UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Reply
 
Thread Tools
Old 16th Oct 2021, 4:44 pm   #21
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,572
Default Re: Lafayette HA-350

Hi folks. The Cheap Chinese meter arrived today and I've fitted it. The scale sizes weren't exactly the same but I managed to photocopy the old dial and print a paper sticky to cover the dial on the new meter. It doesn't look too shabby. Unfortunately there is a large protruberance at the back of the replacement meter that fouls the RF amplifier valve but I've got around that by mounting the meter slightly higher up. I've now found why the old meter might have failed. If you look at the circuit then you can deduce that the meter deflection is caused by the offset between the V6 grid 2 voltage and the cathode voltage (via potential dividers). Because this set has silicon diode HT rectifiers the HT comes up straight away before V6 has warmed up and begun to conduct. Therefore, there's a large positive offset voltage at the meter and it hits the stop. Once V6 conducts, all is well. I don't want to trash another meter so I'm pondering what's best. I could fit a standby switch in the S-meter leads but I have to remember to turn it off when the set is turned off. I can see now why somebody fitted the transistor S-meter driver mentioned earlier; I wouldn't use that circuit but I'm thinking of a single transistor amplifier taking the AGC voltage as control signal. Any thoughts welcomed. Cheers, Jerry
Attached Thumbnails
Click image for larger version

Name:	P1080471.jpg
Views:	42
Size:	71.2 KB
ID:	243537   Click image for larger version

Name:	P1080472.jpg
Views:	33
Size:	88.8 KB
ID:	243538  
Attached Files
File Type: pdf Lafayette S-Meter.pdf (211.6 KB, 16 views)
cathoderay57 is offline   Reply With Quote
Old 17th Oct 2021, 3:36 pm   #22
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,572
Default Re: Lafayette HA-350

I lashed up a circuit using a BC557 PNP with the S-meter in the collector load and the transistor's base taking bias via a 1.2M potential divider off the AGC line. In principle, the S-meter worked but there was too much loading of the AGC line which took the AGC voltage at strong signal input from -6.8v down to about -4.0v which is no good. I've found a few example circuits using FETs so I'll have another go. Cheers, Jerry
cathoderay57 is offline   Reply With Quote
Old 17th Oct 2021, 9:18 pm   #23
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,572
Default Re: Lafayette HA-350

Here's a link to the circuit that I am thinking of using http://www.pan-tex.net/usr/r/receivers/ssmeter.htm. However, I'm not sure that the polarity of the diode protecting the FET gate is correctly shown. Since AGC is negative (in my case about 0v to -6.8v) surely the diode as drawn would simply short circuit the AGC line? Shouldn't it be the other way round, or am I missing something?
cathoderay57 is offline   Reply With Quote
Old 18th Oct 2021, 7:05 am   #24
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,572
Default Re: Lafayette HA-350

Maybe a simpler solution would be to use a 1N4148 diode shunt across the meter to bypass the excess dc at switch-on. The meter has a resistance of approx 1k and FSD 500uA means it needs only 0.5v (i.e. below the diode cut-off at 0.6v) therefore I think this should work and I can keep the S-meter circuit otherwise the same.

Last edited by cathoderay57; 18th Oct 2021 at 7:19 am.
cathoderay57 is offline   Reply With Quote
Old 18th Oct 2021, 9:28 am   #25
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,572
Default Re: Lafayette HA-350

Yep, the diode worked a treat. The meter now swings over to max at switch-on, but only just max, and isn't hard against the stop. After about 15 seconds the valve starts conducting and the meter drops to zero. An adequate fix. I'm still not convinced the FET circuit hyperlinked in post #23 would work properly as-drawn since it permanently limits the AGE line to -0.6v. Jerry
cathoderay57 is offline   Reply With Quote
Old 22nd Oct 2021, 11:51 am   #26
Granitehill
Tetrode
 
Join Date: Mar 2019
Location: York, North Yorkshire, UK.
Posts: 80
Default Re: Lafayette HA-350

Quote:
Originally Posted by cathoderay57 View Post
Yep, the diode worked a treat. The meter now swings over to max at switch-on, but only just max, and isn't hard against the stop. After about 15 seconds the valve starts conducting and the meter drops to zero. An adequate fix. I'm still not convinced the FET circuit hyperlinked in post #23 would work properly as-drawn since it permanently limits the AGE line to -0.6v. Jerry
Now that I think about it - my HA350 did the same on switch on. Another owner also reported the same. The full scale deflection wasn't really alarming - the over voltage wasn't too drastic, apparently.
It looks as if the manufacturers were aware of this, but didn't think it merited some extra components!

The diode across the meter is definitely a good idea though.
__________________
John G4FDD
G-QRP 431
Granitehill is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 2:30 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.