NC-183D engineering error? HRO-60?

[W1VD]

Just got in a very nice unmolested NC-183D receiver. While prepping the receiver for performance measurements, one step is to verify voltages at all tubes per the manual. Measurements starting right with the 1st rf amplifier and continuing through the i-f were significantly off...even though the receiver did function. The reoccurring theme going stage by stage was high plate voltage and very low screen voltage. In each stage the 47k ohm 1/2 watt screen dropping resistors were at fault. The resistors measured anywhere from 120k to 2.9M! Replacing the resistors with new brought each stage back close to factory spec voltages. Virtually all other resistors in the receiver were close enough to not require changing. Could the factory have gotten a bad batch of 47k ohm resistors?

Later I ran across a post by Rodger WQ9E in a different thread commenting on bad 47k ohm screen dropping resistors in his receiver. Coincidence? Using factory voltages the dissipation for each resistor was found to be as follows:

NC-183D

1st rf	.42 w
2nd rf	.42 w
1st i-f	.31 w
2nd i-f	.31 w
3rd i-f	.33 w
phase inverter (plate)	.28 w
phase inverter (cath)	.28 w

Common practice (at least nowadays) is to limit dissipation to 1/2 the rated value. One would guess that the resistors started life as 47 k ohm and over time got 'cooked' driving the values unusually high. Pix of the resistors below.   

This got me wondering about the HRO-60. Again, using factory voltages the dissipation for each resistor was found to be as follows: 

HRO-60

1st rf	.54 w
2nd rf	.26 w (100k ohm)
1st i-f	.10 w
2nd i-f	.31 w
3rd i-f	.31 w
phase inverter (plate)	.10 w
phase inverter (cath)	.10 w

Think I'll go back and take another look especially at the 1st rf stage in the HRO-60!

The 183D did work as received...however the changes made a significant difference in performance. A few other minor repairs were required. Next up is an alignment (although it's probably pretty close as is) followed by a trip to the 'torture rack' for receiver performance testing.

A bunch of 47k ohm 1 watters are on order...

   



 

             

[Jim, W5JO]

What you found is typical of both receivers.  I have had two of each model and found that other resistors have changed significantly like the screen resistors.  The 183D that I have now didn't have that many bad screen resisors.  Another problem is that if you try to unwind the wire around the tube socket pins, they are easy to break. 

Watch out on any National product.

[The Slab Bacon]

What you found is typical of both receivers.  I have had two of each model and found that other resistors have changed significantly like the screen resistors.  The 183D that I have now didn't have that many bad screen resisors.  Another problem is that if you try to unwind the wire around the tube socket pins, they are easy to break. 

Watch out on any National product.

Yea, what he said! ! !

I have run into the same problem on jst about EVERY National receiver from that era that I have worked on. Either the screen dropping resistors, and/or the fuzz inverter plate and cathode resistors are trash. I have also found similar problems on NC300s as well. they look fine (not burned or cracked) but they go way high in resistance.

                                                           The Slab Bacon

[k4kyv]

Another problem is that if you try to unwind the wire around the tube socket pins, they are easy to break. 

I assume "they" means the socket pin.

That's why I always just poke the wire through the hole in the pin and maybe form a hook and slightly crimp it before soldering.  None of that "firm mechanical connection before applying solder" B.S.

I have seen some factory construction where they wrapped each wire 5-6 turns.  I don't know what they were trying to accomplish.  Maybe assembly line connections tended to fall apart before they reached the person who did the soldering, but one or two wraps would have easily been enough.  I usually  don't bother to unwrap the wire.  Just clip off the old one and solder to the pin, wrapped wire and all, if there is enough room.

The screen dropping resistor probably handles more power than some of the others, so it might be more prone to change value over time.

[Jim, W5JO]

Brian I don't know why you have sensitivity problems with either the 183D or HRO 60.  I need about -105 db to hear a discernable signal on my 183D and with the RF gain fully on, I see -90 to get S9 on 10 meters.

Maybe you have an IF section misalignment?

[WU2D]

The ones that I have seen that can really get you in trouble are the feed resistors to the oscillators. Lack of HFO injection and a weak BFO are two common complaints. These resistors typically handle more juice and they drift up even more.

I found an ARC-5 with 20 volts on the BFO tube. It actually still oscillated! but the BFO was weak to say the least. 

