I've not had an opportunity to spend time in my workshop for a couple of weeks, but I did find time today to have another look at the Radionette Kurer valve portable.
With the faulty set tuned to 1000 kHz on the medium wave band I was able to pick up a strong 1455 kHz signal on another set next to it on the bench. This confirmed that the Kurer's Local Oscillator was running. I tried injecting a modulated 455 kHz test signal again, coupling the output from my signal generator into the set via a few turns of wire around the DK96 valve. Instead of the pure tone that I expected, there was a beat note from the speaker. This usually indicates an unstable and oscillating IF stage.
Turning again to the internet, I found several references to similar faults with battery sets in various vintage radio forums. I had already changed the decoupling capacitors and checked screen resistor values, which left possible problems with poor contacts in the valve bases. I gave both mixer and IF amplifier valve sockets a liberal application of contact cleaner spray, and again following suggestions in a forum post, tightened up some of the contacts in the sockets. The advice turned out to be spot on, because on powering it up again I could immediately hear received signals.
It's currently sitting on the bench playing my local AM station on 1332 kHz. The sensitivity isn't great, and i'ts pretty deaf on long wave as well. I'm assuming that this is because the loop aerials aren't connected, so I'll try it again once its back in the case.
It's been an interesting repair job, but I can't honestly say that there is anything about this set that I like. I'll be quite happy to hand it back to my friend.
Keep listening!
Showing posts with label IF instability. Show all posts
Showing posts with label IF instability. Show all posts
Wednesday, February 18, 2015
Tuesday, March 18, 2014
Wartime Civilian Receiver - Part 4: Bringing it back to life
With the chassis work completed, the time had finally come to see if my WCR would work.
The restoration had included replacement of all potentially faulty capacitors and wiring, so the possibility of anything going bang when the power was applied was minimal. Some safety checks were still necessary though before power was applied.
I used my Robin 3111 tester to check the continuity between the earth pin of the plug and the chassis, and the isolation between chassis and the mains. This showed a fraction of an Ohm for the earth continuity, and infinite resistance between both sides of the mains and earth (after checking that the set's mains switch was on!)
A multimeter could be used for this, but a proper insulation tester or Megger is preferable as it applies a higher voltage for the insulation test.
A WCR is unlikely to receive anything without an aerial, so I connected a couple of metres of wire to the A1 socket, more than enough to pick up the local AM Gold transmitter around 5 miles away.
I use a lamp limiter when powering unknown sets for the first time. A 100W 240V lamp connected in series with the live supply wire has negligible effect if the radio is working correctly; the current drawn is insufficient to heat the filament and it's resistance is very low. However, if a fault causes the radio to draw a lot of current the lamp lights, the filament resistance increases and the fault current is safely limited...well that's the theory anyway!
Power was applied, valves began to glow and the lamp limiter didn't...and after 30 seconds or so I could hear noise from the loudspeaker.
It lived!
After removing the lamp limiter I tried again. Tuning around I was pleased to hear three or four stations, but something wasn't right. There was a loud whistle that decreased in frequency as a station was tuned in, dropping to almost zero, but then rising in frequency again as I carried on tuning past the station.
If you read my earlier post you might remember that I had been a bit clumsy in removing the BVA246 IF amplifier valve early in the restoration. It turned out that I had broken the earth connection to the red metalised paint on the glass envelope, and in this receiver this can cause IF instability.
Fitting a good EF39 borrowed from another radio confirmed that this was indeed happening, and a New Old Stock CV1053 (a military equivalent) was duly bought and installed.
It was great to hear the set working again!
All that remained now was to sort out the rear panel and cabinet labels, and take some pictures of the finished radio. I'll put this all in the final part, coming soon.
Keep Listening!
 |
| We can bring it back to life! |
The restoration had included replacement of all potentially faulty capacitors and wiring, so the possibility of anything going bang when the power was applied was minimal. Some safety checks were still necessary though before power was applied.
I used my Robin 3111 tester to check the continuity between the earth pin of the plug and the chassis, and the isolation between chassis and the mains. This showed a fraction of an Ohm for the earth continuity, and infinite resistance between both sides of the mains and earth (after checking that the set's mains switch was on!)
A multimeter could be used for this, but a proper insulation tester or Megger is preferable as it applies a higher voltage for the insulation test.
 |
| Robin Model 3111 tester |
A WCR is unlikely to receive anything without an aerial, so I connected a couple of metres of wire to the A1 socket, more than enough to pick up the local AM Gold transmitter around 5 miles away.
I use a lamp limiter when powering unknown sets for the first time. A 100W 240V lamp connected in series with the live supply wire has negligible effect if the radio is working correctly; the current drawn is insufficient to heat the filament and it's resistance is very low. However, if a fault causes the radio to draw a lot of current the lamp lights, the filament resistance increases and the fault current is safely limited...well that's the theory anyway!
 |
| Chassis re-fitted ready for testing |
Power was applied, valves began to glow and the lamp limiter didn't...and after 30 seconds or so I could hear noise from the loudspeaker.
It lived!
After removing the lamp limiter I tried again. Tuning around I was pleased to hear three or four stations, but something wasn't right. There was a loud whistle that decreased in frequency as a station was tuned in, dropping to almost zero, but then rising in frequency again as I carried on tuning past the station.
If you read my earlier post you might remember that I had been a bit clumsy in removing the BVA246 IF amplifier valve early in the restoration. It turned out that I had broken the earth connection to the red metalised paint on the glass envelope, and in this receiver this can cause IF instability.
 |
| Original BVA246 (EF39) with damaged screening |
Fitting a good EF39 borrowed from another radio confirmed that this was indeed happening, and a New Old Stock CV1053 (a military equivalent) was duly bought and installed.
It was great to hear the set working again!
All that remained now was to sort out the rear panel and cabinet labels, and take some pictures of the finished radio. I'll put this all in the final part, coming soon.
Keep Listening!
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