| Posted by Dave | September 26, 2014 - 08:51 pm |
| Joe -- It would be helpful to have more detailed information. For example, is the noise in both channels or just one? Is it loud or very soft in nature? You call it a high pitched hum, which is somewhat confusing: Hum implies a low frequency signal, but high pitched is a very wide open description. More accurate information then would be a big help. Normally, connecting the 16 ohm feedback connection would not cause the production of any noise, unless the leads at pins 7 and 9 of the output tubes are connected incorrectly. If your modifications included disturbing those, you might check for accuracy of these connections. The only other item would be lead dress. Input and output connections that come too close together can cause feedback problems as well. As a test, place a direct short across the inputs to your PC-10A boards. If the noise is still present, then at lease it has been confined to the power amplifier section. Dave |
| Posted by Joe Janus | September 26, 2014 - 05:50 pm |
| After redoing the ground and 16 ohm connections, the oscilating hight pitched hum remains. It would be nice to have a 2nd sca35 to compare readings at various undocumented points... but... it will teach me not to make to many changes at once... which in this case i did...(replace origial selector switch and mod to pc11, new output binding posts, 3 prong ground which I removed) but even with input to pc10 removed and tubes changed it persists... wondering if I did not cause harm with 3 prong AC attempt.. the hum does disappear when pc10 connections to 16 ohm output taps are removed which leads me to suspect the pc10 board components. ;Perhaps there are some readings and or method for isolating the fault(yea mine) here rather than by disconnecting and testing individual pieces and/or purchasing and substituting a new pc10.... If not, then so be it and let the work begin... |
| Posted by Dave | September 23, 2014 - 12:27 pm |
| Stephen -- You should be good to go! Dave |
| Posted by stephen zaneteas | September 23, 2014 - 05:00 am |
| Hi Dave....almost ready to power up..... ...Just a question or two about the pinouts if you wouldn't mind.... about 5k pot and LM337. Viewing the pot from above with the shaft facing me with the taps from RIGHT TO LEFT, are taps 3,2 and then 1. Tap 3 is the 2 watt connected to the hi-power side (red wire) on the amplifier. Tap 2 has the capacitor and connection from the adjustment(lm337) coming from the LM337's first pin.(adjustment pin). Tap 1 is 15k with the neg end of cap to ground along with a connection to pin 2 on lm337. I hope so far so good!! The lm337 with the front facing you and the three leads right below. FROM LEFT TO RIGHT the leads are: ADJUSTMENT....INPUT......OUTPUT. in that order. I thought I was scrupulous with the pinouts for the 5k and the lm337 and so maybe you can verify this. |
| Posted by Dave | September 22, 2014 - 03:57 pm |
| Hi Stephen -- it sounds perfect! Dave |
| Posted by Joe Janus | September 22, 2014 - 02:12 pm |
| Thanks Dave... I have since removed the (friend recommended) 3-prong grounding cord as many posts have noted it as un-necessary and may in fact contribute to additional "noise" issues, but the feedback thru both channels remain.. If I remove the 2 wires from the PC-10 board that connect to the 16 ohm output lugs OR remove both 6U8A tubes the Hum is no longer. I now intend to follow your advice on ground checks using test equipment rather than my keenly bad old eyes... Thanks very much for your immediate reply... Joe |