Mike Wu2D

[WQ9E]

A properly working NC-183D definitely has plenty of sensitivity for even a fairly quiet location.  Page 15 of the manual provides stage gains for the converter and IF stages so with an output meter and/or scope and signal generator you should be able to narrow down the problem area pretty quickly. 

After I took care of the screen resistors on mine the only remaining issue was low sensitivity on the top range which was due to insufficient oscillator injection.  That took a while to find but it turned out to be a kinked wire on the oscillator coil for that band which apparently reduced the Q a bit.

[w3jn]

Several 50;s national receivers I've had (183, 183D, HRO) have had 22K resistors actually all read 220K.  I suspect National might have had an issue with its supplier.

[Jim, W5JO]

I have never been a fan of IRC resistors.  If memory serves, that is what National used.  Back when I used the new ones, I found them to be very loose in the tolerance department compared to Allen Bradley.

[W1VD]

Closer view of the faulty NC-183D 47k ohm resistor next to a new AB. The 183D 47k resistors have a rougher molded body surface and the color bands are not as bright and as well defined. Some of the resistors in this 183D are AB looking types though. Wonder which type is in your receiver Brian? The 2.9M resistor was on the 2nd rf amplifier.





 

[k4kyv]

I have never run across one, but some metallic film resistors are said to be moulded into a  case that looks identical to carbon composition resistors.  I have found film resistors to be very fragile and susceptible to self-destruction from only moderate overloads - perfect companions for solid state devices.  Metallic film fuses might be a better description.

Maybe someone replaced the original carbon comps with those films, which later blew.

[N3DRB The Derb]

I'm going to look hard at the resistors in my RAO-7 when I get to it - one thing I did notice is that they are very very large for 1/2 watt size. closer to normal 1 watt sized. (I've never seen a schiz of it, so I dont know anything about what might be going on.)

[Jim, W5JO]

I have never run across one, but some metallic film resistors are said to be moulded into a  case that looks identical to carbon composition resistors. 

Some years back the body color of a resistor denoted what kind it was, blue, green, beige, etc.  Has that been abandonded or does it still stand?

[W1VD]

Okay...now we're making some progress...

First, as I noted earlier this is an unmolested receiver except for a couple black beauties. All of the resistors are (were) original with original solder joints intact. There is a mixture of the 47k looking resistors and AB looking resistors.

Rechecked the resistors and one of them indeed measures 2.9M, Brian. Looks like your receiver got the 'good' resistors.

I cracked open a standard AB resistor and it is shown on the left in the pix below. The core is a solid carbon slug as one would expect. Cracking open the defective 47k, shown on the right, presented a problem as they tend to disintegrate. The core appears to be thin and hollow. Between the core and the outer coating is a chincy filler material. These resistors are considerably less robust than the AB types...which probably explains why they went so high in value with the power they were dissipating. Other resistors of this type that are not dissipating much power have retained their value pretty well.       

 

[Tom WA3KLR]

My dad's NC-183D I worked on had 2 distinct vendors of the 47k resistors.  The one vendor's resistors were all o.k.  The other vendor's 47k's were all bad.

When I overhauled my old Heathkit HW-101 it had many resistors high in value.  Many of them turned out to be under-rated in wattage when I measured the voltage drop with the new resistor and did the math.  So  I had to replace many of them again with higher wattage.

[KM1H]

In my experience it wasnt only National. Ive had bad 22K, 24K, 47K especially across the board. Even in a 75A2 there were a few.

In the last HRO-60 I did about 3-4 months ago I replaced 18 resistors if I remember correctly. After that a SX-42 was about as bad as was an earlier HQ-129X. Toss in several Zeniths, Philcos, Silvertones, etc and all have similar problems. Between my own and customers I do typically 1-2 radios a week.

Ive found many radios are quite sensitive to tube quality. Using only NOS tubes I could see a big difference on the output GR meter. I selected the best performers and easily exceeded stage gain specs on the HRO-60 for example and using 10M for the RF and conversion stages. I went thru maybe 4-5 6BE6's in the first mixer. a couple of 6BA6's, and even a few 6SG7's in the IF.

Also note that socket voltages are specified at a 115VAC line.

As far as breaking socket pins there are plenty of spares to choose from   I snip the component close to the socket or tie strip and carefully unwrap the lead. Never cared for the TV repair shop look of splicing. Thats fine if its a S-38 but not in a real radio.

