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Old 15th Nov 2019, 12:48 pm   #1
cathoderay57
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Default Trio Lafayette KT-320 Question

Morning all, I am busy restoring a Trio Lafayette KT-320 communications receiver. The rectifier is a Brimar 5Y3GT and the data book (A.M. Ball) specifies a max res capacitance of 32uF and min series resistance of 30R. The Valve Museum data sheet (maker GE) specifies lower max res capacitance of 20uF and typ series resistance of 50R. In the Lafayette, the rectifier anode voltages are 200-0-200v and the dc resistances of the transformer HT windings are 136R and 140R to centre-tap respectively. the value of res cap specified in the Trio Manual is 40uF. The original cap has been replaced in the past with a 50uF. Since the anode voltage is relatively low compared to the 5Y3GT spec and the dc resistance of the windings provides well above minimum I am incline to leave the 50uF as-is (providing it reforms OK), in the belief that the rectifier life is unlikely to be seriously compromised; the set is unlikely to be running all day every day. The alternative would be to stick in a 32uF replacement. Thoughts? Cheers, Jerry
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Old 15th Nov 2019, 7:53 pm   #2
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Default Re: Trio Lafayette KT-320 Question

TBH I wouldn't get excited about this - just go with what you've got.

If the 5Y3GT does suffer any 'issues' you can always replace it with one of the packaged silicon-diode equivalents [historically sold by "International Rectifier" but nowadays easily-achieved with a couple of UF4007 diodes in an old Octal valve-base] - which would have the additional benefits of reducing the heat-dissipation inside the case [meaning less heat-induced frequency-drift] and the total power-demand on the transformer.
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Old 15th Nov 2019, 8:23 pm   #3
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Default Re: Trio Lafayette KT-320 Question

The secondary AC voltage is low and the series resistance is high compared to what a 5Y3GT is rated to cope with, so they obviously felt they could get away with a relatively generous amount of reservoir capacitance. I'm sure that if it was a problem that Trio would have had a rash of early failures and a lasting reputation for it. Some folk take "maximum capacitance" from one website or another (who may only have got their information from short-form "guide" data anyway) very seriously without considering ameliorating factors such as effective series resistance or secondary voltage- if you dig down far enough into rectifier data-sheets, there will often be derating guides or even a graph of recommended maximum reservoir capacitance vs. other factors.

Without detailed information, it's only wise to be cautious, though, and directly-heated rectifiers like this will be more vulnerable to excess cathode surge current.

Come to think of it, 32uF and 30 ohms is quite generous for a mid '30s design (arguably earlier....) directly-heated rectifier- it might be that the Brimar produced ones were from fairly late in the day and benefited from a couple of decades of development of the technology.

Last edited by turretslug; 15th Nov 2019 at 8:29 pm.
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Old 15th Nov 2019, 11:29 pm   #4
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Default Re: Trio Lafayette KT-320 Question

Thanks for the advice, duly taken. The 50uF reformed OK and the radio is beginning to work, no hum. Measured HT at the rect cathode is down about 10v from spec which is probably due to a slightly tired 5Y3GT. On the VCM163 it measured barely 50% emission (although directly heated rectifiers always seem to measure a bit low compared to indirectly heated ones). I'll recheck with a better 5Y3GT. The AF output valve 6AQ5 is showing commensurately lower anode and G2 volts and all coupling and decoupling caps checked and in good order (mostly mustards). Cheers, Jerry
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Old 16th Nov 2019, 4:20 pm   #5
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Default Re: Trio Lafayette KT-320 Question

Hello again, the lower than expected anode voltages were caused by an issue for which the 5Y3GT was blameless; I had got caught out by something I should have been aware of I measured all the valve voltages and compared to the manual. Two of the 6BA6 (IF amp and RF amp) should have had 19v on the cathode and both measured zero. Metering the cathodes showed short circuit to chassis. It wasn't heater cathode leakage either. On the circuit diagram g3 was strapped to chassis earth while the cathodes were earthed via bias resistors (which is how it had been wired). The 6BA6 valves from Bentley Acoustic Corporation had internal g3-cathode connections so both valves had cathodes direct to chassis and therefore were biased fully on taking excess anode current. The third 6BA6 from a different maker had separate g3 and cathode connections so was OK. For future interchangeability I rewired all 3 valves with g3 strapped externally to cathode and the radio now works well, with all valve voltages within tolerable limits. I should add that this is a kit-version of the radio and so Trio are fairly blameless (although IMHO the circuit in the manual should have been drawn with g3 to cathodes not chassis). Cheers, Jerry
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Old 16th Nov 2019, 4:51 pm   #6
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Default Re: Trio Lafayette KT-320 Question

