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Help with Bosch 48
#23

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.

"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



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