BOL question - Printable Version

BOL question - Printable Version

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BOL question - Radioroslyn - 08-29-2017

I'm working on a 41-608 for Sam. I just have the chassis and spk. Was doing the rf alignment and noticed that the BOL is coupled to the receiver's local oscillator. There is a trimmer to adjust the frequency but I didn't see any spec as to what to set it for? The coupling coil is the bottom one on #9 and trimmer is #11. Not quite sure why the pickup needs a rf signal to operate?

RE: BOL question - Mondial - 08-29-2017

Terry, the RF signal is used to power the lamp in the pickup head. It can't be powered by AC because the photocell would pick up the 60 cycle hum and low voltage high current filtered DC was hard to obtain. So the solution was to power the lamp with RF from the oscillator operating from HV filtered DC. Therefore no hum from the lamp.

I believe the RF osc freq adjustment sets the output level and therefore the brightness of the lamp.

RE: BOL question - Radioroslyn - 08-30-2017

Tnx Mike,
That solves a couple of mysteries. [Image: http://philcoradio.com/phorum/images/smilies/icon_clap.gif]
Why it uses a large oscillator tube, What a rf signal is doing in the BOL, and maybe why I can't get it to tune below about 630kc.
I put a jumper across that pickup winding and it shifted the lo up about so maybe with the BOL plugged in there is some resistive (lamp) and (cable) capacitive effect that will correct the alignment. One can hope.

RE: BOL question - Mondial - 08-30-2017

I don't think having the BOL disconnected will affect the osc freq on the BC band. The pickup coil on the osc trans is only connected when in phono mode. Otherwise the coil is disconnected by the bandswitch on all other bands.

RE: BOL question - Radioroslyn - 08-30-2017

I got it to go down to 580 by adding a .003mfd across 7A. Below that it just want to conk out (both w/push buttons and main tuning). When I got it Sam told me that he couldn't pick up his sstran on it at 1200kc and wouldn't pick up anything at high end of the band.

After giving it a cursory look over I noticed that when the pointer was all the way over to the left the tuning cap @ min capacity. Thought it was a little odd but some sets are set up in this fashion. After looking online at some pics of the cabinet I found that it was indeed tuning backwards. Somewhere in it's sorted past someone restrung the dial cord backwards. Fixed that and the intermittent off/on switch wasn't intermittent any more. One of those p/b style jobs. Replaced it.

IF peaked up nicely at 455kc and the SW band was working fine but the broadcast band would only tune down to about 740kc and die. Didn't have a 7B5 so used a 7C5 and that helped it would tune down to 650kc. By adding the 3000mmfd I got to 580kc. Sam is sending a replacement osc tube and I'll see how it does.

RE: BOL question - sam - 08-30-2017

I'm not saying anything I know I'll get blamed for it all anyway Icon_sad

RE: BOL question - Radioroslyn - 08-30-2017

You did a good job on it!
I only found one wiring error and it was sort of invisible in the sense that it didn't make any smoke or much of an audible difference. All the other stuff is just old age and a dufuss that restrung the dial.
Obtw what is the highest frequency shown on the dial for the bcb and sw??
See it still looks thje same!

RE: BOL question - sam - 08-31-2017

looks great Icon_thumbup