National SW-54 Thoughts

[Electronic M]

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.

[Dubis7]

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.

[Dubis7]

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?

[jr_tech]

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

jr

[Dubis7]

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.

[jr_tech]

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.

jr

[Dubis7]

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

[jr_tech]

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

[Dubis7]

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?

[Dubis7]

Originally Posted by jr_tech

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

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.

[jr_tech]

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.

jr

[Dubis7]

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.

[Dubis7]

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

[jr_tech]

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

[Dubis7]

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