R390A Problems

[W9BHI]

I just acquired a very clean and unmolested EAC made R390A.
After cleaning it up, It looks nearly new.
It was deaf as a post when I first tried to run it.
I tested the tubes and recapped all of the decks but no joy.
AF deck is working and RF gain control has an effect on noise level.
BFO is also working.
I decided to go the troubleshooting rout per the manual.
The conversion oscillators have the right voltage at test points (E209, E210, E211, EXCEPT E402 on
the crystal deck.
It is supposed to have -4 to -11 volts at test point E402 but it is next to nothing (-.158).
Putting a freq. counter on the output shows nothing.
I checked the voltages on the grid and plate as well as continuity on the cathode choke.
I checked continuity on both sections of the selector switch and all is well.
None of the crystals will oscillate.
Any ideas?

Don W9BHI

[WU2D]

So this is the second oscillator and you have proper voltage on the plate and cathode but no oscillation? Lets try some troubleshooting.

The 2nd crystal oscillator is an electron coupled Colpitts with a 6AK5 pentode. It is not likely that the crystal switch, trimmers and crystals are all bad so we have to concentrate on the common parts. Clean the socket and swap the tube just for fun. Then check the screen feed and plate feed resistor voltage drops. Does the screen have enough voltage on it? The most likely component to fail and kill an oscillator is a capacitor so next you will have to change out caps on the grids, screen and plate circuits.

SecondCrystal Oscillator output as measured at pin 7 of V-203:
Normal is approx 3.2V P-P

73's Mike

[W4AMV]

Hi Don, Mike. I am working through the 2nd LO deck as well. At this point crystals that are off frequency, not dead. As I have the deck out as well as functioning, I can pass along some of the same V and R values as Mike. I agree, check the presence of voltage on the tube points, the test pinE402 is the control grid and that problem is evident. You might reseat the plug to J410. Rotate to unlock, pull and reseat and lock. As apoint of reference on this unit on an operational band, I measured the RF peak to peak LO voltage at J145 at 6.5V p-p.There is no adjustment for oscillator frequency, only adjustemnts peak the fundamental, 2nd, or 3rd harmonic output. Common to all is T207. If open, plate V would be zero. With the schematic in front of you and no voltages present, you can make quite a few resistance measurements and quickly find what IS NOT WRONG with the unit. The loss of either of the feedback caps, 12pf, 150pf would certainly kill the LO. If the subchassis is out, you might take a visual on the caps and look for anything obvious, cracked cap, broken wire. 

Alan

[KA3EKH]

Don’t forget the mechanical alignment! If someone had any of the decks removed and turned the controls then all the little cogs and switches will not be in the right place and you have a dead receiver for no apparent reason. They sometimes work at some frequencies and not others or will do other weird things. You can drop the front panel and I forget where but at one frequency all the little lines on the wheels are supposed to be set to their corresponding marks. Think it’s 5.0 or 2.0 or something like that but remember from years ago when working on them that if it was a receiver that others had worked on and you did not know the history to always check the mechanical alignment first.

[W9BHI]

Alan,
The tube has been swapped out with another.
All the resistors in the circuit are within 5%.
The B+ to the deck is about 250v and the regulated screen supply is 148v.
The voltage at the screen grid pin is about 100v.
Cathode is zero.
Cathode choke is not open.
T-207 is not open.
Mechanical alignment is correct.
Maybe a leaky screen bypass or the 12pf or 150pf is open?
I need to do more digging.
Thanks,
Don W9BHI

[W4AMV]

