Hoping some experienced Leak fundis might weigh in!
I have an older Stereo 20 from 1959. This has the older 3921 OPTs. I just recently acquired it and as it was quite messy and had been fiddled with I put in all new caps and resistors. I changed some values along the way which resulted in the amp being way too bright.. I then went back to all original values as per the schematic and all seemed good. I then noticed that the Right channel output was lower than the Left. I went to my test CD and started putting test tones through the amp, this confirmed Right channel was lower but also noticed that as soon as the input signal was raised a little I started getting quite nasty distortion in the Right channel easily audible on top of the sine waves.
I have swapped valves from L to R, swapped cables, speakers and have checked all my solder joints. Also have taken out the links on the OPTs and cleaned them and made sure they have continuity. Also have tried the 4 ohm and 16 ohm taps on the OPTS and find that I get no output at all on the Right channel with these settings!
I am quite new to valve amps and electronics but this is all starting to point me towards a bad OPT on the Right Channel.
Is there an easy test to make sure this is what my problem is? Anything else I may have missed?
I have an older Stereo 20 from 1959. This has the older 3921 OPTs. I just recently acquired it and as it was quite messy and had been fiddled with I put in all new caps and resistors. I changed some values along the way which resulted in the amp being way too bright.. I then went back to all original values as per the schematic and all seemed good. I then noticed that the Right channel output was lower than the Left. I went to my test CD and started putting test tones through the amp, this confirmed Right channel was lower but also noticed that as soon as the input signal was raised a little I started getting quite nasty distortion in the Right channel easily audible on top of the sine waves.
I have swapped valves from L to R, swapped cables, speakers and have checked all my solder joints. Also have taken out the links on the OPTs and cleaned them and made sure they have continuity. Also have tried the 4 ohm and 16 ohm taps on the OPTS and find that I get no output at all on the Right channel with these settings!
I am quite new to valve amps and electronics but this is all starting to point me towards a bad OPT on the Right Channel.
Is there an easy test to make sure this is what my problem is? Anything else I may have missed?
I would start with writing down the voltages on the tube pins and sharing them. There is no real way to up and say a problem is with a certain component without first analyzing. That you may think is a faulty transformer may end up to be faulty operating points because of a faulty cap, resistor or solder joint.
Thanks gents, I will need to sit down with a friend and go through all the voltages. I also have read elsewhere that apparently this is a good test to check the OPT:
"Apply just a few volts of 60Hz AC to the OPT secondary with the amp cold. Measure the stepped-up voltage on the primary side, and compare the two channels. If one is substantially weaker, then you might very well have shorted turns. This test is far more sensitive than anything you can do with an ohmeter."
Also what would explain the fact that I get no output when using the 4ohm and 16 ohm taps on the R channel only? I have opened these transformers during my rebuild and given all contacts a good clean and checked internal wires and solder joints.
"Apply just a few volts of 60Hz AC to the OPT secondary with the amp cold. Measure the stepped-up voltage on the primary side, and compare the two channels. If one is substantially weaker, then you might very well have shorted turns. This test is far more sensitive than anything you can do with an ohmeter."
Also what would explain the fact that I get no output when using the 4ohm and 16 ohm taps on the R channel only? I have opened these transformers during my rebuild and given all contacts a good clean and checked internal wires and solder joints.
This method is indeed the best way to test an OPT in situ. During the test it is recommended to remove the two (EL84/6BQ5) output tubes and also check if you have the same AC voltage amplitude at both halves of the primaries (plates connections) referenced to the common B+ transformer connection.
Well thanks again guys... but I have found the problem. Something obvious that wasnt easy to see and I had assumed was good. One of the solder joints to the binding posts of the Right channel was difficult to see properly when I was inspecting for obvious mistakes.. I tugged on the wires today and one came away completely..(!) Have fixed up and both channels are sounding good. I feel very stupid for not noticing before and probably lucky I didnt toast the OPT due to intermittent connection to the speaker.. but very happy to have a fully functioning stereo 20 to listen to now for the rest of the holidays 
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.