Globe Scout audio gone

[K8WBL]

Good evening group, well, recently acquired a nice little Globe 680A and it seemed to work fine for about a week on 3885 Khz.  Got a good carrier but audio is just barely noticeable now, tried different mics, no difference..replaced the 6U8A (tested ok) and tested the 6L6 (tested OK) but cannot find anything wrong at this point.  Anyone know of an issue like this with the Globe Scouts?  Thanks

73, Tim K8WBL
Cincinnati

[W1EUJ]

Main Problem - they are old.





Are your resistors and capacitors still the original values?

[N3DRB The Derb]

they are old.

ever pull a 45 year old car out of a barn and drive it as found?

it breaks down 99% of the time.

[W1EUJ]

Is it restored or no?

If not:

Tim, you were very lucky that the radio worked OK as found. But, really, you need to put some work into a radio. Paper caps NEED to come out. Resistors checked and replaced as needed. Electrolytics replaced. You can't really do troubleshooting on a radio when those parts may not be what they are labeled anymore.

From there, you can make life easy by cleaning controls and adjustable components, check solder joints and touch-up as needed, loosen and retighten nuts and bolts. Transformers, chokes, coils - these are harder to find replacements for, and failures of those silly old resistors and caps could blow them.

Then you've got a fighting chance for a good daily run-about. As the issue come up and they do, you know where to find the help.

If yes:

Solder joints OK? How's the quality of the work? What's the voltages in the audio circuit?

Dave Goncalves

[w1vtp]

Tim

My first question is what kind of instrumentation do you have?  I'm assuming a VOM, but do you have a scope with some sort of 100:1 divider for use with the scope?

I did a search for a manual for your particular model.  Came up dry -- closest thing I could come up with is that it uses Heising modulation. 
Crispy Critter check.  First rule of thumb  is to unplug it -- short out a couple of HV points to be sure you don't have any HV lurking around.  Then take a GOOD look around for crispy critters -- burnt resistors etc.  Over the years as a calibration / repair tech I've used that approach successfully with the old tube stuff.  That is not the end-all for some parts can be toast but not show it.

Old component replacement.  As has been pointed out -- you were lucky to get a week's use out of it given the age of the unit.  Resistors, especially caps can operate for a while and than quite after years of "rest."  So, let us know what you have for test eq...  and if you have a schematic.

Others on the forum can verify if it uses Heising mod or not.  If so, there should be a reactor choke - shorted turns might be a possibility.   Here is a web page about the Scout

http://www.w8ji.com/globe_scout_page.htm

At this point I guess I really need to have a peek at a schematic before I say much more.

GL es 73, Al VTP

[wb1ead]

Hi Tim..sounds like ya got ur troubles there..as both Dave and Al have said it will require some input too from yourself to track the no audio mod down..for sure a schematic will help..voltage test spots sometimes are given..a basic VOM will get some stuff done for ya..by the way Al it is Heissing mod with the mod choke supposed to be the same as the power supply one..looks like ya got a tube tester..a point in ur favor..suggest an audio generator too..but hang tite here..this forum is well known for the help they extend to AMers and others..like Al i couldn't find a thing on the Globe just memory of what i knew back when..Pete or JN or one of the technical guys will be around most likely in short order..my recent re-store involved caps/resistors/variables/coils and tubes and one heck of a clean up to the wafer switches..tedious but worth it..you'll see     GL 73 de DAVE

[K8WBL]

Well, I always know where to come for help...thanks guys, no - it has not been restored.  I found one of the micas broke and I replaced it, brought the unit slowly with a variac and went from there.  I guess next stop if to take a close look at resistors and replace all the  HV caps...I have HV and a good carrier, so nothing serious went out, like the transformer, so I will start with all the suggestions made here.  I have some basic instruments and more...scope, DVM, freq counter, sig. gen. but I am not exactly proficient at them..hi hi ... background in psychology...ooops....hi hi  anyway pretty good with a signal tracer I have too.  So thanks much for the start and suggestions...I guess I was just throwing a stone in the dark and seeing if there were similar known problems with the Scout...

73, Tim K8WBL

PS> Back off 3885 for now...

[Carl WA1KPD]

You can find the full manual at:http://bama.edebris.com/manuals/wrl/680/
Mod end is below.

I'd put audio in and follow it down the chain with your scope. You'll see where it fails to amplify.

