The AM Forum

I have an nc-183d that runs very hot.  If I keep the lid down after an hour, the transformer is too hot to keep my hand on.  I have replaced all electrolytics, paper caps.  I even popped in a 5r4 rectumfier sintead of the 5u4 to lower the B+ and take a few watts of heater current away.  The HV does measure 260V as it should.  The heater voltage is closer to 6.5V instead of 6.3V.

The bias is very high(low).  It should be around -39V, but I am getting -55V.  Is this too much?  If so, how can I decrease(increase) it to -39V.  The two 1K resistors do read 510 ohms together.

Otherwise it works great.  Also the 6v6GTs read a little weak on my tester, but it gets plenty loud

thanks

Tom - AB3FL

Tom,
       after having several 183s and 183Ds They all seem to do that. Apparently the power transformer National used was a bit undersized for the job at hand.
If you have recapped it and dont have any leaky caps sucking the life out of the power supply, I'd say this is somewhat normal for them.

the increased biass could just be from slightly aged 6V6s. Or it could be caused by excessive draw on the B+ line. IIRC they have a voltage divider on the B- line to create the biass voltage. If it has been recapped and the values of the critical resistors checked. (that generation of Nationals is known for crappy, out of tollerance resistors) If all else is good, consider it normal.

The 2 that I have now (1 183 and 1 183D) both have replacement power transfomas in them. Noticably larger than the originals. So have the majority of them that I have repaired for other people. IIRC, this is a rather common problem.

Just my $.02 worth

My 183D also has a replacement transformer.  As long as your are sure C98,99, and 100 have all been replaced I would also consider it normal.

Thanks!   I will try new 6V6s and maybe lower the resistors from 500 ohm to maybe 400 ohm and see what I get.

Here is the transformer

Tom - AB3FL

How uncanny! ! ! ! !  That looks exactly like the transfoma in my 183D!

Another thing I noticed on the schizmatic is that the screen grids are about 5 volts higher that the plate.  I though it was bad to have the screen at a more positive potential than the plate.  Also on the voltages page on the manual there are a lot of voltages that are positive but when I measure them, they are negative.  For example the diagram says 19V on the cathodes but I have more like -29V on them (6V6s).  IS there a proper voltage diagram anywhere?

Tom

The screens being a few volts higher could just be a product of low emission in the tubes. (the screens are just not drawing enough current to get enough drop out of the screen dropping resistors.)

But the cathodes being negative is a bit odd........ IIRC they have cathode biass resistors which should place the cathodes at a positive voltage. (making the grids more negative in respect to the chassis ground and the cathodes as well.) If you have a negative voltage on the cathodes this makes the rest of the tube's elements more positive.

First, I would verify your meter and the pins that you are checking the voltage on. If all is good so far, I would closely examine the wiring for the "new" power transfoma and make sure that the wiring isn't "hamboned".

The bias scheme for the NC-173, 183, and 183D will provide a negative output tube cathode bias reading when you read from terminal to ground.  I think they just left out the minus sign by accident.  In this area, the NC-183 is very similar and they specify -19 volts between tube terminal and ground.  The NC-173 uses a similar circuit but with a single output tube and specifies a reading of -21.5 to ground.

Measure the actual resistance and then the voltage drop across R-66 and this will let you calculate the actual idling current for the 6V6 stage to see if it is normal.

The screen voltage is going to be a bit higher than the plate; full B+ is fed directly to the screens and the center tap of the output transformer so the plate voltage is going to be reduced by the resistance of the transformer winding.

I would have to look but that power transformer looks mighty familiar.  I thought mine was a replacement but maybe National built some with this style transformer.  Carl???

If the caps have all been changed, I'd consider adding a line bucking transformer.
Since you have 6.5VAC on the filaments adding a line-bucking transformer is a good
avenue to consider since you have a bit of headroom!

I'd buck the voltage down using a 2-amp 12-volt filament transformer and go from there.  The radio and transformer will run a lot cooler, and you will notice no
difference in performance.

Peter

Peter,
         that is a very good idea, but.................. He has a problem somewhere if his voltages are that far out of whack. Once he figgers out what is wrong with it, that will be a viable life-insurence policy.

