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Help with Bosch 48 - Printable Version

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RE: Help with Bosch 48 - daveone23 - 07-13-2024

Took some readings of the inductors L6 and L7. L7 is 440 ohms and has resistance to ground on the c21 side @ 2.8kohms, the other side of L7 to ground is 2.3kohms.
L6 is 1.5kohms and has 2.3kohms to ground at the L7 junction and 800ohms to ground at the T1 transformer.

RE: Help with Bosch 48 - Chas - 07-13-2024

To check the resistance (leakage) to the frame/core all connections must be removed... A sensitive DVM may give a clue, but a leakage test circuit in some capacitor testers that outputs 600 volts is far better at registering leakage. Not every bench is equipped with a "Hi-Pot" for such a test...

Chas

RE: Help with Bosch 48 - daveone23 - 07-13-2024

I measured the resistance of the speaker field to ground and it was 1.5kohms. I was told it should be over 100kohms

RE: Help with Bosch 48 - Chas - 07-13-2024

Be sure the speaker field is not connected to anything. Measure with a high resistance VOM like a Simpson 260. If there is still an indication of leakage then the speaker field coil needs to be insulated from the frame of the speaker.

The method to insulate will be predicated in being able to remove the field coil from the speaker. Cleanup the pole piece and wrap on layer of Kapton tape. Too much tape and the coil will not fit.

It is not a good idea to float the frame of the speaker and allow it to be live at full B+...

GL

Chas

RE: Help with Bosch 48 - MrFixr55 - 07-13-2024

My $0.02:

+1 on the other comments, especially on pulling all the tubes and powering up.  the transformer should run cold.  If hot, then you are dealing with shorted or grounded coils.

First off, with all connected, does the radio play?

Secondly, what values did you use for the filter caps (C20, C21, C19)?  They should be rather low (Preferably the values called out in the diagram but no more than 10 uF).  if much more than 20 uF, the current will be too high.  Working voltage should be at least 450V.  Any lower and the cap may short.

If you can, break the circuit put a DVM set for 2Amp scale in in series between rectifier filament and junction of L7 and C21.  Plug everything in.  Per the RCA Receiving Tube Manual, max output current for a Type 80 rectifier with an expected output of 350V must be less than 125mA (0.125A on the 2A scale).  Anything above that indicates excess current draw.

Power down, remove all tubes except the rectifier.  If possible, place a second meter across C21, leaving the ammeter connected as above.  Power up, monitoring voltage and shut down if the voltage at C21 exceeds its rating.  Current draw caused by the filter caps, field coil, voltage divider (r11) should be about 30 mA or less.

Power down, and reinstall the output tubes.  Current draw should increase, but no more 30 mA per tube of plate current.  each of the other tubes (the 3 24As and the 27 should add no more than 5mA each.

Hope this helps.

RE: Help with Bosch 48 - daveone23 - 07-13-2024

Chas, the i dont think the speaker field is an issue. The first speaker i used was from an atwater 55c, 1000ohm field. The field plugs into the back of the radio. Then i tried another speaker with 100ohm field. Yielded the same results, 150watt drain. Thats too much to be experimenting with, i can burn up the transformer with that kind of wattage. Mrfixr55, radio does not play or make any sounds. I recapped the set with identical value caps. I would think since there is resistance on the field to ground, 1.5kohms) there is a short in the b+

RE: Help with Bosch 48 - daveone23 - 07-13-2024

I found the source of the short. When i replaced the interstage transformer i put it in backwards. Now the radio runs at about 97 watts but its still not working. I need to check all the voltages again.

RE: Help with Bosch 48 - MrFixr55 - 07-14-2024

Hi Dave,

That now makes a lot of sense. If you accidentally had the 2 secondary legs connected as if they were the primary, but still had the center tap grounded, that would certainly load down the power supply. Recheck the transformer to ensure that you didn't smoke it.

Do you at least get a hum? You should. If not, power down, unplug the speaker and check the following resistances:
Speaker voice coil. The reading should be very low.
Output transformer secondary. It should also be very low.
Output transformer primary plate to plate. This should be between 300 and 600 Ohm.
Output transformer either plate to junction of L6, L7, C20 and field coil jack. Reading should be half that of the plate to plate reading.
Interstage (driver) transformer grid to grid, should be rather high, between 2K Ohm and 5 K Ohm
Interstage transformer, either grid to ground should be about half the grid to grid reading.
Detector plate to junction of L6 and C19 (Checks continuity of interstage transformer primary nad detector RF chokes) Should be rather high, between 5K Ohm and 15K Ohm
Plate of each 24 RF Amp and terminal board connection 5 (or the other pin on the field coil socket). The reading should be low for each one, likely 100 Ohm or less.

Repair any "opens"

Try to inject a signal into the grid of the 27, even if it is just touching with a screwdriver. You should get clicks or a hum, or possibly a strong radio station. If you do, troubleshoot the 3 RF stages.

Try the following:
Make sure that there is a voltage drop across the field by measuring voltage between ground and each side of the field coil. There should be a significant voltage drop.

This is a TRF (Tuned Radio Frequency) Receiver with a "Biased" detector. This detector also amplifies. Some refer to biased detectors as "plate detectors". RCA referred to these as "power" detectors. Any copy of the RCA Receiving Tube Manual will have a good discussion on biased detectors, as well as the earlier "grid leak detector and diode detectors. Touching the grid of the detector and each RF amp tube with an antenna lead or just a long wire while tuning should produce a hum or better yet a radio signal, especially if there is a strong station nearby.

