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Thread: Ampeg V4 wierd channel 1 & 2 interacting

  1. #36
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    Quote Originally Posted by boroman View Post
    If the plug is inserted, everything is shorted too.
    That's why I dont get it.

    All lugs are connected to each other, so there will be shorting with the jack inserted and without it too. Am I missing something?
    Seems you wired the jack wrong. Lug #3 (hot input) must not be connected to any other lug. I should be shorted to ground by the switch contact only. Inserting a (non-shortig) plug opens the switch and should allow you to measure the grid leak (5.6M) from lug #3 to ground.

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    Last edited by Helmholtz; 12-12-2019 at 03:32 PM.
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  2. #37
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    And I'm in the beginning. The same behaviour like in the first post. I don't get it. I have tried to measure all the caps and resistors near V1/V2 and all looks/measures close to its specs. No bad traces, no overlapping solder joints. I have got some photos of previous work on other V4's and it PCB/wiring looks the same. I have no idea what to do now. Maybe it'll be "spare parts" V4 for some new projects.
    As already recommended, disassemble the Ultra Hi switch and look for internal shorts between the 2 channel contact rows. If you can't repair/replace it, you can always disengage it and hardwire your preferred setting. There is a good chance this will cure your problem.

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  3. #38
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    Thanks for the tip with the Ultra-Hi. Will try to mess with it today.

    About the jacks - this is a closer shot of factory (I mean, looks like...) wiring. You can see that all lugs are connected to each other. So they will short with each other in any in/out operation, right?

    Click image for larger version. 

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  4. #39
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    You can see that all lugs are connected to each other. So they will short with each other in any in/out operation, right?
    Lug 3 and lug 2 are not connected. There is a 5.6M resistor between.

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  5. #40
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    Quote Originally Posted by boroman View Post
    Am I missing something?
    In your diagram you called lug #2 the tip. Lug #3 is the tip. #2 is the switch. So #3 should only measure grounded when no plug inserted.

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    OK, let's go ahead - it's not the input jack fault. Tested it right now and all seems OK.
    What should I do to disconnect Ultra Hi switch leaving it on neutral setting? Looking at the schematic it seems it passes throught C3 (0,01uf), C102 (0,00012uf) and C101 (0,001uf). I thought neutral setting on this switch was not going through any cap (at least I remember it from V4B). It seems that the mid one from these caps is C101 so this is the neutral? (the same for CH2 = C104)

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  7. #42
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    What should I do to disconnect Ultra Hi switch leaving it on neutral setting?
    As said you might have a short between CH1 and CH2 contacts inside the Ultra Hi switch. Without disconnecting the switch you might check with an Ohmmeter if there is low resistance between any of the CH1 contacts and the CH2 contacts. It's best to have both vol pots fully up for this test.

    For further advice, please post close-up pictures of the switch and vol board to see the physical layout. We also need to agree on a way of numbering the switch terminals.

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  8. #43
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    Quote Originally Posted by boroman View Post
    I thought neutral setting on this switch was not going through any cap (at least I remember it from V4B). It seems that the mid one from these caps is C101 so this is the neutral? (the same for CH2 = C104)
    It's a little different from the V4B here.
    (+) setting puts C102 as bright cap across volume.
    Neutral setting puts C101 & C102 in series but shorts the 2 ends together. However it is still connected to the wiper so could cause interference.
    (-) setting puts C101 from vol. wiper to ground.

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    Last edited by g1; 12-13-2019 at 07:48 PM. Reason: neutral not higher value bright cap
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    Thank you guys.

    The story goes on:
    I have disassembled and cleaned Ultra Hi and Midrange switches, cleaned contacts and now I see how it works another thing learned!
    Inside switches is all OK, does not look strange at all. I have raised the two "contact elements" to have better contact with the points/lugs on the switch

    speaking of Ultra Hi switch these are my measurements:

    Pinout:

    1A - 2A - 3A - 4A
    1B - 2B - 3B - 4B

    When MINUS is engaged: 3A shorts with 3B / 4A shorts with 4B
    When NEUTRAL is engaged: 4A shorts with 4B
    When PLUS in engaged: 4A shorts with 4B

    Resistance between 1A and 4A is 175k
    Resistance between 1B and 4B is 850k
    No sound change between NEUTRAL and PLUS (usually when volue is up and treble is fully up going from minus by neutral to plus raises buzz and noise floor. plus seems the same as neutral)

    Pots fully up (but these values not change when they are turned down)

