pm   #28
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by Edward Huggins View Post
You can get replacement styli for the Ronette BF40 and the Garrard GC8. But why would you use the GC8 in a Stereo unit? Maybe just for 78s?
Be aware then that the tracking weight of these 2 cartridges will be different. You can buy these (if they have stock) from Mantra Audio, The Stylus Lady, Musonic and others.
Thank you for your recommendation Edward. I will contact them and see what they can supply.
Yes spot on there, I intend to use the GC8 for 78's but it has a reversible cartridge for 45's. In terms of tracking weight i intend to start around 5 grams and go from there, but considering what i need to do this is a way off. I just need to get these stylus here ready.
This radiogram came supplied from new with two interchangeable head shells.
One for a mono set up, one for stereo. They are both 3 pin units but of course only the two pins are wired on the mono GC8 cartridge, the Ronette BF40 has of course 3 pins wired.
I appreciate the considerations for the the stereo BF40 application especially in terms of tracking weight thanks.
The gram has a piano key selector for the mono/stereo applications.
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Old 26th Oct 2020, 5:50 pm   #29
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by paulsherwin View Post
No externally accessible metalwork should become live in the event of L and N being reversed. It should be insulated, either by physical separation or by isolation capacitors. Live chassis construction techniques were very widely used in the valve era, though less commonly for radiograms. Appliances had to remain safe with L and N switched because non polarised 2 pin mains plugs were still very common.

You should think carefully before modifying the original safety arrangements in any way, even if you think you are making things safer, as there are legal and liability implications.

The easiest and safest way to add a line level input to a live chassis receiver is to use a Bluetooth module.
Thanks Paul, I am satisfied with my knowledge base to ensure this gram is safe when I have finished with it.
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Old 26th Oct 2020, 6:05 pm   #30
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by AC/HL View Post
Beware references to the SRG300. It is the same basic chassis, and it is not isolated as noted.
The SRG600 should have the additional isolation transformer, not illustrated on the trader notes because it is in the cabinet.
Hi Bill
Many thanks for your reply, there has been some debate about the appearance of an isolation transformer, but interestingly although this gram is wired directly to the deck then goes to the chassis with no isolation transformer, I have have just used a torch to look in the rear of the LH speaker void and there is a disused cable in there....

It could well be this went down and was removed. Very disappointing.

Either way I need to fit one to be safe, and again if anyone has any leads on one i can fit, it would be much appreciated.
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Old 26th Oct 2020, 6:09 pm   #31
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Default Re: Adding a selectable line input to valve radiogram

It's not down to your level of competence, it's a matter of type approval. This radiogram satisfied electrical regs at the time it was made, and inherits grandfather rights today. If you modify it significantly, it will need to satisfy the regs in place *now*, which is a real can of worms.

It's obviously yours to modify as you wish though.
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Old 26th Oct 2020, 6:19 pm   #32
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Thanks Paul

First priority is to locate a suitable isolation transformer.
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Old 27th Oct 2020, 1:18 pm   #33
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Default Re: Adding a selectable line input to valve radiogram

Looking at the schematic, this is not an 'AC/DC circuit', where the tubes are in series with the mains wiring, but there is a transformer that is just not connected as we would want in this day and age.

If you wired the radiogram for 3 wires, create a chassis ground, then only supply the primary of the power transformer from the mains, breaking the connection of the neutral from the chassis, and take a amplifier ground from the last smoothing capacitor, then it would be no more treacherous than any other vintage tube amp.

I have read lots of comments about isolation transformers, but they are tools for professionals, and there is just the same opportunity to be shocked on the other side of the isolation transformer; their primary purpose is to ensure the equipment is galvanically isolated, so you can't kill yourself when you working on a piece of equipment and you hold the radiator, for example.

Whilst an isolation transformer would improve the safety of the device, it does not solve the chassis ground issue, which is their to protect you from the device if it has a failure that causes a breakdown in isolation.
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Old 27th Oct 2020, 1:48 pm   #34
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Default Re: Adding a selectable line input to valve radiogram

Presumably if an isolation transformer was part of the OEM spec, then replacing it with any suitably rated and approved modern isolation transformer would simply constitute a repair not a modification and the unit would then be as safe as intended and any potential legal complications avoided.

The issue of any further mods for your own use is entirely up to yourself but it would be prudent to put everything back to standard if the unit were passed on to anyone else to avoid problems if they subsequently passed on as a result of using it
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Old 27th Oct 2020, 3:04 pm   #35
Rich400
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Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by Richardgr View Post
Looking at the schematic, this is not an 'AC/DC circuit', where the tubes are in series with the mains wiring, but there is a transformer that is just not connected as we would want in this day and age.

If you wired the radiogram for 3 wires, create a chassis ground, then only supply the primary of the power transformer from the mains, breaking the connection of the neutral from the chassis, and take a amplifier ground from the last smoothing capacitor, then it would be no more treacherous than any other vintage tube amp.

I have read lots of comments about isolation transformers, but they are tools for professionals, and there is just the same opportunity to be shocked on the other side of the isolation transformer; their primary purpose is to ensure the equipment is galvanically isolated, so you can't kill yourself when you working on a piece of equipment and you hold the radiator, for example.

