R1155 oscillator problem

[a223bfg]

Hi, I have just finished rebuilding an R1155 including a complete rewire and all new capacitors and resistors (never again !!), however I now have a strange problem in that while range 1 & 2 (HF) are fine ranges 3,4 & 5 (MF)are ok at switch on but after variable ammounts of time stop working. I have changed V4 for several others that are out of other receivers and known good and changed R39 & C35 again even though they were new, I have also cleaned the Z wafer switch several times with no improvement.
I have discovered that if I short pin 5 the oscillator grid to pin 6 the occilator anode of V4 it will burst into life again only to stop again within 30secs to 30 mins.
Any ideas as it is starting to drive me crazy !

Thanks in advance.

Phil

[HamishBoxer]

silly question how good are the valve bases ie sockets?

David GM8JET

[a223bfg]

Hi David, the bases are very good, I checked them thoroughly while they were off the chassis, the pins are nice and tight and clean and the paxolin is like new.

Phil

[HamishBoxer]

Hi Phil,this sounds a good one!It possible its an intermitent res or a dodgy earth tag,guess that when in fault condition you have tapped the chassis.

Having got one of these myself i would be round if i wasnt up here in the "frozen north" these days.

David

[HamishBoxer]

Would guess theres something common to the three duff ranges,not sat with the circuit in front of me at present so not at my best of suggestions.

[a223bfg]

Hi David,
Yes the chassis has had a good "tapping" while the fault persists, as far as I can make out from the circuit the only components shared by the 3 ranges are C100 on the master switch so I changed again that and C40 one of the 0.1 metal cased capacitors in the coil box and I have also replaced that again. Just for peace of mind ill change the base on V4 to rule that out. Thanks for your thoughts.

Phil

[frsimen]

Phil,
The oscillator circuit changes between the upper two bands and the other three. C34 is important to the operation on the lower bands, but will have less influence on the higher frequency bands, due to L17 and L18.. Maybe worth checking?

Paula

[a223bfg]

Hi Paula, thanks you may have hit the nail on the head. C34 is another of the metal cased capacitors in the coil box, (the most awkwad to get to of course) it is connected to the chassis with a single nut it appears I may have been a numpty and only tightened it finger tight !! I have now fully tightened it and the receiver has run for 10 mins so far ok. I will leave it on for a couple of hours to make sure but hopefully it's that simple.

Thanks again.

Phil

[a223bfg]

A couple of pictures of C34 (top l/h corner) and a overall view of the rewired coil box.

Phil

[HamishBoxer]

Can i just add? i think you have done a superb job of it.Mine hasnt been touched and still works last time out.Wonder what power supply you are using.

David

[a223bfg]

Thanks David, it is still belting out radio 4 so it looks as though Paula was dead on.
I dont think I would like to do another one in the near future (I found doing the Hallicrafters RF section much easier) but the result has been worth it. I to have a completly original one that I switch on occasionally but every time I look at the degrading PVC wiring on the multi section capacitiors I put it away again with the intention of sorting it out soon ! (fortunatley the remainder of the wiring on that one is cotton covered0.

The power supply is a traditional external 5Y3 rectified circuit with a 6V6 as an audio amplifier that while the audio isn't in the Quad league is certainly good enough for a com's receiver.

Phil

[ms660]

Hello Phil...well done on the RF box refurb. looks very good...these sections are hardly ever easy to work on, well done.

Lawrence.