Hi all,
After recapping my NC-183D has been playing normally for several months but has now developed some problems. The volume is low and there is some distortion although it can be tuned clearly with VERY careful adjustment. The s-meter is non-responsive. Right now I would appreciate any suggestions as to what area would be best to start looking at. When I initially capped this set I had this fault show up a couple of times but then it cleared and hasn't come back until now.
thanks,
Sandy
PS: I need to find a better schematic. This set uses a 6SN7 and not the 6J5 which I think was in the earlier versions.

Sandy,

PM me with your email and I will send you a scan of the 6SN7 version schematic.

Is the VR tube still glowing? Low B+ voltage?

Rodger WQ9E

If all the out of specs resistors were replaced...at least 20 or so in a typical 183D...then Id suspect a tube with severe heater cathode leakage that is disabling the AGC.

How does it perform with the RF gain turned down with AGC on and off?

Carl

B+ checks out ok. If RF gain is turned down even a little there is no audible signal. All tubes have been subbed 1 at a time with no change in symptoms.
thanks,
Sandy
PS: Rodger PM on the way.
PPS: After re-reading Carl's post it sounds as though I should be doing resistance checks. Any particular area to start with?
thanks,
Sandy

All the screen droppers and plate loads in RF, IF and mixers. All AVC resistors as well as all in the audio circuits.

Carl

Will do resistance checks and report back.
thanks,
Sandy

Sandy:
After changing all the caps and 70% of the resistors, I'm still plugging away at mine.
The series in ER by KD0ZS provided some interesting food for thought.
Email me for some moral support - (mycall)@rac.ca

Ralph, VE7XF
Tsawwassen

Hand in there, Sandy and Ralph.

When you're done, you will have one of the all-time great band-cruisers.

Joe Connor

Hmmm. Checking 6 or 7 of the 47k resistors found all but 2 had gone very high. A couple as much as 3 times correct value. The 2 remaining were open. I don't have a lot of experience but I've never seen anything like this before. At this rate I'll have to order more resistors.
Sandy

That is the same experience as mine, every 47K was way out of spec.

My 183D had many resistors that drifted up in value...some way up.

I've often found 6BA6's in RF and IF sections of receivers to develop grid emission which can cause the problem you are having. Starts out as an intermittent problem that just gets worse and worse. Grid emission doesn't usually show up on tube testers but checking the control grid (pin 1) with a VTVM or DMM while the set is operating will show a positive voltage when the condition occurs.

Dave

By that time it is a severe problem...maybe. It can start showing distortion when the grid bias is just starting to deviate and it gets progressively worse as the leakage increases. That holds for many other radios.

However the 183D isnt always the easiest receiver to hang a DVM on either because of the way the IF stage is biased compared to the RF amps. When in doubt follow the manual voltage table and use the correct instrument and here a DVM or VTVM is correct.

Carl

I started replacing 47k resistors in no particular order other than first doing the ones associated with any of the (many) 6Ba6 tubes. So far this has caused a noticeable increase in volume but the distortion and inactive s-meter remain the same. I also found the RF gain control to be flaky. The resistance is not linear as the control is adjusted. There is a note attached saying that the original 10kohm was replaced with a 20kohm because that was all the previous repairer had available. Replacing this will be added to the list. Further testing shows that there are a whole bunch more resistors to be replaced. Progress report to follow as I go.
thanks,
Sandy

The one big thing about Nationals is that they are so easy to service. Even the very complex HRO-500 doesnt require becoming a contortionist.

Just keep at it and consider every resistor over 10K a suspect; the cheapest possible resistors were purchased during that ownership reign...but they did a good job for the expected life of the set....not for a bunch of nut cases almost 60 years later

Carl

Some progress. Replaced a lot of resistors and tightened 3 or 4 grounds where the grounding lug was bolted to the chassis. Now am able to tune in a clear signal. I have to get a 10k 25 watt ww pot to replace the rf gain control as the control is obviously flaky. I intend to go back and replace more resistors. Volume is good but not as strong as I think it should be.

My immediate concern is trying to figure out why the s-meter doesn't work. It is not completely dead but only moves about 1/8 inch regardless of increase in signal strength. The antenna trimmer which did not previously work now appears to work normally as tested by my ear.

Any suggestions gratefully received.
Sandy

Test the meter separately by using a battery, fixed resistor, and pot in series with the meter.
The value of the fixed resistor should be the one to give you full scale of the meter with the battery voltage. If the meter deflects OK, it is OK.then the values of the resistors in the metering bridge circuit should be checked.
If they are OK, check the tube being monitored.
Pat

You don't need a 25-watt pot for the RF gain control. An ordinary "volume control" sized pot will work fine.

Sandy,

What effect does R39 (S meter sensitivity control) have on the meter reading? It is possible V12 is defective (as I recall you have version B with the 6SN7 phase inverter and S meter amp).

Check the values of resistors R40 and R69. Did you check/replace C105?

I believe the original RF gain control is wire wound but specified at 1.5 watts, not 25. Usually wire wound controls clean up pretty well, remove the pot cover and give it a gentle shot of WD-40 and then run it back and forth through its range a few times. The AGC circuit in the NC-183D is operational with the BFO on and you will probably find that the RF gain control will be left at full gain in normal operation.

Rodger WQ9E

R40, R69 and C-105 have all been replaced. The meter swings when power is first turned on and it is adjustable by the back panel adjustment. I subbed the 6SN7 and there was no change. Slight movement of the s-meter is seen as the signal increases but it is very slight. Maybe about 1/8 inch. I accidentally knocked my long wire antenna connection off while testing .Although the reception was weaker there were still strong signals on the am broadcast band with no antenna connected. This was not the case before the resistor changes were started.
Sandy

Once your thru replacing all the parts do the 455 kc IF and crystal filter alignment per the manual. Have a handful of NOS tubes and swap IF tubes until you get the best output on an external meter. Then swap audio tubes for the same results.

Next align the V-4 stage and select the hottest tube.

Now you can concentrate on the RF and both oscillator alignments. I prefer to do a first pass front end alignment on Band B and then do the V1, 2, 3 tube swaps. You now have a radio full of good tubes and can now concentrate in the final alignment.

If in doubt check the stage gain measurements Ive mentioned before, that will keep you from trying to second guess and go off on tangents as some seem to do.

With new components and possibly many new tubes you can likely enjoy years of fun without a problem.

Carl