UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

G6Tanuki · 22nd Jan 2019, 4:55 pm

The tantalums may 'test' OK with a typical capacitor-tester that only puts a few volts on them - they can still be defective when shown 120V or so.

Apart from failing short-circuit [which does bad things to the transistor in the +120V inverter which works itself to death trying to pull the rail back up to +120V] the problematic failure-mode is minute and continually-varying leakage - which causes the synth VCO-rail to have a random wobble superimposed on it. The VCO will remain in-lock (so you don't get the 'unready tone') but will wander about over a few hundreds of Hertz in a way which makes CW/SSB reception impossible, and will get you some nasty comments if you transmit.

Replacement 'official spec' Tantalum capacitors are available but at a price of something like £80+VAT each! I replace mine with modern 160V 105-degree-rated low-ESR electrolytics which are available from the likes of RS at a much more wallet-friendly price.

22nd Jan 2019, 4:55 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Clansman PRC320 problems The tantalums may 'test' OK with a typical capacitor-tester that only puts a few volts on them - they can still be defective when shown 120V or so. Apart from failing short-circuit [which does bad things to the transistor in the +120V inverter which works itself to death trying to pull the rail back up to +120V] the problematic failure-mode is minute and continually-varying leakage - which causes the synth VCO-rail to have a random wobble superimposed on it. The VCO will remain in-lock (so you don't get the 'unready tone') but will wander about over a few hundreds of Hertz in a way which makes CW/SSB reception impossible, and will get you some nasty comments if you transmit. Replacement 'official spec' Tantalum capacitors are available but at a price of something like £80+VAT each! I replace mine with modern 160V 105-degree-rated low-ESR electrolytics which are available from the likes of RS at a much more wallet-friendly price.