LA2A servicing help

[ghiatorino]

Hello guys,
I am servicing an LA2A that went underwater in a studio flood. After drying up and some servicing now it's almost ok but i have still some issues on the compression.

Here's my test results:

Unity gain: Gain Pot 23/100

Max gain: 38dB

Max peak reduction @ 1KHz +4 dBu balanced input (peak reduction pot 100%, stereo/HF pots and C12 removed from circuit. 1Meg resistor added from 6AQ5 grid to ground):

250 Hz: 8VU
500 Hz: 9 VU
1 KHz: 11 VU
2 KHz: 15 VU
5 KHz: 20VU
10 KHz: 20VU
15 KHz: 13 VU

The results makes me think i am missing 20dB of compression, since the manual claims for 40dB max peak reduction.
I am thinking of a low signal driving the T4 or something wrong in the cell (EL or LDR).

El capacitance is 1,7nF.
Exposing the LDR's  at daylight both have around 400 ohm resistance.

with the same 1KHz +4dBu input the 12AX7 sidechain amplifier drives the 6AQ5 with circa 40 Vpp signal, and the pentode output is around 120 Vpp.

Are these typical values for LA2A or they are too low?

Do you have any other suggestion to perform a test on the 12AX/6AQ5 amplifier or on the T4 cell to check who to blame?

many thanks for helping

 

[buildafriend]

check the power supply rails and voltages around the tube and T4B

compare to DRIP LA2A schematic that has voltages listed

 

[ghiatorino]

Checked all the DC voltages, everything is in the ballpark. I would rather think of something wrong in the side chain signal path.
The unit is compressing much more on the high freqs, looks like a hi pass filter on the pentode output, any other hints? Thanks

 

[beatnik]

maybe an obvious suggestion

have you cleaned the sockets ?

have you tried replacing the tubes and / or the t4b ?

 

[ghiatorino]

one of the biggest effect the (dirty) water made to the LA2A was to tube sockets, I changed every socket including the T4 octal. This solved most of the problems, but this compression issue is still there.  Also tried with a 100% working T4 with the same result

 

[pucho812]

did you try known good tubes?

 

[ghiatorino]

I don't have another LA2A to perform a test, so if any owner or tech could do a quick oscilloscope test this could save my day.
With:
1) Input 1KHz, balanced, +4dBu
2) Peak reduction all the way up
3) T4 removed
4) stereo balance and HF pots disabled

Scope the 12ax7 sidechain amp output at C9/C12 (how many Vpp?)
Scope the 6Aq5 output at C11/R8 (how many Vpp?)

Any gentleman who jumps in? Again this can save my day, many thanks

P.s. All tubes are brand new and double tested

 

[johnheath]

I guess the T4B also went swimming and I have noticed that there  are differences between different T4B's and if it have been drenched you could suspect the T4B to be damaged or at least the components inside have beed affected in one way or another...try to change the T4B with a known functioning one and see.

Also the "limit response" trim pot might be under consideration?

Best regards

/John

 

[moamps]

I don't have another LA2A to perform a test, so if any owner or tech could do a quick oscilloscope test this could save my day.

LA2A clone, original UREI T4B

1. without T4B
R5-R6 - 1Vpp 1kHz
R6-R7 - 0.44Vpp
V3 Anode - 8.8Vpp
V4 Anode 168Vpp

2. with T4B
R5-R6 - 8Vpp 1kHz
R6-R7 - 3.4Vpp (reduction zero) drops to 0.39Vpp (reduction at max)
V3 Anode - 8.8Vpp (reduction at max)
V4 Anode 125Vpp (reduction at max)

reduction at max - about 20dB

 

[ghiatorino]

Thanks Moamps, very helpful.  i will make same measurements and post mine, but at a quick look my results look different.
Mine is also a clone, from 1968 schematic:
http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpg
Just to be sure and make a real comparison, what's your input transformer ratio and R7 value?
Are you applying +4dBu balanced at XLR input?
Can you measure the dark resistance of both LDR of your T4?
many thanks

 

[moamps]

R7 is 2k7 but measurements was done with limiter switch engaged, so R7 was shorted.
Input transformer has 1:1 ratio, anyway measurements was done on the secondary, junction R5-R6 vs. gnd.
The dark resistance of cells (pins 7 vs. pin 8, and pin 5 vs. 6) is greater than 20Mohms.

 

[Whoops]

Did you change the pots and trimmers also?

Are those ok?

 

[CJ]

the LA2a is far from a laboratory instrument,  so expecting 1 db this way or that way is probably going to be a long shot,

 

[ghiatorino]

some updates, thanks everybody:
All trimmers and pots were changed together with the tube sockets.
After a complete rewiring i am able to get 20dB of compression, and turning up the HF sidechain pot i can reach the claimed 40dB (almost) on the HF band. Make up gain is also ok.
The only thing that i am still missing is compression on the low band, only 10dB max under 1K. This is pointing me on something wrong in the rather complex (at least to me) sidechain  filter network. i can live with this anyway.