Tom,
       While I'm thinking about it, take a real good lookie at where they tied the center tap of the transformer secondary to. They could have tied it to one of the cathodes of the outpoots thinking it was grounded.

Looking on the schizmatic, The B+ goes to the screens with no resistor then to the center tap of the outpoot xformer then to the plates.  This would cause the plates to be slightly lower than the screens due to DC resistance in the outpoot xformer.  If you follow the B- line which comes from the center tap of the power transformer, it does go to the cathode and the control grids.  I also notice that if I rotate the RF Gain, the amount of B- changes from about -54 to -24.  The RF gain pot measures fine

Tom - AB3FL

I dont have one here at work, but look closely at the skizmatic and take notice of what the CT of the powa transfoma is supposed to be hooked to. That doesn't sound right, but it's been some years since I have looked at the underside of a 183 or D. I just dont remember how the biass divider was hooked up.

The biass changing with the adjustment of the RF gain is being caused by the increasing / decreasing current flow through the biass divider as you vary the biass on the RF tubes. This could be an indication of something not wired properly or a defective part in the biass divider string.

The screens running a bit higher than the plates is normal in many sets. The plate voltage
is lower because of the voltage drop is due to the resistance in  primary winding off
the audio transformer. Usually the screens and the B+ side of the audio primary
are tied together.
 
Higher than normal bias when using back bias or cathode resistance bias usually
indicates weak tubes. Swap in a NOS pair and see what happens.

Pete

I found some 1614s and popped them in.  The voltages are better.  I guess I need to get new 6V6s


Tom

1614s Ought to rock and roll! They are basically 6L6s..............

Weak tubes will cause all of the voltages to be off some, but not the grid and cathode being off that far. Unless, maybe the tubes are totally piss-beat and spent.
The negative voltage on the cathodes still doesnt sound quite right to me. But I just dont remember the audio outpoot circuit on the 183 that well, it's been a while since I worked on one.

Tom,
       through all of this, how does the audio sound? ? is it distorted or clean?
that might just tell the tale.

The audio has always sounded good.  I am just trying to find a way so it doesn't run so hot


Tom - AB3FL

Tom:

This is not to the letter of "hollow state", but on one of my HRO-60's, I unplugged the 6V6's and use an external solid state power amp. I also plugged in a solid state rectifier. This cuts the heat way down and allows the power transformer to loaf. As a rule I always pull the 2a fuse from the rear and install a 1a fuse. I then add an inline 1/4a fuses on the HV secondary leads to save the stock transformer.

The image of the transformer above does not have the 11.5v tap. Wonder if this was a replacement and the filament current regulation was discarded?

Finally the potted transformers that look like the image above have about 1/2 the HV secondary resistance as the older ones with the end bell caps. This means the newer transformers can take the load of an HRO-60 (the NC-183D's are similar).

Dan

Thanks all.  I think I am going to get a 12V transformer to buck the primary to bring the line voltage down.  I don't want to kill a transformer made out of unobtainium


thx de AB3FL

I'd measure the bias right from cathode to the control grid, just be sure.

Pete

I completely redesigned the power supply in my 183-D.  I got rid of the choke and replaced it with a 7hy 200ma choke.  I solid stated the rectifiers and change the filter from a cap input to a choke input filter.  I run the 6V6s of the first choke.  I then added a second 6hy 125ma choke and cap.  The rest of the radio runs off the second choke.  So, the filter went from CLC to a LCLC.   NC-183-Ds run so hot you can fry eggs on them.

The power supply with a choke input filter allows the xfmr to run much cooler.  Getting rid of the 5U4, which runs very hot, helps to cool down the whole receiver and also reduces the VA load on the xfmr.

I made other changes as well, I added a regulated 105V screen supply (using a VR tube in the 5U4 socket)) for the RF and IF tubes and got rid of the screen resistors in those stages as most always those resistor go way high in resistance.  Doing this mod is more involved and may be beyond what the OP is able to do.

I also would not add a bucking xfmr.  The 6.5 volts on the filaments is still within their ratings.  Could be your line voltage is a little high.  With a CLC filter and a 310vac xfmr your B+ should be somewhat higher than 260 volts.  You may have something pulling down the B+, leaky filter caps most likely.  The filter cap in my 183 was leaky and had to be replaced.

