R-390A Audio Modifications


Chuck Rippel, WA4HHG

In troubleshooting my 32V-3, a condition occurred which bears repeating as a warning to other owners of Vintage Equipment. However, I am not an electrician so check your local situation and codes before proceeding at your own risk.

My 32V-3 was blowing HV power fuses due to an internal short. In the circuit, there is a 115V lamp HV indicator lamp across the primary of the HV transformer and its outside case was just touching a near sub-chassis. When the A/C line was plugged in one way, A/C was on the outer lamp socket case. The station was protected from a potential deadly hot chassis condition and/or fire by the station ground. The 120VAC simply went to station ground and took the fuse. With the plug reversed in wall socket, the neutral side of line was on the lamp socket thus, no A/C short occurred.

It was an easy fix to a potentially dangerous situation.

Like a lot of vintage equipment, a "stock" 32V-3 has a non-polarized, 2 wire A/C line plug. It has a pair of fuses on only one side of the A/C line. A 3A protects the LV transformer and the circuits it serves and a 5A protects the HV transformer and its circuits. In looking at the "V-3" print, I noted that a condition could easily exist where the radio would not be protected by any fusing at all!

One side of the A/C input is found on terminal 21 (actually, a feed-thru capacitor) on the rear panel or the transmitter. The other side of terminal #21 is connected to the 3 and 5A fuses and from there, to the transformers. The return to the wall plug is through terminal #22.

If a failure condition in the 32V-3 causes a short across actual A/C line, the fuse(s) will open and protect the equipment as normal.

Were a fault to occur, such as a transformer failure where the primary shorts to the equipment chassis, causing the A/C to be returned VIA the station ground, it is possible that the transmitter's power fuse(s) would not open. The only overload "protection" would be via the circuit breaker in the panel. A good bit of damage may occur before a 20A breaker would open versus a 3A fuse!

Whether your equipment were protected by its fuse would depend on which way the plug was inserted into the wall outlet.

When looking at a 2 or 3 wire A/C outlet wired under the current code, the narrower of the vertical sockets in the receptacle is the hot side of the A/C line.

Referring to the 32V-3 print, note that as long as its A/C plug was inserted such that rear terminal 21 were connected to the hot side of the A/C line, the equipment fuses would open in the event of a failure to chassis ground. If the A/C plug was reversed such that terminal 21 were connected to neutral, the fuses may not fail and any protection would then be provided by the circuit breaker in your main panel. The current path would be through terminal #22 (unfused in the 32V-3) through the short to return through station ground.

It is possible that this condition can also happen in other equipment (regardless of make/model) which has only one side of the A/C line fused and is equipped with a 2 wire, non-polarized A/C plug.

There are 3 "fixes" which come to mind. Two are simple and one, not quite so simple. All involve identifying the hot side the A/C line and making sure that when the equipment is plugged in, the hot side of the A/C line feeds it through the equipments internal fuse(s).

Since the original line cord was in good shape and I wanted to keep my 32V-3 "stock," here is the fix which I applied to my radio:

Remember that the 32V-3 power cord is completely removable where it enters the rear chassis. If your power cord is not easily removed, there are steps in the procedure below to accommodate that design.

A - Identify the fused rear panel input terminal by removing the power fuse and using an ohm meter, see which terminal the fuse block is connected to. Mark that terminal and go to step "B," below.

- If the A/C line cord on the piece of equipment you are checking is not removable, use the procedure above but note and mark the blade of the A/C wall plug which you determine is connected to the fuse. Proceed to step "C."

B - Using some red paint, mark one of the two blades of the actual power plug with a red band, up near where the blade actually enters the molded plastic plug. This does area not tend to contact anything in the recepticle and should not wear off while also staying out of sight when the equipment is plugged in. Again, using the ohm meter, identify which wire at the radio end is connected to the blade you marked. Connect that wire to the terminal you previously identified as the one which is fused.

C - The A/C plug should be inserted into the wall such that the red banded end is in the hot (narrower) receptacle slot. This way, the hot side of the line must go through the power fuse.

The second fix deviates from "stock" and involves adding additional protection via the installation of a 3-wire cord to accommodate a grounded power plug. The procedure is the same as above save for connecting the ground wire in the new cord to the radio chassis. Computer cords, found for around $1 at hamfests work wonderfully for this purpose. Just cut off and strip the end which would normally insert into the computer.

The third option is to add a second fuse or set of fuses within the equipment to the "as stock," unfused side of the A/C line in conjunction with the 3-wire cord installation above. As this involves possibly drilling or punching out the chassis to fit a fuse socket(s), some prior thought should be given this option. Depending on the situation, the installer would most likely want to replicate the electrical "position" of the new fuse(s) with that of the existing fuse. That is, most likely the fuse should be located before the primary of the transformer. Just install the new, additional fuse on the now unfused side of the A/C line.

Again, research your situation and check your local building codes before proceeding at your own risk. If there is the slightest question, consider hiring a licensed electrician to help.

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20 Jannuary 2000