http://recordist.com/ampex/schematics/600man/600schem.pdf
So someone gave me an ampex 600 to fix the other day. It wasn't powering up, the choke measured open circuit. Took it appart, fixed the broken break out wire. Easy fix. Powered it up to test it, amplifier was working fine but a bit too much low end noise, 50-100hz stuff. Now this unit has had some work done on it before to convert it to a mic pre. Input transformer added, all signal path 0.1 etc caps replaced, unecessary bits of circuit and valves removed (V103, 104, 105, 107), and the recitifier valve was replaced with 2 diodes. But none of the electrolytic caps had been replaced.
So I went through and done the usual recapping job. Vishay ASM/ASH caps. Powered it back up, the hum had dropped a bit but it was cycling. I stuck the scope on the output of the pi filter and saw the ripple voltage being low for ~ 2 seconds, then rising for ~ 6 seconds, over and over.
So i remembered I'd read choke circuits need a minimum load to filter properly. This unit has half its valves missing, so I stuck in a dummy resistive load to simulate the missing valves. Didn't get rid of the problem. Next thing was the rectifier. The output of the pi filter should be 315V, i was getting 425V with the solid state rectifier. Maybe this circuit needs the impedance of the valve rectifier to filter properly? So I stuck one in, still getting the cycling ripple thing (but the right HT volts obviously!). The attached scope trace shows what's going on.
Where to next? The choke measures 5H 1k DC resistance. The caps, I'd assume they're ok (bad idea?). The relevant resistors (R148, 149, 108....) measure fine and the voltage drops accross them are about right, no drifiting with temperature. I've checked for dry joints. What's causing this low frequency cycling!? Any ideas people?
So someone gave me an ampex 600 to fix the other day. It wasn't powering up, the choke measured open circuit. Took it appart, fixed the broken break out wire. Easy fix. Powered it up to test it, amplifier was working fine but a bit too much low end noise, 50-100hz stuff. Now this unit has had some work done on it before to convert it to a mic pre. Input transformer added, all signal path 0.1 etc caps replaced, unecessary bits of circuit and valves removed (V103, 104, 105, 107), and the recitifier valve was replaced with 2 diodes. But none of the electrolytic caps had been replaced.
So I went through and done the usual recapping job. Vishay ASM/ASH caps. Powered it back up, the hum had dropped a bit but it was cycling. I stuck the scope on the output of the pi filter and saw the ripple voltage being low for ~ 2 seconds, then rising for ~ 6 seconds, over and over.
So i remembered I'd read choke circuits need a minimum load to filter properly. This unit has half its valves missing, so I stuck in a dummy resistive load to simulate the missing valves. Didn't get rid of the problem. Next thing was the rectifier. The output of the pi filter should be 315V, i was getting 425V with the solid state rectifier. Maybe this circuit needs the impedance of the valve rectifier to filter properly? So I stuck one in, still getting the cycling ripple thing (but the right HT volts obviously!). The attached scope trace shows what's going on.
Where to next? The choke measures 5H 1k DC resistance. The caps, I'd assume they're ok (bad idea?). The relevant resistors (R148, 149, 108....) measure fine and the voltage drops accross them are about right, no drifiting with temperature. I've checked for dry joints. What's causing this low frequency cycling!? Any ideas people?
