9 knob BOL Restoration

[Stephen Scheel]

Hello All,

I’m finally getting around to starting the resto/repair process on my 9-knob, AM-only BOL Philharmonic. I’ve done this many times on ordinary radios, but for this particular set is there a tried-and-true process to power up and check things as I go? Looks like the amp/PS has had some work done to it at some point in the past already.

Thanks, Steve

[David C. Poland]

caps, not papers. I gotta watch spell check more carefully !!

[Stephen Scheel]

Paper Caps :)

David C. Poland said:

caps, not papers. I gotta watch spell check more carefully !!

[Stephen Scheel]

Hi folks,

I’ve recapped the PS/amp and given the rest of the components a general once-over. Everything looks to be in order.

I’d like to fire the amp up connected to the speaker, feed it an audio signal, and put it through its paces before I call it done.

It looks like the −27 V is derived on the receiver chassis. Is there a way to power up the amp by itself for testing? I’d also expect to need to limit the input voltage, since the overall load would be lower without the receiver.

The output tubes test from 4000 to 4600 on the old Hickok 800. How sensitive is this circuit to imbalance in the output tubes?

Thanks

Steve

[Kent King]

Steve -

Do NOT power the amp without a tuner attached. If you do, the B+ voltages will rise extremely high without the load of the tuner. The filter caps will break down and you'll have a very big mess on your hands. Scott explicitly warns against this in the service data and owner manuals. It might be possible to bring it up part way with a variac, but you'd have to monitor the DC voltages and not exceed the normal operating levels...but then your tests are not full power tests. I would just wait until you have the tuner ready and bring the entire set up. The Scott service data addresses tests for the power supply when connected to the tuner.

Kent

[Stephen Scheel]

ok.. i guess i'll not temp fate.  this will probably be the last post for a while.   Now that the supply is done i need to get onto another project for the local club before I start on the Receiver.   THanks for all of yours help so far.  I'll back :).

[Alex R. Whitaker]

At least plug the tuner chassis into the amp and test it to see if it works.  It may work great, marginally, or not at all.  You'll probably hurt nothing if you try it for a short time.  It would be best to monitor the current going to the receiver to see if it is in range, just to be safe.  

[Stephen Scheel]

Thanks Alex.    I want to give it a visual inspection first which in my case requires physical test environment setup first. I'd rather wait a while so i don't have to lug the big chassis around too much to keep the risk to it to a minimum.

It probably only a month or two delay.

Steve

[David C. Poland]

While you are at this point, do continuity tests on the speaker - 

The B+ field which has a tap about midpoint. And the output transformer. the voice coil impedence is 38 ohm (about 32 ohms on a VTVM.)

The Philly speakers seem pretty rugged and are usually ok in my experience. 

The Standard speaker for the standard BOL AM/SW 30 tube Philharmonic is a special Magnavox 15 inch pedestal speaker, with optional PM tweeters that plug into pedestal base, where the output transformer is housed. The crossover is a 2MFD cap on one of the two tweeters wired in series.

But later production (sometime in 1941) may be a single flange mount 15 inch speaker with no tweeter option OR a Hi Fidelity optional system intended for the FM Philharmonic with an outboard crossover into which the main and also PM (or field coil) tweeter(s) separately plug into.