New guy here, scratching my head with radio problem.

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Re: New guy here, scratching my head with radio problem.

Post by bobwilson1977 on Mon Feb 09, 2009 4:51 pm

Thanks again for all of the help thus far. Will be checking those voltages out once I procure a multimeter.

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Re: New guy here, scratching my head with radio problem.

Post by Switcher on Sun Feb 22, 2009 10:44 am

So ah how did you make out with the voltage checks Bob?
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Re: New guy here, scratching my head with radio problem.

Post by simplex1040 on Sun Feb 22, 2009 12:18 pm

have you tested the speaker to see if it works?
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Re: New guy here, scratching my head with radio problem.

Post by Resistance is Futile on Fri May 01, 2009 4:03 am

bobwilson1977 wrote:Apologies, its actually a 6 tube unit. The model is a A300 Hoffman. Now, when I touch the center tab of the Vol. control, I assume I would also be touching the chassis? Thanks in advance.
No just touch the center tab on the volume control, if the set has DC then the stray AC voltage would be amplified to produce a buzz. If it buzzes then the audio section is OK. That is a first quick step.

If it doesn't buzz then I would question the High voltage at pins 3 and 5 of tube 6X5 for 290VAC
then if thats present,
then the voltage at the cathode, it should be 325VDC.
If that checks OK,
then the next location to check would be the field coil winding or its replacement R14 of 1500 ohms. If it has an open then you would not see a voltage on C10,C19, or pin 4 of the tube 6K6 of 227vdc.
If you have that voltage then check to voltage of the plate of 6K6 (pin 3) of 217v
if you get that then the primary winding of the audio transformer is OK.
I would then suspect either the secondary winding is bad,
or the voice coil is bad.
If not then you will have to continue by signal tracing each stage in turn.
But these are the most obvious steps I would try first. Cool
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Re: New guy here, scratching my head with radio problem.

Post by jewel's old time radio's on Sat Feb 27, 2010 11:20 pm

It sounds like the output trans to me

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Re: New guy here, scratching my head with radio problem.

Post by Alfred Corbin on Tue May 25, 2010 9:37 am

Bob: When you check the audio part by touching the volume control, you touch it with your finger, perhaps while holding a probe or screwdriver.

You must NEVER, NEVER touch the chassis with the other hand while working. That's inviting a serious shock right across your heart. In the Navy we were taught to keep the idle hand in your pocket. Unmilitary, but smart!

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Re: New guy here, scratching my head with radio problem.

Post by Neon27 on Sun Aug 01, 2010 6:21 pm

Solution from the resident Electrical Engineer (ME! hehe)

Quick Fault-finding Process:
1. Check the voltages on all the filament pins on the tubes.
2. Filament voltages good? Check the HV on respective plate pins on all tubes (there's a nice look-up chart on the second page of the schematic you've referenced)

3. HV also good? Connect your oscilloscope to the output of the first audio stage (the anode of 6SQ7). If you have a nice audio waveform bouncing all over the place on your screen (you need to tune your radio up and down until you see one), you know that your problem resides in the final Output stage (6K6GT). Now you know where to look for the problem.

4. No nice bouncy audio signal? Connect your Oscilloscope to the output of the IF stages (6SA7 and back), and make sure you're getting a nice AM envelope (you're going to have to tune up and down the dial again to find a radio station).

5. No signals present? Check tuning caps aren't shorted (all the funny little tuning caps on the gangs, on the chassis etc...) You can do this with a continuity tester.

I can't get into too much detail here, but you're going to have to repair your local oscillator.

DISCLAIMER: I'm not sure of the level of your electronic expertise, the processes I've mentioned above may cause damage to your equipment if not carried out correctly.

As an additional note, I'm not sure the condition of your radio's power supply, so make sure to isolate your oscilloscope's supply with an isolation transformer. If not, you could still perform your tests by not connecting the Probe's ground pin to any part of the radio. Also make sure to use voltage-divider probes, and set them to the highest value when performing initial tests. An isolation capacitor in series with the test probe may be necessary in some tests as well.

let me know how goes the repairs!
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Re: New guy here, scratching my head with radio problem.

Post by Resistance is Futile on Mon Aug 02, 2010 4:07 pm

If you touch it with your finger, just use your index finger, and not touching any other part of the radio at the same time, you should get a loud 60 cycle hum or buzz. this would indicate your audio section is working. If no noise then the audio section is needing troubleshooting.
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Re: New guy here, scratching my head with radio problem.

Post by Nick666 on Mon Dec 06, 2010 7:07 pm

Start testing resistors and test ALL of them, not just the ones that "look bad". I've had many resistors that were open that had no visual flaws to make me suspect them. If the radio has been compleatly re capped and all of the tubes test good i would suspect a bad (probably open) resistor and i would think that it is in the output section of the radio.I had a RCA that had a problem that sounds the same as yours and i fought with that radio off and on for 2 years before i decided to do as everyone had been telling me and test the resistors and shure enough there was a bad one on the grid of the 6k6 output tube.

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