National SW-54 Thoughts
 

I think I'm actually on the right track with this one as is, but I wanted to get some thoughts from anyone who has more experience than me.

I'm rebuilding a National SW-54 shortwave radio. It's a pretty standard restoration so far, but this set has a few major issues. The cheater cord on the back was physically melted, and the shunt resistor across the rectifier tube was burned in half. As near as I could tell, this was due to the set being run with dead electrolytics for long enough to cause a meltdown. There's a scorch mark on the underside of the cabinet where it failed.

Anyway, I've replaced most of the caps and rebuilt the power supply. I picked up a kit from Hayseed Hamfest and apparently my set is a later revision with some additional .002 uf caps between the antenna inputs and one of the coils. No biggy, I've done the rest and those won't impact a basic power up.

Well, anyway, I got it to come to life for a moment, but it died again just as quickly. Checked under the chassis, and there's two resistors in series. One goes to a ground connection through Pin 1 of the 50C5, and the other acts as a bridge between two of the filter cap sections. The one providing the bridge was smoking and hot to the touch (my finger proves it, oh well) so I measured both. Bridge resistor (looks to be a 1K from the schematic) looks to be in tolerance, but I'm getting some odd behavior from the one going to the 50C5. It measures 1K when the set is cold (should be 15K if I'm reading the schematic right), but I noticed that the resistance dropped to 0 when I applied power.

A dead short to ground would explain the smoking 1K resistor, and probably my dead reception as well. I mostly wanted to know if having a resistor become a dead short when power is applied sounded like something anyone has any experience with.

I'll attach the schematic as well, although it's not a perfect match to my set. On mine there's an additional resistor between the 1K bridge and the 60uf filter section that I can't seem to find on the the diagram, and the resistor that's marked as being in parallel with the C1D section coming off of Pin 1 on the 50C5 is in series with it instead. I'm assuming mine is a different revision since it also has a network that isn't listed here, but I'm no expert.

Since both resistors were getting hot, I have parts on order to replace them. I'm hoping removing that wonky 15K will get me back up and running. I have to believe the meltdown I mentioned previously took that out as well.

Okay, a couple corrections.

C1 D is in parallel with that resistor. It didn't register with me that that line is just a straight bridge to ground through the filter capacitor on the other end of the chassis.

The reproduction electrolytic can from Hayseed Hamfest used stranded wire that was too thick for the holes in the tub socket terminals. I ended up cutting half of the wires off to make it fit and feeding that through. In poking around tonight, I discovered that a single strand on the 60 uF section had separated from the rest and was hiding underneath, shorting that section to ground. I've removed that, but now I have to see what damage that caused.

I did try powering up since then, and it looks like the resistors are no longer overheating. I still plan to replace them, of course, but I'm now reasonably confident that that short was the source of my issue.

Basic symptom at the moment is I have faint hum through the speaker (not filter hum so much, more like volume up too high faint distortion) but no signal besides a faint chirping every once in awhile on all bands. So I'll have to figure that out. I plan to check my B+ voltage coming off the rectifier tomorrow (I think that's going to be 115VDC coming off of pin 8 on the 35Z5, but correct me if I'm wrong on that) and trace it up to the 50C5. It looks like it has to go through those resistors that were overheating, so I still have some hope that that's the source of my issue.

I'm still trying to wrap my head around the resistor pattern coming off of pin 8 on the 35Z5. It doesn't match what I'm understanding from looking at the chassis. Pin 8 has two resistors. One is definitely the 15K ohm going to Pin 1 on the 50C5 and connecting the the D section of C1. The other (the one that was burning hot) is connected to pin 8, but goes off to a terminal strip, hooks onto one of the 40 uF sections on the electrolytic can, and then runs through an additional resistor to the 60 uF section in the can mounted to another terminal on the strip. At least that's what I think is happening, I'll retrace it tomorrow to double check.

There's also some stuff wired to pin 1 of the 35Z5. I don't even see that pin listed on the schematic. I know it's factory, as far as I can tell the underside was untouched so there must be some reason, but I'm not seeing where all that is happening. Is Pin 1 on the 35Z5 a no connect internally? The metal is present but perhaps a look at the tube diagram will clear that up.

I'm just spitballing here. Hopefully some of this makes sense. I'm sure another glance over the layout will clear some of this up, but in the meantime any thoughts are appreciated.

