Low volume - Videokarma.org TV - Video

andrefranc · #1 03-22-2009, 04:27 PM

My Philco Model 20 has low volume. I have recapped and rebuilt the capacitor blocks and replaced some resistors but still has low volume, also, have checked tubes. Thanks for any help.

toxcrusadr · #2 03-23-2009, 03:10 PM

I had (still have) a radio that will drop in volume all of a sudden. I believe it has a bad tube socket - one of the pin connections is intermittent - OR it could have a bad solder joint that is sometimes OK but other times not, OR a resistor that reads OK when cold but drifts or opens up when it gets warm. Among other places I would also look around the AVC circuit if there is one, and make sure the resistors there are OK.

Is it constant or intermittent?

Have you swapped tubes out in spite of the fact that they read OK? If it does it regardless of what tubes you have in it, it is not the tubes.

Reece · #3 03-23-2009, 04:45 PM

This is a TRF set and needs a good antenna to operate properly. Did you connect to an outdoor antenna? Check that the audio stages are OK: touch the bare end of a five foot long piece of insulated wire to the grid terminal of the 27; a loud hum means the audio section is OK. Also: good idea to check the tube socket voltages in the chart on the schematic on Nostalgia Air:

http://www.nostalgiaair.org/PagesByM...2/M0029582.pdf

Your voltages will read higher with a DVM as the meters used in the heyday of the model 20 loaded the circuits down more. For example, the diagram says to expect about 215 volts on the plates of the 71-A's, the highest B+ voltage in the radio. Whatever you actually read there will be higher, and you could expect proportionally higher B+ voltages throughout. If you find a voltage that seems "off," look for trouble in that stage, resistor value gone way high ,etc. Have fun poking around in there!

Reece

andrefranc · #4 03-23-2009, 07:00 PM

I have about a 10 ft wire for an antenna. Touched a 5 ft wire to pin 3 of the 27 tube and got a hum but not a real loud hum. Both 71's checked weak but had one good one and when subbed it made no difference.

radio63 · #5 03-23-2009, 11:08 PM

Check the large multi-tap power resistor in the circuit. I beleive it is a large ceramic unit. One of the first radios I worked on was my Model 20 and this resistor had an open in one section that made the whole set play weak. As I recall, voltages were OK when I was testing, but one section of that resistor was the culprit.

Of course you are aware that the volume has to be turned up in order to receive weaker stations. This set has no AVC. Good luck and keep us posted.

Gilbert

andrefranc · #6 03-24-2009, 05:49 AM

I checked the ceramic resistor in circuit and no opens. Should I disconnect it to check or is in circuit good enough? I was told that between 2 and 3 it should be 2450 ohms and I only have 180 ohms there. How do I know what it should be? One 24 has a plate voltage of 273, 1st tube, and the next one only has a plate of 103 volts. Can't see why though. I replaced the caps inn the power supply with electrolytics, 10's, but the cap across the choke I left as is, would it be the problem?

radio63 · #7 03-24-2009, 06:13 PM

I'll have to see if I still have data on what the voltages/resistance of each section should be, but yes, you should check each section with at least one lead disconnected from the circuit. Otherwise you might have erroneous readings.

I do believe the 24s have different plate voltages in that set. I'll have to look up my notes. It has been many years since I worked on my set. The cap across the choke could be a problem, but I would check the resistance in the power resistor first.

Gilbert

jeyurkon · #8 03-24-2009, 10:23 PM

Quote:

Originally Posted by andrefranc

I checked the ceramic resistor in circuit and no opens. Should I disconnect it to check or is in circuit good enough? I was told that between 2 and 3 it should be 2450 ohms and I only have 180 ohms there. How do I know what it should be? One 24 has a plate voltage of 273, 1st tube, and the next one only has a plate of 103 volts. Can't see why though. I replaced the caps inn the power supply with electrolytics, 10's, but the cap across the choke I left as is, would it be the problem?

The schematic on nostalgiaair shows that it should be 187 ohms between 2-3. It would be 2470 between 5-6. Your reading seems correct. The plate voltages are off a bit, but not unreasonably so, in my opinion. The schematic gives conflicting information here. In a table it shows they should both be 250V but on the schematic it shows 225V for the first 24 and 130V for the second 24. And as was noted by Reece the table shows 215V for the 71's but 190V on the schematic.

Knowing the readings for the other tubes might help.

John

radio63 · #9 03-25-2009, 03:01 AM

The data on Nostalgia Air is very helpful if you don't have a hard copy of the schematic in front of you. However, I would measure the resistance as I had mentioned earlier, by disconnecting one of the leads of each section from the circuit.

Here's some additional info from my notes of 20 some odd years back, voltages I measured at the resistor. These are approximate, and just to give you an idea what another Model 20 once measured. The resistor has 7 terminals with number 1 toward the front of the chassis. Voltages were measured to the chassis if I recall correctly.

terminal 1 +50 volts (varies with volume control)
terminal 2 -6.8 volts
terminal 3 chassis ground
terminal 4 +2.4 volts
terminal 5 +12 volts
terminal 6 +92 volts
terminal 7 +120 volts

Again please remember these voltages are approximate, but if you measure something very different from the above voltages, it might be an indication of possible trouble. I worked on this set in 1985, so it's been a long time but this is what I have in my notes. Another trouble spot on these early Philco sets are coils that corrode and open. That may be another thing you wish to check out. Good luck and let us know how it goes.

Gilbert

andrefranc · #10 03-25-2009, 01:17 PM

Hi Gilbert,
Found my problem. The plate voltage on the second 24 was 102 volts and on the first 24 it was much higher like 210 so I started checking around with my ohmmeter and found that the primary winding leading to the plate of the second 24 was OPEN. I don't know how the radio could work at all with that open. Took it out completely and measured it and still wide open so rewound it and reinstalled and it works perfect. All kinds of volume now. Thanks for your help.

radio63 · #11 03-25-2009, 05:51 PM

Hi Andrefranc,

Great! Glad you figured it out. Yeah sometimes there is just enough gain that even with an open stage, the signal squeaks through, either by coupling or some other way.

As I had mentioned perviously, open coils are a fairly common problem on early Philco sets. Corrosion sets in between windings and causes the coil to open.

Congratulations, glad it's playing. Enjoy, it's a neat radio!

Gilbert