UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio 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 20th Oct 2011, 7:43 pm   #1
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Question Jiskra Sonoretta midget valve radio

Anyone know anything about these Czech sets of the mid/late 1950s? The
design is what I term a 'short' superhet, since it only uses 4 valves, including the rectifier. The lineup is EK90, EF 93 or 96, EL95, and EZ90(should this be EY90, since only a half wave rectifier is used? Only one double tuned 452kHz
IFT is fitted, and I cannot work out how the audio signal is obtained since there is no detector diode as such, the AF being taken from the anode of the EF93 to the 'hot' side of the Volume pot., the wiper of which goes to the control grid of the EL95 via a coupling cap. in the usual way
'LIVEWIRE?' is online now  
Old 20th Oct 2011, 7:53 pm   #2
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,267
Default Re: Jiskra Sonoretta midget valve radio

Sounds like a leaky-grid (or possibly anode-bend) detector, using the EF93.

How many IF transformers does it have? That might give a clue - if only one, then almost certainly it's between the EK90 and the EF93, and the EF93 is not working as an IF amplifier.
kalee20 is online now  
Old 21st Oct 2011, 10:02 am   #3
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,541
Default Re: Jiskra Sonoretta midget valve radio

Only one IF transformer suggests it doesn't have in IF amplifier so you've probably got the mixer feeding straight into the EF93 as a leaky-grid or anode bend detector as suggested. I think Sobell used a similar idea in the 'Sobellette'. Only really any good for local station reception. Better than a TRF but not quite as good as a full superhet.



SB
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now  
Old 21st Oct 2011, 10:36 am   #4
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

Yes, there is only one (double tuned) IFT, connected, as you both say, between the EK90
anode & the control grid of the EF93. I guessed it was probably an 'anode bend' detector,
but have yet to find out how well(or badly!)it performs, since, at the moment I get no sound at all from it.
'LIVEWIRE?' is online now  
Old 21st Oct 2011, 1:26 pm   #5
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,541
Default Re: Jiskra Sonoretta midget valve radio

Any chance of some pictures? Sometimes helps identifying the chassis.


SB
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now  
Old 29th Oct 2011, 9:34 am   #6
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

Hopefully I will now be able to upload and post a couple of photos which may help to ID the
radio. They were taken after some repairs, but enough of the original, particularly above
chassis, survives, hopefully. The mains transformer is not the original, however the other above chassis components are. The missing valves are meant to be an EF93, an EL95, and an EZ90 or 91
According to some notes appended to a hand-drawn diagram(which I may also try to post on here)
equivalents are supposed to be6BA6(EF93)6BE6(EK90),6DL5(EL95), & 6X4(EZ90 or 91) A 6F32(EL95?)and 12AT6 are also mentioned, but, as the Ex9.. series are all B7G, whilst the 6BA6 &
6BE6 are B9A this can't be right. This particular radio came with a 6H31(==EK90??) and a 12AT6
(6AT6/EBC90, but with 12.6v heater, AFAIK), which doesn't seem to belong to it at all.
Hopefully someone can shed some light on all this, possibly including an 'official' circuit diagram
Attached Thumbnails
Click image for larger version

Name:	Copy of P1060282.jpg
Views:	182
Size:	121.7 KB
ID:	57920   Click image for larger version

Name:	Copy of P1060283.jpg
Views:	162
Size:	120.4 KB
ID:	57921  

Last edited by 'LIVEWIRE?'; 29th Oct 2011 at 9:55 am. Reason: To add further info. about valves, etc.
'LIVEWIRE?' is online now  
Old 29th Oct 2011, 9:56 am   #7
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

Following on from the above, can someone explain how an 'anode bend' or 'leaky grid' detector works, please?
'LIVEWIRE?' is online now  
Old 29th Oct 2011, 11:20 am   #8
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,364
Default Re: Jiskra Sonoretta midget valve radio

A leaky grid detector rectifies the RF on the grid of the valve. A high value grid-leak resistor sits between the grid and earth. The top half of the signal causes the grid to conduct (a diode) and pushes the grid negative which is smoothed by the coupling capacitor.

An anode bend detector deploys the non-linear characteristics of the valve. The valve is run near cut-off typically using cathode bias so that only the top half of the signals are amplified.

Leaky grid detectors are lower gain and have a lower output impedance but exhibit less distortion.

Last edited by PJL; 29th Oct 2011 at 11:32 am.
PJL is offline  
Old 29th Oct 2011, 5:57 pm   #9
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,541
Default Re: Jiskra Sonoretta midget valve radio

Quote:
Originally Posted by 'LIVEWIRE?' View Post
equivalents are supposed to be6BA6(EF93)6BE6(EK90),6DL5(EL95), & 6X4(EZ90 or 91) A 6F32(EL95?)and 12AT6 are also mentioned, but, as the Ex9.. series are all B7G, whilst the 6BA6 &
6BE6 are B9A this can't be right.