I winder if the 6BA6 Bentley acoustic were Russian EF93’s
http://oldradio.qrz.ru/tubes/russian/short/ef93.shtml

West European EF93’s had the g3 and k separate.
http://www.r-type.org/exhib/aaa0604.htm
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Old 16th Nov 2019, 4:58 pm   #7
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Default Re: Trio Lafayette KT-320 Question

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The 6BA6 valves from Bentley Acoustic Corporation had internal g3-cathode connections so both valves had cathodes direct to chassis and therefore were biased fully on taking excess anode current.
How bizarre! According to the Valve museum & my Brimar Teletube handbook, g3 is strapped to the internal screen.
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Old 16th Nov 2019, 5:51 pm   #8
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Default Re: Trio Lafayette KT-320 Question

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Originally Posted by cathoderay57 View Post
Morning all, I am busy restoring a Trio Lafayette KT-320 communications receiver. The rectifier is a Brimar 5Y3GT and the data book (A.M. Ball) specifies a max res capacitance of 32uF and min series resistance of 30R. The Valve Museum data sheet (maker GE) specifies lower max res capacitance of 20uF and typ series resistance of 50R. In the Lafayette, the rectifier anode voltages are 200-0-200v and the dc resistances of the transformer HT windings are 136R and 140R to centre-tap respectively. the value of res cap specified in the Trio Manual is 40uF. The original cap has been replaced in the past with a 50uF. Since the anode voltage is relatively low compared to the 5Y3GT spec and the dc resistance of the windings provides well above minimum I am incline to leave the 50uF as-is (providing it reforms OK), in the belief that the rectifier life is unlikely to be seriously compromised; the set is unlikely to be running all day every day. The alternative would be to stick in a 32uF replacement. Thoughts? Cheers, Jerry
My Trio-built Lafayette receiver uses a Japanese made 5CG4 rectifier valve, the original one.
It's quite a neat receiver!
Dave, USradcoll1.
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Old 16th Nov 2019, 11:09 pm   #9
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Default Re: Trio Lafayette KT-320 Question

That "6BA6" source discrepancy is a useful thing to be wary of- it sounds as if some importers were a little cavalier in their re-labelling. It seems that the 6BA6 was effectively the miniature all-glass successor to the 6SG7, but the latter had the advantage of an 8-pin base, so it could have both two cathode connections and a separate suppressor grid connection, whereas the 6BA6 was restricted by a 7-pin base and could have offered one or other of these features but not both.

The 5CG4 appears to have been indirectly-heated, so maybe nearer the 5Z4GT in characteristics (warm-up time, forward voltage drop) than the 5Y3GT?
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Old 17th Nov 2019, 6:05 pm   #10
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Default Re: Trio Lafayette KT-320 Question

Help! I have been trying to finish resto of this receiver. It's a bit deaf on a couple of bands but I will do a realignment later. The problem that is driving me nuts is I can't get the BFO or Q-Multiplier to work. I've attached a sketch of the circuit. In BFO mode the switch S4 is open so it uses stray capacitance coupling. I followed the set-up in accordance with the Trio construction manual but I don't think the BFO is oscillating. When I tune to an SSB station the BFO capacitor has no effect. Maybe a clue is when I close S4 (for Q-Multiplier mode) I get total silence. The way the Q-Multiplier is supposed to be operated is to turn on S4 then advance R26 (the selectivity rheostat) up to just short of the point of oscillation. Rotating R26 from one end to the other just gives complete silence until S4 is turned off. The receiver works normally when the selector switch is set to REC AM (in this mode the BFO cannot operate). I have tested the BFO triode oscillator valve 6AV6 and it tests excellent. Tried a substitute; still no joy. Removed and tested the mica capacitors on an Avo bridge C10 measures 0.001uF (spot on), C11 measures 0.003uF (spot on), C12 measures 550pF (should be 500pF but within 10% so should be OK). Resistance of T6 (BFO coil) is about 4 Ohms. R9 measures 2.2M. With the 6BE6 oscillator valve removed and a 455kHz unmodulated signal applied to the grid of the mixer and switched to BFO there is no heterodyne tone which is what makes me think the BFO isn't oscillating (or for some unknown reason is miles off frequency). Turning on modulation on the 455kHz sig gen signal produces the expected tone (as you'd expect because the receiver works fine on A.M. and this also proves the IF is set to the correct freq). Voltage measurements as follows (with the 6BE6 oscillator valve removed): BFO 6AV6 anode 133.8v. HT feed to R8 (22k) 142v (indicates anode current of approx. 0.4mA). 6AV6 cathode voltage with R26 s/c 1.13v (indicating cathode current of approx. 0.34mA). Any ideas folks? Cheers, Jerry
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Old 17th Nov 2019, 7:00 pm   #11
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Default Re: Trio Lafayette KT-320 Question

'scope?

