Welcome Guest! Be sure you know and follow the Phorum Rules before posting. Thank you and Enjoy! (January 12) x

Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Help with Bosch 48
#28

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!

"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis

Best Regards, 

MrFixr55


Messages In This Thread
Help with Bosch 48 - by daveone23 - 07-12-2024, 09:27 AM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 10:43 AM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 12:19 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 12:52 PM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 03:33 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 03:42 PM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 03:56 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 04:07 PM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 04:58 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 05:24 PM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 05:57 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 06:52 PM
RE: Help with Bosch 48 - by Chas - 07-12-2024, 07:15 PM
RE: Help with Bosch 48 - by daveone23 - 07-12-2024, 07:24 PM
RE: Help with Bosch 48 - by morzh - 07-12-2024, 09:01 PM
RE: Help with Bosch 48 - by daveone23 - 07-13-2024, 12:12 PM
RE: Help with Bosch 48 - by Chas - 07-13-2024, 12:56 PM
RE: Help with Bosch 48 - by daveone23 - 07-13-2024, 02:51 PM
RE: Help with Bosch 48 - by Chas - 07-13-2024, 03:07 PM
RE: Help with Bosch 48 - by MrFixr55 - 07-13-2024, 04:06 PM
RE: Help with Bosch 48 - by daveone23 - 07-13-2024, 05:07 PM
RE: Help with Bosch 48 - by daveone23 - 07-13-2024, 08:50 PM
RE: Help with Bosch 48 - by MrFixr55 - 07-14-2024, 12:17 AM
RE: Help with Bosch 48 - by morzh - 07-14-2024, 12:21 AM
RE: Help with Bosch 48 - by daveone23 - 07-17-2024, 08:26 PM
RE: Help with Bosch 48 - by daveone23 - 07-18-2024, 12:39 PM
RE: Help with Bosch 48 - by morzh - 07-18-2024, 09:19 PM
RE: Help with Bosch 48 - by MrFixr55 - 07-18-2024, 09:36 PM
RE: Help with Bosch 48 - by daveone23 - 07-19-2024, 01:13 PM
RE: Help with Bosch 48 - by Eliot Ness - 07-19-2024, 04:01 PM
RE: Help with Bosch 48 - by daveone23 - 07-19-2024, 04:16 PM
RE: Help with Bosch 48 - by MrFixr55 - 07-19-2024, 10:35 PM
RE: Help with Bosch 48 - by morzh - 07-20-2024, 12:15 AM
RE: Help with Bosch 48 - by MrFixr55 - 07-20-2024, 08:43 AM
RE: Help with Bosch 48 - by daveone23 - 07-20-2024, 11:15 AM
RE: Help with Bosch 48 - by morzh - 07-20-2024, 08:14 PM
RE: Help with Bosch 48 - by daveone23 - 07-21-2024, 11:39 AM
RE: Help with Bosch 48 - by morzh - 07-21-2024, 02:37 PM
RE: Help with Bosch 48 - by daveone23 - 07-21-2024, 03:50 PM
RE: Help with Bosch 48 - by morzh - 07-21-2024, 08:55 PM
RE: Help with Bosch 48 - by daveone23 - 07-22-2024, 12:54 PM
RE: Help with Bosch 48 - by morzh - 07-22-2024, 09:20 PM
RE: Help with Bosch 48 - by RodB - 07-23-2024, 10:36 AM
RE: Help with Bosch 48 - by daveone23 - 07-25-2024, 03:39 PM



Users browsing this thread: 2 Guest(s)
[-]
Recent Posts
Philco 39-7C Restore
A little more work today. I got the first IF transformer back together after replacing the wiring going to the 78 tube ...klondike98 — 12:06 AM
Two small radios from France. Restoration and review.
And here they are, the French beauties ready: In summary, the suspicion that radio transmits the national cha...Vlad95 — 10:46 PM
Two small radios from France. Restoration and review.
One is ready for washing, the other for painting: ATTENTION Faint-hearted and heart patients, please d...Vlad95 — 10:36 PM
Two small radios from France. Restoration and review.
Range Switch Block: I thought that mica capacitors of this type died out in the 30s, but no, they just moved ...Vlad95 — 10:29 PM
Two small radios from France. Restoration and review.
Chassis bottom view: Again, like twins, especially in the range switch part. Front panels: And...Vlad95 — 10:25 PM
Two small radios from France. Restoration and review.
Chassis removed, front view: Chassis rear view: It will be enough for the first device to prescribe a bath. Bu...Vlad95 — 10:11 PM
Two small radios from France. Restoration and review.
France Peculiarities of national character. Excelsior-52 threw me off track. It was too far outside my established...Vlad95 — 10:06 PM
Philco 16b no shortwave reception
First let thank everyone who helped ID this radio a few weeks ago! I have replaced all the capacitors and was able to r...Ken D. — 08:07 PM
Philco model 38 code 121 not receiving signal.
Ah yes, the Philco "Green Death" strikes again, someone suspected that cellulose nitrate based insulation migh...Arran — 06:23 PM
Philco model 38 code 121 not receiving signal.
Not sure if it matters much, but measure where the winding sits on the tube and try to rewind as close as possible. Tak...GarySP — 05:45 PM

[-]
Who's Online
There are currently no members online.

>