Aion comet (DS-1) conversion from modern to vintage, no signal.

[eh la bas ma]

Hello,

I am trying to turn a Comet, built on modern specifications, into a vintage one (using a single-in-line op-amp) . I removed the previous IC and D6,R23, R24, I added R11 and C6 as instructed (I hope I didn't miss anything).

Instructions mentions TA7136P. I have a TA7136AP in my hands.

I tried it on this circuit but all I get is statics (IC notch on pin 7), or no sound (notch on pin 1, the square pad).

I wonder if I really need the TA7136P to make it work ? Or if maybe the chip I ordered is a bad one, or if there's something wrong with my build ( I left the big muff mod described in the instructions)

Comet instructions :
https://aionfx.com/app/files/docs/comet_legacy_documentation.pdf

Ta7136 source :
https://www.retroamplis.com/IC-TA7136AP

Thanks  for your help !

Edit : http://www.datasheetcafe.com/ta7136p-datasheet-pdf-amplifier/

         http://www.datasheetcafe.com/ta7136ap-pdf-21045/

Both seem to be the same to me, it should work I guess...I can't see what's wrong here. Corner cut means pin 1, right ?

[eh la bas ma]

I tried to audio probe the circuit to find the issue (Ic oriented with the cut on square pad) :

I have a strong signal until C4, here the signal is quite loud (just before TA7136AP), it gets distorted and the volume decreases a lot on TA7136 pin 6. From there, it stays present but very quiet until it's barely audible on Tone pin1, 3, it disappears on pin 2. No signal on Volume pads...


There is an other important volume loss from IC pin 6 to the Tone pin 1. I guess it's because the signal is compressed on the way. Is it to be expected to have a volume loss on TA7136 pin 6 ?

[duck_arse]

voltages, always voltages. give us IC pin voltages, and supply voltage and VA voltage, please. and the transistor pin voltages.

signal should be loudest at pin 6, but if you had a short anywhere on the clip switch or the diodes there, it would eat all your signal level.

[eh la bas ma]

Right, voltages :

IC TA7136 :

pin 1, square  = 7.65 v
2 = 6.98 v
3 = 5.69 v
4 = 01.1 mV
5 = 7.21 v
6 = 7.22 v
7 = 9.45 v





Q1 : E = 3.750 V
        B = 4.12 V
       C = 9.45 V

Q2 : E = 12.6 mV
       B =0.629 V
       C = 4.12 V

Q3 : E = 3.296 V
       B = 3.581 V
       C = 9.46 V

DC jack = 9.46 V

VA (on R8) = 4.68 V

Edit : Do you think the "big muff mod" is compatible with vintage specs ? I notice there's no sound with this switch on one position, and a faint dirty signal on the other position...

Edit 2 : I desoldered it, CX1, DX1, DX2 are empty. Not even a faint signal now.
I should mention that this circuit worked perfectly with a TL72. I am looking for shorts anyway.

[duck_arse]

well, voltages. you can see from the circuit that the transistor voltages are near enough, so they look ok. but your opamp - if you apply bias from VA, the opamp should sit at VA voltage. your pin 2, fed from VA, is somehow higher than VA. that pin 5 bias arrangement is beyond me, I dunno what effect that will have on the internals.

pin 3 should follow pin 6, why isn't it? (I haven't read the aion docs.)

oh, and you have a 68nF cap where circuit sez 470nF, C4 is that one of the mods? CX and DX should not affect the DC on that stage,  removing them should not kill the sound.


edit - hmmm, check very carefully your R11. the IC datasheet says there should be 300uA thru that R, but from your measures, if you have 27k fitted, you have only 81uA.

[eh la bas ma]

Thanks !

I just realized some cap values were off ( C7, C4, C5). All has been corrected according to vintage specs now. I also replaced the half-burnt 10n cap at C9. It's all standard values now, no mods exept on diodes (red + green 3mm led + D9E).

Result is that I do have a faint dirty signal when the circuit is switched on.

I am checking R11 right now...

[duck_arse]

it keeps nagging at me, and I keep looking at it. the datasheet shows the internal circuit, and that says there is two series diodes between pin 5 and pin 4, so that should be, I think, setting pin 5 at 1V2 or so. and if you have your supply voltage and a 27k feeding a genuine part pin 5, you should get the bias as shown and a drop of about 7 volts, leaving ...... about 1V2, give or take.

is that your only TA on hand? can you do a diode test across pins 5 and 4, power off?

[eh la bas ma]

Diode test powered off on pin 4 and 5 reads 1.635 v. 

Yes, that's the only one I ordered.

R11 is a 22k soldered to a 4.7k, reading 26.32k together.

Edit : I am going to take fresh voltage measurements on TA7136...

[eh la bas ma]

1 = 7.64 v
2 = 5.93 v
3 = 5.67 v
4 = 01.1 mV
5 = 7.25 v
6 = 7.09 v
7 = 9.45 v

Edit : VA still at 4.82 v

Here are some more pics, just in case you can spot something that I've missed (including diode mod) :






Blue wire goes to green led negative side, D9E goes to red led negative side :



Diode mod was working in the modern version, green led was lighting up when I played.

[duck_arse]

you need someone with a better brain than mine to help decipher/confirm the correct pin voltages I'm afraid.

one thing about your D9 diode - Russians mark the Anode with a band whereas we conventionally expect Kathode to be banded. you've taken that into consideration tho, I'm sure.

out of interest, if you still have that burnt-case box cap, cut it up and look inside. unless it is the lowest of low quality manufacture, it will have a plastic outer potting-box housing an epoxied inner capacitor unit. crack off some epoxy and you will see the stacked layers of film and foil. the outer plastic can sustain quite a lot of damage before the innards of the cap will be damaged. doesn't mean we go about burning our box caps, or leaving them on board for others to see.

[Slowpoke101]

IC1 is most likely your problem. I suggest that you replace it. But do buy one from a reputable supplier. The voltages that you have shown reveal that, unless you have a solder bridge somewhere, the IC must be unwell.