UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 22nd Oct 2020, 3:47 pm   #1
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default Adding a selectable line input to valve radiogram

I have recently acquired a DECCA SRG600 radiogram, which although operates is in need of some TLC, which is a nice little project.

Ultimately I would like to retain its radio and turntable stereo functions but add a rotary switch selecting an additional more modern line input to the amplifier. (for the option to connect a CD player or similar)

I have acquired new old stock of matching knobs for the existing fascia so the originality will not be compromised too much, and this is within the aesthetic limits I am happy with.

I have performed a little investigation, but although I understand the basics of what is involved I wish to ask for help with this modification on the forum as a newbie.

What I do know is the following:

1. The existing stereo input to the valve amplifier is taken direct from the Garrard 121 mkII deck (This has interchangeable head shells but i intend to use it mainly with the stereo magnetic cartridge). This is connected to the amp chassis via a round 3 pin plug. The mains is connected through the deck to the valve chassis.

2. I am aware that i need to standardise any input to the amplifier by reducing the signal of the modern line input to that of phono level. I have seen that these exist in the form that they input/output via RCA connectors which is suitable for the modern line inputs i wish to input.

3. I am aware that when this signal is reduced the RIAA signal should be considered and if not this may result in a higher component of bass as compared to the deck input. If this is not excessive I am prepared to live with a little trimming on changeover on the existing bass and treble controls of the 'gram'.

4. The valve chassis has 'E' valves and its own transformer so the chassis is not live.

This is what I am considering with my limited investigations.

A. Use the existing 3 pin stereo plug input into the amplifier, and rewire the opposite end to connect to the output of a stereo rotary switch with RCA connections. (connect to the output of the 2ch in 1 out rotary switch)

B. Use a simple line to phono board/converter to equalise the modern input signal, and connect to the newly fitted rotary switch.

C. The stereo output wires from the deck would then be re-terminated to RCA connectors and connected to the rotary switch.

I am suggesting the use of RCA connectors as I have seen on eBay a kit with a remote rotary switch which links to a separate input/output box (this can be mounted away from the amp to eliminate interference) which accepts the RCA connectors. Its output channel is actually a screw type connection and would eliminate the need to connect RCA plugs to the final amp connection as described in A.

Using this i would connect the external line to phono converter directly by RCA's and mount it on the rear panel to facilitate the external line connection.

My concerns with all this are picking up hum, but to mitigate this I would ensure each unit is grounded and ran at a distance from other live items.

I would appreciate any constructive advice in relation to if there is a simpler way to achieve my aim, or indeed if there is a unit available that would save me the bother of piecing this all together.

Also with the conditioning of the line signal to phono level would it be prudent to have an adjustment for level to trim the deck and external inputs to be the same on changeover.

I am not an electronics enthusiast so if I could achieve my goal with buying a unit to achieve the functionality or a simpler method that would be preferable.

Thanks in advance

Richard
Rich400 is offline  
Old 22nd Oct 2020, 4:47 pm   #2
Edward Huggins
Dekatron
 
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,302
Default Re: Adding a selectable line input to valve radiogram

I think you are trying to make this into something it can never effectively be. And you may well be dissapointed with the end result. As to fitting a stereo magnetic cartridge, there is more to this than meets the eye as it will need pre-amplification, it will not track very well in that heavy Garrard tone arm, and, yes, there definately a high risk of introducing hum however well it might be earthed. I'm sure other Members may take a different view.
__________________
Edward.
Edward Huggins is offline  
Old 22nd Oct 2020, 6:18 pm   #3
regentone001
Heptode
 
regentone001's Avatar
 
Join Date: Sep 2014
Location: Coventry, West Midlands, UK.
Posts: 518
Default Re: Adding a selectable line input to valve radiogram

please be very careful with this radiogram. In my radio and tv servicing books someone has written "same as SRG600" next to the circuit of the SRG300. If this is correct then it has a live chassis. It uses E range valves, including 2xEZ80 but one side of the mains is connected to chassis and the transformer is used as an autotransformer for HT and just for the valve heaters.
Please check this out to make sure you are safe.
Steve
regentone001 is offline  
Old 22nd Oct 2020, 6:36 pm   #4
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,798
Default Re: Adding a selectable line input to valve radiogram

That safety warning is very important. Some of these things have nasty surprises that are deceiving unless you go looking for them.

Those period record decks are pretty brutal to cartridges. Modern magnetic ones are decidedly fragile in comparison to the old piezo-electric jobs. You're better off sticking to a piezo type if you want to keep the deck. If you want to go magnetic, then you'll need to find a gentler deck that will fit in the space.

Yes, RIAA circuits have a lot more gain at the low frequency end, but this compensates the falling voltage from magnetic transducers at lower frequencies. It doesn't give you excess bass, but it does mean you need to be more careful about isolating paths where hum can get in because it does exaggerate that.

First things first, check the isolation between the mains and the chassis. If it's not isolated, this might not be the best candidate for your intended changes.

Oh, and welcome!

