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 Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 1st Jan 2017, 4:31 pm   #1
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Defiant RGSH 755 EL84 problem

The plate on the back of this radio says it is an RGSH 755 but it is not actually a radiogram

Anyway, I tried it on full mains as the seller said he had plugged it in and got "white noise" and the smoothing caps seemed ok. There is something coming in through FM so I started at the AF end. The AF coupling caps to the EL84s look like some sort of ceramic disk types. The EL84s were getting very hot and there was a positive voltage on the control grids so I replaced the coupling caps with .022uf ones which is the value shown on teh sheet "up top". When I got the old ones out I found that they were actually .005 but I assume this will make no difference. R39 and R40 are not there but there are 1M resistors from the grid tags to chassis.

Anyway one of the EL84s is still getting hot and there is a "pulsing" overlaying a signal fed into the PU sockets. The red visible inside the valve pulses in time to this. If I swap the valves the fault stays at the same socket and if I briefly remove the offending valve the pulsing disappears.

On checking the HT on the offending valve there is only 40 odd volts and I cannot get a resistance reading between the anode and the HT rail. I am assuming therefore that the output transformer must be open circuit on that side but I cannot understand why the valve glows so hot or pulses. Can anyone enlighten me please?

Thanks

Paul
PaulR is online now  
Old 1st Jan 2017, 4:40 pm   #2
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Defiant RGSH 755 EL84 problem

Don't have a schematic but screen grid?

Lawrence.
ms660 is offline  
Old 1st Jan 2017, 4:52 pm   #3
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Defiant RGSH 755 EL84 problem

Extract from circuit.

Check the anode and screen voltages with the EL84's removed. You'll soon see if the O/P transformer is o/c.

Could the caps from anode to ground be leaking?

Is the common cathode resistor reading the correct resistance?
Attached Thumbnails
Click image for larger version

Name:	defiant.jpg
Views:	264
Size:	23.7 KB
ID:	135205  
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline  
Old 1st Jan 2017, 5:05 pm   #4
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Defiant RGSH 755 EL84 problem

I'll buy the CD one of these days.

Lawrence.
ms660 is offline  
Old 1st Jan 2017, 5:05 pm   #5
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Yes the screen voltage would explain it. The anode of one is at 300 odd volts and the other around 40. I replaced the caps from the anode to ground.

It seemed a promising set as well!
Paul
PaulR is online now  
Old 1st Jan 2017, 5:15 pm   #6
crackle
Rest in Peace
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: Defiant RGSH 755 EL84 problem

Quote:
Originally Posted by ms660 View Post
I'll buy the CD one of these days.
Well worth it.

Mike
crackle is offline  
Old 1st Jan 2017, 5:26 pm   #7
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

I keep thinking that, but then each set I buy is going to be the last - until i see the next one of course

PS The common cathode resistor is a little high but nothing troubling
PaulR is online now  
Old 1st Jan 2017, 6:06 pm   #8
Edward Huggins
Dekatron
 
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 8,302
Default Re: Defiant RGSH 755 EL84 problem

The larger value coupling caps fitted by the OP might improve the bass repsonse as those removed were quite a low value.
__________________
Edward.
Edward Huggins is offline  
Old 1st Jan 2017, 9:07 pm   #9
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
Default Re: Defiant RGSH 755 EL84 problem

Yes, but it makes following other's problems much more engaging if you have the circuit available for reference.......

As does the almost complete run of R&TVS books (anyone got any after 1986/87 they'd like to shift?) and the BVWS Trader sheet CDs, not to mention Radiomuseum and Nostalgiaair etc etc.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is online now  
Old 1st Jan 2017, 10:09 pm   #10
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Defiant RGSH 755 EL84 problem

I don't do repairs at the moment but I might put the CD on my birthday...I want one o'them list in a month or two.

I assume it'll work with windows 10?

I know zilch about PC/program compatibility etc.

Lawrence.
ms660 is offline  
Old 1st Jan 2017, 10:13 pm   #11
Cobaltblue
Moderator
 
Cobaltblue's Avatar
 
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,823
Default Re: Defiant RGSH 755 EL84 problem

Works my Windows 10 machines
__________________
Invisible airwaves crackle with life or at least they used to
Mike T BVWS member.
www.cossor.co.uk
Cobaltblue is online now  
Old 1st Jan 2017, 10:26 pm   #12
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Quote:
Originally Posted by Herald1360 View Post
Yes, but it makes following other's problems much more engaging if you have the circuit available for reference.......

As does the almost complete run of R&TVS books (anyone got any after 1986/87 they'd like to shift?) and the BVWS Trader sheet CDs, not to mention Radiomuseum and Nostalgiaair etc etc.
That reminds me I actually have the sheet for this set in a R&TVS book. I forgot to look in it. I always buy a copy from here if I get a new set even though they may be available free elsewhere as it helps to support the site and is a small price to pay for all the help I get.
PaulR is online now  
Old 2nd Jan 2017, 12:29 pm   #13
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: Defiant RGSH 755 EL84 problem

Quote:
Originally Posted by ms660 View Post
I don't do repairs at the moment but I might put the CD on my birthday...I want one o'them list in a month or two.
I don't do repairs anymore but it's good to be be able to follow along or try and help. The other thing is that I enjoy reading the old manuals and seeing the different techniques used over the years, ok I need to get a life.
I think Paul is working on an update, it will be on my list when it's available.
Frank
Nuvistor is offline  
Old 2nd Jan 2017, 10:58 pm   #14
Restoration73
Nonode
 
Restoration73's Avatar
 
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
Posts: 2,801
Default Re: Defiant RGSH 755 EL84 problem

If you can accept a slight reduction in output, consider changing to separate EL84 cathode
resistors of 270 or even 390 ohms, with individual capacitors. The valves will run cooler.
Restoration73 is offline  
Old 2nd Jan 2017, 11:14 pm   #15
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Yes I was thinking about that. I have to do something about the transformer though. I think I have one from a hi fi ultra linear amp somewhere. I assume it would work if I just left the tap disconnected
PaulR is online now  
Old 18th Mar 2017, 1:42 pm   #16
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

I bought a new output transformer advertised as coming from a Fender EL84 amp.

I duly fitted this yesterday but just get a bad "feedback" howling when the set has warmed up. Is this likely to be something to do with the new transformer or should I be looking for something else that has failed or come adrift? The original soldering is not very robust on this set and several joints have come apart.

Thanks

Paul
__________________
Paul
PaulR is online now  
Old 18th Mar 2017, 1:58 pm   #17
vidjoman
Dekatron
 
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,315
Default Re: Defiant RGSH 755 EL84 problem

This suggests that you have the anode connections reversed and therefor are getting positive feedback.
vidjoman is offline  
Old 18th Mar 2017, 2:59 pm   #18
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Thank you. I did try reversing the connections to the valves but it made no difference. Sorry to be dense but how would the way they are connected produce feedback?
__________________
Paul
PaulR is online now  
Old 18th Mar 2017, 4:21 pm   #19
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Just trying to get my head round this positive feedback.

I have tried to associate the earthy end of the secondary with the lead from V8 so that the end of the secondary that supplies the negative feedback is associated with V7. I did this by simply observing the leads that were adjacent on the transformer. That didn't help. The howling continues if I disconnect R43 and the feedback loop from the secondary.

Paul
__________________
Paul
PaulR is online now  
Old 19th Mar 2017, 10:47 am   #20
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: Defiant RGSH 755 EL84 problem

Another snippet if it helps - it doesn't do it with the ECC83 pulled.
__________________
Paul
PaulR is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:04 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.