SIR Tom. . . . . It seems that 60’s vintage of unit was fully capable of the stereo handling aspect when using the phono or tape input portion. As far as the radio aspect it was able to handle FM stereo if its output was fed into a separate stereo decoder and then being fed back into the unit. Now as per your queried AM aspect, I remember just a wee bit of a select few hours that a station would devote to the transmission of one channel of stereo thru FM transmission and its companion other channel thru AM radio, thus acquiring the stereo effect. That was just to prove out to be a novelty of the time. That was soon to be done again with AM stereo radio many years later, with it also never catching on. Then as for your REAL info, in consulting your units attached schematic, one sees that in the AM radio mode the switching is diverting the AM audio solely to the CHAN 2 output. Read my far right [ YELLOW HIGHLIGHTING ] and its red referencing. I have also placed in the AM radio [GREEN-DOTS] and the FM radio [BLUE-DOTS], input routings to the selector switch S14—sections 2 and 4 rears. Now if you really want it, you could provide additional switching action to route the AM signal additionally into CHAN 1, so you would then have a more pleasant mono source filling the room . . . .BUT. . . with it then being binaural mono ! 73's de Edd

Bogen SRB-40 Tuner –Amp Referencing Info:

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: : : : : : :SIR Tom. . . . . : : : : :It seems that 60’s vintage of unit was fully capable of the stereo handling aspect when using the phono or tape input portion. : :As far as the radio aspect it was able to handle FM stereo if its output was fed into a separate stereo decoder and then being fed back into the unit. : : :Now as per your queried AM aspect, I remember just a wee bit of a select few hours that a station would devote to the transmission of one channel of stereo thru FM transmission and its companion other channel thru AM radio, thus acquiring the stereo effect. : :That was just to prove out to be a novelty of the time. That was soon to be done again with AM stereo radio many years later, with it also never catching on. : : :Then as for your REAL info, in consulting your units attached schematic, one sees that in the AM radio mode the switching is diverting the AM audio solely to the CHAN 2 output. : Read my far right [ YELLOW HIGHLIGHTING ] and its red referencing. : :I have also placed in the AM radio [GREEN-DOTS] and the FM radio [BLUE-DOTS], input routings to the selector switch S14—sections 2 and 4 rears. : :Now if you really want it, you could provide additional switching action to route the AM signal additionally into CHAN 1, so you would then have a more pleasant mono source filling the room . . . .BUT. . . with it then being binaural mono ! : : : : : : : :73's de Edd : : : :

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I am looking for the Bogen SRB40 schematic.
Would you be able to email it to me?

Thanks so much!
David Dill

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: : : : : : :SIR Tom. . . . . : : : : :It seems that 60’s vintage of unit was fully capable of the stereo handling aspect when using the phono or tape input portion. : :As far as the radio aspect it was able to handle FM stereo if its output was fed into a separate stereo decoder and then being fed back into the unit. : : :Now as per your queried AM aspect, I remember just a wee bit of a select few hours that a station would devote to the transmission of one channel of stereo thru FM transmission and its companion other channel thru AM radio, thus acquiring the stereo effect. : :That was just to prove out to be a novelty of the time. That was soon to be done again with AM stereo radio many years later, with it also never catching on. : : :Then as for your REAL info, in consulting your units attached schematic, one sees that in the AM radio mode the switching is diverting the AM audio solely to the CHAN 2 output. : Read my far right [ YELLOW HIGHLIGHTING ] and its red referencing. : :I have also placed in the AM radio [GREEN-DOTS] and the FM radio [BLUE-DOTS], input routings to the selector switch S14—sections 2 and 4 rears. : :Now if you really want it, you could provide additional switching action to route the AM signal additionally into CHAN 1, so you would then have a more pleasant mono source filling the room . . . .BUT. . . with it then being binaural mono ! : : : : : : : :73's de Edd : : : :

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: : : : : : : : : :Sir John K k k k k k k k k . . . . . : : : : : :Why . . . .’soitanly . . . . their function / explanation follows: : : : : :Those jumper links make for an uber convenient access for the testing of the bias operation parameters of : : that channels pair of output tubes. : : :If you open the links, that accesses some low value (***see later :comment***) series resistors in the tube : :cathode to ground loop. : : :That measurement then correlates to the current passing thru each output tube, when performing a DC : : voltage monitoring across either of the resistors. : : :The spec for the operational negative grid bias to the 1st grid of them is set at ~-10V and is initially acquired : : from the sets -14 bias supply. : : :It is then further divided down to that final value by the R50/51(R69/70) and R54/55(R73/74) voltage divider : : resistors. : : :Initial dropping down to the -14 VDC intermediary supply value is being provided by R88 in the power supply. : : :That supply then feeds into the R4(R5) wiper, where the weaker of the outputs can be compensated for by : : shifting the wipers adjustment, to favor its side, and balance the pairs current pull. : : :When all is properly balanced out for the tube pairs quiescent current pull, move on to the other channel and : : do the same. : : :When finished , swing the jumpers back in place to be shorting out the current shunt resistors. : : : : :And n o o o o o ow, since the party of the first party . . . .twice removed . . .has not shown up to lay claim to : : his schematic . . . . just yet. : : : : :If this was my unit to rework . . . . in amongst all of its checkouts, a high priority would be to pull the selenium : :rectifier and replace it with a 1N4007 (a little overkill there !) : : :Also, one needs to replace that 100 ufd 50VDC C5 filter with a fresh unit, one certainly does not want any : :problems with the 7189 AF output tubes bias supply. : : :Then, in order get the more efficient rectifiers higher output voltage, on back down to that -14 VDC median, : : tack in a 500k linear pot from the output of R88 to ground with it initially set at max resistance. : : :Then one adjusts it down in value, to see what value is needed to attain -14VDC output again. : : Then replace that required value with a fixed resistor and the result will be a stable voltage divider. : : :A voltage correction procedure done by diddling with R 88’s value is not the route to take, with this only being : :a <1 ma supply line. : : :When it comes to the final balancing of the output stages, a procedure that I use, is to set it by using my : : voltmeter in a DC “ quasi differential ” mode. : : :That is being whereupon I place one meter probe on one of the 7189 tubes cathode and the other probe on : :the companion 7189 tubes cathode and then adjust the R4/R5 for a null. : : :(Instead of walking measurements from across R53(R72) resistor / R57(R76) resistor . . . many-many-many times.) : : : : :BTW . . . . as per that . . . (***see later comment***). : : : :They were using 15 ohm current shunt metering resistors, I would change over to sets of 10 ohms resistors, and might : :as well make them more modern metal film 2% tolerance unit’s, vice the old 5% CARBON COMPOSITION units. : : :With those values now being used in monitoring current flow, it then more easily mentally transposes into a : : voltage/to/current conversion reading. : : : :Thasssit . . . . . : : : : : : : : : : : : :73's de Edd : : : : : : : : : : : : :My crazy dog . . . . when I get up very close and blow in his face, he gets super mad at me, but : : anytime I take him along for a car ride, the first thing that he does, is to stick his head out the window ? : : : : : :: :

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::Muchos gracias, Edd, for the fine schematic. I don't have a Bogen stereo tuner, but I appreciate the schematic nonetheless.
::Now could you please be de-scribing for us the biasing of the output stages and the nifty brass hook jumpers on the bias transformers?
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