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.

Reply
 
Thread Tools
Old 22nd Nov 2017, 10:53 pm   #21
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default Re: Unknown make radiogram (Identified as an MRG)

The motor board was cutout today. The Garrard RC120H has been test fitted in the radiogram. Of course it didn't fit first time and various cutouts had to be made to clear the many parts around the outer edges of the mechanics. It's a perfect fit now so the next stage is fit the three suspension springs and screws which locate the deck in it's correct position.

DFWB.
Attached Thumbnails
Click image for larger version

Name:	MRG_RC120H_motor_board4.jpg
Views:	91
Size:	110.5 KB
ID:	152731   Click image for larger version

Name:	MRG_RC120H_motor_board5.jpg
Views:	87
Size:	174.0 KB
ID:	152732   Click image for larger version

Name:	MRG_RC120H4.jpg
Views:	106
Size:	108.4 KB
ID:	152733  
FERNSEH is offline   Reply With Quote
Old 22nd Nov 2017, 11:00 pm   #22
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default Re: Unknown make radiogram (Identified as an MRG)

An FC13C octode has been fitted in lieu of the 15D1 pentagrid frequency changer valve. The medium aerial and oscillator trimmers tweeked up so now the stations can be tuned in on the correct positions on the tuning scale. The receiver is giving a good account of itself, lots of distant MW stations coming in now. I'm waiting for delivery of a Brimar 15D2. Like the 15D1 and FC13C it also has a 13 volt heater except the heater current is 0.15amp which doesn't matter in this set because the valve heaters are wired in parallel. Let's find out which is the better valve. The MRG radio chassis has a bandpass front end, that is with two tuned circuits at aerial frequency and the tuned local oscillator. This would lead one to think that the receiver has a low IF, say 128Kc/s but by 1937 most radio receivers use the standard 465Kc/s IF, which in fact is the IF in this receiver.

DFWB.
FERNSEH is offline   Reply With Quote
Old 23rd Nov 2017, 8:22 am   #23
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,660
Default Re: Unknown make radiogram (Identified as an MRG)

I begin to think you're designing a new period radiogram from just the cabinet and chassis tray, David.

The inner papers of the mains transformer look a bit carbonised in your photos

It should look impressive and have a decent sound when done. To keep with the Bradford theme, I'd suggest a Wharfedale super 10 RSDD speaker? With the tuned RF stage, I wonder if there was, or was intended, a twin with a shortwave band for the export market?

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 23rd Nov 2017, 4:23 pm   #24
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default Re: Unknown make radiogram (Identified as an MRG)

Hi David, the mains transformer is in excellent condition, what looks like carbonising is the black pitch insulation material. Apart from the addition of a HT smoothing choke, fitted to replace the loudspeaker field coil the chassis and circuit hasn't been altered at all, except for the valves employed as replacements for certain hard to find types. A Tungsram VP13K replaces the VP13C IF amplifier. A 12H6 double-diode replaces the 2D13C, a special adaptor made for the octal valve. A Brimar 7D5 replaces the impossible to find Mullard PEN13C, the Brimar valve has proven to be an excellent replacement. The HT rectifier is a Tungsram APV4 = Mullard IW4-350
It would have been nice to fit a Wharfedale loudspeaker but the 10" unit now fitted as the replacement was made in France. No modifications required to the baffle board.
The Garrard RC120H is the type of record deck that would have been fitted in the 1950s as a replacement for the single speed RC4.

DFWB.
FERNSEH is offline   Reply With Quote
Old 23rd Nov 2017, 7:46 pm   #25
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,139
Default Re: Unknown make radiogram (Identified as an MRG)

To be honest David I think you have done a pretty good job with what was a very decrepid MRG radiogram.
It now works very well and the replacement autochanger will have given it a new lease of life.
The valve line up is fine and it will probably work for decades! John.
Heatercathodeshort is offline   Reply With Quote
Old 22nd Sep 2023, 11:01 am   #26
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default 1937 MRG radiogram.

The radio works OK but more audio power is needed to make the radiogram deliver a better sound from it's 10" loudspeaker.
The audio amplifier in it's present state consists of a Mullard HL13 triode and a Brimar 7D5 output valve. The characteristics of the HL13 are similar to the 6C5 and the 7D5 to the 6F6.
The plan is to to replace the triode with a pentode. Because this receiver uses 13volt heater valves the only valve available from my stock is a Brimar 8D2, which is electrically similar to the 6J7.
The attachment shows the circuit diagram of a amplifier which appeared in the 1949 Mullard Valve and Service Manual.
It should be possible to incorporate the feedback tone control instead of the simple top cut circuit.

DFWB.
Attached Thumbnails
Click image for larger version

Name:	Mullard_3watt_amplifier.jpg
Views:	60
Size:	71.0 KB
ID:	285378  

Last edited by Station X; 22nd Sep 2023 at 3:57 pm. Reason: Threads merged.
FERNSEH is offline   Reply With Quote
Old 22nd Sep 2023, 12:08 pm   #27
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,660
Default Re: 1937 MRG radiogram.

Not loud enough?

It could be a case of not enough gain in the two audio stages, or of the output valve not being capable of enough power, or a bit of both.

At full volume, is there much distortion? If not, then it's low gain.

With 13v heaters, that triode driver could become 2 stages of ECC82 with one stage as a tone control giving a bit of extra gain.

You could also play games with the output valve using a pair of small beam tetrodes in parallel with their heaters in series. EL91, 6BW6. Limiting factor may be saturation current of single-ended output transformer.

There's no end of dirty underhand trickery possible depending on how far you deviate from original.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 22nd Sep 2023, 12:48 pm   #28
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default Re: 1937 MRG radiogram.

The Brimar 7D5 has a gm of only 2.5mA/V and has similar characteristics as the old type 42 which dates back to 1934.
Like the HL13 the Brimar 8D2 has a B7 base so it is just a simple matter of fitting a few additional components to the unused valveholder tags.
I'll report the results from the modified amplifier later today.

DFWB.
FERNSEH is offline   Reply With Quote
Old 22nd Sep 2023, 2:07 pm   #29
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,660
Default Re: 1937 MRG radiogram.

Yes, that's much less invasive.

As well as checking Gm, look also at Ra especially with triodes. If an increase in Gm is met with a decrease in Ra, then the lower Ra acts in parallel with the intentional anode loads and you don't see quite the gain increase the change in Gm would suggest.

I take it that gram volume and radio volume are similar, so it's not a case of low IF voltage at the detector?

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now   Reply With Quote
Old 22nd Sep 2023, 5:36 pm   #30
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,438
Default Re: Unknown make radiogram (Identified as an MRG)

Replacing the triode audio amplifier with a pentode has proved to be a worthwhile exercise. I'd be tempted to employ the negative feedback tone control but the cathodes of the double-diode detector and the audio amplifier are connected together to provide delayed AVC. The fed back audio will have an effect on the functioning of the detector.

DFWB.
Attached Thumbnails
Click image for larger version

Name:	MRG_Radiogram_Pentode_Amp.jpg
Views:	39
Size:	83.4 KB
ID:	285396  
FERNSEH is offline   Reply With Quote
Old 23rd Sep 2023, 8:51 am   #31
Edward Huggins
Dekatron
 
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,258
Default Re: Unknown make radiogram (Identified as an MRG)

Well done for the work so far. To get it working as well as this with so few cap/resistor to its design changes is some testament. Your choice of replacement autochanger is most appropriate. If you did end up fitting a double-cone 10" driver, it would be interesting to see how this sounds given it's an AM set.
__________________
Edward.
Edward Huggins is online now   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:15 pm.


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.