ADA microtube anode power supply problem

[Shiro]

 Hi! I apologize in advance if something is unclear. I use an automatic translator into English.

I bought on Ebay amplifier ADA microtube 200. but he came to me a broken. it was easier to repair it, but do not open a dispute with the seller. 'cause similar lots in the sale was no more.

the problem was the lack of anode supply +170 v. I found and replaced the faulty resistor in the power supply, then the anode voltage appears.

but I suspect that the amplifier is not working properly. no-load voltage is 170 volts, but when connected to the channels of the amplifier voltage drops sharply. up to 20 volts.

question: what should be the anode voltage in good amplifier when connected to the power supply channel amplifier

thanks :)

[rnolan]

Hello Shiro, hopefully MJMP and/or Sparker  or others here can help/answer you question  :thumb-up:
Regards Richard

[MarshallJMP]

Did you replace resistor R34?

[Shiro]

Did you replace resistor R34?

Yes

[MarshallJMP]

What is the input voltage (should be around 220Vdc) of the anode voltage?Does it also drop?

[Shiro]

 it is no drop, voltage 211V

[MarshallJMP]

Now i can't be sure if this is normal or not,maybe someone with a MT200 can check this?

[Shiro]

I'll try to check the voltage at transistor q8 q9 q10, if this information is useful for diagnosing

[Shiro]

I applied to the anode voltage to bypass transistor stabilizer, voltage is 210 volts, the sound became louder and richer bass. apparently transistor stabilizer is faulty.

I'm not familiar with the transistor voltage stabilizers, but I think that if in turn replace all diodes and transistors, I eliminate breakage anyway =)

I will be grateful to any advice =)

[MarshallJMP]

Wel give it shot,like you said it will mosy likely eliminate the problem.

[Sparker]

Sorry, late reply here (holidays): Yes as MJMP says - change the lot and see what happens.   :thumb-up:

Most likely fault now is Q9 + Q10 have gone 'soft' - their combined gain has decreased due to fault.

Some checks you can do as well, if you want :-

R34 sets the current through D23 + D24 = (220V - 7V / 220k) = approx 1mA
- voltage across D24(6.2V) + D23(0.7V) = approx 7V DC

Q8 is a current source programmed to give 1mA through R32 (47R)
- voltage across R35 = approx 180V DC
- voltage across R33 + D22 = approx 10V DC

I would also change the HV e-cap C31 as well.

Hope this helps   :)