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    Problems with ST70

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    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Problems with ST70

    Post by Pillo69 on Sun Apr 15, 2012 4:14 pm

    In the section of photographs you can see my ST70 amplifier.
    I bought some Sprague Orange Drop capacitors to replace those brought my amp, before deciding to place jumped the fuse, I opened the bottom cover of the amplifier and detected the tracks of the PCB were detached from the plate and some about to split, I decided make a new plate and replace all components, thinking it could have been a short break and cause the fuse.
    With the new board and put it into operation again to jump the fuse. I took tensions and apparently everything is correct.
    With the valves 7199 and GZ34 rectifier does not pop the fuse, I detected a failure in the rectifier tube I replaced and rectifier diode (1N4007) and two capacitors 47uF-100V.
    The amplifier works again, but one day after the fuse blows again.
    The Choke is wax on the outside support, so I'm hoping to send a new Kevin (Dynakitparts) because I think it can ocaasionar shorts.
    The filter capacitor gives me the following measures with 200k scale:
    - Semicircle --- 185.9
    - Square ---- 159.6
    - Triangle ----- 179.1
    - No mark ---- 185.0

    If I turn the unit to heat the fuse blows, if I have placed the outlet valves (EL34) does not pop the fuse.
    ¿¿The problem I can come capacitor four, perhaps the outlet valves (I have no valve tester), or I have another problem?

    NOTE.- I have to report that my voltage is 220V., Which are real 240v., I have placed external transformer 220/125v. and 1000VA., I picked input voltage and there amplificcador 130 to 140 v.

    Thanks again for your attention.

    Pepe.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Problems with ST70

    Post by tubes4hifi on Mon Apr 16, 2012 12:57 am

    just to make sure I'm understanding you correctly, you are using a 240v to 120v converter, but you are actually getting around 130vac input to the amp? That's too high. You may need to use a variac.
    Also it's difficult to tell from the info you gave, but I would guess your quad capacitor is bad, especially if that input voltage is too high. Max DC voltage on that quad cap would be around 480-500v right at startup, then drift down to around 420-430v after about 30 seconds. New quad caps are rated at 550v rather than the old 525v.

    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Problems with ST70

    Post by Pillo69 on Sat Jun 02, 2012 2:32 pm

    The problem has been solved by replacing the capacitor quad, I put the new F&T 550V. and uf 80-40-30-20., I've also changed the four output tubes EL-34 with new HD. Everything went great until about 100 hours of use. At this time I detected a change in the volume, falling at once and do not even have the same balance the two channels. I changed the connectors on the previous CD to Aux. and continues to do, I tried the tape deck and so does.
    A that can occur?.
    A greeting.


    Last edited by Pillo69 on Tue Jun 05, 2012 6:37 pm; edited 1 time in total

    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Problems with ST70

    Post by Pillo69 on Tue Jun 05, 2012 6:36 pm

    I found the problem, replace the selenium rectifier with a diode rectifier (1N4007), I caused a mismatch of bias that following the recommendations of the forum I solved including resistors 10 and 20K in seven terminal strip. Now my Dynaco has returned to its full splendor.
    Take this opportunity to announce the purchase of a PAS-3 preamp which I hope is the perfect complement to my amp.
    A salute to the comrades.

    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Problems with ST70

    Post by Pillo69 on Sat Jun 30, 2012 7:56 pm

    As mentioned earlier I purchased a PAS preamp.
    I've changed the front plate and buttons for a black version and the selenium rectifier diodes changed by two 1N4007 (amending Curcio-audio).
    I've also changed the capacitors and resistors as I read in a thread for the RIAA Bob.
    But .... I have a problem, when I use the phone function PAS (turntables), after a half hour of use, the ST70 BIAS begins to raise a lot, which does not occur when using the line function (cd) of PAS.
    You know you what could it be?
    A greeting.
    José



    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Problems with ST70

    Post by Bob Latino on Sun Jul 01, 2012 8:32 am

    Hi Jose,

    I don't think that the use of the phono on your PAS preamp could have any affect on the bias of your ST-70. As the amp warms up, the bias will rise during the first half hour or so until all the tubes warm up. After a half hour the bias should be stabilized. If after a half hour the bias does wander a bit, then check ...

    1. all the components in the bias system
    2. the rectifier tube
    3. the output tubes
    4. your line voltage

    If ALL OUTPUT TUBES show a bias rise then probably your line voltage has gone up. What I would do is keep a meter plugged in to an AC outlet and see if the line voltage goes up. Even a half volt of line voltage up or down can change the bias voltage. If your line voltage does go UP, then your bias voltage WILL ALSO GO UP. Small up and down movement of the bias has no real affect on the sound of the amp.

    Bob

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: Problems with ST70

    Post by tubes4hifi on Sun Jul 01, 2012 1:55 pm

    replace the resistors and the capacitors in the bias circuit, they are probably changing value as the amp heats up and thus changing the bias voltage.
    Also change the four 270K resistors on the far left and right side of the board, replace with 220K resistors.

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