La Rata Problems

[dpresley58]

Hi.
I've built a couple of Rat clones with the tonepad.com layout (La Rata) with good results, but this one has me stumped. The sound is muffled and fades more quickly than it should. It sort of fizzes on the way out.  Here's the schem and layout for it... http://www.tonepad.com/getFile.asp?id=89

I'm using the LM308AN with pin readings as follows:

Boss PSA supply @ 9.73vdc
1 = 8.81
2 = 4.58
3 = 4.20
4 = 0.0
5 = 0.0
6 = 4.75
7 = 9.50
8 = 6.05

any ideas why this muffling might be happening? I've searched the forum, but haven't found anything relevant to this.

Thanks in advance.

[petemoore]

  Other than I don't know what voltages the compensations pins 1 and 8 should read voltage wise, the inputs and output and PS voltages look ok.
  Boss PSA supply @ 9.73vdc
1 = 8.81
2 = 4.58
3 = 4.20
4 = 0.0
5 = 0.0
6 = 4.75
7 = 9.50
8 = 6.05
  Looks like LM308 pin5 isn't assigned on the RAT circuit..
  I could open mine and see, this house has been turned upside down so many times...PS adapter cable is...somewhere...someone probably just knows what the comp pins should bias at...
  I should look at the 308 data sheet, isn't there a really small internal comp cap in the 308..which would mean the DC voltages look high?
  'dogmatic but you could test that that pins 1 and 8 go to only the comp cap by process of elimination..connect to pin1...test all conductive points, repeat for pin 8.

[dpresley58]

Tested positive that pins 1 and 8 only go to the compensation cap... Could it be the cap is bad?

[petemoore]

  Could it be the cap is bad?
  8 Ball says: 'a possiblity exists' ..but it is so doubtful I hesitate to mention that cap could be lifted from one end to eliminate the possibility that's what's causing the misbehaviour, but then expect more HF gain/content, which could cause oscillation?...dunno...probably something else, especially if it's a new, non-polarized capacitor.

[dpresley58]

The cap was fine, and all continuity in that area was fine. Due to the nature of the problem (sustain not there and dying in a fizzy manner) I checked the diodes. The clipping diodes tested out at about 0.5 volts and the other one at about 1.3 or so.

Unless someone has any other ideas, I'm going back to check for bad solder joints.

[petemoore]

1 = 8.81  *seems high V for this pin  [compensation pin].
2 = 4.58  ~1/2v looks pretty good  [-input]
3 = 4.20  ~1/2v again..good enough I think  [+input]
4 = 0.0  Gnd., correct.
5 = 0.0   Dunno..probly ok
6 = 4.75  [output, near 1/2v is good]
7 = 9.50  [V+, battery is fresh.]
8 = 6.05  [dunno, these compensation pin voltages]
  *check that my memory is consistant with the data sheet before heeding any voltage info...
  *Pins 1 and 8 are compensation pins for rolling off treble/eliminating HF oscillation as I understand it, but I'm not familiar enough with Rats to say if these V readings are within operational parameters.
  All the other pins look pretty good V wise, pin five according to schematic is not connected?[  ] if not, 0.0v is fine.
   

[oskar]

The clipping diodes tested out at about 0.5 volts and the other one at about 1.3 or so.

Chewing my nails while suggesting one of the clipping diods... ( 1.3v )   ?