All things LA2a related

[sr1200]

Yes youre right about the clockwise thing... was trying to remember it off the top of my head.  Not sure about the results of a miswired meter.  I miswired mine the first time but just put the main leads backwards so the +4 was the +10 and vice versa so nothing really "off" there.  Not sure about the trafos either, i used cinemags in the units I built, (building a 3rd now with cinemags also).

 

[CJ]

don't sweat it, this is a piece of cake to troubleshoot,

test #1

1) insert a signal again at the front of the transformer with compression knob on zero
2) read the VU meter in Output position,
3) don't touch the input gain, now remove the T4 module to see if the Output reading on the VU meter goes up a bunch,

if it  does, then maybe the T4 LDR is dragging the signal down too much,

if you get a big jump in output, then look to see if any outside light has a way to get inside the module,

test #2

1) insert a 1 volt signal into the primary of the input transformer
2) measure the secondary side of the input xfmr, s/b about 5 times higher,

if not, then the input xfmr is dragging down the signal,

most of the time, a 1:10 nickel core xfmr is used for the input transformer,

you have a 1:5 steel core transformer which means less signal getting to the input tube,

however, the high gain circuit of the LA2a might make up for this,

test #3

1) apply a 6 volt AC signal at 500 Hz to the sec of the input transformer and try the gain knob to see if you can peg out the VU meter when set to the Output position,


we can dig deeper after we get this stuff eliminated as a cause of the problem,

 

[orangechili]

Thanks CJ for the test ideas! Here's my results:
Test 1: It doesn't appear the opto is dragging down the signal, I get about the same reading on the VU meter whether the opto is in or not on the +4 and -10 mode on meter. I have a Kenetek T4b so I can't imagine much light would get in that black case.

Test 2: I don't have a scope but used a signal generator coming out of my daw and measured 1 volts on the input. The Input transformer did indeed have 5 times more output on the secondaries.

Test 3:
I can peg the meter out.

I have read all LA2a related threads recently and people mention removing R6(68k) which is a pad of sorts. I was thinking of trying this but it seems like putting a band aid on a gunshot wound… I'd rather fix the issue of why its inputting such a low signal in the first place. The output section I suspect is fine. I can get my daw to clip at higher settings on the gain knob. I'm also using Alpha pots and the taper of the pot is probably off a bit and the gain speak controls seem 'bunched up' at the end of the pots rotation(clockwise end). Would a 1:10 ratio input transformer instead of 1:5 help the issue here?

 

[CJ]

1:5 should be fine, i used a 600:600 as an input as a crazy idea and it still compressed, only you had to turn the compress knob up all the way,

now that we have checked off the input circuit, let's verify the signal amp,

pump in a 1 volt signal, say at 500 Hz,

adjust Gain control so that you have 50 mv AC on the control grid of V1a, 12AX7a,

measure AC volts after C1-.02 uf cap, s/b about 500 mv (tube has a gain of 10 with negative feedback)

measure AC volts after C2 - .1 uf cap, s/b about 7.5

measure AC volts after C5 - 10 uf, s/b about 7.5

if using a 4:1 OPT, AC volts should be about 2.25  volts,

report back immediately... 

 

[orangechili]

Hi CJ,
Reporting back as soon as I could 

I sent a 1 volt ac signal at 50hz(even measured on the input transformer leads themselves to confirm this). There's definitely something weird going on here in this section…

I had to dime my gain control(meter pegged) and can only get up to 37mv on control grid(pin 2) of v1a.
On pin7 after C1 I get 317mv, about 5 times higher like you stated.
After c2 I get 11.8vac
After 10uf cap I get 11.08vac

Weird that i have to completely crank my gain control and still can't get the desired 50mv… I'm using the Edcor WSM 15k/600 for the output transformer. I'm officially stumped. The only tube I wasn't able to swap out was the 12bh7a as I don't have an extra one laying around. The tube looks NOS and the pin voltages seemed ballpark so I wasn't too concerned.