Carl
KM1H

[k4kyv]

I have seen real carbon composition resistors modified to make higher values by filing notches in the side of the resistor, through the case material, to remove some of the carbon material from the slug.

[KM1H]

Ive done the filing a notch thing many times Don when maintaining original appearance. Notch to desired value, a bit of Bondo and repaint which is real easy on the 30's dogbones.

I always save the pulls from radios that get carbon or metal films.

Carl
KM1H

[KM1H]

I vote design error on this one…

Why?

Carl
KM1H

[W1VD]

Using factory voltages the dissipation for each resistor was found to be as follows:

NC-183D

1st rf      .42 w
2nd rf      .42 w
1st i-f      .31 w
2nd i-f      .31 w
3rd i-f      .33 w
phase inverter (plate)      .28 w
phase inverter (cath)      .28 w

A bit much for 1/2 watt resistors considering they're in a receiver that runs 'continuous'...no?

[KM1H]

Calculations are fine but did you actually measure at the required 115VAC?

The reason I ask is that after rebuilding the last HRO-60 and wondering similar about the resistors I did the old touch test on both RF stages with the AC at 111V thru a bucking transformer. The 1/2W replacements were barely above room temperature and IMO operating well within ratings.

As I mentioned earlier I replaced 18 out of tolerance resistors and many had no appreciable current thru them.

Of the five 47K RF and IF screen droppers I just measured them at 59K, 69K, 114K, 325K and 1.3M. Should I assume that the 2 highest came from the RF amps? Was the increase due to DC or RF heating or both?

Before Id run out and blame engineering remember that they had no control over the vendor. National was as cost conscious (cheap) as Hallicrafters and the Purchasing Dept scraped the bottom. I saw the result of that thinking in other products while at National.

During the years I worked in the Service Dept it included hundreds of postwar receivers in the NC-173 to HRO-60 era which all used similar vendor components. Except for resistors cooked by leaky caps it was a non issue at the time when some of the sets were over 20 years old. That certainly disproves any reliability guessing.

What happened to components during the next 40 years in no way can be blamed on engineering.

Carl
KM1H
National Radio 1963-69

[K1ZJH]

Closer view of the faulty NC-183D 47k ohm resistor next to a new AB. The 183D 47k resistors have a rougher molded body surface and the color bands are not as bright and as well defined. Some of the resistors in this 183D are AB looking types though. Wonder which type is in your receiver Brian? The 2.9M resistor was on the 2nd rf amplifier.

ABs are the pick of the litter. Almost all carbon comps go high in value with age. Those particular resistors with the rough finish are especially prone to going high or being found open.  Most of us that restore antique receivers from the 30s and 40s routinely check every resistor for value.  Wax caps are either replaced or rebuilt with new components hidden inside. Engineers never anticipated that a 1930s communication or home appliance would still be used 60 or 70 years later. 

Pete

[K1ZJH]

Almost all carbon comps go high in value with age.

Yes, if used in a HV circuit for long periods of time. I dug around a found two that looked just like the ones I pulled out and they still were within 1 to 2 percent of 47K. They were probably just as old as the ones I pulled out, but they had never been used.

It’s just going to be worse when the ratings are borderlined for the circuit they are in.

No, actually they will, and do, go up in value sitting a drawer, New Old Stock. They aren't stable devices.

A few isolated examples that have held value doesn't prove much.  High voltage may affect them, but
how much use have many of these radios over the past 60 or so years?? Most overheated (discolored resistors
tend to cook downward in value, in my experience.

Allen Bradley manufactured resistors are generally more stable than those of other manufacturers.
This subject has been beaten to death on other Forums. Here is one recent discussion:

http://antiqueradios.com/forums/viewtopic.php?t=115577

and a good website for further reference:

http://www.cliftonlaboratories.com/carbon_composition_resistors.htm

I've pretty much abandoned carbon comps and now prefer to use carbon film and metal oxides.

Peter k1zjh

[WZ1M]

After readind all this. I just happen to have a Collins 75A-4 on the bench and changing out the 7 deadly caps along with other bad caps. I snipped one side of a few screen resistors and guess what, two to four times there normal value. Looks like a good project for tomorrow.
Regards,
Gary

[KM1H]

Repeated photos in the forum body just waste bandwidth for those on dialup. If its of any use to the thread please make it as an attachment.

Carl
KM1H