07-20-2024, 07:43 AM
Hi Dave and Folksfollowing,
My bad, I modified my post after Morzh posted his post, so my new thoughts (or brain droppings?) are out of sequence. Dittos on Morzh's last post.
Looking at the "Total" voltage divider from approximately 250V supplied to the output transformer, through the speaker field coil, through R11 to ground, based on the measurements that Dave posted:
Field coil = 1,000 Ohm
Total resistance of R11 from field connection (terminal 5) to ground is 3,800 Ohm
Total resistance from 250V supply through the field coil and the 2 sections of R11 to ground is 4,800 Ohm
Using Ohm's Law, I=E/R
250V/4,800 Ohm = 0.053A
To calculate power (wattage) dissipation of the voltage divider, P=EI
0.053A X 250V = 13W This is greater than the 10 W dissipation capacity of the resistor. Nonetheless, this resistor, and the other section between terminal 4 and ground are going to get hot no matter the rated wattage of the resistor!
Unfortunately the N/A service noted for the Bosch 48 do not provide correct resistance values for the field coil and the voltage divider
A B+ to ground voltage divider tends to draw more current than the circuits connected to it. This is why they tended to fall out of favor as the technology advanced. However, look at the somewhat similar Philco 20. There are differences. The detector is a 24A vs a 27, there is an extra stage of audio amplification, the output tubes are 71As instead of 45s, and plate voltage for the 3 24As do not go through the divider in the Philco 20. The B+ at the field connection to B+ is still near 250V. The total voltage divider resistance for the Philco 20 including Field Coil is approximately 6,870 Ohm, much more than that of the Bosch. Current through the Philco voltage divider is 0.036A, power dissipation by the voltage divider of the Philco 20 is 9 Watts.
You may not be far off, but current draw through the divider in the Bosch is about 40% more than that in the Philco. It's a shae that there are no specs for the Bosch.
Yes, I would probably replace both resistors that are part of R11 for field to ground, updating them to 20 watts. The 175V at terminal 5 vs the stated 150V indicates that there may be either too low a resistance in the speaker voice coil or there may be shorted turns.
Hope this helps
My bad, I modified my post after Morzh posted his post, so my new thoughts (or brain droppings?) are out of sequence. Dittos on Morzh's last post.
Looking at the "Total" voltage divider from approximately 250V supplied to the output transformer, through the speaker field coil, through R11 to ground, based on the measurements that Dave posted:
Field coil = 1,000 Ohm
Total resistance of R11 from field connection (terminal 5) to ground is 3,800 Ohm
Total resistance from 250V supply through the field coil and the 2 sections of R11 to ground is 4,800 Ohm
Using Ohm's Law, I=E/R
250V/4,800 Ohm = 0.053A
To calculate power (wattage) dissipation of the voltage divider, P=EI
0.053A X 250V = 13W This is greater than the 10 W dissipation capacity of the resistor. Nonetheless, this resistor, and the other section between terminal 4 and ground are going to get hot no matter the rated wattage of the resistor!
Unfortunately the N/A service noted for the Bosch 48 do not provide correct resistance values for the field coil and the voltage divider
A B+ to ground voltage divider tends to draw more current than the circuits connected to it. This is why they tended to fall out of favor as the technology advanced. However, look at the somewhat similar Philco 20. There are differences. The detector is a 24A vs a 27, there is an extra stage of audio amplification, the output tubes are 71As instead of 45s, and plate voltage for the 3 24As do not go through the divider in the Philco 20. The B+ at the field connection to B+ is still near 250V. The total voltage divider resistance for the Philco 20 including Field Coil is approximately 6,870 Ohm, much more than that of the Bosch. Current through the Philco voltage divider is 0.036A, power dissipation by the voltage divider of the Philco 20 is 9 Watts.
You may not be far off, but current draw through the divider in the Bosch is about 40% more than that in the Philco. It's a shae that there are no specs for the Bosch.
Yes, I would probably replace both resistors that are part of R11 for field to ground, updating them to 20 watts. The 175V at terminal 5 vs the stated 150V indicates that there may be either too low a resistance in the speaker voice coil or there may be shorted turns.
Hope this helps
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
