Victor R-32 too loud

[JohnCT]

First, excuse my complete and total ignorance with TRF radios. This is the first I ever worked on.

This radio had a shorted section of the filter block on the amp/power chassis, so I gutted it the big brown can and filled it with plastic replacements. All other caps rebuilt in the same manner including the small filter block in the tuner chassis. All resistors in spec. So far so good.

The problem is that this thing has ridiculous gain. With no antenna, the local AM radio station (5KW daytime about 3 miles away) overpowers the radio. I can pull the first three RF tubes out of the radio (Ant RF, 1st and 2nd RF) and the radio is still loud, and this is with the top shield in place.

The only parts not replaced other than the resistors are the mica caps.

The problem is that the volume control has to be at zero and it's still audible. Just crack it a bit and it's too loud. The control is new and is a double 5K linear (all I could find). One section is on the antenna input line and the second controls a bucking coil on the 2nd RF coil - it does not handle either audio or B+.

I carefully neutralized all RF sections with a dead 26 tube per the instructions and I was easily able to null out each stage.

The radio plays much better with either the Ant RF or the 1st RF tube removed.

My next plan is to turn the front panel radio/phono switch to a pseudo "local distant" switch and disable the 1st RF tube with it unless someone else has an idea. I suspect the radio is operating properly and maybe the local AM tower is too close. The owner of the radio lives farther away from the tower than my shop.

Any advice greatly appreciated.

https://www.tubesandmore.com/sites/d...r_32_pg1-1.png


John

[old_coot88]

Putting the volume control at the antenna input was common practice on those old TRF setups. A strong local station would still bleed through unless the whole set could be electrically shielded. Just part of the charm of the old beast. Your idea of disabling one or more stages is probably the most workable solution. Or maybe a rheostat on the filaments of the RF tubes (as was done earlier on some battery-operated designs).

[jr_tech]

Perhaps a trap in the antenna circuit to attenuate the offending nearby station, without reducing the gain needed for weaker stations.

jr

[old_coot88]

Originally Posted by jr_tech

Perhaps a trap in the antenna circuit...

Problem is, the offending signal is getting in 'downstream' directly into the RF stages, chassis wiring etc. Total RF shielding would be the only fix for this.

[Electronic M]

Given you're fixing this for a customer you should probably try opperating it in a public place the same distance from the stations as the customer lives...If you modify it to not overload in your excessive signal area then deliver it to the customer if may prove to be completely deaf to all stations in your customers home.
TRF sets are not very selective and often have inadequate shielding for as close to the towers as you are.
If this was your set to keep I'd understand modifying it, but since it's a customer's set you need to make sure it works right where they're at (presumably under more normal conditions).

TRF sets often have no audio gain control and only an antenna attenuator pot so running them in strong signal locations is tricky....Also if any of the factory shielding is missing they'll tend to feedback and oscillate (make funny noises).

[old_coot88]

Originally Posted by Electronic M

Given you're fixing this for a customer you should probably try operating it in a public place the same distance from the stations as the customer lives...

Good call.

[JohnCT]

Originally Posted by Electronic M

If this was your set to keep I'd understand modifying it, but since it's a customer's set you need to make sure it works right where they're at (presumably under more normal conditions).

I agree, which is why my plan is to use the radio/phono switch to switch any mod in and out. I used that switch to simply add a resistor between the last RF and the audio amplifier, and it kind of works, but I'm not happy with it. I have another plan to try later today.

I honestly don't know how the signal is getting in. I can remove the first three RF 26 tubes and it still plays and tunes. Because the volume control is a bucking coil on the second RF stage, removing that tube eliminates the volume control

The radio chassis is fully shielded and only about 2/3 of each of the tubes are above the shielding pan. I placed soup cans over the tubes and it has no effect whatsoever. There is a longish umbilical cable from the power chassis to the radio chassis, but it's bypassed both at the power chassis and the radio chassis, so I'm not sure what's going on.

John

[JohnCT]

I wasn't happy with using the radio/phono switch to add a resistor between the last RF to the audio amp, so I ended up using the switch to drop a capacitor on the grid of the second RF tube (1st RF because there's also an antenna RF).

It attenuates the audio by about 30% and it sounds better because it's not overloading the later RF stages. The switch will allow full gain for weaker stations when needed.

The customer said he would never add a phono to the back of this radio and told me to go ahead with that plan.

John