Hi,

I gotta a terrible firestorm on this radio and I figure it's one or both of the the IF cans. I have nothing to read the capacitors with and was hoping someone might know what size the capacitor(s) might be in can #2 of my DeWald D-519 http://www.nostalgiaair.org/PagesByModel/368/M0004368.pdf? It appears that one side of #2 will require a larger capacitor, right?

Heck, if you'd know the size of the mica caps in can #1 that'd be great, too!

Just do a search for silver mica disease and you'll get more information than you'll ever need. The caps are typically ~ 100 mmf. Do the cans have 119 on them? That supplier made, I'd guess, a high percentage of the IF's in '50's radios.

Yes, can #1 has a 119 on it. I have used 100pf on that can. But # 2 is an anomaly 'cause there's just a few tunrs on the left side while the other side is the same as Can 1 (119).

Oh, can #2 has onlt 16-6758 stampled on it. Any clue there?

Can #2 has been replaced at some time, with a Meissner #16-6758

Okay, but...is there any way to determine what size the caps are inside - if you don't have a digital capacitor meter? I looked at Nostalgia Air and Meissner was a sub of Maguire Industries and I see nothing there except a couple of cans w/ 20/55mmf micas inside.

If this can is a replacement (although it does not look like a replacement looks original). Do I just try various size pf caps till one removes the firestorm inside and is tunable?

Does anybody have a Meissner IF Transformer 16-6758? I would like to get one.

I saw it listed here
http://www.oldradioparts.com/2a6fla.txt

Thank you all!!! That'll do'er! Play things of the Past! Okay!

Here is how you can determine the cap values without measuring the inductance.

viewtopic.php?f=6&t=190583&p=1591470#p1591470

I am still wrestling with this DeWald D519...http://www.nostalgiaair.org/PagesByModel/368/M0004368.pdf and the voltages are wacked out!

There's a couplate CRL - PC 70 - it has 4 leads. I can't firgure out which is which on the schematic? If it's bad would it - could cause such a mess??

Initially, I had a mica disease-type firestorm so I replaced can 1 & 2 capacitors. Now I have no sound from the speaker.
Voltages;
BE6 - pin 1/-.98 should be -10v
Pin 2 - 0 should be 0
pin 3 - -.008 should be 10v
Pin 4 - 55.5v should be 80v
Pin 5 - 55.5v should be 80v
Pin 7 - 53v should be -1
BA6
pin 1 -7.25 should be-1.1v
Pin 2 0 should be 0
pin 3 -.009 should be 0
pin 4 -.002 should be 20v
pin 5 - 53v should be 80v
pin 6 - 53v should be 80v
pin 7 - 53v should be 0
AT6
Pin 1 - .003 should be -0.7
pin2 - 0 should be 0
pin 3 - -.003v should be 0
pin 4 - -.001 should be 10v
pin 5 - -.002 should be -0.5v
pin 6 - 56v should be 0v
pin 7 - 51v should be 30v
50B5
Pin 1 - 5v should be 0
pin 2 - 12v should be 4.5v
pin 3 - -.012 should be 32v
pin 4 -.913 should be 78v
pin 5 - .294 should be 110
pin 6 - 58v should be 80
35W4
Pin 3 - no voltage
Pins 4 & 5 are soldered together??????????????????? erratic readiing - no reading
pin 7 113v should be 110v

Can you give me any clues as to what the heack is going on? Thanks.

JDPII,

Your voltages do not make sense.

Was your meter working properly? What sort of meter?

Are you sure you are counting the pins on the tubes correctly?

Were the measurements made while running on AC power?

If so, were the measurements for pins 3 and 4 of each tube AC volts?

You failed to record a value for pins 4 & 5 on the 35W4. This would have revealed the power line voltage. Was the power source erratic?

Most of your " should be " values are the same as on the chart on the last page of the linked Rider file, but not all.

Do you denote negative voltage measurements with two dashes, and negative should-be values with one dash?

