USMC Spike
Well-Known Member
BACKGROUND:
Got a SA-7800 from neighbor (15 years ago) as its wood cabinet was 1/2 broken off and
it had seen better days. That said I used it periodically for TV duty. It kept getting worse and
worse in the one channel, so it was removed from service placed in the back room for a year.
Recently, opened her up and took a look inside, figured I'd go through the manual and give
the amp a tune up. I followed the instructions for adjusting the output transistor bias and that
is when the fun began.
Setting the left channel was without incident performed as specified.
Setting the right channel resulted in the sacred smoke coming out of
R58, R60, the 150 ohm emitter resistors, that appear to be the drivers
for Q2,Q4.
Well shame on me, Operator Error. So I put in a pair of metal films
and brought her back up...ta dah. More sacred smoke...I felt powerful!
NOT
So I brought her back up with a variac to 40 volts AC on the main line.
This resulted in a B+ in the amp of 20.6 Vdc measured at pin 1 to chassis ground.
Total current draw measured 0.1 amp current draw through the Variac.
Right channel Q2, Q4 readings not good, strange reading on the one emitter
resistor, other well it is what it is.
So I then measured the two transistor rows below Q2, Q4, and compared
them to the two row below Q1, Q3. and they were the same. Okay, then what?
Did a search and found some pretty good stuff here.
So, I pulled the outputs, Q2, Q4, and their emmitter resistors R62, R64.
Tested the pulls, Q2, Q4 are dead, short BCEs on both.
R62 short, R64 changed value, higher measurement.
Time to order replacements for all for output device and all four
emitter resisters. Trying to find what exactly to order though
and there appears to be some going back and forth about which
sets to use.
I can get standard non inductive wirewound in .22 2W...any one source the Nobel resistors?
Then question what is the failure mode of standard Vs. Nobel? Nobel seems to short out.
I would think standard wirewound should open? Would that be better in this amp?
Finally, I have most of the small signal transistors here at the bench along with most of
the 'lytic caps, except those specific to this amp.
Once I know which outputs will fit, I'll order them...and emitter resistors.
The goal is to get the Right side working first, before doing any of the other
updates to the amp.
So, just for the record I have the following specs & measurements.
These were measurement take with the faulty component in and
then with them removed.
GRW 112, Measurements (HP34401A) from the following pins:
Pin BEFORE REM AFTER REM
Pin 15,16,17: 48.6 / 48.6
Pin 19,20,21: -47.4 / -47.4
Pin 29,30: 32.8 / 44.7
Pin 31,32: na / -44.7
Pin 22: -1.9 / -1.6
These AFTER REM values are all within the expected tolerances.
The following were measured (HP34401A) on the power supply board GWR-112
Pin Number, expected / measured values:
Pin 21: - 48 / 13.9....will check again. This is the circuit with the failed components.
Pin 15: +48 / 48.5
Pin 29: +46 / 45.4
Pin 32: - 46 / -45.7
Thanks,
Spike
Got a SA-7800 from neighbor (15 years ago) as its wood cabinet was 1/2 broken off and
it had seen better days. That said I used it periodically for TV duty. It kept getting worse and
worse in the one channel, so it was removed from service placed in the back room for a year.
Recently, opened her up and took a look inside, figured I'd go through the manual and give
the amp a tune up. I followed the instructions for adjusting the output transistor bias and that
is when the fun began.
Setting the left channel was without incident performed as specified.
Setting the right channel resulted in the sacred smoke coming out of
R58, R60, the 150 ohm emitter resistors, that appear to be the drivers
for Q2,Q4.
Well shame on me, Operator Error. So I put in a pair of metal films
and brought her back up...ta dah. More sacred smoke...I felt powerful!
NOT
So I brought her back up with a variac to 40 volts AC on the main line.
This resulted in a B+ in the amp of 20.6 Vdc measured at pin 1 to chassis ground.
Total current draw measured 0.1 amp current draw through the Variac.
Right channel Q2, Q4 readings not good, strange reading on the one emitter
resistor, other well it is what it is.
So I then measured the two transistor rows below Q2, Q4, and compared
them to the two row below Q1, Q3. and they were the same. Okay, then what?
Did a search and found some pretty good stuff here.
So, I pulled the outputs, Q2, Q4, and their emmitter resistors R62, R64.
Tested the pulls, Q2, Q4 are dead, short BCEs on both.
R62 short, R64 changed value, higher measurement.
Time to order replacements for all for output device and all four
emitter resisters. Trying to find what exactly to order though
and there appears to be some going back and forth about which
sets to use.
I can get standard non inductive wirewound in .22 2W...any one source the Nobel resistors?
Then question what is the failure mode of standard Vs. Nobel? Nobel seems to short out.
I would think standard wirewound should open? Would that be better in this amp?
Finally, I have most of the small signal transistors here at the bench along with most of
the 'lytic caps, except those specific to this amp.
Once I know which outputs will fit, I'll order them...and emitter resistors.
The goal is to get the Right side working first, before doing any of the other
updates to the amp.
So, just for the record I have the following specs & measurements.
These were measurement take with the faulty component in and
then with them removed.
GRW 112, Measurements (HP34401A) from the following pins:
Pin BEFORE REM AFTER REM
Pin 15,16,17: 48.6 / 48.6
Pin 19,20,21: -47.4 / -47.4
Pin 29,30: 32.8 / 44.7
Pin 31,32: na / -44.7
Pin 22: -1.9 / -1.6
These AFTER REM values are all within the expected tolerances.
The following were measured (HP34401A) on the power supply board GWR-112
Pin Number, expected / measured values:
Pin 21: - 48 / 13.9....will check again. This is the circuit with the failed components.
Pin 15: +48 / 48.5
Pin 29: +46 / 45.4
Pin 32: - 46 / -45.7
Thanks,
Spike