03-08-2020, 07:51 PM
(This post was last modified: 03-08-2020, 07:54 PM by Mike Watterson.)
Now that I'm convinced it's not the power switch or the wave change I've been simply gently clicking it on to normal volume.
1) If it's been off for a while there is always an instant of normal sound and it fades to nothing, but in a few minutes gradually comes back, distorted at first.
2) If it's been running recently, then it turns on normally,
So I looked at the current just clicking on. It would be a little over 18mA and drop eventually to a bit under 17mA. At 16.8mA it's always working when the volume is turned up.
Next I took it out and found the jumper wire. Someone has been here before. Lots of burns on wires. The insulation split back on the jumper wire. The service info says 3.5mA and it was 2.5mA, at least it wasn't loads high. Adjusted to 3.5mA with volume at zero.
The capacitors are a pig to get out. The tracks poor adhesion and flimsy. Also neither PCB, R&TVS or Trader note polarity, so mark on the diagram before you take one out.
I shorted cap and tested on 9V on PSU with meter in series to see leakage. Then shorted out twice, to be sure to be sure and tested on the magic Chinese tester.
C20, 16 uF was OK, a bias decoupling.
C23, 640 uF to speaker was OK (I'd not expect it to cause this issue)
C15, 400 uF IF amp psu decoupling was OK
C21, 200uF 6.4V seemed dubious. It would take 1mA rapidly dropping to microamps, everytime it was shorted. ESR 1.9 Ohms, but capacitance OK at 230uF. I replaced it with ancient 220uF 16V that passed tests OK.
C19, 16 uF preamp supply decoupling was OK.
On testing the set the zero volume quiescent current is now 13.5 mA rather than 16.8mA. I'll leave it for a couple of hours and see if this was the fix. I didn't look at the balance preset as when it's good, there is no distortion even when loud. I may check it tomorrow if the repair is real rather than imaginary.
Back in the 70s & 80s I hated the intermittent fault and so did everyone else. Especially if it was in the Belfast workshop and from Sligo or Letterkenny and it was a N1500. I think sometimes we swapped with something not intermittent nearer to base. It's also good if the faulty part goes on fire.
I hope this is the fault, I'm not confident, but it's more like a capacitor symptom than a transistor or dry joint, though there isn't even a his at full volume when it's dead, though taking over 17 mA.
1) If it's been off for a while there is always an instant of normal sound and it fades to nothing, but in a few minutes gradually comes back, distorted at first.
2) If it's been running recently, then it turns on normally,
So I looked at the current just clicking on. It would be a little over 18mA and drop eventually to a bit under 17mA. At 16.8mA it's always working when the volume is turned up.
Next I took it out and found the jumper wire. Someone has been here before. Lots of burns on wires. The insulation split back on the jumper wire. The service info says 3.5mA and it was 2.5mA, at least it wasn't loads high. Adjusted to 3.5mA with volume at zero.
The capacitors are a pig to get out. The tracks poor adhesion and flimsy. Also neither PCB, R&TVS or Trader note polarity, so mark on the diagram before you take one out.
I shorted cap and tested on 9V on PSU with meter in series to see leakage. Then shorted out twice, to be sure to be sure and tested on the magic Chinese tester.
C20, 16 uF was OK, a bias decoupling.
C23, 640 uF to speaker was OK (I'd not expect it to cause this issue)
C15, 400 uF IF amp psu decoupling was OK
C21, 200uF 6.4V seemed dubious. It would take 1mA rapidly dropping to microamps, everytime it was shorted. ESR 1.9 Ohms, but capacitance OK at 230uF. I replaced it with ancient 220uF 16V that passed tests OK.
C19, 16 uF preamp supply decoupling was OK.
On testing the set the zero volume quiescent current is now 13.5 mA rather than 16.8mA. I'll leave it for a couple of hours and see if this was the fix. I didn't look at the balance preset as when it's good, there is no distortion even when loud. I may check it tomorrow if the repair is real rather than imaginary.
Back in the 70s & 80s I hated the intermittent fault and so did everyone else. Especially if it was in the Belfast workshop and from Sligo or Letterkenny and it was a N1500. I think sometimes we swapped with something not intermittent nearer to base. It's also good if the faulty part goes on fire.
I hope this is the fault, I'm not confident, but it's more like a capacitor symptom than a transistor or dry joint, though there isn't even a his at full volume when it's dead, though taking over 17 mA.