 

[CJ]

try 500 hz and see if the gain pot comrs down a bit to get 50 mv
xfmr might be cutting your bass freqs

 

[orangechili]

Hi CJ,
Sorry I meant to type 500hz but typed 50hz instead. That was what happened with a 500hz 1 volt ac signal. I can't get it past 32 mv on pin 2 with the gain control maxed out…

 

[CJ]

sounds like the signal amp is OK,

adjust the input wave to get you 2.5 volts on the output transformer secondary,

then see what the VU meter reads when set to +10 db,

check the AC volts on pin 3 of the T4 socket,

do yo have a stereo adj pot on this thing?

if so, try turning it if you get low compression with 2 volts on OPT sec,

we are almost done localizing the trouble,

pretty soon it will feel like Date Night at Disneyland! 

you will feel as cool as Steve McGarrett of Hawaii 5-0,

even while wearing a Sears sucker suit  in balmy Oahu, polyester of course, 

 

[orangechili]

Ok CJ, I'll settle for a fully working LA2a but feeling as cool as Steve McGarrett does sound nice ;D Thank you so much for the help!

First off, to get 2.5vac on the secondary of my OT I have to fully crank my signal generator and the Gain control on the LA2a. I notice if I raise the frequency on the signal generator from 500hz to 1000k to 5000khz I can get more voltage on the secondaries of the OT. Here's the readings though with 500hz 1 volt signal in and 2.5vac on the secondaries of the OT:
Meter +10 mode(not calibrated to agree with GR mode yet): 2.5 is the reading on the meter
Ac volts on pin 3 of T4B socket with peak reduction cranked fully up: 17 volts
Tried adjusting stereo and limiter adjust a little counter-clockwise but got lower numbers on the T4B pin 3 socket so I put them back to fully clockwise.

It just seems weird to me, I've checked every connection numerous times with a continuity tester and nothing looks wrong…

 

[CJ]

try 2 volts into the input transformer, see if you now can turn down the pot and still have 50 mv,

remember your turns ratio will decrease signal by 1/2, as opposed to stock A-10

2.5 on the VU is in the ball park, i bet the zero adj pot would get you to 0

you can pull the cover off the T4 and turn off the lights, see how much the EL panel glows with 17 volts driving it,

do you have a flip down face plate?

if so make sure the grounds are solid, these units will go into oscillation sometimes because of grounds and cables to the pot, this causes the meter to peg, having a face plate connected to the main box by a hinge does not help matters as does any paint that is present,

you can "blueprint" the unit by getting a schematic and a hi-lighter pen, trace out the whole circuit, does not take as long as you think, this is how pros troubleshoot really stubborn problems at work,

 

[orangechili]

Hi CJ,
I can't get past 1.3 volts AC on the input transformer coming from the signal generator coming out from my DAW(measured on primary side of Input transformer with probes attached). If I take the signal generator probes off the input transformer and measure them directly I get 2.5v ac with the DAW sending max signal, very weird… I could get a variance on the output transformer secondary by changing the frequency range but it doesn't affect the volts coming in the input transformer. Therefore i still have to dime my gain control. I know that 17 volts isn't causing any Gain reduction because I don't see the meter moving in GR mode. I'm sure the Kenetek unit is fine, its something to do with signal getting to it. I have gotten the unit to compress quite a bit when I send a very loud signal from a preamp and condenser mic into it. I even measured 100 volts AC when sending a loud signal to it on pin 3. The GR meter was making large movements with the peak reduction cranked then.

I don't have a flip down panel, I have a standard bolt on panel. The front panel is grounded nice. When I first started testing the unit the meter would peg bad as soon as I turned the gain control up(oscillate bad) but the grid stoppers on the 12bh7a solved that issue completely and the meter worked fine afterwards. I did spend a lot of time when building it checking grounds and make sure everything had a very low ohm reading and great continuity.

I am highly considering just buying an vintage A-10 input transformer to see if it buys me any more signal going in….

 

[CJ]

with 2.5 V-ac on that OPT sec you should be able to get max compression/GR,

i will take some voltage measurements on my LA2a side chain amp and post so you can compare,

there were a few changes on that 6aq5 tube over the years,

do you  have a schematic for that unit?

can you hear compression even when the needle is not moving in GR mode?

you might have a meter wiring error, it is the easiest place to make a mistake on the whole circuit,

you do not have to buy an A-10 and pay those crazy prices, the Jensen input is really nice and only about 79 bucks,

you can use the VU meter in Gain mode to see if the unit is compressing,

inject a signal so you get 0 db and then turn up the compression and see if the VU drops,

 

[orangechili]

I do have the schematic from the recproaudio site, I'm using the Cayacosta PTP layout. I've checked every connection on the layout for continuity numerous times… I'll look through the schem though and see if something pops out at me.

