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Old 24th Nov 2021, 5:42 pm   #1
Ernest1
Triode
 
Join Date: Mar 2021
Location: North Devon, UK.
Posts: 43
Default BRT 400 shorting?

I’ve some basic work on a Brt 400 . However, the selectivity band 6 appears to short and no sounds , the signal strength meter goes full. Any ideas ? I really haven’t delved into yet so thought someone may have had similar. My version also has a ‘cal’ button. I think this must be a later model? Thanks, Ernest.
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Old 25th Nov 2021, 9:28 pm   #2
G3PIJpeter
Hexode
 
Join Date: Nov 2011
Location: Chippenham, Wiltshire, UK.
Posts: 319
Default Re: Brt 400 shorting?

Does the problem arise when you switch to Band 6 - or when you operate the selectivity control? The S-meter circuit is given at Figure 5 in the handbook. If the S-meter is pinning against the right hand stop, then something is happening in the 2nd IF amplifier V6. Possibly a problem with S11, which switches HT to the anode load of V6.

I don't have a BRT400 but, looking at the circuit, the above would seem to be a place to start looking for shorts, burned contacts etc.

Peter G3PIJ
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Old 30th Nov 2021, 8:55 am   #3
Ernest1
Triode
 
Join Date: Mar 2021
Location: North Devon, UK.
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Default Re: Brt 400 shorting?

Hi , Peter G3PIJ thanks for the reply. It happens when selecting IF 6 on the ‘selectivity’ dial.
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Old 30th Nov 2021, 5:44 pm   #4
G3PIJpeter
Hexode
 
Join Date: Nov 2011
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Posts: 319
Default Re: BRT 400 shorting?

Ernest

I am looking at the circuit diagram on the final three pages of the BRT400 manual available from http://www.vmarsmanuals.co.uk/archiv...g_Handbook.pdf . The S-meter is connected between a potential divider in the cathode of V6 (2nd IF amplifier) and a potential divider across the HT supply. Therefore, I suppose that the overall S-meter circuit is the usual sort of bridge design.

You say that switching to 6 on the selectivity control pins the S-meter needle. Position 6 on the selectivity control corresponds to 9 kc/s, the widest position. If position 9.0 (as marked on S11 in the diagram) does not make contact with the wiper, then the 2nd IF will not be connected to anode volts via IFT2 and the valve will draw an abnormal current. That abnormal current will flow through the potential divider in the V6 cathode that is connected to the S-meter and will unbalance the bridge beyond its design parameters. The needle will whack the right-hand stop.

I suggest you look at S11 for signs of burning or breakdown or other mechanical failure. With the set on, check the voltage on each of the relevant tags (against ground) as you rotate the selectivity switch from 0.5 through 1.0, 2.0, 5.5, 7.0 and 9.0 (each should be roughly similar, at around the HT value). With the set off, you could also check the resistance between the wiper on S11 and the tags as you rotate the selectivity switch (should be very low).

As I said before, I do not own a BRT400 but that's where I’d start - S11.

Peter
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