01-11-2020, 07:13 PM
I have been repairing things all my life.
Now I have to grovel and admit I have failed.
And what has beaten me? A KB OB10 Minuet radio.
Its a simple 5 valve set, nothing odd or clever.
I'll use the Trader sheet 1345 as its clearer to follow.
Started out making a tuning knob from a broken one and an incorrect one, simple.
Tuning cap was falling about so it was refitted with new grommets.
So far no problems. No signs of the Twiddler, just an audio coupling cap C17 and resistor R10 triode anode had been changed correctly by previous fixer.
Powered up, no drama, the audio works from the volume control to the speaker.
Off tune static on both bands across the tuning range with slight alterations in sound but no reception at all on either band. I have a signal path from the aerial through to the speaker, no broken tracks or bad connections.
Immediately suggests that the local oscillator is not running, My scope has had a bad brew-up so I try to find the heterodyne on another radio without any result.
Now I have changed or checked as OK every component in the V1 grid 1 circuit. The V1 and V2 have been substituted. Switch is cleaned and is OK, oscillator coil is OK. Core is secure in grease and unmarked.
Smoothing caps are OK, HT is a little down but rectifier checks out. So V1 and V2 anode voltage is correspondingly lower, by around 7%.
All non mica caps have been changed, Mica ones have been removed and check out OK.
Still no oscillator.
R3 runs warm, but C4 is changed, certainly OK.
The only caps not checked are the ones inside the IF cans, getting at those will involve removing the core as they protrude through the holes in the cans.
Can anyone see why the oscillator doesn't run? Not got much hair left!
Sam.
Now I have to grovel and admit I have failed.
And what has beaten me? A KB OB10 Minuet radio.
Its a simple 5 valve set, nothing odd or clever.
I'll use the Trader sheet 1345 as its clearer to follow.
Started out making a tuning knob from a broken one and an incorrect one, simple.
Tuning cap was falling about so it was refitted with new grommets.
So far no problems. No signs of the Twiddler, just an audio coupling cap C17 and resistor R10 triode anode had been changed correctly by previous fixer.
Powered up, no drama, the audio works from the volume control to the speaker.
Off tune static on both bands across the tuning range with slight alterations in sound but no reception at all on either band. I have a signal path from the aerial through to the speaker, no broken tracks or bad connections.
Immediately suggests that the local oscillator is not running, My scope has had a bad brew-up so I try to find the heterodyne on another radio without any result.
Now I have changed or checked as OK every component in the V1 grid 1 circuit. The V1 and V2 have been substituted. Switch is cleaned and is OK, oscillator coil is OK. Core is secure in grease and unmarked.
Smoothing caps are OK, HT is a little down but rectifier checks out. So V1 and V2 anode voltage is correspondingly lower, by around 7%.
All non mica caps have been changed, Mica ones have been removed and check out OK.
Still no oscillator.
R3 runs warm, but C4 is changed, certainly OK.
The only caps not checked are the ones inside the IF cans, getting at those will involve removing the core as they protrude through the holes in the cans.
Can anyone see why the oscillator doesn't run? Not got much hair left!
Sam.
Boater Sam.
