Hum in left channer - rogers cadet 3

[cr0wl3y]

Hi there,

I have been slowly refurbishing my Rogers Cadet 3 (I have the integrated all in one box version) and have hit a bit of a bottleneck. The left channel has a rather loud hum, even when the volume is down to zero. More worryingly, there is an ocassional scratching noise on the left channel, again even when the volume is down to zero. Right channel in contrast is completely clean. Neither the hum or the noise really change in loudness when the volume is turned up (although by that point a sublte and acceptable for a budget amp hum develops on the right channel too).

Does anyone have an idea of what could be causing this? Things that may be important:

1. One of the original capacitors going into one of the power amp valves of the left channel was leaky, resulting in the valve running hot (grid was bright orange). Replaced it (and a bunch of other capacitors in the amp) and this stopped
2. Balance circuit was messed up, resulting in left channel being much louder than right. Changed a few electrolytics and it's now fine.
3. Changing the valves around between left and right channels does not change anything - left channel is still humming and scratching.
4. A while back I used this amplifier for a house party. At some point in the night, one of the speakers died (the coil broke and circuit measured open when tested earlier). Unfortunately I cannot remember if it was the left or the right speaker that died and I am not sure when this happened, but one of the two channels were certainly on with no load for some time during that evening.
5. Despite point 4, it is worth pointing out that the hum and scratching on the left channel were present before the party. Hum seems more prominent even since I have replaced various components, but this may merely reflect sound improvements elsewhere bringing it out more.

Thanks for the help in advance and happy new year to everyone!

Nikos

 

[pointy]

there is only two things that i know of that will go wrong on most of the int rogers mk3's

1. replace the main volume pot.

2.the heater supply runs down a step on the left hand side at the back of the chassis and there it can short. so they can need that part of the heater supply rerouted. (can be done but not that easy)

 

[rotaspec]

While I haven't any experience with the Rogers amp, I have done a fair bit of fault tracing in my time.
Looking at the diagram someone posted for the Rogers that uses ECL86s, your best bet is to isolate the stages starting with the output pentodes and working your way back until you identify the stage generating the noise. I'm assuming the power supply electrolytics are shared between channels, so a fault there would affect both channels.
On the faulty channel, try the following:
Lift the signal coupling capacitors that feed the final grids (both capacitors for this test). If the noise is gone, the problem is further back. If not, then you need to look at the components in the output stage. Some voltage measurements to compare between channels would help here too.
If the noise went away when you lifted them, resolder them and lift the capacitor that couples the first two triode stages. Same deal, it will isolate the faulty stage.
Given the age of the amp, I'd be replacing all the electrolytics and the high value resistors (470k and up) as a matter of course, and probably the signal coupling capacitors as well. I did that with my old SE ECL86 amp and it made the world of difference.

Gary

 

[cr0wl3y]

Thanks for the advice to both - will try rotaspec's suggested debugging approach and hopefully I'll figure out what causes the problem. As a general point, I have already replaced most coupling caps and most electrolytic capacitors other than power supply. Would appreciate a suggestion on what I should get for the power supply, most of the capacitors suggested by audiophiles are prohibitively expensive considering this is a low cost amp after all.

Pointy - Regarding the volume pot, planning to replace that and the balance pot with Alps pots (need to order some in anyway for another project so thought why not). As for the heater supply, would this not affect both channels if the fault was there?

 

[barretter]

If you have a "scratching" noise, it's always a good idea to look for a bad solder joint. If the bad solder joint is connected with the earthing of the left channel, you might solve both problems at once.

 

[Loren42]

Start by swapping all tubes between both channels and see if the problem migrates with the tubes.

If it does, swap tubes back one at a time until the suspect is cornered.

If that doesn't do the trick, the other advice given here is a good route for action.

 

[cr0wl3y]

Success!

Many thanks to Rotaspec for the excellent advice - followed your suggestion and isolated the problem in power amplifier stage. Then (also following your suggestion) measured voltage difference between various points in the final stage and earth.

As indicated in the image attached, I got some unexpected figures in at the first triode's anode pin and also at the primary of the output transformer. Everything else was virtually identical between the two channels, so I surmised that the problem was at the anode of the triode. The capacitor that connected that with the grid of the second triode(C2 in chematic) was one of the ones I had not replaced, so I guessed that it could be dodgy and causing the trouble. So I replaced them with whatever I could find in Maplin in the right size and voltage and the hum is now completely gone!

So, once again, many thanks Rotaspec, you saved me from a great deal of head-scratching and prompted me to learn a couple of things in the process too!

Nikos