The AM Forum

Good morning.  I bought a "working" Globe Scout 65B - not really.  After trouble shooting switches - grid/plate meter switch open to plate position, CW Phone switch open - I shorted position for CW for testing.  Switch connections verified prior to testing. 

On transmit, I get a oscillator tone at the crystal frequency (tried several different crystals both 40 and 80) in a monitor - none or very low (.5 ma) grid current once the meter switch was replaced.  This is intermittent as well and most is to zero.  I get about 5 watts out with the grid current at .5 ma and 50 ma (plate does dip when I get some grid current), but mostly no joy.  The 6V6 oscillator and 6146 final tested in the green on a Hickok 539a.  I have checked out of circuit and verified values of the oscillator coupling capacitor, blocking capacitor, etc.  Still need to test or just replace the .005 uf bypass caps.

Power supply is low - I have replaced original filter capacitors with equivalent originals (new 16 uf - 800 volts).  Open circuit voltage of transformer is near 800vac, but loaded it drops to about 425 - should be 500.

Most resistance values around oscillator and final are within tolerances.

Tuning of oscillator control can decrease any output heard in the receiver (stops oscillating) - no meter reading most of the time.   Plate meter reading can dip when there is some grid current - does not dip when no drive at all.

I should be getting at least 3 ma and perhaps more on some bands - there is a caution in the manual to keep the drive below 3 ma.

I would like this to operate as designed.  Any help out there in the Globe owner community?

Those familiar with the Scout circuits using the 6V6, any thing I am missing?

Update on Globe 65B - the problem is in the power supply.  Open circuit voltage measured is 800 vac on transformer without the 5U4 (full wave choke input) and with tubes in the circuit, the output voltage is 400 vac and DC output is about the same.  On transmit, these fall about 100 volts.  Screen and plate voltages for the other tubes are fed from the dc supply through dropping resistors.  Plate voltage on transmit to a 6146 is 200 volts.  Poor and erratic oscillation and very low if any output.

The electrolytics were replaced....  The manual (With original components in power supply) says I should have 600 VAC at the rectifier and 500 volts DC supply output.  The voltage from the existing supply seems very low and drops significantly with load.   After providing power to the B+ from an independent supply (HP-23A) and adjusting HV output to 500 VDC - I got about 30 watts output and sufficient grid current of at least 2 ma.  It also worked on AM (needs some more work) as well as a cw signal that needs some work.  BUT it works with the right voltages.  It appears I may need a new transformer but I am still confused as to a failure mode.  The transformer does appear original as do the wiring connections from the transformer and there is no overheating, smoke, or vibration - just the failure to deliver at load.  The transformer is rated at 180 watts.  The choke is not open but I have not taken it out of circuit and measured RLC.

Any similar experiences?

Could the 5U4 be leaky/gassy?

What happens to the secondary voltage if you put a suitably-sized resistor across the secondary leads (with the rectifier removed)?
Have you measured the winding resistances of the transformer itself?

I could think of a "zebra" where the HV winding is open inside the transformer, but a carbon track burned across the bad spot is providing a high resistance.
But I'd look for horses first (bad tube, bad solder connections, etc.)  ;)

A quick and dirty test if you don’t have a spare 5U4 (and don’t want to spend on one until you’re sure) is to tack solder a pair of diodes to the socket in its place, a cheesy ‘solid-stating’ if you will. Even though you may not want to run it that way, it’ll tell you quickly if that’s the problem.

Ed

This is the circuit I use when replacing a 5U4GB tube rectifier with solid state diodes.

This should also help with your testing.

Phil - AC0OB

Thanks for the replies.  I have replaced the filter capacitors with the Antique Radio 16 uf, 800 volt units and the circuit retained the original 50K, 10 watt resistor shunt across the capacitor to ground.  I will try another SS replacement to see if the voltage drop reduces.   The Hammond catalog has a suitable replacement - their guidance indicates an expected  .45 V sec of the transformer output with a full wave rectifier.  That is consistent with the 800 VAC HV sec (no rectifier in place) to the 400 VAC V sec I measure with the rectifier in place.  BUT the manual indicates I should be getting 600 VAC on the V sec with the rectifier in place and 500 VDC as the HV output of the filter.  Removal of the rectifier and feeding 500- 550 VDC to the HV buss as indicated for the Scout in portable service, I get grid drive and sufficient plate current to load and dip to about 30 watts.

My experience in failed transformers generally is that you get smoke or no output and this replacement is going to wait until I have a better understanding of what failure has taken place.  Not rocket science, but a little frustrating to a rusty, 74 year old brain.

Thanks again for your replies.  No real problem with low grid drive if you have adequate voltage!

Bob, NS7H

Question to you happy Globe Scout owner-operators:  What is your measured HV at the filter?  The manual says 600 VAC at the plates of the 5U4 and 500 volts DC as the filter output.   Has anyone the specs for the original Globe Scout transformer.  I have a Globe Scout 65B, supposedly factory wired, but will not operate reliably and only at reduced power consistent with having a low HV at load. 

I am trying to determine if and how I may have a transformer failure give all the things I have tried.

Thanks, Bob NS7H

For PS problems one should always lift the total load at first to see what the voltage rises to, that is, lift anything after the PS filtering circuit.

I no longer have mine but HVB+ on Keydown was 550-620 volts at 124Vac input with the solid state replacement of the 5U4 as per the schematic I posted.

What is your AC voltage on Keydown at the input to the transformer?

Have you replaced C15, C18, and C19?

Don't expect those old transformers or chokes to make any sense in terms of their failure modes. Filtering Choke resistance can vary but many had about 100 ohms DC resistance.

If you don't have any good 5U4 replacements for substitution, then remove the tube and solder in those diodes and see if the voltage rises. Older 5U4's usually have low electron emission and so the space charge current will be weak resulting in low output voltages.

Phil - AC0B

Keep in mind that OP said, several posts ago: "That is consistent with the 800 VAC HV sec (no rectifier in place) to the 400 VAC V sec I measure with the rectifier in place".

Something is seriously wrong if just inserting the 5U4 in the socket causes the secondary voltage to drop by 50%. That should not be difficult to find...

gotta remember Leo and his efforts to make affordable ham gear.  this reminds me of a Viking 1 with low modulation in am mode.  it successfully resisted all efforts to fix until problem was isolated to mod xfmr by subbing in a replacement.  turned (bad pun) out that one of the primary windings was wound out of wrong sized wire(too small) and would not allow proper current flow  ugly