Do the tubes light-up? If they do, the heater voltages may be actually correct. The tubes could not light-up correctly with the measured voltages shown. The heaters for all of the tubes are on pins 3 and 4. The Rider chart failed to mention that the values for these two pins are all AC volts when powered from AC.

Have you verified all of the coils and transformer windings for continuity? This particular schematic fails to show expected DC resistance values, but you can just use another AA5 schematic that does. Look for reasonably close values.

Generally, the audio output transformer primary may measure 200 - 300 ohms. All other windings will run lower than this.

There is no Couplate shown on the Rider schematic. Sometimes these were used in later production units and don't get into the documentation we have now. The circuit should be very similar; the parts are just "integrated" on a ceramic plate under the brown coating. You will have to see where the four Couplate leads go in the circuit.

Couplates in AA5 sets were commonly used between the 12AT6 plate and 50B5 grid. If this is the case, C8 on Rider's would likely be in the couplate. A bad C8 can mess-up your voltages, your 50B5, and your output transformer. The 50B5 grid 1, on pin 1, must not measure more than a fraction of a volt positive. If C8 is leaky, it can make this voltage go positive. That will cause the 50B5 to draw far too much current.

So check your windings for continuity. Then figure out what went wrong with the voltage measurements, and post those again.

There can't be all that much wrong with an AA5!

Ted

Thank you very much for your thoughtful insight and help! I plan to get back to working on the DeWald today after this ol' guy's daily chores...pickin' honey-dos, etc.

I have to admit that taking the voltages on this cheapo radio was kinda surreal. I knew it was way too bizaar, but I had to go thru with it.

The meter is very good @ Fluke...it ain't the meter it's me! I'm sure I was counting the pins correctly, but after my readings...I ain't sure. I mean, there's only 7 pins, right? I will do it again. Yes, the AC plugged in.

Pin value on 4 & 5 35W4 was unreadable (erratic) as they are solderd together and the meter was whacko & would not read the voltage.
Well, the 'should be' values was taken from the chart. I may have gotten mixed up while doing it, but I don't think so.
No, I denote negative voltage w/one dash...the other was just a dask to separation
Yes, all the tubes light up!
Dumb me, obviously took reading thinking DC when NOT thinking AC.
No I have not verified the coils...that' something I need to do along w/resistance values.

Nostalgia Air tube reference does not appear to be working..this radio has a 50C5 in it not a 50B5 which schematic call for, I have switched tubes to a B5 several times with no change. Are they interchangeable? I have also changed the entire line of tubes w/o any luck.

I will check the couplate and C-8 and continue in a forward direction!

Thank you!

JDPII,

No, the 50C5 and 50B5 cannot be interchanged. The guts are the same, but the pins are wired differently.
Be sure you use the tube the radio was wired-for.

You can trace the wiring to see which tube is correct. If the radio you have had a 50C5 originally, the wiring at that socket will not match the schematic that shows a 50B5. You can just pencil-in the correct pin numbers on your copy of the schematic.

Do not operate the radio with the incorrect tube. I think a 50B5 in a 50C5 socket might smoke the output transformer or cathode resistor. Or 35W4.

Do check the coils and windings with your ohmmeter before you apply power again. There is little point in taking voltage measurements if a coil is open.

Remember that some voltage readings must be made using the meter's AC voltage ranges. All readings on pins 3 and 4 of the tubes are AC voltages.

If your set uses a 50C5, the Rider voltage chart will have some of the voltages for V4 switched-around.

With a modern meter, you may need to add a series resistor at your probe end when measuring the voltage at pin 1 of V1. 47k or 100k would be good.

This is the local oscillator grid, and it has both RF and DC voltages. The meter can load-down the RF, which will change the DC reading. The Precision VTVM specified more than likely had a resistor in the DC probe.

Rider specifies two different VTVMs, one for voltage readings, the other for resistances. It is not clear why.