Okay, yep. Pin 1 on the 35Z5 isn't connected to anything internally. That threw me off. Pins 4 and 6 are also unused as well, but they didn't include any physical pins for those. I figured that meant that any pin that was present was connected. That may very well clear up some of my confusion.

Wow that's a lot of info...

One thing that jumps out to me is your statement that resistance of a resistor changes when power is applied. How did you determine that? If you left your DMM in resistance mode connected across the resistor and powered up the set then you are making a BIG mistake....DMMs measure resistance by either sending a fixed current into the resistor and measuring voltage drop or sending a fixed voltage into the resistor and measuring current flow....If the radio is powered and sending power through the resistor the resistor then has an additional power source across it that changes current flow and voltage across the resistor (which the DMM don't expect)....Best case scenario that power the meter doesn't expect makes your reading substantially inaccurate and worst case scenario the voltage and current it sends into your meter destroys the DMM...
I've had the later happen by accident multiple times...I tend to switch between measuring resistance with the set off and voltage with the set on and sometimes forget to change mode on the DMM.
If you like your DMM and want accurate readings NEVER measure resistance of a powered resistor!

Okay, thanks for the heads up. I think that may be my issue. The multimeter is fine, at least. I've tested it since then and everything is behaving as far as I can tell.

I'm not seeing any voltage on my B+ (Pin 8 of the 35Z5 if I'm correct on that) and I think a few of my electrolytic sections have opened. I read that you can check electrolytics with a multimeter by reading resistance across them. If they spike once and go down to OL very quickly, they're good. If they retain value, they're bad. Two of my sections are retaining value and two are dropping, so I'll just go ahead and replace the whole thing and those two resistors.

Of course, if anyone else has any thoughts or that all sounds completely off the mark, please let me know.

I would also check continuity (measure the ohms resistance) of the pilot lamp, the 330ohm resistor and the heater pins of the 35Z5 2-3, 2-7 and 3-7.

jr

;) Absolutely 101% not true ....

The only infallible test for an electrolytic capacitor is to either see it work in it's circuit without it or it's other associated components getting hot , or to test it using a capacitor tester that tests it at the actual voltage it's gonna see when it's in use . A cap that won't break down at the tiny (less than 9 volts) voltage your meter applies to it for the test could very well break down at the 100 or so volts your 35Z5 is gonna feed it .

My personal policy is "take no prisoners:pistols: with electrolytics , if I'm gonna get a piece of vintage tube electronics running , the first thing I do is to replace them before the first power up .

These are replacements. The concern is that the positive of the 60 uF section was shorted to ground, which also has a direct connection to the negative of the same side. I know it caused the 1K and 15K resistors to overheat, and I'm concerned it may have also damaged the electrolytics.

I've ordered a replacement set for safety's sake. I'll try substituting that so I know those and the new resistors I ordered are good. I'll have to start doing continuity tests from there.

I know the pilot lamp and heaters are good because the lamp is still lighting and the series string is all coming on. So the problem is somewhere further into the set.

Does anyone have any specific ideas on checking the B+? Am I accurate in measuring between pin 8 and ground to see if that's coming off of the 35Z5?

Okay, I've got no B+ coming off of pin 8 on the 35Z5. For sake of testing, I've gone ahead and ordered a spare tube. My heater should be good since my tubes are all lighting and the filament tested with continuity, but I checked it in my tester and it came back as no emission so perhaps that's my problem?

I want to verify that my voltmeter is good, though. I know the resistance checker part is fine, but to be safe I'm going to check the voltage on the heater pins. That should give me a good indication on what's going on.

Any other suggestions? I have a new filter cap set on the way to see if I burned out the old one, and the new resistors are installed and not getting hot. This has to be a power supply problem, whatever it is.

Okay, now I'm really confused.

I used the voltmeter function on my multimeter (on AC setting) with the positive lead on Pin 7 of the rectifier, and the negative lead on ground. 0 volts. I then moved it to the negative lead to pin 3 on the 50C5 and measured voltage between pin 7 on the rectifier to pin 3 on the 50C5. That should be filament. I know it's lighting. I get hum from the speaker and there's a pop when I turn the bandswitch, plus the dial light is lighting. But 0 volts on the filament!

