| Author | Message | ||
| Le Basseur |
Hi, I'm finishing a repair job on a MM 210-HD "One-Fifty" 2275 chassis model (actually,it was more like a restoring job...too many amateurish interventions,but we're already familiarized with this,aren't we?). There are some things that don't follow the book,though... First,the driver section.I replaced the driver transistors with a paired pair of fresh transistors and the 3,9 Ohm resistors and I measured every other component in this section,but the bias is further unbalanced,this is 25 mV on one side and 39 mV on the other.The output tubes I put in this amp are 5881 NOS Russian military wich I selected myself with the utmost care (within 2% regarding emission,transconductance and vacuum quality) so it doesn't have sense to swap them in their sockets as a foolproof measure.Also,I checked the driver's electrolytics' integrity and they appear to be fine (as opposed to the HT electrolytics wich were worn out and I replaced them). After I eliminated every variable as stated in this board,what could possibly cause the driver's imbalance? The second issue is about a difference between levels of the two preamp channels.I checked (scoped) every stage in these areas and there's no sign of leaking cap or any other misfunction.Though,the second channel (the one wich has the mid correction) is quieter than the first channel.What should I look for? ...and,in case that the second channel is supposed to play quieter "by default",did anybody tried a mod in order to bring both channels to the same level? Thanks, Le Basseur | ||
| Anonymous |
Had similar bias issue. Replaced out of spec grid resistors. | ||
| Le Basseur |
You mean the control grids' 220 Ohms,or the control grids' 470 Ohms? Oh yeah,I did it in the first place because there were some burned marks and odd substitutions (the amp had a quartet of 6V6's instead of 6L6's...geee!). Anyway,thanks for your input! Regards, | ||
| Le Basseur |
Ouch...Dept.Of Correction: read "control grids'220 Ohms or screen grids' 470 Ohms". Thanks, |
Tue, 03/25/2014 - 09:26
#1