Whilst an isolation transformer would improve the safety of the device, it does not solve the chassis ground issue, which is their to protect you from the device if it has a failure that causes a breakdown in isolation.
Hi Richard
I have just scanned through your post and I am very much on board with what you are saying. I will look into it deeper shortly.
I am sure this will be contentious but it appears to fit in with my preference for electrical safety.
My main intention is restore this gram and make it acceptable and safe to operate in my household, as opposed to providing another layer of safety when fault finding. (Providing you have one hand in your pocket in a safe environment etc etc).
It is my opinion that Isolation transformers mainly Provide a recognised method of protection for a trained audio engineer when the chassis is exposed and live fault finding is underway.
Once this unit is up to spec the next time the chassis will be exposed is if it develops a fault, in which case it will be removed and passed into the hands of a competent person to repair in a suitable environment of their choice, along with attached details of potential mods.

I was thinking of locating a safety isolating transformer and fitting it, And exchanging metal pot spindles for ABS versions to mitigate exposure to mains voltage in the event the knobs fell off!

Now in reality it makes much more sense To provide a modern earth trip on board and earth the exposed metalwork. There are of course caveats to this but if your circuit modification is viable I am happy to modify it and provide additional Cabinet internal shielding with warnings of live conductors and enclose details of these circuit mods onboard.
I do not accept the rationale that earth trips should not be used as they may fail and only operate in the event of a shock so should not be fitted. It enables metalwork to be earthed and provides a better level of protection in normal use. In the event of An on board earth trip failure, My house (and I’d suggest many other houses) have another earth trip at the consumer unit albeit with a slightly longer disconnection time due to the slight increase in cabling. So redundancy there.
I am sure there will very good audio engineers with different views, and I look forward to hearing any pitfalls I have not considered in case my rationale is incorrect. I would only ask that safety schemes are considered as protection for the user rather than a repair engineer. An engineer of course would be armed with the mod information with the chassis and makes his own arrangements for safety.
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Old 27th Oct 2020, 3:17 pm   #36
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Default Re: Adding a selectable line input to valve radiogram

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Originally Posted by Herald1360 View Post
Presumably if an isolation transformer was part of the OEM spec, then replacing it with any suitably rated and approved modern isolation transformer would simply constitute a repair not a modification and the unit would then be as safe as intended and any potential legal complications avoided.

The issue of any further mods for your own use is entirely up to yourself but it would be prudent to put everything back to standard if the unit were passed on to anyone else to avoid problems if they subsequently passed on as a result of using it
Hi thanks for your thoughtful comments I have the original service manual for the SRG600, and there is no mention on the chassis diagram or parts list of an isolation transformer. There is a question of whether one was fitted to the SRG700 speaker void however, but this is only shown in ‘trader notes’ and manufacturer documentation usually is considered reference. I suppose it could be argued that adding an isolation transformer would actually be classed as a modification and should not take place!
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Old 27th Oct 2020, 5:15 pm   #37
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Default Re: Adding a selectable line input to valve radiogram

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Originally Posted by Richardgr View Post

If you wired the radiogram for 3 wires, create a chassis ground, then only supply the primary of the power transformer from the mains, breaking the connection of the neutral from the chassis, and take a amplifier ground from the last smoothing capacitor, then it would be no more treacherous than any other vintage tube amp.
The secondary mains transformer T3 is only supplying heater current. The HT is derived from the mains side of the transformer.
So I don't think that suggestion is going to work.
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Old 27th Oct 2020, 5:43 pm   #38
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Default Re: Adding a selectable line input to valve radiogram

This is from my manufacturers service notes
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Old 27th Oct 2020, 7:32 pm   #39
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Default Re: Adding a selectable line input to valve radiogram

Given that you have a professional background and a strong sense of responsibility Richard, I can see why you are concerned but this almost belt and braces" approach could be hazardous in itself. I can't give you a technical analysis but members often do start trying to add an earth and it's usually not a good idea. The really interesting aspect of live chassis design era is that there don't seem to be any examples of fatality or series injury that can be located [except in Repair Workshop's that is]. This applied to the more basic designs, where the valve heaters would be "daisy chained" at mains potential'. You seem have an Auto transformer included here. Historically, there were two reasons for the adoption of AC/DC techniques [a] Cheaper production costs [the mains transformer was a costly item] and [b] post war flexibility when many areas still had DC mains. Both types of power systems could be simply covered without a transformer in place.

It wouldn't look so good now but it was safe and it worked. People were careful and got the benefit of cheap consumer goods, using highly insulated casings and electrically separated links. Even the, perhaps higher risk, metal record deck or a valve mains/battery portable could be isolated, via a suitable capacitors, as already mentioned. Only Tape Recorders were a no go for obvious reasons perhaps. If your RG was returned to it's original standard I'm sure you would ensure that the negative lead went to chassis and not vice versa for example and be in sole charge. Often the two core lead had the same colours. That's easily updated without making a significant change I'd say but even a "live chassis is only a problem if it is live and can be accessed.

Of course I'm not making any recommendations here. It's up to you in the end as stated. I hope you ultimately get to safely enjoy your carefully refurbished Radiogram.

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Old 27th Oct 2020, 10:21 pm   #40
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Originally Posted by crackle View Post
The secondary mains transformer T3 is only supplying heater current. The HT is derived from the mains side of the transformer.
So I don't think that suggestion is going to work.
Mike
Yes, you are correct. Sorry for going off in the wrong direction. That is a tricky one to solve, if you were to try and earth the chassis! I think I’d be on the lookout for an old transformer with the extra winding.
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