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Old 23rd Mar 2017, 12:17 pm   #1
Guydio123
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Default Another dead Roberts RT22.

I've just joined the forum after reading the posts on here going for inspiration for some time.

I've recently acquired an RT22 for a fiver, cleaned off the nicotine and dust, installed a fresh battery, but no go. I managed to separate the chassis and there appears to be several dodgy looking components that may need replacing.

A BC328/338 pair look split, a few if the capacitors look like they've swollen and split their plastic coat.

I looked at replacing C18 and C19 first as they were mentioned and highlighted in an old post as a possible problem, but I have been having a problem matching a replacement as the writing on the side states 4u7 (u being micro) does this make it a 4.7 micro farad?

Thanks for any pointers you can give me.
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Old 23rd Mar 2017, 12:43 pm   #2
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Default Re: Another dead Roberts RT22.

Hello and welcome to the forums.

The service data for your radio can be downloaded here:-

http://www.service-data.com/product....16/7360/m15716

Changing components because they might be faulty is not a good method of repair. You need to do some fault finding.

Do you have a test meter of some kind?
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Old 23rd Mar 2017, 12:54 pm   #3
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Default Re: Another dead Roberts RT22.

4u7 is indeed 4.7 microfarads. This is a common value, readily available from all component suppliers. Which capacitors, other than C18 & 19, do you feel need replacing? BC328 & 338 transistors are common easily available types, but other transistors in the audio amplifier may also have failed. As Graham says, you need to do some fault-finding, preferably with the aid of the service sheet, which costs 1.99 and helps to fund the forum. You also need to be able to understand circuit diagrams, and interpret the radings you get with a multimeter.
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Old 23rd Mar 2017, 11:53 pm   #4
Guydio123
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Default Re: Another dead Roberts RT22.

Thanks for the warm welcome and initial help.

So far I've managed to do one test. I've got 9v at T1.
I think I've got the service sheet somewhere so I'll have to try and work my way through.

I've had another look at the service sheet but I'm not sure how much I'll be able to get done with just my multi meter as there seems to be a lot of testing using an oscilloscope. I've got 9v across C45 and 6.5v at R1.
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Old 24th Mar 2017, 8:46 am   #5
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Default Re: Another dead Roberts RT22.

I think you'll get there with just a meter
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Old 24th Mar 2017, 9:25 am   #6
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Default Re: Another dead Roberts RT22.

Get the DC conditions correct to start, it will not work with no current or volts. Measure the set current consumption.

Measure the voltage on all the transistors relative to battery negative. Be careful not to short base to collector when doing this.
Make a table of e,b,c and calculate transistor currents.
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Old 24th Mar 2017, 10:13 am   #7
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Default Re: Another dead Roberts RT22.

Concentrate of the Audio Frequency (AF) stages ie TR7 to TR11 first. They're much simpler than the preceding IF and RF stages.
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Old 25th Mar 2017, 12:59 am   #8
Guydio123
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Default Re: Another dead Roberts RT22.

Thanks for the suggestions. This evening I worked through the testing the voltages from the service sheet. All were fine (some within 10% variance) except for the test point at r33 which reads 0 volts when the vhf button is depressed and then reads 2.64 when in mf or lf. I assume it's a faulty vhf switch but I still have no output from the other switches either?
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Old 25th Mar 2017, 10:00 am   #9
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Default Re: Another dead Roberts RT22.

That's OK. Whether TR6 conducts or not, causing a volt drop across R33, depends on a voltage being applied to its base via S4 contacts a and b. When you operate this switch you should see the voltage on TR6 base change.

Check the speaker for continuity.

Do you hear anything from the speaker if you touch the wiper tag of the volume control with a metallic object?
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Old 26th Mar 2017, 12:05 am   #10
Guydio123
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Default Re: Another dead Roberts RT22.

Thanks Graham, it was the speaker. It turns out that both radios I'm working on have dead speakers. Replacement have been ordered. Thanks for everyone's input.
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