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 22nd Oct 2020, 7:06 pm   #5
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,425
Default Re: Adding a selectable line input to valve radiogram

Service data for the SRG600
https://www.service-data.com/product...43/1686/t11243

The service sheet describes T4 as an isolation transformer, that feeds the motor and another transformer.

Confirm it’s the same radio gram and T4 is serviceable and has not been removed
Attached Thumbnails
Click image for larger version

Name:	F9CDBDEE-711F-43FC-A1CF-6275331D9B31.jpg
Views:	207
Size:	54.7 KB
ID:	218631  
__________________
Frank
Nuvistor is online now  
Old 22nd Oct 2020, 7:25 pm   #6
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,783
Default Re: Adding a selectable line input to valve radiogram

Rich, if you want to modify this radiogram along the lines you suggest you will have to do a *lot* of work. It's not impossible by any means, but may be beyond you given your admitted level of expertise.

A more practical solution may be to keep the radiogram more or less as it is, and feed it with an external mono source, which is how it was designed to be used. You can parallel the stereo channels of an external device to mono with just a couple of resistors.

These radiograms were never intended to work as hifi devices.
paulsherwin is online now  
Old 22nd Oct 2020, 7:32 pm   #7
emeritus
Dekatron
 
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,316
Default Re: Adding a selectable line input to valve radiogram

In the 1960's I added a line input for my EL3585 tape recorder to dad's mid-1950's RGD radiogram (mono) by wiring a DIN socket in parallel with the cartridge via a 2 way switch to select either source. The gram was not live chassis.

My 1980's Philips music centre seems to simply connects its line input in parallel with the turntable cartridge, no separate switch being provided. The relevant selector button is marked "Phono/TV", envisaging its use for reproducing stereo TV sound. We don't use it for that, but it's useful for playing my wife's iPad through it.
emeritus is offline  
Old 22nd Oct 2020, 8:04 pm   #8
JohnBHanson
Heptode
 
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 661
Default Re: Adding a selectable line input to valve radiogram

I would consider using a switched 3.5mm stereo socket which can be wired in the gram lead.

connect the left and right pins of the socket each via a resistor (2k ohms is fine) to
make it mono and connect this to the amp. Connect the gram hot lead to one or both of the switch connectors.

That way the socket can placed in a hidden position and inserting a plug would automatically change from gram to external input.
JohnBHanson is offline  
Old 22nd Oct 2020, 8:14 pm   #9
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,783
Default Re: Adding a selectable line input to valve radiogram

This is not easily possible if the OP wants to fit a magnetic cartridge to the record deck though.
paulsherwin is online now  
Old 22nd Oct 2020, 8:21 pm   #10
JohnBHanson
Heptode
 
Join Date: Aug 2009
Location: Worthing, Sussex, UK.
Posts: 661
Default Re: Adding a selectable line input to valve radiogram

In that case the 3.5 mm socket would need to be between the output of the new phono pre-amplifier and the original gram input to the amplifier
JohnBHanson is offline  
Old 22nd Oct 2020, 9:48 pm   #11
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default

Quote:
Originally Posted by emeritus View Post
In the 1960's I added a line input for my EL3585 tape recorder to dad's mid-1950's RGD radiogram (mono) by wiring a DIN socket in parallel with the cartridge via a 2 way switch to select either source. The gram was not live chassis.

My 1980's Philips music centre seems to simply connects its line input in parallel with the turntable cartridge, no separate switch being provided. The relevant selector button is marked "Phono/TV", envisaging its use for reproducing stereo TV sound. We don't use it for that, but it's useful for playing my wife's iPad through it.
Thanks Emeritus and many thanks everyone who has contributed, all very interesting.
I perhaps have not been entirely clear, the gram has two headshell's one for stereo with a Ronette BF 40 cartridge which is wired with the three pins as you would expect. The other is a mono GC8 crystal cartridge for mono playback and is a dual (twist head) that plays 78's and 45's from memory (Red/green coding).

The front of the 'stereogram' has piano keys to make radio selections and there are two for the deck one marked gram and the other stereo. (the radio being mono) So we already have a stereo way into this amplifier which i failed to make obvious when i mentioned the 3 pin connection to the amp in my description.

Emeritus, this is just the solution I am looking for a simple addition which will give me my functionality. I will check to see if the chassis is live as in another comment by testing the chassis to earth through my fluke. I have it running through an earth trip on the incomer but the test is completely relevant and of course i will check this is not live before proceeding.

I still wish to select the external option via a rotary switch as in your modification though. and recommendations as to a quality unit to use again would be welcomed.

I am still a little concerned that the phono input the amp expects is much smaller than the line in from modern equipment though. As the line to phono converter boards are relatively inexpensive it may be prudent to connect through this on the onset.

I acquired this SRG600 from the family of the original purchaser and have had extended conversations regarding its history most of which has just been a piece of furniture. So quietly confidant ahead of the checks that it is stock. It certainly looks entirely original inside and out.

Many thanks again for everyones contributions, and for welcoming me into this site. It will be a while before i am able to start this modification as the turntable is gummed up and requires a good cleaning at the minimum, and im pretty sure i saw it try to spin in reverse when i had a go at operating it initially!

