ADA Microtube 200 Help Needed

[davec69]

Any ideas on how i should proceed, what I should check or replace?

[davec69]

Well, I think I've found the source of the problem.  I pullout the pcb (this time, all the way), so I could replace the neon, and was greeted with a mess on the back of the pcb.  Looks like there was a rubber foot on the back, which was placed right under the C37,D25,D26,R42, which melted/disinigrated, and created a mess.  (See pics)

I used some goof off to clean off all the black goo, then sprayed some Deoxit to remove the corrosion.  The PCB is now clean again.  I also cut out a new card to shield the PCB from the metal enclosure.

I also since I had the pcb out, I replaced R42 (200ohm/.5w).

Same symptoms still persist.  Unit powers on, protect light off, no sound when taken off standby.

[MarshallJMP]

What happends with the voltages if you disconnect 1 or 2 of those big connectors carrying the voltages, do they stay the same?

[Sparker]

Ok.  Here's the measurements from the other connector on the preamp boards.  There are four wires.  Red, Green, Black, White.  I used the black wire as the ground.

Red / Black = 3.736v
Green / Black = 11.89
White / Black = 092.2v

The measurements for both channels were pretty much the same.

Can you measure the PSU voltages again, using the Green wire as the 0V/Ground ref as follows -

1 = White = +75V
2 = Black = -12V
3 = Green = 0V
4 = Red = +170V

Let us know

[davec69]

New measurements below. 

I also tried it with one of the connectors, disconnected, and the reading was the same on the other channel. 
FYI.  I did notice on the front panel, that all the leds for one side were lit, while the connector was disconnected.

1 = White = +75V (80.5v)
2 = Black = -12V (11.88v)
3 = Green = 0V
4 = Red = +170V (08.10v)

Let us know

[Sparker]

Looks like there could be a problem around Q9 (TIP48) in the PSU - this is part of the HV regulator (buy a couple). This might not be the only fault though, but I would start with trying to get that +170V rail back up first. Would also be a good idea to get a couple of the Q10 as well - MPSA42.

[MarshallJMP]

Also check R34 should be 220k, sometimes these go up in value.

[davec69]

So, I swapped out the TIP48 last night, and noticed that one of the legs was broken on theold part.  After the swap, I still get the same voltages on the 4 pin connector.

Tonight, I will replace R34, and the MPSA42, and report back.

[davec69]

Tonight I swapped out the 220K R34 resistor, and Q10 (MPSA42).  No change.  I'm still getting -8v between the Green and Red connector.

[Sparker]

Just to make sure, this is with both PSU connectors pulled from the power amp boards and not in standby?

[MarshallJMP]

Can you measure the voltage on the standby switch, one side to ground (green), when the standby is off both sides should have around 220V

[davec69]

I checked the voltage to the two connectors on the standby switch, to the green ground on the 4 pin connector. 

First connector on standby switch to ground = 6.9v
Second connector on the standby switch to ground = 50v

When I flipped the standby switch on, I measured 6.9v on both connectors.

Also, I am now hearing a crackling sound from something.  Very faint, and sporadic crackling.  I couldn't track it down.

[rnolan]

Hmm crackling often = bad solder joins, a good thing to do (regardless) is re-flow (remelt) all the solder joins particularly around the power supply (was suggested by MJMP and fixed my (crackling) B200s :thumb-up: ).

[MarshallJMP]

Can you measure the voltage over R30 and R40?

[davec69]

Measurements from R30 & R40 were a little weird.

Amp powered on in standby, and I got:

R30 = 182v (1st measurement) then I checked again and got 19v.
R40 = -7.92v

Measurement when not in standby

R30 = 7v
R40 = 7.92v

Not sure if it matters, but the led in the power switch also flickers on and off on occassion.