Am I just using this wrong? Maybe my meter is dead after all? I can't believe I'm not getting any voltage there.

Oh, and I almost forgot. I did check this. Each pin set showed continuity.

Try between pin 3 and 4 of the 50C5, those are the heater pins.

jr

Voltage or continuity?

Also would that heater be bad if I'm getting sound from the speaker, even if it's just a very low hum?

Either one... continuity with the tube out of the radio, or voltage with the tube in the socket and the radio turned on.

not likely.

jr

Gotcha. I'll test both.

Series string rectifiers typically have a dial lamp heater tap...the heater can go open on either side of the tap. During WWII when replacement tubes were scarce if the section of rect heater in parallel with the dial lamp blew (likely because the dial lamp died first) instead of changing the rect they would install a higher current dial lamp to allow it to soldier on with a poor tube.

If it is AC voltage scale you want to check on your meter just stick the probes into the wall outlet and you should see somewhere between 110 and 130VAC...Just check your positive lead is in the voltage connector and not the current before jamming it in the outlet. The current jacks on a DMM typically have a short (thick 1ohm piece of wire) between them...I killed my first meter by measuring line voltage with the probe in the 10A current jack...the meter leads melted, but the breaker never tripped.

DC voltage I typically check with a 9V battery. Remember to have the meter set to AC voltage if checking heater and DC voltage if checking B+...If you get that backwards you'll get near 0 volt readings.....Some folks prefer to measure voltage with a DC coupled oscilloscope because it will measure AC and DC at the same time.

I'll have to pump the brakes on this. My multimeter isn't working properly.

I'm showing 0 VDC across multiple brand new 9V batteries, and nearly 0VAC across known good wall outlets.

I'll order a new one (eventually I need to get my Heathkit VTVM restored but it's nice to have a digital one as well) and wait for that to come in before moving on.

Okay, so I'm back at it with a new multimeter. This one has a capacitor tester, which will hopefully make my life a little easier.

So I did a few voltage tests, and my VDC on Pin 8 of the 35Z5 looks okay. It's actually a little higher than what they listed in the Sams. So as far as I can tell, I have B+, but my signal is getting lost somewhere.

Does anyone have any thoughts on where I should be looking next? I tested voltages around the 35Z5, and they all looked okay, so I'm thinking this isn't a voltage problem.

I did notice that my capacitance readings on C18 and C19 look off, but I'm also not sure if those can be reliably measured in circuit or not. This is all with power off, of course.

Where would you all start looking?

Voltage and resistance measurements at the tube sockets and compare to the charts ?

jr

Gotcha. I sort of started that but I'll go through and get readings for each pin on each tube.

I did notice that my set is a different revision from the SAMS. It has a network where the one SAMS listed doesn't and the position of the 60 and 40uf sections of the filter can are swapped. Mine came with a schematic that shows those revisions, but it doesn't list the pin measurements for reference.

Do you think, barring any other information, the SAMS info will be safe to go off of for this? I can list any variations, I know tube gear tends to have a pretty wide tolerance.

Likely the parts in the network are very similar to the discreet parts that the network replaced... I suspect that the Sams charts would be close.

A more basic question... as the set warms up, do you hear anything from the speaker? a slight hum would be normal also slight static as the volume control is turned up and down. :scratch2:

jr

Yes to the slight hum and maybe a little static, and it clicks when I switch bands, but that's about it.

So it would seem that the speaker and audio stages are at least alive... Have you tried a fairly long antenna wire, perhaps 20 feet or so, to perhaps get some signal through the converter and if stages?

jr

Okay, sorry for the delay. I work in healthcare research so things have been crazy.

Anyway, I grabbed voltage and resistance measurements. Here's the breakdown:

V1:
1: 24.5K | -9.3 VDC
2: 2.1 Ohms | 0 VAC
3: 14 Ohms | 13 VAC
4: 26 Ohms | 25 VAC
5: 16.9K | 107 VDC
6: 16.9K | 107 VDC
7: 3.7M | 0 VAC

V2:
1: 3.1M | 0 VAC
2: 1 Ohm | 0 VAC
3: 26 Ohms | 25.7 VAC
4: 37.5 Ohms| 39 VAC
5: 16.9K | 107 VDC
6: 16.9K | 107 VDC
7: 182 Ohms | 252 VDC

