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Fixing UA La-610 Compressor (need schematics with voltage points)

Discussion in 'Electronic Repair Modifications DIY' started by pcrecord, Jun 20, 2016.

  1. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    Hi guys,

    Like the title says, I got one of my two LA-610 who doesn't compress anymore.. the action faded over a few months.
    I tried to change the tubes and even exchanged the opto cell with the other LA-610 and it did no good.

    So I guess a cap or a resistor failed.. I'd need some guidance and some precise schematics

    Thanks in advance.
  2. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    BTW it's a mk 1
  3. Boswell

    Boswell Moderator Distinguished Member

    Depends on your definition of "precise"! There are several schematics on the web if you Google for "LA-610 schematics" and select Images, but you have to beware that one or two of the ones it brings up contain glaring errors (such as a cathode connected to ground via a capacitor only).

    It was many years ago, but I seem to remember that in the last one of these that I had in for repair with the symptoms you describe I found the cathode resistor of V4 had gone high resistance, biassing V4 almost off, and hence not able to supply any change in the drive to the photocell. It's worth unplugging V4 and checking that the resistance to chassis from pin 2 of the valveholder is 1K Ohm or less. The fitted value of this resistor varied from issue to issue of the LA610 - on some it was 1K Ohm and on others it was 470 Ohms.

    If this resistor checks out OK, then I think you are going to need a signal generator and a scope to start looking for the problem elsewhere in the circuit. I'm happy to guide you through a checklist. Fortunately, there are not that many components in the gain reduction control path.

    I'm assuming that the quality of the signal passing though the unit is OK with no distortion.
    kmetal and pcrecord like this.
  4. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    Exactly ! in fact that unit sounds better than the other one but no more compressor.
    Thanks for the good info, I check that when I get time.
  5. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    Got 470 Ohms like you said @Boswell
    I guess I'm gonna research someone I can trust to check it.. or I may keep it like that.. it's not all sources that need a comp in the way in...
    Thank you very much for the info !
    kmetal likes this.
  6. Boswell

    Boswell Moderator Distinguished Member

    It was worth trying, as that would have been an easy hit.

    If you can get hold of a scope and a signal generator at some stage, I could suggest a list of point-by-point signal checks to carry out.
  7. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    You are very kind Bos ! I think I'm gonna use it without compression for a while.
    A scope would be a big investment to repair just one unit.. but I keep an eye open for a used one. ;)
  8. rmburrow

    rmburrow Active Member

    I don't own a LA610, but I own LA 2 units. My guess is you should be able to repair the LA610 with an audio generator and a good digital multimeter ("DMM"). My guess is the no compression problem is associated with the T4C (opto cell) or associated circuitry. I don't have any schematic for the LA610, but my schematic for the LA-2 (from 21 Dec 1965) shows a 470K resistor (R28) between 275 volt B plus to a NE2 neon bulb (used to regulate voltage) and the wiper of a 1 meg ohm potentiometer (zero adj). The schematic shows +65 volts at this junction. When the unit is fired up, and the neon bulb is NOT lit and you don't find +65 volts at the "hot" end of the neon bulb, you have a problem. (The ground side of the neon bulb is grounded.) Also test the 1 megohm zero adjust potentiometer (R4) to make certain it is not open to either side or the wiper. The value of R25 was chosen to work with the particular photo cell in use; don't change this unless it measures open.

    The T4C is driven by a 6AQ5 in the LA-2, so DC signal varying at audio rate at the "hot" input of the T4C should be found there...

    PLEASE NOTE that I took my information from a LA-2 manual; the part numbers and values may not correspond to the LA610, but the operation principle should be similar.

    If you don't have an audio generator, do you have an old test record or CD with clean audio tones (like 1 kHz) recorded on it? That should serve as a audio source for testing. BTW a good DMM, audio generator, and a scope are good to have around. Look around at local "hamfests" or electronic surplus dealers.

    ADDENDUM: Noted your comment about a 470 ohm resistor. Wonder if that is a misprint? 470 ohm resistor is yellow purple brown (and either gold or silver) color code (in that order), or the value is printed on the component. 470 K ohms is yellow purple yellow (and either gold and silver). Check carefully, if similar circuitry is used in the LA610 as used in the LA2, the 470 K ohm resistor may be open, the NE2 lamp blown, and hopefully the T4C (light cell) wasn't damaged by excessive voltage....
    kmetal and pcrecord like this.
  9. DonnyThompson

    DonnyThompson Distinguished Member

    I hate you. LOL ;)
  10. rmburrow

    rmburrow Active Member

    LOL...Just looked at the schematic of the LA610 compressor online; the 470k resistor is there and still designated as R28.

    BTW, if you have to change R28 to the correct value of 470K, the NE2 neon bulb, or other components, MAKE CERTAIN your LA610 is UNPLUGGED and there is NO DC on the 275 volt B plus rail. You can get a healthy shock off the stored energy in the capacitors, even with the unit turned off!
    kmetal and pcrecord like this.
  11. dvdhawk

    dvdhawk Well-Known Member

    How lucky are we to have guys contributing here on a regular basis, with this level of knowledge and experience, who are so generous with their time and talents??

