19-06-2020, 05:49 AM
(This post was last modified: 19-06-2020, 05:50 AM by Diabolical Artificer.)
With the valves in place (just a quick check before dismantling), the lamp was very dim but grew brighter as the hum grew (does this suggest anything or nothing?). Yes, it means that as the valves warm up either a short or other fault kicks in.testing ESR is ok for a quick yes/no answer but you want to test cap with full voltage (or near) applied, they then show their true condition.
Make yourself this circuit, see attached. As you have a variac all you need to is an isolation tfmr, or two 240v/12v or similar tfmr connected back to back, then build the circuit on a tag board, vis the voltage doubler, IE D1/D2 & C1/C2, then connect that to R11, NE1 and C3. Connect +Ve to your meter set to mA first, then uA, the OP from your meter goes to the CUT positive, connect Ve - to the caps same. Use another meter to monitor V. Wind up the varaic slowly to full V, the neon will glow if the cap is leaky, also the ammeter should show a linear response to voltage. EG 250v 25uA or 2.5uA. if it doesn't the cap is bad.
Another easier way is to power up and scope or meter (set to AC V ) the HT. If the ripple is over something like a few hundred mV, the cap may be bad. Using your kit will improve your knowledge, I think you worry too much, have a go even if your not sure, as long as you've taken safety precautions try things out. You'll make mistakes but that's good, we learn more that way.
Start off as I said, valves out, resistance check, then when and only then EZ81 in, check HT. You want to know what the hum is. Is it 50 or 100hz? After the checks mentioned in the previous post, power the amp up but at half or 3/4 mains voltage, scope, the HT, but check your scopes Vin max rating or just tack on a 0.1u 630v cap onto the reservoir cap, then have a look, also scope the first IP valves grid, then anode.
Hope that helps, Andy.
Make yourself this circuit, see attached. As you have a variac all you need to is an isolation tfmr, or two 240v/12v or similar tfmr connected back to back, then build the circuit on a tag board, vis the voltage doubler, IE D1/D2 & C1/C2, then connect that to R11, NE1 and C3. Connect +Ve to your meter set to mA first, then uA, the OP from your meter goes to the CUT positive, connect Ve - to the caps same. Use another meter to monitor V. Wind up the varaic slowly to full V, the neon will glow if the cap is leaky, also the ammeter should show a linear response to voltage. EG 250v 25uA or 2.5uA. if it doesn't the cap is bad.
Another easier way is to power up and scope or meter (set to AC V ) the HT. If the ripple is over something like a few hundred mV, the cap may be bad. Using your kit will improve your knowledge, I think you worry too much, have a go even if your not sure, as long as you've taken safety precautions try things out. You'll make mistakes but that's good, we learn more that way.
Start off as I said, valves out, resistance check, then when and only then EZ81 in, check HT. You want to know what the hum is. Is it 50 or 100hz? After the checks mentioned in the previous post, power the amp up but at half or 3/4 mains voltage, scope, the HT, but check your scopes Vin max rating or just tack on a 0.1u 630v cap onto the reservoir cap, then have a look, also scope the first IP valves grid, then anode.
Hope that helps, Andy.