Fred

There is nothing wrong with the 183D as designed and there were incremental minor changes in each run.



That is because it is a factory original as used in later production and as a replacement for the earlier one.

The reason for the heat is most likely 6V6's with leakage which will give strange voltage readings due to the way the bias is configured.....you already found that out with 1614's which may fry the filament winding due to excessive current. Stick to the correct tubes and PS and forget hammy hambone mods.
Leakage in any of the 5 6BA6's will also cause them to draw more current as well as affect the AGC.

The bucking transformer is absolutely your best bet in most tube radios that were built when the AC line was lower; dont let anyone tell you different. The manual voltage chart CLEARLY states they were measured at 115V. That set will work fine down to 105V and I run everything here at 111-113V with a high current transformer at each operating bench plus the repair benches. Nothing runs hot including the HT-32B and lots of Hallidrifters.

Carl

I measure the current draw on the B+ line and it was 140ma which I think is close to what it should be.  This is with a 5R4WGB (Potato Masher).  The B+ V is about 260V which is about right.  I am going to get 2 new 6V6GTs and a 12V xformer to buck the AC down.  Are any 6V6s good for this as I can get a pair of Chinese ones for $20 up to over $100 for a pair for AudioPhools

thanks

Tom - AB3FL

Get some NOS on eBay.  You'll never notice the difference between the audiophool brands and any other.  There were GM and Ford branded 6V6s for auto radios that the audiophools shun, they're as good as any and dirt cheap.

Tom,

I took a look at my manual for the 183D.  The 6V6s use cathode bias in my set.  The negative bias is not used for the 6V6s even though it looks like it is being used for grid bias.  The current drop on the two 1000 ohm resistors set the bias voltage.  The current through the 6V6s does not pass through the two 1K bias resistors as the cathode resistor is returned to the neg bias line.  The current through the phase inverter (1/2 6SN7) is also returned to the neg bias line.  The bias is used for the RF GAIN control.

So, the remaining current load from the remaining circuits are returned to the chassis.  This current is returned from the chassis through the two 1K resistors back to the xfmr CT and the filter caps' negative terminal.  That's how the neg bias is developed.

The higher (more neg) the bias is, the greater the current drain from the circuits in the set (except for the 6V6s and PH INV)  The adjustment of the RF GAIN control will probably cause a change in the bias voltage as it controls the current through some of the stages.

When bias is developed in this fashion in a power supply, the bias voltage reduces the B+ voltage measured from ground.  The reason the bias is applied to the 6V6s grids is to avoid this reduction in B+ on the 6V6 output stage.  This is true as the cathode bias resistor is returned to the bias line and not to chassis ground.  So, to measure the true total B+ on the 6V6 plates, it should be measured from the bias line to the plates.

I mentioned in my last post that I thought the B+ from your supply should be more than the 260 volts you measured.  In fact it is because you have to add the B+ (measured from chassis ground) to the magnitude of the bias voltage.

I also see that my power xfmr was designed for 110 volt.  I think yours is a 115 volt primary.  The use of the bucking xfmr might not hurt, especially with my set (110 volts).   My line voltage, at times, is 125 volts probably way too much on a 110 volt xfmr.  But, I use variacs in my station to keep the line voltage under control.

Hope some of this helps.

Fred

PS,  Don't make any of the HAMMY HAMBONE mods that I did to my 183D even though it made the receiver work better than designed.

Measure the 6V6 cathode voltage from the bias line to the cathode.  The bias you said was about -55 volts, cathode voltage -29.  Do the math and you get about 26 volts cathode voltage with respect to the bias line which is also connected to the grids.

Fred

my NC-300 runs 24-7 and it runs hot been on 8 months now but i use turn it off now i just leave in on but even if i turn it it on bout 40 min. the tranny gets really warm almost can't touch very long!

                                                                                                      Carl KB3RDT

I dint knooooooo dat!! I just assumed that it was some mil surplus transfoma that someone stuck in there after the original one crapped out. I would have expected a non potted unit like the one in a 183. the other 183Ds I have repaired had the more conventional looking transfoma in them.

