Dynaco Stereo 400 - where to start?

Discussion in 'Solid State' started by AdamAnt316, Oct 8, 2017.

  1. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Hello everyone. While visiting a local audio store, I spotted a Dynaco Stereo 400 in their used gear section. I'd never seen one 'in the flesh', but had heard a fair number of good things about it and the rest of the series over the years. The amp had a mystery wire sticking out of the side, which I later realized had once been connected to a fan of some sort. After some discussion with the store owners, and after a listening test that showed some distortion in the right channel, they sold it to me for $150, which I think was a pretty good deal. :D

    I've owned numerous Dynaco equipment over the years, but other than a couple of PAT-4 preamps, all of it has been tubed (ST-70, Mark IIIs, ST-35). Also, the vast majority of the component-level work I've done on vintage gear has involved tube equipment. Additionally, this ranks as the largest amplifier I've owned thus far, displacing a Pioneer SPEC-4 in that position. Regardless, I feel good about my prospects of being able to fix this ST-400, since it was designed to be built as a kit, and lots of documentation is available about it. Also, I have a fair amount of test equipment around here, including a transistor checker.

    Anyway, as mentioned earlier, I was able to give it a listening test before purchase, which showed some (hopefully) minor faults. When powered up for the first time, the pilot lamp came on, as did both of the lamps behind the smoked plastic window, which I later learned are the DynaGuard indicators (the switch was turned to OFF, and cycling the switch through the settings has no effect on their illumination), and the relay clicked. Once fed with a signal, I got audio from both channels, though some distortion could be heard in the right channel. When I checked the DC offset, the right channel showed -22mV when first powered up, which later fell below -1mV, while the left channel shows a pretty steady 18-19mV. Granted, I didn't give it a full 15 minutes to warm up, but things seemed to get settled in fairly quickly.

    In summary, my question is as quoted in the title. As mentioned before, I have little experience working on solid-state equipment in general, though I think it's time I learned how. My guess is that new capacitors are needed, along with whatever drifted resistors there may be, but I'd rather not just shotgun everything or replace all the boards with new ones (as shown in my ST-70 restoration article, I like to give the stock circuitry in these things a fair shake if at all possible). Anyway, here are some pictures of the amplifier:

    [​IMG]
    The front panel. Seems to be in pretty good shape overall.

    [​IMG]
    The rear panel. The presence of a serial number sticker makes me think that it was factory-wired, as was the case with Dynaco's tube equipment, but I don't know if that's the case for the ST-400.

    [​IMG]
    The inside of the cage. Seems decently clean overall, and I didn't see any obviously-burnt components or whatnot. The copper-colored coils near the fuse holders make me think it's an early version, as discussed in this article about a ST-400 modification (unless it was referring to a different coil-wrapped resistor).

    [​IMG]
    The 'mystery wire' mentioned above. Both leads of the zip cord are connected to a terminal strip near the power switch; one to a primary lead of the power transformer, and the other to one of the leads from the power switch. I removed some of the electrical tape, and the two bare ends are isolated from one another.

    Anyway, thanks in advance!
    -Adam
     

     

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  2. Jim Marantz

    Jim Marantz Active Member

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    She's a looker. Look up my "Let's Restore a Dynaco St400" thread. Lots of good info there. Also, download the service manual. If you can't find it I can email you one.
    Jim
     
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  3. mhardy6647

    mhardy6647 AK Subscriber Subscriber

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    There's an ST-410 sittin' down in the basement. I am terrified at the thought of plugging it in. Heck, I am terrified at the thought of taking the cover(s) off. Dang solid state stuff is so complicated, and -- best I can tell -- it is pretty much guaranteed not to work at this late date.
     
