Doctor Olds
Active Member
I am not here all that often but I have another problem and I am pretty sure someone here can tell me where to look. I now have a problem with the ST-70 I have owned for about 25 years. The first 15 years or so were trouble free and other than checking the bias every once in a while this unit was trouble free. Then it died and I found the 5AR4 rectifier tube went and while testing the tubes the rest were weak so I replaced them all. The new 5AR4 failed within a couple of days so I began a thread here and removed 2 huge external caps added to help the bass response then replaced the can cap on the amp with a new CP one and installed new orange drops on the original board. It ran fine and sounded good but began to burn out the 5AR4 occasionally (every other year or so.) I installed a NOS (ouch) 5AR4 a few years ago which solved that problem. The unit developed a "hum" / both speakers in stereo mode so now I am back here......all the tubes are lit normally so I tried a spare 5AR4 just in case which did not help. When I checked the bias I found it was very low @ .44 VDC. I tried turning up the bias set (for just a second or two) and could only achieve .70 and the hum did not change either louder or quieter (I turned it down to where it was.) The unit is now down on my bench and I am wondering where to look first?
Any help would be appreciated. Doc
Any help would be appreciated. Doc