Atlas 210x Audio Circuit

Discussion in '"Boat Anchor" & Classic Equipment' started by K0OKS, Nov 1, 2019.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I recently acquired an Atlas 210x. This was one of the first fully solid state ham transceivers from the early 70s.

    It had no audio output. Using a scope I narrowed this lack of audio down to the output amplifier, an LM380.

    I replaced the LM380 with a NOS one I got off ebay (major suppliers either had a long lead time or minimum purchase limits that made this cheaper).

    This caused me to get SOME audio output, but it appears to be incorrectly biased, which is weird since the LM380 is internally biased. The output waveform is chopped off at the bottom and only shows tiny tips of the sinewave output with a line at the bottom. The DC voltage on pin 8 output is about 1.8V, whereas the voltage chart says it should be about 5.2V. The input waveform is relatively clean, and sounds fine if I drive it into an external audio amp.

    My first guess and Ockham's Razor would suggest the replacement LM380 is bad or got too hot when soldering it in. It gets soldered to the ground plane for heat sinking purposes and takes a lot of heat. I probably should have waited a few minutes between soldering each pin.

    You can see the schematic attached. Note the cap indicated with magenta is not present on the board (maybe a revision of PCB). I tried adding the 0.1 uF cap from pin 14 to ground as suggested in the Lm380 App notes. This is for decoupling to reduce oscillation. However, my waveform didn't look like oscillation; it looks chopped off, like incorrect bias. This additional cap had no effect.

    The input signal to the LM380 looks OK on the scope and sounds OK going into another audio amp. I had replaced C316 the 2.2uF on pin 6. So that is new. C317 checks out on an LCR meter. C315 has also been replaced with a fresh cap.

    I have ordered some more LM380 chips. I am guessing this is the issue, but any insight is appreciated.

    Also, are MLCC caps acceptable for audio coupling caps (e.g. C311)? I will probably swap it out for a film cap, but the little MLCCs fit better. (I know that MLCCs reduce their capacitance as their voltage approaches their rated voltage, and thus they should be overrated in terms of working voltage.)

    Thanks in advance.
     

    Attached Files:

  2. WB2QLL

    WB2QLL Ham Member QRZ Page

    You say you replaced C316 on pin 6, but the schematic shows it connected to pin 8. I have found several bad tantalums in Atlas rigs. Is Vcc on pin 14 and voltage on pin 1 correct? Might replace C314. I wouldn"t place too much faith in Atlas' schematic and voltage accuracy if they differ from the 380 data sheet.
     
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, if you had some dodgy coupling capacitors I would imagine that would throw the bias off . . .

    Personally I find electrolytics much more reliable than tantalums !

    Roger G3YRO
     
  4. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thanks for the reply.

    The schematic is a little hard to read. Yes C316 on pin 8.

    I have not replaced C314. I can. Don’t think I have 15uF here but likely 22.

    The other voltages on all pins except pin 8 are correct and nothing appears to have much ripple or be oscillating when viewed on the scope.
     
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    A guy on the TekScopes mailing list referred to tantalum students as “match simulators.” I have seen this first .
     
  6. WA9UAA

    WA9UAA Ham Member QRZ Page

    Check your voltage at pin one, R327 and R328 supply the internal bias for the bottom rail so to speak. C314 could be bad as well pulling that voltage down.
    73,
    Rob
     
  7. K0OKS

    K0OKS Premium Subscriber QRZ Page

    The voltage on pin one matches the Atlas voltage chart. +7V

    The only DC voltage that does not match is pin 8 add above. This is why I am suspecting the lm380, but C314 is the one cap I did not specifically test or replace, and it is a tantalum. However, for it to be causing a problem I am pretty sure Pin 1 would NOT be at +7.

    When I get back to this the week after this coming week I will go ahead and replace it.
     
  8. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Pin 1 should always be at ~ half supply voltage since it is supplied by a voltage divider. Only if a short or something that pulled the supply down. The symptoms point to C316. Old tantalums are often bad. Even new old stock.

    73,
    Mark
     
  9. K0OKS

    K0OKS Premium Subscriber QRZ Page

    That was what I was alluding to, a shorted or leaky C314 pulling down pin 1. However, pin 1 is at the correct voltage

    This was my very first thought before I even replaced the LM380 to begin with. I replaced C316 at the start of this whole thing. It is working fine. I tested it before installation (and after in parallel with the film cap C317).
     
  10. N7EKU

    N7EKU Ham Member QRZ Page

    Ah,

    I see now -- had to read the other posts more carefully. Yep then maybe the lm380 got fried from too much heat.

    For soldering to big ground planes, you need to crank the iron temp up, and have a good broad tip. Then first heat the ground plane until solder melts well for the whole area where the pins will go, then place the IC down and it will only take a second for the pins to wet (especially if they are pre-tinned) and the iron can be taken off almost immediately. Do one side first, let it cool down fully, then do the other side. The non ground plane pins can be soldered later after the whole thing has cooled down.

    If you have a breadboard you may want to check the IC for function first before you put it in (you can just input a signal from your phone or an mp3 player).

    73,


    Mark
     

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