NC-183D VR tube dark

[WB4AQL]

Hello everyone. Switched on the rcvr today and sounded like the antenna was disconnected. Looked inside and the OB2 tube was dark. Checked the tube (I`ve never found a bad VR tube) and the 5U4 rectifier. All OK.

I can adjust audio volume, but no receiver sensitivity. Looks like B+ has gone away. I`m gonna wrestle this thing to the bench and warm up the VTVM.

Anyone with previous experience I would be pleased to hear. Thanks!

Michael  WB4AQL

[N4LTA]

Sounds like you have lost the regulated voltage to the LO. If you are hearing a hiss in the speaker then you must have unregulated B+ and that is good. Look at the dropping resistors and filter caps upstream of the VR tube. You may have an open dropping resistor.

[K3ZS]

I had problems with my original NC-183 (not D).    A paper cap had shorted in a screen bypass circit and eventually the current draw took out the power transformer.    Also may resistors had changed value.    After changing the paper caps and a new transformer, the RF sensitivity was low.  After changing the screen dropping resistors in the RF and IF sections that had changed value, the sensitivity was restored.    I would check all bypass caps for shorts to make sure you are not loading down the B+ supply beyond its limit.   If the NC-183D has an external power plug like mine, you should be able the measure the current draw.   

[WQ9E]

Michael,

First, check to see if your receiver is one where there is a capacitor to ground (.1 or .01 as I recall) from one of the rectifier plate terminals.  If so cut it out and don't replace it, it isn't needed.  This one component has killed a lot of HRO and NC-183 series power transformers.  Then make sure that the fuse installed is the correct value so as to provide some power supply protection during troubleshooting.

If a cap to ground off one of the rectifier plate terminals is not the issue then proceed cautiously because it could be something as simple as a bad dropping resistor to the VR tube.  But more likely something is drawing excessive current which can lead to significant damage.  I would measure resistance to ground at the main B+ output and also at the VR tube to see how those measurements compare to what is called for in the manual.  An unusual result here will help steer you towards the trouble.

For older receivers, components I always replace are the power supply filter capacitors, cathode bypass capacitor for the audio output stage, and blocking/coupling cap to the grid (s) of the output tubes.  Failure in any of these capacitors will lead to increased current draw potentially damaging the power transformer, filter choke, and audio output transformer.

[WB4AQL]

Thanks guys. The plate dropping resistor calls for 3.9K ohms (10W) and I`m getting a reading of 400K ohms with it in the circuit. This may be the problem.

The original owner was an engineer and had re-capped a large portion of the rcvr. I didn`t find the cap to ground at the rectifier plate terminals, so that is a good thing. I`ll make sure those other caps mentioned have been renewed, find a 3.9K resistor and go from there.

73,

Michael WB4AQL

[ke7trp]

If you are wondering if something is pulling current you can simply take a current reading on the primary power input to the Reciever. Power input is listed on the back or in the manual. I have an old eico test set that I use all the time. It has a 120 volt socket and a magic eye tube calibrated to watts. You can use any current meter you may have.  I had an SX100 that was showing 90 watts when running, We found bad caps and resistors where changed. Once restored it was down to 45 or so and the thing ran cool.

Worth a check.

[WB4AQL]

Very good idea.

I replaced the plate dropping resistor for the VR tube with a new 3.9K ohm and the receiver is working again. It has been re-capped and looks good underneath. With that resistor opened up, there was little or no voltage on the tube.

Thanks for all the help guys!

73,  Michael