An unpleasant little surprise

[AF9J]

My mega modded HQ-129X arrived yesterday via USPS Priority mail.  It was boxed up decently, and externally was in good physical shape.  I decided to fire it up, to see how things were.  Unfortunately, as soon as power was applied, I heard arcing.  I still had dial lamp lights (of course - more than likely they're on a different circuit), but I had zip for audio output.  The HQ-129X uses a 5U4G rectifier tube. I wanted to give the HQ-129X a possible test drive during Saturday morning's Midwest Classic Radio net.  So I took a gamble, and assuming that possibly the rectifier was gone, I went to a music store, and bought a 5U4GB today, after work.

I opened up the top of my HQ-129X, and what did I find installed in rectifer socket, a 5V4GA!!!  I don't know if Bob (Robert Hess - the SK who owned & modded the HQ-129X) was smoking something, or put in the 5V4GA, becuse of all of the mods he did.  Neither tube (the 5U4GB, or 5V4GA) are considered substitutes for each other.  As it is, I just discvered that the 5U4GB, isn't even really a substitute for the 5U4G.  I don't have the parts on-hand to make up a solid state rectifier, before tomorrow morning.  I'll have to pull the big, heavy monstrosity off of the table, and put the SX-96 back on.

Feeling frustrated,
Ellen - AF9J

P.S. - I wouldn't be at all surprised, if the failure occurred elsewhere (other than the rectifier).  Hello Mouser.

[Pete, WA2CWA]

5U4GB can be used as replacements for 5U4GA, and 5U4G. Been doing it for years. 5V4GA will work as a replacement for 5U4G but it depends upon the power supply and over all circuit requirements.
For starters, check the B+ line for shorts. If you don't see any tube arcing, turn the chassis upside down, dim the lights in the room, and watch for arcing. With all the mods, it could be a piece of loose solder that decided to migrate between two contacts during the shipment.

[AF9J]

Hi Pete,

I've a lot later than I originally planned to, fiddling with the HQ-129X.  Here are some odd obsevations:
 
1.  The speaker terminals on the back panel were bridged with a wire.  Very strange.
 
2.   Removing the wire bridge from the speaker terminals, I still had no audio out of the headphones.
 
3.  I hooked up a speaker I had laying around from a modular (detachable speakers) boombox system. 
 
    A.  I got audio, but it was with S9+ hum.  Turning down the volume/audio gain had no effect on the hum's volume.
    B.  Disconnecting the antenna had no effect on the hum's volume.
    C.  The hum seems to sound more like 120 Hz hum, than your garden variety 60 Hz hum.
 
I know dried out caps can cause hum, but on this scale?
 
73,
Ellen - AF9J
 

[W3RSW]

Hi Ellen.  Tough luck on the new purchase but as Derb says, your opportunity!
Anyway, yes the pwr. supply caps seem to be dried out and can easily cause hum at that level.  They're probably drawing excessive power and hard on the pwr. transformer, 5U4 /v4 rectifier, etc. But the good news is you apparently have B+ so don't operate it much more without checking for shorts and then changing out the filter caps before powering it up.  You might find a Sam's photofac or get the old manual and run the resistance checks before changing out the filter caps. I'd sure run res. checks now before doing anything else.

At any rate you've gotten a good carcas and sounds like without too much work a great receiver. You've gotten neat offers such as a set of knobs n' stuff to get it working and loookin' good. Some of the mods look pretty good rf wise if not very photogenic.

Good luck and think of the fun your having.

[k4kyv]

The 5U4G and 5U4GB should be interchangeable unless you are running them right at the ragged edge of their ratings.  The socket connections and filament ratings are the same.  If you run them conservatively, there should be negligible difference in performance.

The 5V4 is the octal version of the 83V.  The 83V suggests that it is a vacuum version of the 83, but it is not.  The vacuum version of the 83 is the 5Z3. The 5Z3 is the older 4-pin version of the 5U4G.  The 83V and 5V4 are not directly interchangeable with the 83 and 5U4G.  The pin connections are compatible, but the 83V and 5V4 have only a 2-amp filament and won't take as high a plate voltage.  The 5V4 is more like a high-current version of the 5Y3, and has less voltage drop.  I use one of those in my outboard receiver audio amplifier for better power supply regulation.  Just pulled the original 5Y3GT and substituted.

If you substitute solid state diodes, it would be a good idea to install a dropping resistor to simulate the voltage drop in the tube type rectifier.  Otherwise, you may have a little too much B+ voltage, which could shorten the life of tubes and other components.

Also, some equipment is designed to depend on the warm-up time of a tube type rectifier to delay the application of B+ to the rest of the circuit.