Check resistors as they tend to drift up in value with age. I would also suspect some of the cathode caps as well as the filtering they are putting in between stages
How is the wiper on the mic gain  control?
73
Carl
/KPD

[N3DRB The Derb]

Pet Peeve:

bringing a old radio up on a variac is like sucking on a baby pacifier - it makes you feel better but you dont get any milk.

'reforming caps' by doing so belongs with other myths as well.  If you 'reform' them you still have 40+ year old electrolite solution in there. How much energy do you think that will store?

BL: variacs are good to to have on the bench but not for that. The radio needs to be LOOKED AT before you turn it on.

[W1EUJ]

Don't need a variac when you change out the parts that will fail. Unless it's a kit.

The restoration mantra - paper caps - electrolytics - bad resistors. This is the path to radio nirvana.

[N3DRB The Derb]

and of course removing any hammy hambone JS mods.

[w1vtp]

I'm for "cutting to the chase" as it were.

I'm going to assume that the carrier does not have a significant hum on it so for the time being we can divert our attention from the HV PS.  The schematic helps a lot.  As I suspected, the modulation method is a Heising circuit.  Three items of interest catch my attention.  First the two multi-component circuit packs (in this case PC91, PC81) are notorious for failure.  I would prepare the xmtr for some resistance testing before plugging it in.  Turn the unit up so that you can inspect the bottom circuitry carefully.  Note the pin connections on the audio chain (6U8 and 6L6) -- particularly PC91 and PC81 and where the connections go (to the plates and to VC1 / pin 9 on the 6U8) (grid connection to 6L6).  Also the wiring of SW2 should be familiar to you -- trace it out.  Note that the mod circuit goes through the keying jack K1.  When the keying plug is pulled out there is a contact that has to complete the cathode circuit of the 6L6 to ground when in the AM mode - is that OK?

Pay careful attention to SW2 (AM CW).  Is it making contact in both positions?  At this point contact integrity of SW2 would be a good thing to check with your VOM in the resistance mode (remember, the unit is unplugged).  In the AM position, there should be some resistance across the mod reactor (CH2) and the cathode of the 6L6 should go to near ground (except for R18, the cathode resistor).  In the CW position there should be essentially a short across CH2 and the cathode of the 6L6 should go to high resistance.

Next, (still unplugged) turn your attention to the connection between the plate of the 6U8 pin 6 and pin 9 (the grid) should be very high if there is any resistance (megohms).  Also, the resistance between 1 of the 6U8 and pin 5 of the 6L6 should be very high (megohms if any).

Now put your VOM on pin 9 of the 6U8 and ground.  Vary the pot VC1 throughout its range.  There should be a smooth transition from 0 ohms and the maximum value of VC1.  Any jump to no resistance puts VC1 under suspicion.  OK, you can see what I'm doing with this.  Check resistors around the grid and plate circuit if you have the values somewhere in the manual of "couplate PC91, PC81, you can check those. I could not find any values for those damn things.  One other thing.  Carefully inspect solder connections -- these kits are notorious for bad solder joints.  Re solder connections if necessary.  Don't just reheat the connection.  Either reheat the connection with some rosin flux or use some rosin solder -- just a little!!

I'm going to stop at this point 'cause this is beginning to be an epistle.  Note all this testing was done with the unit unplugged.  Next we can go to the beastie plugged in -- that's for later.

GL and let us know how things go

73, Al VTP

[K8WBL]

Thanks Al, this is a good start for me, might take me into the weekend to work on this Scout...as far as the Couplates are concerned, I have no idea of the values, but will do a close inspection before starting the measurements..unplugged....I know on my Globe King 400B someone removed all the couplates but I also have a 400C which has not had any mods, except its missing the HV transformer...looking for a suitable replacement...not too tall, about 5.5 inches..I have a pretty good parts build up, so if the choke is gone, might have one.  Also, I mentioned there is "some" audio, but very very weak...will check those switches..thanks so much.

73, Tim K8WBL

[WQ9E]

Tim,

Also check any cathode bypass caps on the audio stages, electrolytics that have lost most of their capacity can create this issue.

Rodger WQ9E

[K8WBL]

Thanks to everyone for all the good suggestions, rig is fixed, either a cold solder joint or the plug in the back has a poor connection, but is working well now.  See you on 3885 Khz.

73, Tim K8WBL