Velly intelesting....... ;)  ;)

My NC-183 (not D) had many resistors that changed in value.  Especially screen dropping resistors and all the ones in the phase inverter, and as you have done, change all paper caps.   I learned all this after replacing the power transformer three times.

Thats debatable.

Carl,

Probably is debatable,  I put a VR 105 in there for the screens on the RF and IF stages.  Had to add 2200ohm decoupling resistors and .01uf bypass caps.  I thought it seemed to work better, but who knows.  I thought the S-meter worked better, seemed to give more accurate readings.  The mods I did to the power supply brought the xfmr temperature way down.  I didn't want to lose the xfmr as I had heard that they were prone to failure, even though I have probably a dozen xfmrs that could replace the original xfmr.

Fred

Hey Tom, do you need a pair of 6V6's? I'd be more than happy to send you a pair of good testing old stock for the price of postage.

KC2TAU: Yes, the only 6v6s I have are in the nc-183D

All:  I bucked my nc-183d and it likes it...hihi.  It runs much cooler now and has around 108V on the primary of the power transformer. 

thanks

Tom - AB3FL

sacreligiously.. maybe using thermal epoxy and gluing an aluminum heatsink on the side might help (some TV sets had a heatsink built onto the transformer core)? 
Or just using a small fan to cool the iron. I've used fans on several transformers and it really does work to carry the heat away.

As an example, the NCL-2000 iron will run pretty darn hot if the unit is keyed down for a couple hours at a time which I was doing using it for a source of 200-500W RF power.

One concern for that style is that transformer there looks potted, so that's a heat issue right there. Perhaps the core actually touches the outer case on a couple sides which would be helpful if you direct airflow at it. There's no need to drill holes, any airflow will help. Must be taboo though because no one seems to want to do it, or admit to doing it.

I don't subscribe to the belief that a very hot transformer is normal, no matter what the rating. I'll try to cool it if possible to increase life.

It is still a lot easier, and much less intrusive, to add an external
bucking transformer on the AC line voltage. Redesigning the power supply
seems to be a rather extreme approach to a problem with a simple cure.
None of these mods address the issue that many manufacturers may have
used xformers that are bit shy in iron/inductance and that may be going
into saturation when run at higher AC line voltages.

I'm not arguing that choke or resistive input is easier on any transformer,
but there are other considerations that need to made.

Pete

I have used Weber Copper Top SS replacement rectifiers on occasion. They're just a convenient way to use solid state rectifiers and add an appropriate dropping resistor with no mods to the equipment. The dropping resistor is built into the plug-in. That would at least take the 5V filament load off the transformer.  The B+ should work out the same and the dropping resistor helps with inrush current somewhat like a real tube rectifier does.  A big part of inrush current comes from the filaments which have low resistance when cold and, of course, this is one less filament.

I run all boat anchors at 115V.

Interesting but I wonder.  I have owned 3 NC 183D receivers and run them by plugging into the line socket for years and never had a failure.  Yes they get hot and yes heat will hurt them, but if you replace out of spec components, tubes and be sure the ventilation is good then it should last.  All the old design criteria was 115 volts +/- 10%.  That would equal to +11.5 added to 115 so it should work ok.  Don't block the vents and if something in the audio section or power supply doesn' short, the transformer should be ok.  I always put a thread of RG 58 shield wire in the HV lead right where it comes out of the rectifier to prevent a short from ruining the transformer.

I have to agree with Jim, Although nice, a bucking transfoma, is it really necessary? ?
I run EVERYTHING right out of the wall socket. My line voltage varies from 115 to 125v, depending on the time of year. My stuff, especially the recievers will easily run sometimes for 12 or more hours at a clip. Yea, they get a little warm at times. but it helps bake the moisture out of everything.  As long as you dont have leaky caps, to overload it, it shouldn't be a problem. I also have never had a power transformer failure in my low power stuff. Plate transformers in high powered stuff is another story.

Class-A P/P 6V6s do get brutally hot, and that is just the way it is. As long as the plates arent showing any color, they're prolly OK-Fine. I have gotten some nasty burns off of class-A audio outpoot tubes over the years.

When someone on here describes something as "getting hot" you really don't know what to think. Especially if they describe the voltages as being way off, you start to expect to find a problem..