The AM Forum

So I came down to the hamshack and turned on the R390A.    No lights, nothing.    Check 3A fuse - blown.   Not sure why, but put in a new fuse, and it lights up, but no radio stuff is happening, not even any hiss in speaker.    OK, maybe no B+.    Check power supply rectifiers, no filaments glowing.    BOTH 26Z5 tubes are dark.    Put in two new 26Z5 tubes.    Filaments light, OK for about 15 seconds, then go nuclear.     Fireworks inside both rectifier tubes.     Fuse blows again.   

Major power supply issue is at hand.    Is it salvageable, or do I just get a new PS subchassis and pitch the old one?    I suspect a short in the transformer.

Thanks, Ron

The filament winding for the rectifiers also feeds the 3TF7 regulator and crystal ovens. Maybe check the filament line connections, but the fireworks could mean a HV short. How is F102, the 1/4A fuse?
LB

It's easy enough to pull the connections from the power supply chassis.  Do that first.  Measure the voltages. 

It could easily be something else, somewhere else.

Hi Ron,

Sometimes a squirrelly short that comes and goes can mean filter capacitors coming and going as the chemicals change inside.
A good test is to disconnect them and clip lead  in another set.  Testing the originals with HV on them can be unreliable at times. They will not all age together. Heat is a big killer of electrolytic caps.

As mentioned in the last post, divide and conquer - first disconnect the load from the supply and see which general module/area is suspect.

T

OK, so I disconnected the B+ line at the power supply and no more fireworks, so the problem does not appear to be in the power supply itself. I'll be looking at the subchassis modules one at a time to see if I can find any obvious burnt components or other signs of shorts. My receiver does not have an F102 which I believe should be on the rear panel, right? I think the earlier production runs did not have F102.

Off to California today to meet my new grandson. Will resume testing when I get back.

Ron

indeed sparks inside vacuum rectifiers means a severe overload.

This is an easy one, pull the connectors from the audio deck and fire the 390A up, my bet is that one of the plug in electrolytics are bad. If the fuse doesn't blow, check the plug in electrolytics in the audio deck for shorts. Don't put in bigger fuses as you will cook 1 of the inductors on the audio deck. 26Z5's are getting pricey, if you don't have any spare, just tack 2 diodes in place in the PS module until you get the issue fixed.

Bingo Ed!  8)

I spent much of the weekend working on the R-390, and finally got to the root of the problem on Saturday afternoon, although somewhat by pure luck. I was looking for a short on the IF deck. It had been previously modified by the former owner. I was not able to find any bad components or other problems on the IF deck however. But after studying the schematic, I realized the B+ line and the power supply electrolytics were on the AF deck, so I took it out and found a burnt 2200 ohm resistor, R606.  The can electrolytics looked pristine, and I did not really suspect that they were bad. But I took them out of their sockets anyway and to my surprise the three section filter cap was nothing but corrosion at its base. I have temporarily put in a three section 10uf plug in that I found on another piece of equipment, and it works OK, but I'd like to get an exact replacement if I can find one.

Fortunately, I have a few spare 26Z5W's.

Does anyone know where I can get a new replacement for the two electrolytic plug in caps?

Thanks, Ron W8ACR

Not cheap.  https://www.ebay.com/itm/Collins-R-390A-multi-section-plug-in-chassis-mounted-electrolytic-capacitor-set/333474334696?hash=item4da49e87e8:g:2BsAAOSw1kxdkxR5

Circumsize the can just above the lip with a Dremel cutoff wheel.  Pull out the guts and solder in new caps to the base, then glue the top back on with JBweld.