19-09-2024, 11:19 AM
I'd say those seem OK. Not in brilliant form, but good enough. You can always double-check by tacking a new low-ESR cap across the back of each.
And, as the hum is only on AM, that probably rules out all of the audio amp caps. I'd be looking further downstream, such as C48 and C49.
Have you had a poke around with a 'scope yet? It might be worth making sure there isn't too much ripple on the -12V rail, just in case the zener is playing up (unlikely, but not impossible).
And is it definitely mains hum, locked to the "line" trigger source on your 'scope? I've had instability that sounded like hum, only it just happened to be near-ish to mains frequency, as luck(!) would have it.
In lieu of a scope, the hum disappearing the instant the mains is pulled - assuming the radio plays for a few seconds before dying - can be a useful indicator.
Another test might be to lift the left hand end of R55 (the end coming from C64) and apply a voltage from an external bench power supply - whatever voltage is needed to result in -12V at the zener. If you don't have a bench PSU, a PP3 across the zener might do - the radio likely would be happy enough at the lower voltage.
(Sorry - can't remember what you've got in your workshop. But then, I can't remember what I was doing yesterday
Do you still have those Telequipment scopes?)
Cheers,
Mark
And, as the hum is only on AM, that probably rules out all of the audio amp caps. I'd be looking further downstream, such as C48 and C49.
Have you had a poke around with a 'scope yet? It might be worth making sure there isn't too much ripple on the -12V rail, just in case the zener is playing up (unlikely, but not impossible).
And is it definitely mains hum, locked to the "line" trigger source on your 'scope? I've had instability that sounded like hum, only it just happened to be near-ish to mains frequency, as luck(!) would have it.
In lieu of a scope, the hum disappearing the instant the mains is pulled - assuming the radio plays for a few seconds before dying - can be a useful indicator.
Another test might be to lift the left hand end of R55 (the end coming from C64) and apply a voltage from an external bench power supply - whatever voltage is needed to result in -12V at the zener. If you don't have a bench PSU, a PP3 across the zener might do - the radio likely would be happy enough at the lower voltage.
(Sorry - can't remember what you've got in your workshop. But then, I can't remember what I was doing yesterday
Do you still have those Telequipment scopes?)Cheers,
Mark
