Cant find what's wrong with my Ampeg Micro CL

[Alex3001]

Hi all

I'm new here and i come with a question about my Ampeg Micro CL. Few weeks ago, it blew up. So i tried to see if i could repair it. I know it's not a great amp, but i dont want to spend more money if i can save it without buying so much replacement parts. I got the schematics and some infos on this forum, but nothing related exactly to my problem.

So here's the thing. I located a blown resistor (R117). I tried to change it (and R118 too, in order to keep the same tech for the resistors (originals are carbon ones, mine are metal). It's still blew up. So i unsoldered and tried every transistors of the power amp with a multimeter. I saw that the Q109 & Q110 where shorted. It explain the blown resistor.

I managed to change every broken transistor. So i changed Q109, Q110, (with equivalent parts 2SC5198 & 2SA1941),Q105 & Q106. I also changed resistors around the damaged zone on the board and all related resistors which could have and overflow of current
R109
R118
R134
R113
R117
R123


I also noticed that R121 was shorted too, so i replaced the 2 ceramic resistors (R121 & R122).

After all this changes, the amp does power up. I can get sound from the Send/Line Out output, but nothing on the HP output. Only a low frequency hum. I thought maybe an untested transistor or diode could be the problem, i changed Q101, and the 3 diodes on the poweramp stage and now, still nothing. I think i can smell a strange thing when it's powered on, but cant locate the source of the smell. It's doesn't smell like a burning resistor.

I also make thoses measures :

U_R121 = 0V while U_R122 = 0.36V
U_R119 : 0v !
U_R115 : 0V!

Voltage between the HP out and the GND = -42V (approx = VEE)
Voltage between collector & emitter of Q109 : ~90v (approx VCC-VEE)

I dont have probes to check the signal or any frequency generator. So, maybe someone could help ? Thanks a lot

[Enzo]

Resistors don't short, R121,122 are not likely shorted, they only have 0,22 ohm resistance as it is. Pretty much either they are open or they are OK.

Q107,108 are automatically suspect since the outputs were bad.

So you have V- on the output, that explains the full V=/V- voltage across Q109.

Q110 is either shorted or being turned on hard.

Don't connect a load to the output until you have got rid of the V- on the output.

[Alex3001]

Resistors don't short, R121,122 are not likely shorted, they only have 0,22 ohm resistance as it is. Pretty much either they are open or they are OK.

Sorry, i mixed things up haha, i should have said that one of them was like an open circuit, not shorted.

Q107,108 are automatically suspect since the outputs were bad.

Yeah, i agree, my next idea was to buy replacement parts, even if they were fine with the multimeter.

So you have V- on the output, that explains the full V=/V- voltage across Q109.

Yup that's it.

Q110 is either shorted or being turned on hard.

How can i check this using only my multi-meter ? Check the voltage on between R118 & Q108 ?

Don't connect a load to the output until you have got rid of the V- on the output.

Okay, it was connected to a Torpedo Captor, but it seems okay

[Enzo]

I don't know what a torpedo captor might be, but when an amplifier puts DC on the output, all a load does is draw more current from the outputs. Not desirable.

Q110 might be shorted? SO turn the unit off and measure resistance from collector to emitter. If it is vry low, then the part is likely bad and should be removed for further test.

Q104 is the bias transistor, and whatever it says, the output follows. SO if there is -42v on it, then so will there be at output. Normally it centers on zero, Q103 is the VAS - voltage amplifier stage. And it is what tells Q104 where to steer tjhings.

Q104 has one real job, to provide a steady voltage difference between the bases of Q107.108. ROUGHLY 2 volts. So when I say it might have -42v on it, it should be more like -41 on one end and -43 on the other.

So if Q110 is not shorted, check Q103 for EC short. Of course trouble in Q101 land can tell Q103 to go over hard and stay.

[Alex3001]

I don't know what a torpedo captor might be, but when an amplifier puts DC on the output, all a load does is draw more current from the outputs. Not desirable.