Lawrence.
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Old 17th Nov 2019, 7:50 pm   #12
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'scope?

Lawrence.
Yes,
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Old 17th Nov 2019, 9:03 pm   #13
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Default Re: Trio Lafayette KT-320 Question

One thought, are you switching the AGC to manual? If it is oscillating it will produce a high AGC voltage and desensitise the radio.
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Old 17th Nov 2019, 9:33 pm   #14
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Default Re: Trio Lafayette KT-320 Question

Thanks for the replies. I have a slightly dodgy scope that runs up to about 1MHz and no RF probe but I'll give it a go tomorrow and see if the BFO has taken off at some obscure frequency. Not sure why it should do so though because the components and layout are as per Trio manual. I did try a 10nF decoupler on the HT feed but it didn't make any difference. Cheers, Jerry
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Old 17th Nov 2019, 10:06 pm   #15
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Default Re: Trio Lafayette KT-320 Question

Correction, it's a 10MHz scope Tektronix T912
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Old 18th Nov 2019, 12:22 pm   #16
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Default Re: Trio Lafayette KT-320 Question

Got the scope on this morning. Luckily the timebase decided to work properly; sometimes it doesn't. On the BFO output I got a nice fat signal, sinewave period about 2.2 microseconds which given the scale accuracy is close enough for 455kHz. As I tweaked the tuning slug in the BFO coil the freq shifted as it should. The problem was I had tweaked the slug last night to try to get the BFO to work but the sweet spot is quite narrow and I must have missed it. Being more careful today I found the right point (it had only been slightly out) and now the BFO and Q-Mult functions both work correctly. pulled in a German SSB on 40m no problem. Many thanks for the advice. Cheers!
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Old 18th Nov 2019, 4:41 pm   #17
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Default Re: Trio Lafayette KT-320 Question

Excellent, I had the built version 50 odd years ago.
The LO was not the most stable but ok, it had harmonics as well which didn’t matter too much apart from the highest frequency band that would receive the local VHF FM stations that were in Band 2, they were tuneable so didn’t cause a problem.
Had it a good number of years, good for SWL but not really good enough for amateur station.
I did try various things to improve the LO but nothing worked, could even be the dial cord system that didn’t help.
When I got my license in the 70’s I bought a KW2000, that was a nice rig.
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Old 21st Nov 2019, 5:05 pm   #18
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Default Re: Trio Lafayette KT-320 Question

Hello again, unfortunately the BFO problem has returned - not running. I put the scope on again and this time definitely no oscillation. 6AV6 heater on, HT on anode, cathode about 1v so passing current. I touched a 4uF electrolytic across grid end of R8 (22k) - see circuit image in post #10, and the BFO oscillator started running and stayed running after I removed the 4uF. Any suggestions to ensure the BFO oscillator starts reliably? I'm thinking HT decoupling maybe, or else replace C10, C11 & C12 (although they tested OK previously - see post #10). Cheers, Jerry
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Old 21st Nov 2019, 7:00 pm   #19
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Default Re: Trio Lafayette KT-320 Question

What type of capacitors are C10,11,12, if the are the MicaMold type I would replace them.
Is R26 in good condition? That being intermittent could stop the osc.
Is there signs of damp in the BFO coil?
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Old 21st Nov 2019, 9:58 pm   #20
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Default Re: Trio Lafayette KT-320 Question

Hi Frank, R26 is a pot wired as a rheostat. In the BFO position the control is at minimum resistance i.e. short circuit with slider connected to chassis. I will check resistance from cathode to earth to make sure though, then whip the BFO sub chassis out again and change the 3 caps. They do look like micamold types but I thought as they are 1960s vintage they would be OK as I couldn't measure any leakage on the Avo8 megohms range (albeit at only 15v) and the capacitance values measured OK on the Avo bridge. At least if I eliminate the caps I can then look for something else if the problem persists. I wondered if the iron dust plug had come off the end of the adjuster screw in the BFO coil but it must be OK as the BFO freq still changes as you turn it (when the BFO is running, that is). As found, there was a 10nF cap wired in parallel with R10 the cathode resistor. Since it wasn't there in the circuit I removed it as it was evidently a later fitment. Not sure if it would do much to help? I haven't opened the coil can but there is no evidence of damp anywhere else in the set. Cheers, Jerry
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