While getting 2.5 on the output secondary I can't get the needle moving in GR mode even with the peak reduction control maxed out. While in +4 mode with needle at zero(gain at 90) when i crank the peak reduction full the meter doesn't move at all. It appears I need more signal going in than my signal generator can send. I know it can compress but with a very very loud signal going in(preamp dimed with my loudest condenser mic going into it). The signal is being attenuated somewhere in the unit before it can get to the side chain(or in the side chain) i presume… Also, I'll try to get a picture of my meter switch wiring up here soon. It's hard to tell if compression is occurring without the needle moving to my ears because of how transparent the compression of the LA2a is.

Thanks for the tip on the Jensen, i'll look into it!

 

[CJ]

there is a problem with the side chain amp,

only 2 tubes, check first tube for DC voltage of 0.9 on cathodes and 105 on plates,

if good measure AC volts on down stream side of C9 - .02 cap with GR cranked with 2.5 V-ac on OPT sec,

 

[orangechili]

I agree there's an issue with the side chain, it does seem to function properly though when a ton of signal is net into it though so I'm not sure if something before it is attenuating signal? Every else complains about the ton of gain from these units and I've seen a few here who seem to lack overall gain like myself. It'd be great to get to the bottom of this.

Here's my voltages for V3:
.822 DC volts on cathodes and 88.5 vdc on plates of V3(12ax7)

With 2.5 volts AC on OPT Secondary I get 1.65 volts ac on either side of C9

 

[CJ]

lets go back to the input circuit for a moment,

inject a 500 hz signal into the pri of the input transformer so you get 5 or 6 volts across the input transformer secondary,

measure the AC volts on top of the 100K gain control, not at the wiper,

you should have 2.2 volts AC on top of the gain pot regardless of the lim/comp sw setting, and regardless of where the Gain pot is set,

for this test we want the Peak Reduction pot all the way off, T4 module installed,

you should also have 2.2 volts on top of the Peak Reduction pot,

i think you are right, there is something going on with the input section which is causing low voltage into the side chain which is keeping this circuit from working like it should,

make sure you do not have your input transformer wired wrong,

here is a side chain test if you get the front end solved:

inject a 500 hz signal into the input trans,
turn Gain knob so that you get 1.23 volts AC on the OPT sec.
set meter to 4 db
meter should be reading 0 db, trim if necessary with 1 meg zero adj pot
turn Peak Reduction pot so that meter reads -10 db
switch meter to monitor Gain Reduction - it should still read -10 db
voltage at Peak Reduction 100K pot wiper should be 128 mv AC
C9 should have about 2.5 volts AC on the down stream side,
6AQ5 - C11 - .1 cap should have 45 V-ac upstream, 39 V-ac down stream,


report back immediately,

 

[orangechili]

Ok,
I sent 1.3 volts ac into the primary side and measured 6.63 volts on the secondary side of my input transformer. I can only measure .062 volts on the top of the gain and peak reduction controls(set how you mentioned). Something is wrong here….

Here's the wiring I have for my input transformer. It's only four wires coming off it so its pretty easy to verify.
Input transformer WSM600:15k
Pin 1: Direct to XLR socket pin 2
Pin 2 CT: No connection
Pin 4: Direct to XLR socket Pin 3
Pin 5: Goes to junction of R5 and R6(68k resistors on Cayacosta layout)
Pin 6 CT: No connection
Pin 8: Ground

 

[CJ]

input trans is wired right (edcor  site)

verify edcor assemblers by checking DCR of pri vs sec, pri s/b lower,

sec is shunted by 68 k which will drop the DCR a bit but not much,

looks like we have the problem confined to a small area

check lim/comp sw wiring for possible shorts to ground,

disconnect wire to top of Gain pot and measure pot resistance, s/b 100 K...

leave off the pot wire and do the input voltage test again,

measure volts AC at the comp/lim sw.

 

[orangechili]

Ok,
Comp/limit switch shows no shorts. I even see a little drop in the meter when i turn the switch to limit mode when i'm sending a hot signal in. I did measure gain and peak reduction pots before i soldered them in and they were around 100k I remember. I'll try to desolder them in a bit and see if anything's changed. They do seem to work fine within their whole sweep when sending a hot signal in. They're brand new Alpha Pots

I measured resistance on the primaries and secondaries of input transformer when sending an 1.3vac signal in,
Primary has around 63k
Secondary has around 17k

 

[CJ]

measure the transformer again only with no signal generator