Regards Richard

Quote:
Originally Posted by Nuvistor View Post
Service data for the SRG600
https://www.service-data.com/product...43/1686/t11243

The service sheet describes T4 as an isolation transformer, that feeds the motor and another transformer.

Confirm it’s the same radio gram and T4 is serviceable and has not been removed
Hi Frank

Many thanks for your diagram, i will check the gram tomorrow for T4.
I have the SRG600 service manual that only gives the details of the chassis amp and tuner, i would love the diagram you have there.

I have attached a shot i made from my manual regarding the stereo input to the amp. this is where i intend to enter the circuit via a new external rotary switch.
Attached Thumbnails
Click image for larger version

Name:	SRG 600 Rich400.jpg
Views:	244
Size:	52.2 KB
ID:	218642   Click image for larger version

Name:	SRG 600 pickup socket diagram.jpg
Views:	163
Size:	34.2 KB
ID:	218645  
Rich400 is offline  
Old 23rd Oct 2020, 12:05 am   #12
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Adding a selectable line input to valve radiogram

I've repaired a few of these, but I've never seen T4 the isolation transformer. Also the chassis layout doesn't show it either.

As far as I'm aware, they are not live chassis though I may well stand to be corrected.

They are stereo, using two sets of ECL82's in push pull.

You must have this chassis properly serviced. Most of the resistors will have drifted and it is full of Hunts mouldseal capacitors. It is not a job for the fainthearted!

Theoretically you could modify the switchbank so that the GRAM is for stereo records and STEREO is for an external input. I once thought about it, but never got round to it.

The output from a crystal pick up is similar to the line out of most modern equipment. If you find it too high, you can attenuate it with resistors.

Best of luck.
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Old 23rd Oct 2020, 10:48 am   #13
vidjoman
Dekatron
 
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,315
Default Re: Adding a selectable line input to valve radiogram

The SRG300 was a live chassis, I know because I had one. The T4 isolation transformer shown in the above circuit clip is odd in that it looks like an afterthought as it also feeds the turntable. It would have been cheaper to just fit one transformer. It may say something in the parts list. I suspect it could have been for export models.
vidjoman is offline  
Old 23rd Oct 2020, 10:58 am   #14
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default Re: Adding a selectable line input to valve radiogram

Unfortunately worst case has been proven here, no T4 isolation transformer and 240v to earth testing from the chassis.
It’s either dispose of it or fit a 6A RCBO on board and continue with the plan.
Richard
Rich400 is offline  
Old 23rd Oct 2020, 11:06 am   #15
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,798
Default Re: Adding a selectable line input to valve radiogram

An RCBO doesn't provide isolation. it just kills the power after you've started getting a shock, and like any trip, they are not 100% dependable.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 23rd Oct 2020, 11:11 am   #16
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default Re: Adding a selectable line input to valve radiogram

Hi David yes I appreciate what you say, and of course the earth protection should never be relied on as a functional element, just protection in the event of a earth fault condition.

Regards Richard
Rich400 is offline  
Old 23rd Oct 2020, 11:46 am   #17
rontech
Heptode
 
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
Default Re: Adding a selectable line input to valve radiogram

In the late 1950s I added a tape input ( mono ) To my father's 1937 Marconi 564. I used a P.O. jack socket with screened cable to the top end and earth pin of the volume control. Selected by a simple 2 way switch. Set the radiogram selector to Gram and it worked fine. No hum problems.

The radiogram was an AC only type. I would not have done it with an AC/DC set
__________________
Time flies like an arrow, fruit flies like a banana
rontech is offline  
Old 23rd Oct 2020, 12:34 pm   #18
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default Re: Adding a selectable line input to valve radiogram

Quote:
Originally Posted by Rich400 View Post
Unfortunately worst case has been proven here, no T4 isolation transformer and 240v to earth testing from the chassis.
I wonder if the mains transformer is faulty. Looking at the diagram, it would appear the pick up screen is connected directly to chassis. If this is so, then either its design is flawed (very very unlikely with a quality manufacturer like Decca) or the transformer has a short to the chassis. IF the chassis is live, then so is the lamp in the record player compartment.

I think that transformer needs checking out
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Old 23rd Oct 2020, 12:54 pm   #19
Rich400
Tetrode
 
Join Date: Oct 2020
Location: Lowestoft, Suffolk, UK.
Posts: 71
Default Re: Adding a selectable line input to valve radiogram

Thanks Maurice I’ll do that next, I’m currently french polishing the top in case it remains!
Rich400 is offline  
Old 23rd Oct 2020, 3:24 pm   #20
AC/HL
Dekatron
 
AC/HL's Avatar
 
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
Default Re: Adding a selectable line input to valve radiogram

The problem with Trader sheets is that they are notes for the guidance of experienced Engineers, not workshop manuals. This one covers several models, and if it is fitted the isolation transformer is in one of the loudspeaker compartments. It appears to be in association with external speaker facilities not provided in lesser models.
AC/HL is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:38 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.