It's a loadbox with a lineout output in order to use IR instead of cabinets

Q110 might be shorted? SO turn the unit off and measure resistance from collector to emitter. If it is vry low, then the part is likely bad and should be removed for further test.

Rce for Q110 : ~ 15Mohms
Rce for Q109 : ~ 13Mohms

I tried, unpowered but with the transistors still on the board, so i assume that the real resistance value is not this one, but we only wanted to check if there was a short ? So it seems ok.

Q104 is the bias transistor, and whatever it says, the output follows. SO if there is -42v on it, then so will there be at output. Normally it centers on zero, Q103 is the VAS - voltage amplifier stage. And it is what tells Q104 where to steer tjhings.

Q104 :
Emitter<->GND : -50V
Collector<->GND : -48V
Base <-> GND : -49V

(based on this link http://www.datasheetcafe.com/d600k-d...pn-transistor/ , assuming this is the top view, i mean looking at the side which isnt stick to the heat sink)

Q104 has one real job, to provide a steady voltage difference between the bases of Q107.108. ROUGHLY 2 volts. So when I say it might have -42v on it, it should be more like -41 on one end and -43 on the other.

Vb for Q108 : -50.4V
Vb for Q107 : -48.2V

So if Q110 is not shorted, check Q103 for EC short. Of course trouble in Q101 land can tell Q103 to go over hard and stay.

So it seems that Q110 is not shorted, Resistance between C & E of Q103 is < 1ohms (the multi meter say it's shorted ) (same thing, based on the same link, maybe i read it the wrong way) So i guess i'll go buy a new D600 ?

I'll try a completed explanation to see if i get it right.

-> Q103 is shorted, so there's the ~ VEE (minus the voltage across R134/R113) on the Q104 base. and same on R112. Which leads to a -50 / -48V bias on the C & E of Q104 and then Q108 & Q107 are also wrong biased ?


Thanks for your help & replies. I enjoy learning with all your explanations

[Enzo]

OK, so Q104 is working, it is making that 2v space.

Close enough, Shorted Q103 means V- voltage at the collector, hence also the base of Q108. (Q104 keeps base Q107 at just 2v more [positive.) SInce the rest of the output stages seemto work, they blindly follow along and put V- on the speaker jack.

[Alex3001]

OK, so Q104 is working, it is making that 2v space.

Close enough, Shorted Q103 means V- voltage at the collector, hence also the base of Q108. (Q104 keeps base Q107 at just 2v more [positive.) SInce the rest of the output stages seemto work, they blindly follow along and put V- on the speaker jack.

Okay, i'm going to buy a new D600 and replace the Q103 !

[Budgie]

I saw a post from Enzo about SWR SM 900 power modules and was wondering if he still had them?

[Enzo]

Hi Budgie, welcome to the forum.

However it is bad form to pop into someone's thread about a different amp. I already replied to your own thread, let's keep it over there.

[Alex3001]

Hi Budgie, welcome to the forum.

However it is bad form to pop into someone's thread about a different amp. I already replied to your own thread, let's keep it over there.

Hi i'm back, with my new parts.

So i changed Q103, put a new one, and, i still measure -50v on the output. And the 3 pins of the NEW Q103 are on -51.6V, i guess it's already broke. But cant find why ->

Ohmmeter on R107 gives me 680ohms, the Rbe of a D600 is ~ 30Mohms, so i conclude that mine is not shorted between base & collector, the multi meter says there's a short between C & E of Q103 already

Multimeter says Base & Collector on Q108 are in contact, it this normal ?

I still have this strange smell arround when the amp is turned on for more than 1min, i think it's the same smell than when you put way too much current in a red led and it starts turning yellow

[Enzo]

I'd assume Q103 is fine, and Q108 has a base to collector short.

[Alex3001]

I'd assume Q103 is fine, and Q108 has a base to collector short.

I changed Q108 aaaaaaand it Woooorks ! Thnaks a lot for your help & your explanations Enzo