LA2A voltage issue
« on: October 16, 2020, 01:03:59 PM »
My first post in GDIY. I built an LA2a p2p-I have a problem on V4, (6AQ5a), pin 5-voltage chart suggests I should read 100VDC there, I am getting 197VDC. I do not know enough to know if this can be damaging, but it does seem like it could be an issue. All of my other voltage points read higher than the chart lays out, but this spot is the only spot that exceeds 30% higher...can anyone tell me if this is cause for concern? I've attached a copy of the voltage chart with my readings to the far right...

Thanks for any help!


fallout

Re: LA2A voltage issue
« Reply #1 on: October 16, 2020, 07:01:40 PM »
Your B+ voltage (@ C7B) is too high. Is R29 the correct value (4.7k 2W) and are you sure it's wired to the rest of the circuit correctly?

What are you measuring after the rectifier (diodes CR1 and  CR2)?

I would start there and scrutinize that particular area of the circuit.

« Last Edit: October 16, 2020, 07:28:24 PM by fallout »

Re: LA2A voltage issue
« Reply #2 on: October 17, 2020, 01:28:14 PM »
Thanks for responding. To answer you questions, R29 is correct value, and I've twice checked my wiring against the schematic, and as far I I can tell, it is wired correctly.

CR1 and 2 are reading 380VDC on the cathode end, and 284VAC on the anode end.

My schematic, attached reflects using 1N3196 diodes, and your attachments reflect 1N4007.

I am not a pro at this, do I need to change mine out?

fallout

Re: LA2A voltage issue
« Reply #3 on: October 17, 2020, 02:00:58 PM »
CR1 and 2 are reading 380VDC on the cathode end, and 284VAC on the anode end.

What is your reading on the other side of R29 (the side not connected to the diodes)?

Re: LA2A voltage issue
« Reply #4 on: October 17, 2020, 02:41:06 PM »
298vdc

Re: LA2A voltage issue
« Reply #5 on: October 17, 2020, 03:06:33 PM »
May or may not be of any help but I recall having a similar issue in my very first LA2A years ago and mine related to the fact that there are 470, 470k, 47k, and 4.7k resistors in this build and it's possible to get them mixed up if you're not paying close attention.

Re: LA2A voltage issue
« Reply #6 on: October 17, 2020, 03:13:12 PM »
@Bowie-
Thats for sure, I made that mistake with an 1176. However, this one measures 4.7K

fallout

Re: LA2A voltage issue
« Reply #7 on: October 17, 2020, 10:24:14 PM »
Unless I'm misunderstanding, the initial measurement for C7B was 370V (first post) and now you're measuring 298V? It still seems a bit high.

What power transformer are you using? Is the compressor working?
« Last Edit: October 17, 2020, 11:39:55 PM by fallout »

abbey road d enfer

Re: LA2A voltage issue
« Reply #8 on: October 18, 2020, 05:29:26 AM »
Everything I read here draws the same conclusion. The transformer secondary voltage is way too high. Simplest cure is to increase the value of resistor R29.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

 

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