That's not right. 6BA6 and 6BE6 are also B7G. See here http://www.r-type.org/static/web06.htm You have to scroll down to find them.



Rich
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now  
Old 29th Oct 2011, 7:00 pm   #10
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

I realised that I'd mixed up valve bases later today, Rich, but didn't get around to posting
a correction, but there are still question marks about this radio
'LIVEWIRE?' is online now  
Old 29th Oct 2011, 9:29 pm   #11
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,998
Default Re: Jiskra Sonoretta midget valve radio

Is the marque Jiskra or Iskra? Iskra means 'spark'.

By the mains input cable there seems to be a rather new looking transformer, as are some of the wires.
As is appears to have a 2 core mains input I thought that it was the heater transformer or a replacement for the original. The HT being from the mains with the anodes of the EZ90 being strapped together.
Neil Purling is offline  
Old 30th Oct 2011, 11:29 am   #12
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

The mains transformer, and modern 2-core mains flex & plug were fitted by the present owner, a radio enthusiast of many years standing. The original transformer was faulty when he acquired the radio.

What I have done is replace a number of faulty Rs & Cs under the chassis, the radio being intermittent due to these faults. The A.F. stages are now working, but there is no R.F.(I think I may have rewired one of the connections to the IFT wrongly. This is easily corrected, but I will also recheck other wiring.

As to the make, I don't have the radio in front of me, but I'm certain it's 'Jiskra.' A check onhttp://www.radiohistoria.skreveals several Jiskra radios, though not this one. AFAIK there is no mention of 'Iskra.'

As to the rectifier connections, the radio appears to have been wired from new for an EZ90 or 91 with the anodes paralleled. Incidentally the hand drawn circuit diagram shows the mains connected directly to the anodes without a transformer, and with the mains switch in the earth side of the wiring. (NB I can't scan in any diagrams at the moment, as the PSU for the scanner(which is a sealed plastic 'wall wart' packed up, and I don't have a suitable replacement handy).

Last edited by 'LIVEWIRE?'; 30th Oct 2011 at 11:34 am. Reason: To add extra information
'LIVEWIRE?' is online now  
Old 30th Oct 2011, 12:06 pm   #13
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

As a result of some further 'surfing', I have found that Sonoretta Radios were made by
'Elektra' in Czechoslovakia in the 40s & 50s. They are listed both onhttp:www.radiomuseum.org *andwww.radiohistoria.sk. Both list an earlier model, the RV12, a 2-valve TRF, which I have also seen, but neither list the 3 valve superhet which is the subject of this thread. What caused me some confusion is that the word 'Jiskra' is molded into the inside of the Cabinets.
* Although the URL I've quoted for the Radio Museum site is correct, for some reason a '404 not found' page is displayed when clicking on it, and I don't know why
'LIVEWIRE?' is online now  
Old 30th Oct 2011, 4:18 pm   #14
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 5,541
Default Re: Jiskra Sonoretta midget valve radio

I found this very interesting Czech site a few weeks ago. http://www.oldradio.cz/english.htm

Is there anything on the site that helps identification?



Rich.
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is online now  
Old 30th Oct 2011, 9:24 pm   #15
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

I have had a browse through that site, Rich., and, via a link thereon, visited a French Site.www.miniradios.falgoux.net
The latter has circuit diagrams of several Sonorette/Sonoretta models, none of them being
the one in question, unfortunately.

Last edited by 'LIVEWIRE?'; 30th Oct 2011 at 9:29 pm.
'LIVEWIRE?' is online now  
Old 31st Oct 2011, 6:40 pm   #16
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,998
Default Re: Jiskra Sonoretta midget valve radio

Well, there's nothing wrong with the previous owner isolating the chassis from the mains, as well as replacing the defective transformer.
Have you been able to narrow down what the correct Mixer/osc & IF stage valves should be from looking at the base connections?
Neil Purling is offline  
Old 31st Oct 2011, 8:18 pm   #17
'LIVEWIRE?'
Dekatron
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,291
Default Re: Jiskra Sonoretta midget valve radio

I have now identified that the correct valves are EK90, EF93, EL95, and EZ90 or their
equivalents, and in fact the radio is up and running reasonably well, although I'm fairly certain that the F.C. valve in it is a 12BE6(HK90)though the lettering has almost worn
away, so I may get a 6BE6/EK90 to see if the performance improves
'LIVEWIRE?' is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:22 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 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.