The volume control on these sets do not work the same way as the volume control on more modern radios. It is a ganged unit where R1 varies the signal into the 1st RF tube and R2 varies the bias. an open control, especially the R2 portion will render that stage inoperative. A strong signal will overload the stage and stop operation if the signal is too strong and the control is turned to far up. Start with the control at a low setting during normal operation.

RE: Help with Bosch 48 - morzh - 07-14-2024

...and the transformer is still alive?
Like him. Or her. Or it.

All well that is reconnected well Icon_smile

I will let other folks drive: I have just done 970 mile drive from NJ to FL (15-1/4 hr) and I am a little bit drained. Virginia sucks. The rest rules.

See y'all and youse tomorrow.

RE: Help with Bosch 48 - daveone23 - 07-17-2024

Iam just getting back into it. I had covid. Iam going to print out mrfixr55s info and go thru it. When I put the interstate in right this time the watts came down to 100 which is still high, but better. No sound at all. I will make some checks and report back. Thanks.

RE: Help with Bosch 48 - daveone23 - 07-18-2024

Good news, I hear a faint radio station. Listen below. I noticed a few problems, 1. A replacement voltage dropping resistor, 10watt 2200 ohms is baking and super hot. Its part of the 3 resistor divider. I may need to bump it to 20watts or more. 2. The volume pot is junk and why i could not hear it before. Replacing it would be difficult. Its multi-gang. Whats the next step?
https://www.dropbox.com/scl/fi/mvd2u1n8bcydehnp0ikbk/Bosch-radio.m4a?rlkey=9u8g2ke2lgzqokldy6fljuvxv&st=ztz91rfi&dl=0

RE: Help with Bosch 48 - morzh - 07-18-2024

which section of the divider is it?

RE: Help with Bosch 48 - MrFixr55 - 07-18-2024

Hi Dave, 

I had COVID too Icon_sick

Try this:

Power off and disconnect the screw terminals.  Unplug the speaker field coil.

Measure the total resistance of the R11 "voltage divider" resistor between Terminal 5 and chassis ground.  Measure between Terminal 4 and ground.  That resistance should be a little less than half the resistance between Terminal 5 and ground.  Note these values.  Note the DC resistance of the field coil.  I believe that it should be similar to the value received from measuring between terminal 5 and chassis ground.  My rational for this is that the Plate voltage for the output tubes is 250V. One end of the field coil connects to the point that supplies the output transformer. The plate voltage for the RF amps is supposed to be 120V.   This comes from the other end of the field coil which is essentially part of the voltage divider. (Terminal 5.) The screen voltages of 70V come from Terminal 4. 

Reconnect all after troubleshooting.

Measure voltage at terminal 5.  It should be 150V.  If it is much more than that, the speaker field coil met have shorted turns or be the wrong one.
Measure voltage at terminal 4.  It should be 70V, and about half of the terminal 5 voltage.  If both voltages are high but proportional, then 
the speaker field may be at issue.  If the terminal 4 voltage is low, then something is loading it down.  Check C13, 14 and 15 for shorts. 

Unfortunately, the diagram does not give resistance values for the speaker field or for the voltage divider.

If you touch the antenna to the grid cap of the 2nd RF tube, you should receive a very loud signal, considering that your volume control is trashed.

I don't know of the volume control is wirewound or not. However, you can do one of 2 things.  

There were 2 different volume controls used for this set.  One control had both the antenna and cathode bias pots being 10K. the other control had (I believe) an antenna pot of 50KOhm and the cathode bias pot being 5KOhm.  I believe that these are linear taper, as opposed to audio tape, used in modern amps and receivers. 

The voltage divider that includes the cathode bias pot has a total resistance of 31,500Ohm if the 5K pot is used.  At 70V, current through this total resistance of 31,500Ohm is only 0.0222 Amp.  Power dissipation through the entire resistor network is only 0.15W, so a 0.5W 5K or 10K pot will work just fine for the cathode bias portion of the circuit.  The antenna portion should likely be 50KOhm.  If possible, get 2 pots.  Otherwise, if you can get a ganged 10K 0.5W linear taper pot, this will do just fine.

Hope this helps!

RE: Help with Bosch 48 - daveone23 - 07-19-2024

Morzh, its the resistor that goes to b+(5). Right now its 2200 ohms and 10 watts but its roasting. Radio works though.
Mrfixr55, here are the results of the tests:
-r11 pin 5 to ground 3.8kohms
-r11 pin 4 to ground 1.4k ohms
-field coil = 1000k ohms
-it is not similar to pin 5
-pin 5 voltage 197vdc
-pin 4 voltage 70 volts

At this point i need to cool down that resistor.

RE: Help with Bosch 48 - Eliot Ness - 07-19-2024

Quote:Morzh, its the resistor that goes to b+(5). Right now its 2200 ohms and 10 watts but its roasting.
I wouldn't run it too long with that resistor getting so hot, you're apt to burn something else up because someting is drawing too much current. Is it part of R11? If so could you post a pic of the original resistor? If it is a candohm style one or more sections might be shorted to ground.... they are known to do that, so it is wise to replace all sections at once. Otherwise you need to find out why the resistor is drawing so much current. I take it you have already replaced the original? And that was because it was open?

I'd also measure that transformer you put it in backwards and make sure both sections are OK. Can you tell us exactly which transformer it was? And did you replace it with an original or ???