Sweet Linda came home today.
- Thread starter Oldskoolfan
- Start date
Well, I fired it up on my DBT just to see.... didn't blow any fuses, DBT worked like it should. No magic smoke. All seemed well so I checked rail voltages.
Positive rail was 60.2 vdc. Negative rail was -60.2 vdc.
AC input was 50vac both sides of secondary to center tap.
No dc on the output terminals.
I knew these voltages were low so I took out the rail fuses and read voltages again.
Rail voltages were 75.9 and -75.9vdc
AC voltages were 57.2vac both sides to center tap.
Something is loading it down pretty good. This should be educational.
Feel free to offer suggestions. I think I need to start pulling power transistors so here is my first question.
Do I start with the driver row or start with the top row?
Positive rail was 60.2 vdc. Negative rail was -60.2 vdc.
AC input was 50vac both sides of secondary to center tap.
No dc on the output terminals.
I knew these voltages were low so I took out the rail fuses and read voltages again.
Rail voltages were 75.9 and -75.9vdc
AC voltages were 57.2vac both sides to center tap.
Something is loading it down pretty good. This should be educational.
Feel free to offer suggestions. I think I need to start pulling power transistors so here is my first question.
Do I start with the driver row or start with the top row?
- Joined
- Jan 14, 2011
- Messages
- 75,237
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 75,237
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
Took the DBT out of the circuit and brought it up on the variac.
Rail volts are 75.4vdc and -75.4vdc
Left channel bias at .475vdc. Reset to .350vdc - stable
Right channel bias at .438vdc. unable to set below .384vdc - stable
Right channel output transistors are all RCA D3937-6 with No name C5869 pre-drivers
Left channel PL XPL910 with Motorola SJ2741
I have sufficient old/used transistors to match right to left. Think that might help?
Rail volts are 75.4vdc and -75.4vdc
Left channel bias at .475vdc. Reset to .350vdc - stable
Right channel bias at .438vdc. unable to set below .384vdc - stable
Right channel output transistors are all RCA D3937-6 with No name C5869 pre-drivers
Left channel PL XPL910 with Motorola SJ2741
I have sufficient old/used transistors to match right to left. Think that might help?
Took a chance and hooked up to my cell phone for input and a pair of cheap test speakers - All seemed well for about 15 minutes then I was getting intermittent popcorn noise in the right channel. Enough to peg the meter so I shut it down. I think I'm gonna make sure I got good transistors and replace the right side so it matches the left and test again.
Many folks think that since the B+ and B- voltages are +/- 80V that they only need outputs that can handle 80V Vce. Wrong!
Each has to do 2x that or 160V plus a bunch of extra volts of margin to support current carrying capability when at the 160V extreme (driving capacitive electrostatics for example). Not easy to comprehend and that is why many repair techs go astray...
Each has to do 2x that or 160V plus a bunch of extra volts of margin to support current carrying capability when at the 160V extreme (driving capacitive electrostatics for example). Not easy to comprehend and that is why many repair techs go astray...
Ok, so here is where it stands; Sorted thru 3 full sets of output transistors from previous rebuilds. Came up with 8 FPL 909's, 2 RCA410's, 4 RCA 66546's that were indicated good on my transistor tester. Installed 6 FPL 909's and the RCA 410's in columns 3 & 4. Been running now for 2 hours without a single click or pop. Only outstanding issue is bias won't go lower than 366 millivolts and exhibits some minor fluctuation (+/- .2millivolts). Could this be something with Q6?
14A control board BTW.
14A control board BTW.