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  10. #45
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    Pinout:

    1A - 2A - 3A - 4A
    1B - 2B - 3B - 4B

    When MINUS is engaged: 3A shorts with 3B / 4A shorts with 4B
    When NEUTRAL is engaged: 4A shorts with 4B
    When PLUS in engaged: 4A shorts with 4B
    4A and 4B are ground (please verify) and are jumpered, so they will always be shorted. In MINUS position, the switch connects 3B with 4B and 3A with 4A, so all of them will be shorted with each other and grounded.
    In position NEUTRAL the switch shorts 2A with 3A and 2B with 3B. In position PLUS the switch shorts 1A with 2A and 1B with 2B.

    As row A and row B connect to different channels, it is essential that with the switch in MINUS position there is no leakage between either 1A or 2A and 1B or 2B. Because of the path through the pots you should measure something like 2M between 1A and 1B, but infinite resistance between 1A and 2B, between 1B and 2A and between 2A and 2B.

    Also in PLUS position there should be no connection/leakage between 3A and 3B.

    I hope I got all this right. If the switch was completely disconnected I just could have said: Make sure that none of the row A contacts leaks to any of the row B contacts.

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  11. #46
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    Yes, 4A/4B are grounds.

    What you tell about horizontal shorts is right (checked now)


    1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)
    1A+2B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS
    2A+1B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS

    in the PLUS position 3A and 3B does not short/leak.

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  12. #47
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    Quote Originally Posted by boroman View Post
    Yes, 4A/4B are grounds.

    What you tell about horizontal shorts is right (checked now)


    1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)
    1A+2B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS
    2A+1B measures infinite only in MINUS and NEUTRAL. It shows 1Mohm when it's in PLUS

    in the PLUS position 3A and 3B does not short/leak.
    Looks ok except that I would have expected to see 2M instead of 1M. Reason is that between 1A and 1B there is a DC path through both vol pots in series. Does your amp have 500k vol pots instead of 1M ones?

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    Quote Originally Posted by Helmholtz View Post
    Looks ok except that I would have expected to see 2M instead of 1M. Reason is that between 1A and 1B there is a DC path through both vol pots in series. Does your amp have 500k vol pots instead of 1M ones?
    No, they are original, 1M's

    Can I ask why you addressed Ultra Hi switch for potential problem and not the Midrange one?

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    Quote Originally Posted by Helmholtz View Post
    Because of the path through the pots you should measure something like 2M between 1A and 1B,
    Quote Originally Posted by boroman View Post
    1A+1B measures 1 Mohm on all three rocker sw. settings (-/N/+)
    Checked mine and reading is 1.9Meg so this is a good clue.

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    No, they are original, 1M's
    This would mean a leakage resistance of around 2M.

    I recommend to disconnect the wire from either 1A or 1B and measure resistance between 1A and 1B again.


    Can I ask why you addressed Ultra Hi switch for potential problem and not the Midrange one?
    You described an asymmetrial interaction between the channel volumes. Both channels are mixed at the pin1 plates of V1 and V2. So after this point there is only one single channel and anything happening to the signal must influence both channels in identical manner.

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    Quote Originally Posted by Helmholtz View Post
    This would mean a leakage resistance of around 2M.

    I recommend to disconnect the wire from either 1A or 1B and measure resistance between 1A and 1B again.

    You described an asymmetrial interaction between the channel volumes. Both channels are mixed at the pin1 plates of V1 and V2. So after this point there is only one single channel and anything happening to the signal must influence both channels in identical manner.
    I'll measure that tomorrow. And about the Midrange is the same also for both channels, so it can be somewhere too?

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  17. #52
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    And about the Midrange is the same also for both channels, so it can be somewhere too?
    The way you described your problem, the cause must be before the channel mixing point.

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    Allright. I finally found the solution. It wasn't jacks' fault. It wasn't ultra hi fault. Someone has cut two traces on the pcb near VR102, made a jumper with resistor and jumper to the reverb tubes. Whoever did that, it looks stock (solder joint etc), that's why I haven't noticed in before. Looking at other V4 guts I made shots of over the years, this jumper got my attention. I showed it to my friend and he said that's probably someone wanting the same amount of reverb regardless of the volume setting on the amp but "something went wrong". The jumper was desoldered along with that resistor, traces were jumpered and viola.

    V4 is working great puts 100W RMS and no hum, noises, distortions... Thank you all for the help. Another lesson learned.

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