Help with Bosch 48 - Printable Version
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RE: Help with Bosch 48 - daveone23 - 07-19-2024
The voltage divider looks ok and is not shorted out. Radio works fine, just the b+ resistor gets hot. I think its pulling 20 watts and the resistor is 10 watts. The interstage transformer is fine. Replacement is p-t156 from aes.
RE: Help with Bosch 48 - MrFixr55 - 07-19-2024
Hi Folks,
I have modified this post several times since originally posting it, so read it again. Sorry for the multiple mods.
You may have more than one issue causing excess current draw, which is common when one must do much work to restore a radio this old to functionality.
I've used the P-T156 transformer on more than one occasion, but not on a Push Pull (although it is designed just for that purpose). Most of the time I use them on old battery radios like AK20s, etc. Remember that the primary current for this transformer should not exceed 10 mA. I believe that the DC resistance for this transformer is a lot lower than many transformers that it replaces. Although this transformer is not supplied by the R11 Voltage Divider resistor, it could be contributing to excess current if the DC resistance of the transformer is too low.
Feel the interstage transformer. If it is hot, then it is drawing too much current. The transformer that you are using does have a 10KOhm primary impedance, but I don't know what the DC resistance is. These transformers were primarily used with a B+ voltage of about 90V. This radio has 250V on the 1st audio plate, which may be too much for this transformer (and also more than what is traditionally done with a type 27 tube, especially if it is driving a class A or AB push pull amplifier). OOps, this is not the 1st audio amp, it is the biased detector that I spoke about before. Verify the cathode resistor and bypass cap. Verify that between the cathode of the 27 and ground, there should be a rather high voltage. You can also pull the 27 to see if the current draw of the radio drops. The wrong bias on the detector will also lower the volume or even prevent detection. Per the RCA Receiving Tube Manual Version RC12, current draw in the biased detector circuit should be only about 0.2mA. The voltage drop across the cathode resistor should be about 30V. Sorry, I had Philco Model 20 on my mind, which uses both a biased detector and a first audio amp. However, coupling the transformer by an RC network may help. In addition, if the primary of the original transformer is gone but the secondary is good, you can still use it by "bridging" the transformer. Check out the following article:
https://philcoradio.com/library/index.php/shop-talk/general/tips-on-audio-interstage-transformers/
I think that the 10W resistors should be sufficient for R11. Are the voltages close to the diagram and what I listed in my last post? Oops, BAD JOHN. I did not see your answer. it does seem that the voltage at terminal 5 is rather high, DMM or not, considering the voltage at terminal 4. Is the speaker original? Is the field coil original? Your result was stated as 1000kOhm. Did you mean 1KOhm (1,000 Ohm)?
Is the 2200 Ohm resistor replacing a burned out section of R11? If so, how did you arrive at that value. Is the second leg of that resistor (between terminal 4 and ground) close to 2K Ohm?
Using Ohm's Law, Current = Voltage divided by resistance, or I = E/R
Measure the voltage dropped across the resistor in question Divide that voltage by the value of the resistor. This will give you the current flowing through the resistor. Now, multiply the current by the voltage drop across the resistor. This will give you the power being dissipated by the resistor.
Sorry, this post is getting complicated.
Good luck and keep us posted.
RE: Help with Bosch 48 - morzh - 07-20-2024
Does pulling any of the 1Rf, 2Rf or 3Rf tubes reduce the voltage drop acrorr tthat 2.2K drastically, more that others?
Check C13-C15 for leakage.
What about R3, R2, R6 abd C9?
RE: Help with Bosch 48 - MrFixr55 - 07-20-2024
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
RE: Help with Bosch 48 - daveone23 - 07-20-2024
I did mean 1kohms on the field coil. Its not original. The 2200ohm resistor is not original either, came with the set. I was going to try a 5k resistor in its place.
RE: Help with Bosch 48 - morzh - 07-20-2024
>>-pin 5 voltage 197vdc
-pin 4 voltage 70 volts
OK. -pin 4 voltage is correect, 70V.
The pin 5 voltage is 197V vs 175V shown in the sch.
This makes the difference between 7.3W vs 5W across that 2.2K section.
Are the original specs on that field coil and the resistor the same, 1000 and 2200 Ohms?
RE: Help with Bosch 48 - daveone23 - 07-21-2024
Would a field coil with less resistance create less heat on the 2200ohm voltage divider or do i need higher resistance?
RE: Help with Bosch 48 - morzh - 07-21-2024
I will repeat the question one more time: is the Field Coil original?
The reason I am asking is, trying to change resistance to make the curcuit work is wrong: we need too find out why the curcuit does not work.
If the FC is original and should be 1kOhm, you need to troubleshoot the curcuit, and not try to change the resistance.
This said, yes, in order to lower the dissipation you need to increase the FC resistance. Decreasing it will increase the dissipation.
RE: Help with Bosch 48 - daveone23 - 07-21-2024
Read post #35
RE: Help with Bosch 48 - morzh - 07-21-2024
What I meant was (I guess I need to re-phrase):
Is 1 kOhm resistance original? Or should it be different?
RE: Help with Bosch 48 - daveone23 - 07-22-2024
That is the million dollar question. There is no information for the field coil or the voltage divider. Its going to be trial and error, seat of the pants.
RE: Help with Bosch 48 - morzh - 07-22-2024
OK then.
Let's say, the 1K of FC is not original.
Start increasing the FC resistance (meaning, putting extra resistors in series with it), and at the same time decreasing the 2.2K resistor.
You need bring the 197V to 170V, while trying to keep 70V at the same level.
(Also we should account for you measuring the voltages with today's DMM, which will likely return higher values. But first let's do the first part. )
RE: Help with Bosch 48 - RodB - 07-23-2024
Hi guys. Thought I might shed some light on this subject. I went to Nostalgia Air and found a Bosch model 58 schematic. It has dual 45s in push-pull and is from the same year. The power supply has a 4 section divider resistor and a 2500 ohm field coil. You might want to compare this to your Bosch 48 and see if it gives a clue.
http://www.nostalgiaair.org/Resources/917/M0028917.htm
RE: Help with Bosch 48 - daveone23 - 07-25-2024
Put an atwater f2 speaker on the radio and it works well now. Project finished.
https://www.dropbox.com/scl/fi/8uxhovra174jzhhwarh2h/20240725_152233.mp4?rlkey=nnp6sg99f2pnfg57nakpwyouo&st=hdasdfnf&dl=0