letterbeacon

DIY BA6A not working anymore
« on: May 17, 2019, 03:29:46 PM »
Ages ago, back in 2015 I built an RCA BA6A using original transformers etc. with huge amounts of help from the good people of this forum. It sounded great, but then I had to put it in storage and it hasn't been used in about a year.

The other day I got it out and turned it on. It was working for a bit, but then the purple light on the 0D3 went out and the meter went back to 0.

I've ordered another 0D3, put it in the socket and I'm getting no purple light on the new 0D3.

I've measured the voltages at the pins of all the sockets and I have the following:

6SK7
V1 Pin 8 = 145vdc (135) 299
V2 Pin 8 = 140vdc (135) 299

V1 Pin 6 = 96vdc (103) 1.2
V2 Pin 6 = 96vdc (103) 1.2

V1 Pin 5 = 4.79vdc (5.15) .02
V2 Pin 5 = 4.80vdc (5.15) .02

6J7
V3 Pin 3 = 69.5vdc (71) 56.3
V4 Pin 3 = 43.8vdc (71) 83.4

V3 Pin 4 = 36.3vdc (42) 43.4
V4 Pin 4 = 36.3vdc (42) 43.4

V3 Pin 8 = 1.10vdc (1.18) 1.2
V4 Pin 8 = 1.19vdc (1.18) 1.1

6V6
V5 Pin 3 = 269vdc (275) 292
V6 Pin 3 = 269vdc (275) 291

V5 Pin 4 = 277vdc (285) 301
V6 Pin 4 = 277vdc (285) 301

V5 Pin 8 = 14vdc (13.1) 17.1
V6 Pin 8 = 15.57vdc (13.1) 17.2

6H6
V7 Pin 4 & 8 = 63vdc (66) 1

0D3
V8 Pin 5 = 146.4 (155) 1.49

5R4GY
V9 Pin 4 & 6 = 565vac (315vac) 579

The first number is what I measured back when it worked in 2015, the number in brackets is what RCA says I should expect at that point, and then the third number is what I'm measuring now.

It looks like the 0D3 is definitely not happy, nor are the 6SK7s and the 6H6.

I suppose there is a chance that the new 0D3 is not working - I'm told by the eBay seller that it was tested though.

Does anyone have any ideas?

I've attached the schematic and here's some DIY build porn...







5v333

Re: DIY BA6A not working anymore
« Reply #1 on: May 17, 2019, 04:17:14 PM »
have you checked your heater voltages

vari-mu

Re: DIY BA6A not working anymore
« Reply #2 on: May 17, 2019, 04:26:11 PM »
I think you need to check the resistor R43 6800 Ohm  for a break,  then check OD3/VR150

letterbeacon

Re: DIY BA6A not working anymore
« Reply #3 on: May 17, 2019, 06:14:04 PM »
Thanks both.

Yes heater voltages are all fine. I'll check the R43 tomorrow - are you thinking that maybe it could be open so not supplying enough DC to start the 0D3?

vari-mu

Re: DIY BA6A not working anymore
« Reply #4 on: May 17, 2019, 06:42:19 PM »
I'll check the R43 tomorrow - are you thinking that maybe it could be open so not supplying enough DC to start the 0D3?
Yes .

letterbeacon

Re: DIY BA6A not working anymore
« Reply #5 on: May 18, 2019, 10:18:58 AM »
I think you need to check the resistor R43 6800 Ohm  for a break,  then check OD3/VR150

I've measured it out of circuit and it now weirdly measures -2.1M if I have the probes round one way, and then completely open if the probes are round another way. I presume this means it's kaput.

I'll try replacing it and will see what happens...

letterbeacon

Re: DIY BA6A not working anymore
« Reply #6 on: May 18, 2019, 11:42:25 AM »
I've replaced with a new 6.8k and it now works - thanks for your help!

I had a fairly high wattage power resistor in there, but forgot to scrape the paint away from underneath it so perhaps it wasn't thermally coupling with the case as efficiently as it should have been. Got a 25W resistor in there now so that should do it hopefully.


 

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