Help with LA-2A no compression

[jwhmca]

Passes audio fine. I've replaced the 12AX7 and tested the 6AQ5 with a tube checker.

The T4A module is not lighting up. If I take the case off and shine light on it, the unit compresses.

Pin 6 of the 6AQ5 is at 25 volts. It seems like it is suppose to be higher? (70)?

Can someone help me troubleshoot this? I can report back the findings and paypal you a Guinness!

 

[Brian Roth]

According to the manual:

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-LA-2A%20manual.pdf

...pin 6 (screen grid) is supposed to be at 100 V.

Bri

 

[jwhmca]

According to the manual:

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-LA-2A%20manual.pdf

...pin 6 (screen grid) is supposed to be at 100 V.

Bri

Yep, I see that and am one it right now... So does that mean it's being pulled down someplace?

I have 342VDC on the other side of R35.

 

[Brian Roth]

Hmmmm....the manual shows the B+ to be 275V.  Is your  unit a UREI or a clone?

Regardless, the screen at 25V is WAY off-base.  What do you measure on the plate and cathode?  Keep in mind the voltages all need to be measured with the unit idling with no audio.

Bri

 

[jwhmca]

Hmmmm....the manual shows the B+ to be 275V.  Is your  unit a UREI or a clone?

Regardless, the screen at 25V is WAY off-base.  What do you measure on the plate and cathode?  Keep in mind the voltages all need to be measured with the unit idling with no audio.

Bri

It's a UREI reissue.

No signal - right.

Pin 5 - Plate = .5vdc
Pin 2 - cathode 1.5 vdc

I'm using the Case as ground.

 

[jwhmca]

R34 looks open... pulling to verify.

 

[Brian Roth]

If R34 is open, that explains the non-existent plate voltage (which the manual shows as 135 V), and perhaps why the B+ is also high.

Bri

 

[jwhmca]

Yeah it's totally open... Now I'm trying to lock down the right value. It's toasted in the unit and as such the color codes are PINK, PINK, YELLOW, GOLD...

And per the schematic that makes me think it was a 22k 5%. But I see some with a 10k????

 

[Brian Roth]

The various schemos I have all show it as 10K, 2 Watt.  Perhaps the heat discolored the brown and black bands and made them look pink?????

Bri

 

[CJ]

22K.

it used to be red red orange , so 2 pinks means the first 2 bands are the same,

10K would be brn blk org, hard to get brn and black to turn pink and pink unless you are 007 or somebody,    check out the guitar tab for Goldfinger sometime, have your chord book handy,

hard to bake a 2 watt that bad, might be something else wrong so monitor the smell factor of R34 for a few minutes upon reboot, check 6BQ5 bias voltage,

here is a chart with component differences listed by years of production>

 

[Brian Roth]

Interesting info, CJ...I hadn't seen that list.  It shows the '79 reissue with a 10K (which is on the three schemos I found), and the manual available at JBLpro apparently is for a 1992 reissue.  The schemo in that manual has a 1968 date FWIW, and shows 10K.

Very interesting.....

Bri

EDIT....I just noticed something in CJ's document....when R34 is 10K, cathode R36 is 470.  But with a 22K plate resistor, R36 is 1K.  So, the value of the burnt R34 could be deduced by the value of R36 as shown in the list that CJ posted.

 

[CJ]

i think i procured the last LA2a DIY binder from poor ol David Jahnsen before he popped,

there is a CD out, but the cool color pics are more fun,
OT: here is a radio shack T4b, sounds quicker than stock, alarm clock light panel ( now they use blue LED's darnit!) and rat shack LDR cells from the assortment pac,

 

[jwhmca]

hard to bake a 2 watt that bad, might be something else wrong so monitor the smell factor of R34 for a few minutes upon reboot, check 6BQ5 bias voltage,

The Voltage on pin 2 of the 6BQ5 is 10.5 VDC

R34 does kinds seem like it's running too hot. CJ, is that bias too high?

 

[CJ]

check your screen voltage, pin 6 and report back immediately,

 

[PRR]

If the plate can't suck (because resistor is blown open), the screen will take some of the missing current. Which explains your low screen voltage.

The exact values 22K 10K 470 1K are not TOO critical. You can patch-up with 17.8K, if that's all you have; but it probably has to be a couple Watts to last more than a minute. (A Radio Shed 5-pack of 3K 1/2W seriesed-up would do in a pinch.) If it is not Factory Values, the action will differ slightly, but still it will "work" instead of "dont work", which is a step forward, and may be good-enuff to get work done while you suss-out and order proper replacements.

If the new plate resistor smokes, the 6BQ5 has gone short. If it don't smoke, monitor the plate voltage while you beat the 6BQ5: it can have a loose-bit inside and short-out at random moments. 6BQ6 is EL84 which is available in ANY gitar-shop or high-end toobe-monger. You don't need a super-special tube here, it only blinks the light. Something reliable is wise.

 

[jwhmca]

check your screen voltage, pin 6 and report back immediately,

It's 150VDC

I have replaced the R34 already... if that isn't already apparent.

 

[CJ]

you should be alright,

10 volts across a 1 k cathode resistor is 10 ma,
10 volts across a 470 cathode resistor is 21 ma, still alright,

RCA says you can drag 40 ma out of that tube but that was for a TV vertical deflection amp which beats up tubes pretty quick, maybe RCA likes selling tubes, they got Armstrong to bail from his 13th floor apartment which made quite a mess, so not the nicest company in the world, glad i did not have to clean that mess up, hydrostatic shock probably opened up his torso like a sardine can, ok where were we?

LA2a is running the tube at a more reasonable power rating, you could bring the screen down if you want a cooler plate resistor, put your finger on the plate resistor, maybe power down first, then count 1 mississippi 2 mississippi, if you can count 2 or 3 mississippi's then that resistor is fine, remember, they don't make 2 watters for looks, it is a power resistor which means heat,

here is a voltage chart for the LA2a,
check your screen resistor, is it 220 K? (red red yellow, one of my favorites) 70 volts on the screen would lower your  plate current which means you should see a drop in bias voltage also, they used 1 k and 470 ohms for cathode resistors on that 6AQ5, if you want even lower plate current, change a 470 to 1 K, unless 1 k is already there,

i like 4 mississippi's for all the rack stuff as it lasts longer and smells better, some of these kids now days, if they smell a little carbon comp they get stressed, heck ever smell a 25 inch all tube zenith color TV?  i still remember that smell,

now this JJ stuff sounds great but i notice more plate current than NOS stuff,
their 6V6 drags current like a 6L6, etc,

this has all your other tube voltages if you want to blueprint the whole box, tube stuff runs at a 20% tolerance so don't stress if your voltages are not spot on,