Agree on the mechanical alignment. Check CAMs at 7+000 on the kHz dial. The CAMs point tip or holes in the CAMs must be bisected by the dark pencil like etches viewed on the front of the rack. To be sure the large rotary switch in the second LO is on the proper mark, viewable through the hole in the 2nd LO sub chassis. However, your issue now is really not related to the mechanical tracking. So what if it is off mark, the oscillator needs to run on whatever switch location is in position. You can disconnect the mini coax from the 2nd LO injection module and take a scope measurement right at the center pin of the connector. In my case, the 9 MHz xtal was off by 15 KHz, it was running, but the fundamental, and 3rd harmonic were less than 1 V p-p. So the activity, series equivalent R of this xtal is probably off the chart. Plan on pulling it and record what it looks like for the record. Yes, I would suspect either of the 2- feedback C's or possible the grid leak R, 120 K (?), is history. One other clue, if you have a scope, monitor the 2nd LO RF output port and switch the Rx from STBY to AGC or ON mode. I believe all LO's have their B+ removed in stby and so what I would look for is to see if the LO shows a hint of starting, a transient on the scope and then dies. You may see one or few cycles of oscillation and then die off quickly. That would certainly imply loss of loop gain or feedback.  Look forward to what you find.  Sorry, so the R's are ok... Boy, it sure points to the C feedback elements. OR a bad BYPASS. In my unit, they are all ceramic disks. I don't know if "ugly" bypass caps are used in some units and not in others.

[WD8KDG]

I just acquired a very clean and unmolested EAC made R390A.
After cleaning it up, It looks nearly new.
It was deaf as a post when I first tried to run it.
I tested the tubes and recapped all of the decks but no joy.
AF deck is working and RF gain control has an effect on noise level.
BFO is also working.
I decided to go the troubleshooting rout per the manual.
The conversion oscillators have the right voltage at test points (E209, E210, E211, EXCEPT E402 on
the crystal deck.
It is supposed to have -4 to -11 volts at test point E402 but it is next to nothing (-.158).
Putting a freq. counter on the output shows nothing.
I checked the voltages on the grid and plate as well as continuity on the cathode choke.
I checked continuity on both sections of the selector switch and all is well.
None of the crystals will oscillate.
Any ideas?

Don W9BHI

Don,

I believe I've been down this road before. As a matter of fact, notations were made in my printed Y2K! On page 5-16 of the Y2K we are told to turn the Function switch to Standby. My notation on page 5-19 tells me I got the required voltage if the switch was on AGC. YMMV. Two R-390/A's function this way?? This is just for E402, I agree with the Y2K for the other test points.

Question: Do you have -7VDC at the diode load with 150 microvolts @ 455KC on the IF deck?

Craig,

[W9BHI]

Craig,
There is plate and screen voltage to the crystal deck in both the STAND BY  and AGC positions.
The crystal deck is out of the receiver as well as the IF deck.
The power cable has been reconnected the crystal deck so I can take measurements from the bottom.
I don't see how the IF deck has anything to do with the operation of the crystal deck.(my opinion)
I going  to replace the 12Pf and 150Pf caps and see what happens.

Thanks,
Don W9BHI

[W4AMV]

I agree Don. The 2nd conversion oscillator should stand on its own.
It will be neat and satisfying once you find the problem. This is a long shot, but if the osc plate coil was way out of wack, too low a reactance, you may see nothing on all selected bands. Particularly if you are taking readings at the jack port which is transfomer coupled to the primary of that xmfr. Have you tried tuning the slug on that unit. And to give it a fair shot, choose a band frequency that would operate an xtal on its fundamental. For example, the 11 MHz band uses 14 MHz fundametal.

[W9BHI]

Took the cover off of my working R390A and measured the test point on the crystal deck
and got the same -.154volts.
I also found out that my Fluke 85 DVM's input impedance is low enough to kill the oscillator.
I compared the output to my working unit with a scope and they were almost the same.
SO, the crystal deck is working and I have to borrow a friends HP 606A and start working my way through the decks until I find the problem.
I will post my progress.

Don W9BHI

[W4AMV]

Don, when you made the measurement was the Rx in standby. Early post should have stated when in standby, B+ is removed from all tubes except all LO's. Then the test points are at the mixer(s) grid and the rectification which occurs provides the negative V readings in the manual. I also used a Fluke DVM, although the oscillator is pulled, it is not cutoff. In any case, the scope certainly confirms good LO. Prior post on PTO alignment was confirmed to be a bad 9 MHz xtal. At injection it was over 15 kHz in error. There was no way the PTO zero adjust could compensate. I exchanged the xtal and it is proper now. Now I need to look into alignments.