1935 International Model 52 again

Paper caps all replaced, and a couple out-of-tolerance cathode resistors swapped out for 1 watt carbon film. The 6F6 output stage cathode bypass electrolytic was also replaced; it wasn't in the can as specified in the schematic but was a "Dandee" brand 10uF axial lead type air-wired inside the chassis.

The paper caps were a hodge-podge of different brands - Mallory, Aerovox, Sprague, Tobe Detustchmann. The can electrolytic is an 8uF/8uF unit instead of the specified 6-6-6. Some of the paper caps weren't the correct values e.g. the 0.25uF paper cap was actually a (very large)

0.5uF unit. I'm guessing the last time this radio was serviced might have been during WW2 and perhaps wartime rationing had something to do with it - repair shop made use of whatever they had on hand.

Maybe surprisingly both sections of the can electrolytic reformed OK running a current thru it via stepping up the voltage ranges while on the "leakage" setting of my Heathkit IT-11. It took about a minute for the eye to open at the rated max voltage of 450. I measure a "power factor" of around 3-4% on both sections at that voltage. Don't know if it would be good to leave that as is but I'm using it in-place for testing.

The 5Z4 and 6F6 + power supply circuit seems to be working fine, with the set powered up the voltage on the first capacitor section and the second after the field coil choke look approximately correct. Injecting1kHz into the 6F6 grid and into the grid of the 6J7 detector I get a strong output through the speaker with no hum.

The problem now is that the plate and screen voltages on the 6A8 and 6K7 are way off; the service manual specifies 195 and 210 for plate voltages respectively but I'm actually getting more like 260 and 275. The screen voltages are specified as being 70 and 90 respectively but I'm looking at about 130 for both of them. Pin 5 of the 6A8 looks correct at about

-10.5 volts. Cathode resistors and bypasses were replaced on both of them but the cathode voltages are high, e.g. with the AM broadcast band selected, volume all the way up I'm reading about 4.8 volts on the cathode of the 6A8 instead of 3.

With a 3 meter longwire antenna I'm getting one local strong AM station (50kW about 8 miles down the road) at night but the AM band is otherwise silent. I'm guessing at this point one or both of the 6A8/6K7 has poor emission? I don't own a vintage tube tester but I have both HV and low voltage bench supplies available, any suggestion on how to rig up a quick emissions test?

Reply to
bitrex
Loading thread data ...

Schematic:

Reply to
bitrex

BTW I also replaced the screen resistor feeding both screen circuits as well as the screen bypass cap. No change.

Reply to
bitrex

What is your wallplate voltage?

110:195 = XXX/260 where XXX = 147 VAC. 110:210 = XXX/275 where XXX = 144 VAC. 110:3 = XXX/4.8 where XXX = 179 VAC

These results are reasonably self-consistent as what comes to a volume-cont rol can have many variations. Note that increasing capacitance in many case s will also increase B+ and other down-line voltages, and generally *SHOULD NOT BE DONE*, especially in radios with field-coil speakers such as that o ne.

So, hazarding a guess at long distance, you have a combination of high wall plate voltage, excessive capacitance and measurement variance.

This voltage of 14X seems extreme, but given that these sets were measured with VTVMs back in the day, and VTVMs load very differently than a modern V OM. And the vintage unit was not "true RMS" and so could measure low. Do yo u have an old analog meter? Or, perhaps a VTVM?

I have long-since learned to distrust factory-schematic voltages. Generally . I will bring a radio up to 110V on the Variac and measure secondary volta ges off the transformer (unloaded, then loaded) to see what is what. If I g et consistent discrepancies, I attribute it to measurement variances. If I get inconsistent discrepancies - more than 15% apart - I will look for a ba d transformer winding or some other problem of that nature.

Did you replace the line-filter cap on the line-cord? Do so with a type Y s afety capacitor. That will help on reception.

I have no idea where you are, but if you are anywhere near southeastern PA, USA, I would be glad to test your tubes. I keep both a little Simpson emis sions tester and a Hickok 539B for the heavy-duty stuff.

Otherwise, try swapping tubes with another radio. 6A8s are notoriously pron e to failure, and glass ones are getting quite scarce. More so than even a

1L6 in my experience.

Peter Wieck Melrose Park, PA

Reply to
pfjw

Using my DMM on the "AC" setting, out of the wall it's reading 124 VAC.

Thanks for the offer! I'm currently in Providence, RI most of the time so a little far, unfortunately...:(

Will do. Yeah, gosh, NOS 6A8Gs are expensive! The schematic specifies either the metal or "G" shape enclosure, I guess the GT doesn't have the correct pinout? In any case, all the tubes currently fitted in the set are the metal enclosure type.

Reply to
bitrex

Seems extreme for a 6uF - 8uF swap, but I'm not experienced enough with tube PSUs to know for sure. Weird that the cathode resistor voltage is too high, I'd think that if emission were the problem it would be too low. There's definitely not enough voltage drop across the screen resistor. I'm going to measure every voltage on every pin again and post it to make sure I haven't bungled a measurement, somewhere.

Unfortunately not, all my meters are DMMs. Unlike a tube tester that's probably a tool I can get my hands on readily though

Ok, will do, it's currently just a 630V regular film type.

Reply to
bitrex

The GT is fine, but it is a short tube and often the grid-cap lead will not reach the top of the tube over the shield - required for a glass-type tube. If you have no shield, stick with the metal tubes. \

It may also need an alignment, by the way. Are you equipped to do that? It is certainly simple enough - with the right tools.

Peter Wieck Melrose Park, PA

Reply to
pfjw

Updated voltage measurements, broadcast band, volume control fully on, wallplug voltage 124VAC. All measurements taken ~30 sc

6F6:

Pin 1, enclosure: 0 Pin 2, heater: 3.2 VAC Pin 3, plate: 283 Pin 4, screen grid: 283 Pin 5, control grid: 0.8 Pin 6, NC Pin 7, heater: 3.2 VAC Pin 8, cathode: 18

6J7:

Pin 1, enclosure: 0 Pin 2, heater 3.2VAC Pin 3, plate: 7.7

Reply to
bitrex

30 seconds after power on, rather
Reply to
bitrex

You are getting 6.4V across the filament - which makes sense at 124V at the wallplate.

I expect that your transformer is good. I expect that the elevated B+ is due, in part, to excess capacitance. 6 - 8 uf may not seem like much from 30,000 feet, but it is a 33% increase at gr ound level. And 6 - 10 uF is a 67% increase at ground level. And, of course , an 11+% increase in wallplate voltage will contribute significantly.

Do you have any replacement tubes in your inventory?

If not, contact me off-group. I probably have dozens in that line-up and I could send you some proven-good for testing. Also, can you do an alignment?

Peter Wieck Melrose Park, PA

Reply to
pfjw

Right, that makes sense, but what about the low plate voltage on the

6J7? There's a small RC network there, I don't think I checked all those for tolerance. I should do that.

That'd be great, I can contribute a nominal fee if you like because I'm definitely not thrilled with the prices I'm seeing on even used variants at the usual outlets! Yeah, I have an RF signal gen available to do an alignment, though I've only read about the procedure and never actually done it in practice, doesn't seem too intimidating.

Reply to
bitrex

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.