AW 23 amplifier voltages low

[John Stafford]

I have finished replacing capacitors on my AW 23. Radio has been powered up and is working on all but the Green Band. Reception is okay but nothing to brag about.

Now that it is working I started checking voltages and have found most of the readings for the Amplifier are 10 to 20% low. The Service Data shows expected voltage readings while specifying a line voltage of 112 Volts. The document does not indicate which position the Hi/Low switch should be in but since the specified Voltage is > 110 Volts I have assumed that "Hi" is the correct position.

Filament Voltages are 2.2 VAC and 4.6 VAC compared to the expected 2.5 and 6.5 VAC.

The 2A3 plate Voltage is 253 VDC compared to the expected 310 VDC.

The only Voltage that is near spec is the 2A3 - Bias. It is -62 VDC.

Changing the Hi/Low to the Low setting increased the Voltages slightly but they were still quite low compared to the documentation.

Has anyone seen a similar situation? I don't want to proceed with further investigation into the poor reception issue without first knowing that low filament and plate voltages are not part of the problem.

[Kent King]

John -

That is interesting, with today's high line voltages, I always use the "Hi" setting and still have voltages near the top of the acceptable range. What is your local line voltage (mine is usually around 123VAC)? 

Check the field coil on the speaker, and also the resistor in the speaker (if you are not using tweeters). If the set has tweeters, be sure the fields on them are ok too. Also check the resistance on the two filter chokes, they sometimes (rarely) do go bad. 

Make sure all the resistances of the fields and/or resistor on the speaker are in range first. You are right, these voltages should be higher and I wouldn't troubleshoot other issues until this is found. One last thing that comes to mind...if you put an ammeter in series with the B+ to the tuner, you should see about 50-55 mA draw from the tuner. If it is higher, you've got something in the tuner - but that should not impact the filament voltages. Last (worst case) power transformer is possibly bad.

Kent

[John Stafford]

Thanks Kent.

Line voltage at my house is reliably between 120VAC and 123VAC. I use a variac on the bench to set voltage as necessary for testing.

It is great to have typical current draw for the tuner B+, I will definitely check that. 

I will check the speaker circuits but don't expect to find much. The amplifier and speakers, including tweeters, are all working and sound very good when fed with an external audio source. My only complaint is a very slight background hum. Not sure what is normal here, the hum is not noticeable with low volume audio content present. The hum adjustment has been set to null. There definitely is a minimum setting so that is working.

The evidence seems to point to the power transformer since the secondaries all measure low. Overall AC power input seems normal at 180 - 190 Watts. 200 Watts is listed as typical. 

Not sure what to think. The power transformer runs only slightly warm to the touch after hours of run time. No excess input current as one might expect if there were shorted primary turns. 

John S

[Brad Winder]

John, I thought I would chime in...Maybe Kent could tell us if this is a possibility ...did Scott build power supplies for 25 cycle use? Could that be the issue with the differences in voltages and the fact your transformer only runs warm to the touch?

[David C. Poland]

Yes there are some 25 cycle Scott radios. They have an oversized power transformer (because additional iron mass is needed)., sometimes big black metal box instead of the chrome housing. No problem using a 25 cycle radio on 60 cycle power.

Use a good AC meter to verify you have the Variac set at the desired voltage ( I use a digital for this purpose).

At that house voltage, for the hi-low switch, you want the position that dims the bulb and tube filaments a bit.

[John Stafford]

I have located two problems resulting in low voltage readings. My testing was done with a line voltage of 112 VAC per the Technical Service Data document. I measured this at the AC plug. What I found was that there was excessive voltage drop across the On/Off switch that had been at the back of my bench getting quite warm. As a result the voltage at the input to the Power Transformer was significantly less than 112 VAC.

Once the switch issue was corrected all voltages were in normal range except the 2A3 Plate voltage. This was easily traced to a weak 83-V rectifier tube.

Now that the Amplifier/Power Supply is sorted it is time to move on to the Set Chassis. With regard to the expected B+ current for the Tuner, I found this to be steady at 52 mA. Nicely within the expected range of 50 to 55 mA Kent said to expect. Thanks for all of the suggestions.