Is the chassis the actual circuit ground in your particular radio? The Rider schematic shows this to be the case, but the tube type change and use of the Couplate raise the possibility of other changes. If the chassis is not circuit ground, you should not use the chassis to connect your meter for voltage and resistance measurements.

To reduce the electrocution risk, manufacturers of AC/DC radios stopped using the chassis for the circuit ground connections, and added separate wires to connect the ground points in the circuit. The chassis was connected to this ground bus with a capacitor, and usually, a high value resistor.

The negative lead of the electrolytic filter capacitors should be connected to the circuit ground in either case. ( C10 on the Rider schematic.) You can measure the resistance between this point and the chassis to see if the chassis is grounded directly. Another check would be to turn the AC power switch on, then measure the resistance between both prongs of the AC plug and the chassis. A low reading means the chassis is used as ground. Do this while the set is unplugged, of course!

You can also trace the wiring. Do the points shown as upside-down Christmas tree symbols on the schematic connect to the chassis, or are they wired together to each other, but not to the chassis?

If your set has the separate ground bus, there will be a capacitor not shown on the Rider schematic, connecting the ground bus to the chassis. This capacitor should be replaced with a Y type safety rated capacitor of no more than 0.01uF capacitance, to minimize the hazard.

This thread has not touched on your safety while you work on the set. Old radios generally all have dangerous voltages inside. AC/DC sets such as this one may have the whole chassis connected directly to the AC line. For servicing AC/DC radios you should be using an isolation transformer. The isolation transformer is particularly important if you are using test instruments while working on the radio.

Great caution must be exercised at all times while working on the radio. If you have not done this already, you need to read-up on the safety precautions you should be taking. We don't want you getting zapped!

Ted

Ted, I have to post a reply here to say "thank you so very much"! I am a 'lil-2-moderate under the weather and have been all week. My concentration level stinks!

I plan to pick up this restoration project early in the week. So please don't leave...like ol' Doug MacArthur said, I shall/will return!".

I am coming back to this DeWald. It's been sitting since the early summer.

Without reinventing the wheel...is there any way to determine the size of the mica capacitors that 'were' in cans 1 and 2? I have replaced the mica caps in both cans and am using 100pf caps on the outside. I get a low decible hum.

Is there a way to know what size these caps should/need to be? Thanks.

JDPII,

What test equipment do you have that you can use?

Something like an RF signal generator, and a 'scope, or RF voltmeter, would be useful.

Have you checked the resistance of all the coils and windings in the radio?

Ted

What I usually do in this case is get a couple of 365pF miniature variable caps, the kind you can get form an old transistor radio. Tack solder those in, they nicely line up with the pins. Then you can set your signal generator to 455Kc, couple it to the antenna, turn up the signal level, and then juggle the temporary trimmers for best volume. Most of those variable caps are like 25 to 365pF, and linear too, so you can guesstimate the capacitance from the amount the plates are meshed. They'll probably be somewhat less than half meshed, which suggests a value of around 150 pF. Then you can try substituting a real fixed cap for one of them and see how close that is and if you can adjust the core to peak up that resonant circuit. Repeat with the other coil. It's not as hard as it sounds.

Mica's will not cause these type of voltage problems , you might make sure you have Volt meter ground wire hooled to B- not chassis , then test voltage on plate of audio output tube
edit Looks like your B- is chassis and the schematic does not show any mica's inside of IF cans , hmmm

Daniel Hilderbrand,

Yes, the older posts included some seriously whacko voltage readings. Almost certainly some sort of measurement problem.

The OP was concerned about silver mica disease because of "a firestorm"; I presume he meant that he heard severe static in the speaker.

The radio dates from the SMD era, and the OP mentioned a "119..." part number on one of the IF cans. I think the schematic may be slightly off. The actual set has a couplate in it, and the Rider schematic shows discrete components only. The schematic may be for an earlier version than the OP's radio.

Ted

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