    Gentlemen, you are greatly appreciated.
    kmetal and pcrecord like this.
  12. Boswell

    Boswell Moderator Distinguished Member

    The 470R is the cathode bias resistor in the cell drive stage; I'm told some versions of the LA610 have this as 1K. In a 610 I serviced some while ago this part was marked as 470R but had gone high resistance and was starving the 6AQ5A valve, giving symptoms very similar to the fault Marco was describing.

    I suspect the 470K resistor you describe is the one feeding current from the 275V HT into the slave photo panel and on to the meter circuit. If that part went open circuit, it would sabotage the meter readings when the meter was switched to gain reduction, but the audio compression action would be unaffected.
    kmetal likes this.
  13. rmburrow

    rmburrow Active Member

    Agreed, if R36 (the 470 ohm 6AQ5 cathode resistor) increases value, the operating point of the 6AQ5 changes, and that will "starve" the 6AQ5 as you mention. R34 (10 k, 2 w) shows a 120 v drop across it (275 supply-155 v anode); therefore 12 mA of anode current and 1.44 w dissipated in R34. There is a greater chance R34 could open or change value before R36 due to the power dissipated in those resistors. Look for discoloration of any resistor and test resistors using a reliable DMM.
    kmetal likes this.
  14. Sean G

    Sean G Well-Known Member

    We are very lucky indeed
    I second the motion put forward by the member from Western Pennsylvania :D
    kmetal likes this.
  15. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    Thats very good info guys..
    One thing I should say is that I idendtified the problem: There isn't enough current to light up the opto cell. If I remove the cover and use a flashlight on it, a fair amount of compression is applied. No, the light isn't broken, I tried the Opto cell in an other LA-610 and it works fine.

    There isn't any discoloration or visual indication any part was damaged or blowned. (would have been too easy...)
    I have many projects to attend in the next weeks, I'll get back to it when I get time.

    Thanks again
    kmetal likes this.
  16. Sean G

    Sean G Well-Known Member

    Theres' your fix Marco...

    Just leave the cover off and the lights on in your room :ROFLMAO:

    Just Kidding !
    pcrecord likes this.
  17. Boswell

    Boswell Moderator Distinguished Member

    Great work! That's completely consistent with one or more of the cell driver components having developed a fault. You said in your first post of this thread that you had swapped the valves (tubes) with the ones in your other (working) 610 and the fault did not move, so you have a limited number of passive components to investigate when you next have the time.
    How did I not think of that? A complete set of disco lights should do the trick, and with careful choice you could get multi-band compression for free.
    Sean G and kmetal like this.
  18. rmburrow

    rmburrow Active Member

    OK..the next step is to get out the DMM and check for 155 v at the anode and 100 volts at the screen of the 6AQ5. If R34 (10 k 2w), R35 (220 k 0.5 w), or R36 (470 ohm 0.5 w) change value, these voltages will be incorrect. Next, check C9 (0.02 uF) for leakage or short; leakage through C9 will put DC at the top of R3 (stereo adj), upset the 6AQ5 grid bias, and perhaps "red plate" the tube unless R3 was set to ground. If C9 tests good, then check that R3 (stereo adj) isn't set all the way to the ground end, since this effectively shorts the 6AQ5 control grid to ground and removes drive signal.

    Check C11 (0.01 uF) for correct value and no leakage/short. C11 couples drive from the 6AQ5 anode to the T4C cell. If the 6AQ5 screen voltage is incorrect (low), check C7D and C13 for short, and R38 (22k 0.5 w) for open or changed value.

    C7 is a multisection electrolytic capacitor, in a metal can. These capacitors are expensive and hard to find now, unless UREI stocks them.

    Be careful when measuring the DC voltages...
    pcrecord likes this.
  19. Sean G

    Sean G Well-Known Member

    With one of those drum chaser sensors they use for lighting rigs would do the trick...:D
  20. pcrecord

    pcrecord Don't you want the best recording like I do ? Well-Known Member

    I just thought I'd give you some news about this.
    I finally got it to work last weekend. Opened the beast and inspected all parts.
    I found a small discoloration near a resistor. Tried a few replacement I had that weren't the exact same value and I was able to get compression but at an extreem volumes and with the knob to maximum.

    Intriguing enough, I first thought a smaller value with send more current to the light in the compressor but no it was the other way around. I guess it uses some kind of signal cancellation and the current needs to be lower at one end to let it hit the Opto cell. So after a few tests, I ended up with a 4.99k resistor instead of the original 1.2k. Man these old preamps circuits are alive and changing overtime and that's why we love them !!! As for now, the action of the compressor knob gets better precision that it ever did since I got it. (used) For exemple ; to get the same degree of compression I needed to put it at 7 when I got it and now at 4 (very similar to my other LA-610)
    I've used it everyday all week without a glitch, so I think I'm good ! :love:

    Thing is, this unit sounds better than the other one I have. Clearer, better transients, more in your face. It's a big relief that I got it to work ok.
    When coupled to my T47 DIY mic, it's a blast !!

    Lately I was talking about changing one of them for a ISA220 or 430 but I'm not so sure anymore. . . They sound so different but yet alike.
    I'm thinking that I should wait for something totally different that what I have in my preamp list. Like a Millennia STT-1 Origin. At around 2.1k used.. it will take a while to get money for it...

    Anyway, my LA-610s work so I'm a happy man !!

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