I am just finishing my restoration of a RCA 128.
http://www.nostalgiaair.org/PagesByModel/737/M0014737.pdf

The chassis was completely recapped and gone over head to toe. It was working about a month ago when I shelved it to work on the cabinet. I just finished the cabinet and went to reinstall it and I get no sound or reception. The speaker slightly vibrates to touch. I have listed the socket voltages below. Basically, no cathode or screen voltages, high plate voltages and filament voltages are fine.

The only sound I could get was when I was probing for voltages on the SG of the output I heard a loud hum. Other than that nothing by touch the volume or grid caps.

I have a feeling it has something to do with volume pot or AVC. I took apart and recleaned the volume control and even put another 0.05uF cap (C35) in just for good measure and it didn't help. It was working but I have no idea what could have changed since then????

Any ideas???

Check all your ground connections around the power supply.

For whatever reason it appears that either R21 (10K) is blown or you have lost the the connection on one end or the other end of it. So you need to check from the bottom end of R21 to ground and make sure there is no short. You should also check the bottom end of R22 (5K) to ground to make sure there is no short. If either of these show a short (anything less than 100 ohms) to ground you will need to find it before replacing any blown resistors.

If there is a short, C16 and C17 would be suspect. Also any solder joints where something may be touching ground inadvertently. Make sure C16 (4 mfd) is not installed backwards (should be postive to the screen voltage line).

Basically check everything hooked to the screen voltage lines.

Curtis Eickerman

Additionally, if it turns out that 10,000 ohm R21 is blown (as opposed to merely "disconnected"), and no short is discovered, what wattage resistor was it?
Was it one that you replaced during your restoration efforts? If so, what wattage did you use? Maybe you simply didn't use one high enough and that's why it blew? Regardless, if it's blown, you'll need to determine the correct wattage to use when and if you replace it.

The data sheets don't give the value for some reason, but we can calculate it based on the voltages given in the table.
The table shows 265v on the top side, and 105v on the bottom, giving a drop of 160v.
Using ohm's law (voltage squared/resistance) we get:
160v squared/10,000 ohms = 2.56 watts actual power dissipation.
You'd want to double or triple that value for insurance. Might as well use a 10 watt resistor.

While you're at it, maybe 5,000 ohm R22 and R23 ought to be beefed up if you've replaced those.
The parts list shows them at 1/2 watt, but this seems pretty marginal. Using the same method as above, I calculate actual power dissipation at .605 watts and .5 watts respectively. If I were replacing those, I'd go with 1 or 2 watt resistors there.

UPDATE

I used a 5W resistor for R21 (10K) and all other resistors I replaced were with 2W whether it called for that or not.

I checked R21 (10K) and it meters out good.
I also checked out R22 (5K) and R23 and they also metered out good.
The electrolytics were rechecked and none are reversed.
I poured through the chassis looking for a bad ground and could find none.
I also tacked in different new caps at C46 and C16 just in case one was bad; no difference.

Couple observations:
1) I have 390V at my positive side of C46 (4uF) but no voltage at the positive side of C16 (4uF).

2) With the radio on and the meter in the resistance mode I can hear all kinds of clicks and scratches at various points throughout the radio. When in DC mode I hear no such noises.

Is C16 after your field coil?

Might be worth voltage checks on the high and low side
of R21, to see what you get while powered up. And maybe
do a continuity check between the high side of R21 and C16
without power.
Nelson

Edited for clarity.

What is the voltage at C44? Also have you checked the resistance of the speaker field coil and made sure you have not lost a connection there?

If R21 is good and there is no short at the bottom to ground, the only thing standing between your screens and voltage seems to be the speaker field coil (1290 ohms) and wiring. If the field coil is blown you need to check the resistance to ground at C44 to see if there is a short that blew the field coil.

Curtis Eickerman

I finally got it working!

All of your suggestions seemed to center around R21 (10K 5W) which was a brand new resistor that I made a special trip into the city to get. I pulled it and even though it looks perfectly good it was reading waaaaaay above value???

Anyhow it is now working again and I appreciate the guidance. I will update my start to finish restoration thread on this RCA 128 when I get a few pics of the completed radio tomorrow.

IOW S**T Happens.

This may have been just one of those incidents that despite ones best efforts, happen.

I note that the plate current is given for tubes but non for screens. I did not look at circuit but if that resistor is feeding B+ to the tubes that stopped, a voltage check across it would indicate the current being passed. Screens may not go via it. Cathode voltage is also a way of detirmining what current a tube is passing. It may pay, as suggested to check a few voltages, just in case something is running overloaded.

Design mistakes do happen. I note in an Australian HMV a screen resistor was changed (accounts department?) from two 22K in parallel to one 10K. Brilliant... the sum total of the screen currents is in excess of 10mA for the voltage, so the 1W resistors substituted, have quite a high attrition rate.

Marc