  4. mbz

    mbz AK Subscriber Subscriber

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    Would be good to track down the source of that distortion before investing too much time in it.
    Hopefully you have an oscilloscope, without it you will be in the dark/take pot luck by checking
    solder joints/wires, cleaning any/all switches, checking for known problematic transistors, replace
    caps, maybe diodes etc... hifiengine has the schematic
     
  5. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Thanks for the replies.
    I did indeed see that thread, and have it bookmarked. I did download copies of the service manual from both HiFi Engine and Update My Dynaco, though I'm not sure yet if either one is fully applicable to the unit I have. I'm not sure how many revisions were made to the ST-400 over its production lifespan. I know that there were at least two different versions of the PC-29 "DynaGuard" board, for one thing.
    I know the feeling. The store I bought it from had neither a variac nor a multimeter on hand, so all we could do was hook up a pair of semi-expendable speakers, plug it in, throw the Big Switch™, and hope for the best. Luckily for me, it worked enough that it didn't emit smoke, which is nice. :) In any case, the nice thing about this Dynaco solid-state gear is that it was designed as a kit, so instructions for troubleshooting are widely available (as it was an expected part of assembly).
    I intend to do that. What would be the best place to start in trying to track it down? I was thinking that a run-through of the "Preliminary Tests" in the manual, followed by a check of voltages against whatever chart(s) I can find. I do own an oscilloscope, along with a fair amount of other pieces of test equipment, including signal generators and frequency counters.
    -Adam
     
  6. mbz

    mbz AK Subscriber Subscriber

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    Certainly run through the performance tests in the sm.

    Using revised schematic from hifiengine

    Suggest supplying amp with low level 1kHz sine wave and use CRO to
    identify distortion. Test at transistor bases/adjoining components
    for safety. Dynaguard switched off.

    Test points,
    - Right channel output (speaker post of fuse F301)
    - Right channel input Q1b/R3 (either side)

    PC-28 board
    - Q1b/R2
    - Q4b/R7
    - Q5b/R6
    - Q13b/R45
    - Q14b/R46
     
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  7. djoffe

    djoffe Active Member

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    Best of luck with your restoral efforts. You have a nice clean looking sample there. I'd advise focused efforts to fix what is broken, rather than wholesale renovations. It seems to be mostly working now, and it's fairly easy to let the "magic smoke" out of one of these beasts...
     
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  8. Jim Marantz

    Jim Marantz Active Member

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    The preliminary tests in the service manual is the best place to start. For what it's worth, I've never worked on a 400 series amp that didn't have a power supply issue. The PS runs hotter than anything else in these amps. With 40 years gone by, it's not surprising.
     
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  9. llwhtt

    llwhtt Super Member

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    That wire is for the fan that mounts right there on that end of the heat sink. The most common problem is overheated resistors on the drive boards, two of each 1K, 750, and 300 Ohms on each board. Sometimes they are standing on end, early board, or flat against the board, later version. They run fairly warm and the solder joints go bad.
     
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  10. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Again, thanks for the replies. My intents are to preserve as much of the amplifier's original circuitry as possible, as I've done with my tubed Dynaco gear. While I'm definitely not one for 'shotgunning', there are some parts which are highly likely to require replacement either now or in the future, such as electrolytic and tantalum capacitors. Once I determine the reason for the right channel distortion and repair it, I will likely go ahead and replace all of those parts, plus whatever else might need replacement, like drifted resistors.
    -Adam
     
  11. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Okay, I just ran the preliminary voltage tests on the unit, and there's definitely something that's not not quite right. Tests 3 and 4 on the fuse block check out fine at +73V and -73V respectively, while test 5 (eyelet #18 on PC-29) checks out OK at +13V. However, the reading for test 6 (eyelet #12 on PC-29) is way too high, at -22V (expected reading -13V +/- 1V).

