Could use some tech advice Scott 800B

[sloane freeman]

Took my 800B out of mothballs but could not locate speaker so rigged up a jones plug with a pair of resistors for the field, the jumper to turn it on, and and some speaker wire to a bookshelf speaker.

After working the buttons with some deoxit spray I was able to turn it on, change bands and listen to some FM and a couple of shortwave stations.  However, the Broadcast band was mostly absent except for one weak sounding local.  So back to the deoxit on the SW BC switch with no improvement.  I dug out the RF oscillator and st it to 10 mHz and got a strong signal, then set the radio to BC and could pick up the oscillator weakly at 1290 a local station.  I then set the oscillator and radio for 630 and nothing.  I reset both radio and oscillator back at 1290 and tuned them both towrds the bottom, the signal getting weaker until around 800 it disappeared.  I had recapped the radio and amp except for the bathtub caps in the rf-if section before I retired it to the attic when my wife became ill.  At that time the radio was quite sensitive on all bands.

The FM plays great.  Any ideas on this problem?  Thanks!

[sloane freeman]

Thanks!  It's a shame all that power has to be generated then wasted in heat.

[David C. Poland]

I have encountered a couple failed adjustable air caps. The press fit failed due to the pressure of a brass tension spring.

Encountered on a Philharmonic in the broadcast band oscillator adjustment. Couldn't keep adjustment, then slipped and shorted. Also happened on a Phantom restoration.

Fortunately sourced replacements from a part chassis.

Also encountered similar failure on the little brown AW-23 broadcast trimmer  - the adjustable slug inside had frozen - seemed to turn, but really wasn't adjusting. Again, a parts set solution.

[sloane freeman]

Now I have this problem.  After a minute or two on FM from a cold start, the volume drops a good bit and the eye is no longer responsive on this station that is very close.  I can tune to another station and the eye is responsive.  I have been using this one station for working out the bugs.  I have left the radio on this station after it dropped down in volume to have it come blasting on if I have turned the volume up.  To further complicate things, turning the BC-SW knob to SW diminishes the volume on FM.  I did not think there was any interaction between the two circuits.  I have cleaned the plugs with deoxit and they seem to be making good contact.  I am trying the follow the wiring through the band switch now.  Any thoughts?

[David C. Poland]

Not just stations at different signal strength due to distance and transmitter power?

This radio is early FM technology so stability and drifting during first couple minutes is far short of today's tech.

Be sure you understand certain control combinations are needed to switch between AM and FM. Affects access to the AM/FM relay switch.

Also, the tweeter is FM only, switches off on AM &. SW - so could be cause of an apparent change in volume.

Been some years, but I had to color the diagram with colored pencils to better follow the various circuits.

The station preset wiring greatly complicates the diagram.

A tube failing during warm up might cause this with, for example, a cathode short during warm up.

Next try monitoring the B+ at various circuits with a meter to narrow the problem area. 

Good luck. My 800B restoration years ago for wife's distant in-law ultimately sounded very good.

[sloane freeman]

Thanks for the pointers.  I have some new tubes coming though these were all new before I put it in storage.  I will check the tweeter situation. I have been so focused on the RF-IF that I bunched the change in volume with the BC-SW switch with the radio rather than the amp so I will look there also.  My though was that for the eye sensitivity to diminish the problem would be in those stages.  This used to be fun when I was younger and had good eyesight.  I remember when the older techs would complain about their vision or miss something so obvious to my young eyes that it would never happen to me.  I wish I could take back some of the thoughts I had back then.