V3:
1: 13.7M | -.6 VDC
2: 2 Ohms | 0VAC
3: 14.5 Ohms | 13 VAC
4: 1.7 Ohms | 0 VAC
5: .657M | -2.3 VDC
6: .657M | -2.3 VDC
7: 242K | 69 VDC

V4:
1: 300 Ohms | 9.5 VDC
2: 447K | .02 VAC
3: 85 Ohms | 90 VAC
4: 37 Ohms | 40 VAC
5: 447K | 0 VAC
6: 16K | 115 VDC
7: 15K | 123 VDC

V5:
1: 16.24K | 115 VDC
2: 112.8 Ohms | 121 VAC
3: 110.3 Ohms | 117 VAC
4: NC | 0 VAC
5: 110.3 Ohms | 117 VAC
6: 15.2K | 127 VDC
7: 84.8 Ohms | 90 VAC
8: 15.2K | 127 VDC


I'm noticing a few wonky resistance measurements around the 50C5, and pins 5 and 6 on both V1 and V2. Any idea where I should be looking next?

When I first powered it on after my recap, it was pulling in a station (I have an in-home transmitter) that gradually faded away within the first few minutes of it being turned on. Whatever burned out happened within that time frame. I'm looking to get it back to that point.

Couple of observations:

There seems to be a note missing in the Sams... the ohm table shows a tag for V1 and V2 pins 5&6, V3 pin 7, V4 pins 6&7. I suspect that these resistances should be measured to the 35Z5 cathode (pin8) and not to ground. With this assumption, your “wonky” restance readings look reasonable.

252 volts on V2 pin 7 can’t be correct or the 100 ohm cathode resistor would be smoking. I would double check the voltage and resistance values at that pin, and around that tube. Perhaps the problem is related to the area around V2. :scratch2:

jr

Okay, I rechecked Pin 7 on V2. I wrote it down wrong.

Today it was hovering around 230 mVDC. I forgot to write down the M when I was taking notes. The resistance is measuring the same, though.

In looking at the schematic, that one may be a little too low if anything. Could that difference cause my audio to disappear?

While I suspect it will not be the big red X that we are trying to find, I would go ahead and replace the 100 ohm resistor in the cathode connection of V2... 182 ohms is likely too far off for good performance.

jr

Got it. I'll put one on order and see if it helps.

Okay. I've swapped out the questionable resistor. Not hearing any change.

Any thoughts on what I should check next?

Have you tested/substituted all tubes?

jr

I have not. I ordered some spares a while back and they've been delayed. That being said, since this is an AA5 for all intents and purposes, I might could borrow some known good ones from my AM radio and see if that has any impact.

I think I have a signal generator set up somewhere. Would there be any value in checking my audio section with one? I know my speaker output is good since I'm getting some audio hum from it, so it has to be a breakdown somewhere in the signal line, right?

I thought that we verified that the audio stages were working...do you have a RF generator? Swapping tubes from a working set seems like a good idea since your shipment of new tubes has been delayed.

jr

The audio stage is humming, but I'm not getting any signal through. I can get a video of the noise if that will help.

And yeah, I'll pull some tubes from working sets to see if that makes any difference.

Went ahead and created a video: https://www.youtube.com/watch?v=Nyrr8iUFRzE

I also tried new tubes for everything except the 35Z5. I don't have any spares of that for now, but it also doesn't seem like the problem is in the power supply.

I don't believe I have an RF generator, but I do have a digital signal generator I could set up. I haven't used it before but it looks like it might work for running some tests.

Again, I think the audio section is ok... I hear slight hum which changes with volume control rotation... some volume control scratch ... hum increase with body capacitance... noise from band switching. Do you get noise or change in noise as you connrct a wire to the antenna terminal?

What is the frequency range of the signal generator? can it generate a square wave? (rich in harmonics).

jr

Looks like it can, yeah. I'll have to do a little research into the specific functionality (It's a sort of combo oscilloscope and signal generator) but the interface definitely allows for square waves.

Never seen such a unit, what model is it? Does the oscilloscope have sufficient bandwidth/sensitivity to display RF/IF signals from the radio?

jr

https://store.digilentinc.com/analog...-limited-time/

One of these things, but I've got an adapter to give it BNC connectors for the Scope functions. It plugs into the computer for the display.