Tracing a fault in a Laskys 4 band valve superhet Sky Rover. - UK Vintage Radio Repair and Restoration Discussion Forum

bikerhifinut · 2nd May 2020, 1:28 pm

I put this old radio on the bench to have a look at it. It's been in my possession for over 50 years and it's got great sentimental value as being my first radio, so I have a mind to give it a tart up and get it back in use as a shed/garage set.
It was never that great on the higher SW frequencies, nothing unusual there given its basic circuit. 6BE6 mixer/osc 6BA6 IF amp, 6AV6 detector/first audio and 6AR5 output.
It's got an unusual "BFO" control which basically controls an amount of regeneration in the IF circuit as far as Ii can see, basically I think it gets borderline unstable and the oscillation allows reception of SSB speech. I know I will have got a few things wrong there and happy to be corrected.
Anyway it was silent which baffled me initially as it had generally had a healthy power output and decent reception on MW where it was a good little set with a bit of random wire.
I had a measure round the set and aside from being unable to get a voltage reading on the 6BE6 cathode but the resistances to earth checked out ok, I will need to dig deeper there.
But when I checked the output valve I couldn't measure a voltage on the anode although the HT was a healthy 150V to the output transformer primary. The resistance of the primary indicates its open circuit, I don't know why it went O/C but its very small no more than about 30mm cubed which struck me as being a bit under engineered. So I think I'll need to find a replacement which is going to be tricky, theres not much room to fit a larger replacement. I would guess any SE output transformer would be ok as long as its primary impedance was in the 7k ball park, I doubt theres any critical circuit conditions here. I will as a matter of course replace the old electrolytics and coupling caps which look like electrolytics but could be oil caps, I don't know anything about those Japanese caps but at .005 uF I reckon I'll use some of my polyester .047 uF with a decent working voltage. Theres only the one electrolytic cathode bypass on the output valve and again I have a load of decent 25uF at 25V or better. All the rest look like mica jobs or are polystyrene or ceramic so I shall assume they are all good. So before I do anything else, do I disconnect the output transformer and I wonder if I could just wire any old SE output transformer with flying leads to eliminate that as a possibility?
I'd like to get it working as It has good memories for me. And it looks easy to work on.

Andy

Station X · 2nd May 2020, 1:52 pm

Yes. You can fit a suitable output transformer on flying leads.

Remember it's the turns/impedance ratio that counts. The DC resistance of the windings doesn't matter.

Don't forget to change the audio coupling capacitor, or you may need more than a new output transformer.

toshiba tony · 2nd May 2020, 1:54 pm

Don't know the model but you sound clued up, what about the old favourite, the grid coupling cap. I'd be looking at that if you have an o\c primary. Or perhaps you have been running into an o\c output. Is transformer failure known on that model?

bikerhifinut · 2nd May 2020, 6:01 pm

Quote:

Originally Posted by toshiba tony

Don't know the model but you sound clued up, what about the old favourite, the grid coupling cap. I'd be looking at that if you have an o\c primary. Or perhaps you have been running into an o\c output. Is transformer failure known on that model?

I have no information on it, there were a few similar sets on sale in the 60's/early 70s under different badges. I'll get some photos for tomorrow.
I couldn't measure any DC on the grid side of both grid couplers (6AV6 and 6AR5) but as you say, I will be swapping out all those suspect looking couplers and bypass caps for modern High Voltage plastic films and the output cathode decoupler for a new 25uF electrolytic. I suspect the output transformer may have died a while back, theres a complete lack of background noise so I hope that's where the issue lies.
Thanks for the pointers guys, If it is the output transformer I will be on the hunt for a miniature replacement and without the various jumbles and swapmeets that's going to be a challenge! Anyway I can use a bigger one I have lying around for now and with the suspect capacitors replaced I hope it will diagnose the output transformer. It's dead weedy anyway and I wonder if it just died through a bit of corrosion possibly where the primary/secondary wire leads are soldered to the windings?
A.

Sideband · 2nd May 2020, 7:39 pm

Quote:

Originally Posted by bikerhifinut

I don't know anything about those Japanese caps but at .005 uF I reckon I'll use some of my polyester .047 uF with a decent working voltage.

That might be a typo......005uF will be .0047uf in modern values. .05uF will be .047uF

bikerhifinut · 2nd May 2020, 11:05 pm

Quote:

Originally Posted by Sideband

Quote:

Originally Posted by bikerhifinut

I don't know anything about those Japanese caps but at .005 uF I reckon I'll use some of my polyester .047 uF with a decent working voltage.

That might be a typo......005uF will be .0047uf in modern values. .05uF will be .047uF

Correct! Sorry my mistake. Gotta watch that stray decimal point.

2nd May 2020, 1:28 pm	#1
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Tracing a fault in a Laskys 4 band valve superhet Sky Rover. I put this old radio on the bench to have a look at it. It's been in my possession for over 50 years and it's got great sentimental value as being my first radio, so I have a mind to give it a tart up and get it back in use as a shed/garage set. It was never that great on the higher SW frequencies, nothing unusual there given its basic circuit. 6BE6 mixer/osc 6BA6 IF amp, 6AV6 detector/first audio and 6AR5 output. It's got an unusual "BFO" control which basically controls an amount of regeneration in the IF circuit as far as Ii can see, basically I think it gets borderline unstable and the oscillation allows reception of SSB speech. I know I will have got a few things wrong there and happy to be corrected. Anyway it was silent which baffled me initially as it had generally had a healthy power output and decent reception on MW where it was a good little set with a bit of random wire. I had a measure round the set and aside from being unable to get a voltage reading on the 6BE6 cathode but the resistances to earth checked out ok, I will need to dig deeper there. But when I checked the output valve I couldn't measure a voltage on the anode although the HT was a healthy 150V to the output transformer primary. The resistance of the primary indicates its open circuit, I don't know why it went O/C but its very small no more than about 30mm cubed which struck me as being a bit under engineered. So I think I'll need to find a replacement which is going to be tricky, theres not much room to fit a larger replacement. I would guess any SE output transformer would be ok as long as its primary impedance was in the 7k ball park, I doubt theres any critical circuit conditions here. I will as a matter of course replace the old electrolytics and coupling caps which look like electrolytics but could be oil caps, I don't know anything about those Japanese caps but at .005 uF I reckon I'll use some of my polyester .047 uF with a decent working voltage. Theres only the one electrolytic cathode bypass on the output valve and again I have a load of decent 25uF at 25V or better. All the rest look like mica jobs or are polystyrene or ceramic so I shall assume they are all good. So before I do anything else, do I disconnect the output transformer and I wonder if I could just wire any old SE output transformer with flying leads to eliminate that as a possibility? I'd like to get it working as It has good memories for me. And it looks easy to work on. Andy

2nd May 2020, 1:52 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Tracing a fault in a Laskys 4 band valve superhet Sky Rover. Yes. You can fit a suitable output transformer on flying leads. Remember it's the turns/impedance ratio that counts. The DC resistance of the windings doesn't matter. Don't forget to change the audio coupling capacitor, or you may need more than a new output transformer. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

2nd May 2020, 1:54 pm	#3
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: Tracing a fault in a Laskys 4 band valve superhet Sky Rover. Don't know the model but you sound clued up, what about the old favourite, the grid coupling cap. I'd be looking at that if you have an o\c primary. Or perhaps you have been running into an o\c output. Is transformer failure known on that model?