    What might be the likely cause for this? If I'm reading the schematic correctly, the voltage to eyelet #12 on PC-29 is derived from eyelet #7 of PC-30, which is fed by one end of D305, the 13V 1W zener, plus the negative end of C303 (500µF@15V) and one end of R307 (600Ω@10W). I'm guessing D305 is the most likely culprit, but it's hard to say. I'm guessing it's not the case of the distortion, but could it be causing both DynaGuard lamps to light? Methinks the game is afoot, in any case. :idea:
    -Adam
     

     

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  12. mbz

    mbz AK Subscriber Subscriber

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    typo, derived from eyelet #6 of PC-30.
    "Regulating" from 22V to 13V seems like a big ask for a zener.

    I'm thinking maybe some down stream fault so there is only small current flow through R307 and a smaller than expected voltage drop.
    You could test this theory by measuing dc Voltage drop across R307 (one probe at #5, other at #6) and compare with R308 (probes at #2 and #3) all on PC-30.
     
  13. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    I've double-checked the schematic, and eyelet #7 seems to be correct. Eyelet #6 on PC-30 connects several components on PC-30 together, and then runs to the OFF position of the DynaGuard switch. Eyelet #7, on the other hand, connects directly to eyelet #12 on PC-29, if I'm reading things correctly. The next time I have the amp open, I'll trace the voltages on PC-30 itself, and see if D305 is outputting -13V as it's supposed to be, and how much voltage is going into the receiving end, such as it were.
    -Adam
     
  14. mbz

    mbz AK Subscriber Subscriber

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    OK, looking at different schematics, #7 PC-30 is a GND connection

    dyancc.JPG
     
  15. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Very curious. I know they would've made revisions to the design over the production lifespan, but changing the designation of output points on a board seems a tad extreme. o_O It appears that eyelet #7 in the manual version seen on the UpdateMyDynaco page corresponds to eyelet #6 on the one you have posted, and (possibly) vice-versa. I shall have to examine the PC-30 board in my amplifier carefully to determine which version I have, and to make sure I have the proper manual version for my amplifier.
    -Adam
     
  16. mbz

    mbz AK Subscriber Subscriber

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    Think the point is to measure the voltage drop across each resistor to confirm current draw.

    Also, if you switch the dyna guard off, there should be minimal current draw, are the voltages the same.
    Agree, that zener has probably had a hard time and is near death.
     

     

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  17. Jim Marantz

    Jim Marantz Active Member

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    It's easier to bypass the PC29 to see if it helps your issue. Running the signal through the Dynaguard circuit causes lots of distortion even in it's "off" state and working properly. Just my $.02.
     
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  18. mbz

    mbz AK Subscriber Subscriber

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    Wasn't aware of that, thanks.

    While the OP was about distortion in one channel, think that we all agree that psu voltages should be normal. The -22V may be killing/knocked out an opamp
    so might be related to the distortion.

    Really need a SM/schematic that match the unit under test.
     
  19. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Another update: I decided to take some readings across both D304 and D305. With one lead of my DMM connected to the chassis, D304 reads 0V at the non-striped end, and 12-13V at the striped end. However, D305 reads -23V at the non-striped end, and -10V at the striped end. Given that this still works out to a 13V difference, it would seem that D305 is doing its job to some extent, though perhaps not correctly. What would this seem to indicate? Is the zener at fault, or could the issue lie elsewhere? Again, thanks!
    -Adam
     
  20. mbz

    mbz AK Subscriber Subscriber

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    I think the zeners are there to act like a cheap/simple voltage regulator. Any minor bounce in voltage mains is channelled down the zener. Talking 1-2Volts at most.
    The zeners appear to be 12 or 13Vdc. So -23 volts is a red flag, means channelling 10 volts, they would simply change R307 so it would burn up these volts.

    A greater red flag is that one end of the zener is connected to ground . At D305 you have -23V (input, too high) and -10V, this should be 0V, maybe you have an earth issue.
    With unit powered off/unplugged measure the resistance between D305 stripped end and the chassis, expect maybe 1 ohm. You can repeat for D304 stripped end
    and chassis.

    Maybe a burnt track near D305 stripped end.
     
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