Avalon737 Disassembly

[awesomealex9]

Hey friends!

Quick question. Could anyone give me a run down of how to access the daughterboard of the 737? (The board on the bottom of the unit, underneath the all the other boards, where the tube headers are mounted)
Am I really meant to desolder connections just to get down there?

Background: Fuse blew on 737, replaced all tubes, turned on, fuse blew again. I'd like to investigate components around the tubes but they're buried. Any suggestions on troubleshooting appreciated there as well.

Thanks in advance!

-A novice to studio tech

 

[pucho812]

wow... which daughter board?

o.k. avalon 737, if your blowing a fuse start up stream.  it should be a 1 amp slow blo 250V. if you are outside the u.s. avalon still specs a 1amp 250V slow blo. You can disconnect the PSU from the unit. it's an 8 pin molex.  From there you can  power it on. While you will not get any led action or lights  you can see if the fuse holds and you can measure VDC at the molex in ref to ground which is where the PSU outputs to the unit.

You should see  around +180VDC(B+), 12VDC for relays, 6VDC(heaters), and +/-36VDC(discrete cicuits). Phantom is derived from B+, +/-15VDC for the ic's doing dc offset and meter driver are regulated from the +/-36VDC both on the main board.

I doubt you will get that far. I suspect you will blow a fuse. I suspect you have a faulty regulator.  The kbu6d in circuit for the heaters likes to go out. you should also check c23 in the PSU as that is also part of that circuit. both are usual suspects.

If you have a vari-ac put it on there and feel the KBU6d, it will get super hot to touch if it is wonky.  you can burn yourself so be careful.

From there the tube daughter board has 9 connections to the main board via wires.  They run around the rectangle.  from there you remove 4 screws holding it in place and you can get it out.  they act as pass throughs for signal, ground, and power. each tube is in each stage.  preamp, eq, output, and compressor.  you access this from removing the bottom lid of the chassis.

 

[awesomealex9]

Hey there!

That was super helpful advice and I'm sorry it's taken me so long to get back to you! I sourced some new fuses from RadioShack. I took off the cover, and disconnected the PSU cable from the mainboard. Once I powered it on, the new fuse blew. So, I'm safe to assume the issue is somewhere in the power supply.

I have now removed the power supply entirely from the unit. What should be my next step? There's no obvious signs of damage (burning, popped caps, etc.). Do I lift all resistors and diodes and start measuring? Or should I just recap everything and then power it on?

Thanks for any input!

 

[awesomealex9]

Also, per your suggestions, I'll check C23 and the regulator. I don't have access to an AC PSU, though. Any other way to test that regulator?

 

[awesomealex9]

I just noticed on the bottom of the PCB the solder points for the KBU6D do look slightly black/brown. Unsure if it's a bad solder job or an indication of failure... Think I'll change it out just in case.

 

[awesomealex9]

Also, I'm considering recapping the electrolytics in the PSU since I have it apart. I can easily source the Nichicon capacitors, but I can't tell what the smaller blue ones are? Could anyone loan some insight? They/re light blue/silver and are marked with an M on top.

 

[pucho812]

O.k. I am not sure what you mean by an AC PSU. You can run the PSU without a load(the molex connected into the main board). Are you referring to a variac ac? No matter. running it on a veriac with the amp meter would confirm that the current draw is too much which is why the fuse pops and it would give you a little more time before the fuse pops if you are at less the the set mains voltage.    Be careful to not short the PSU when it is out of the chassis. I usually place a dry towel or  other thing to keep the PSU away from touching anything.  I may have mentioned this previously but you can measure VDC at molex. green and blue are both ground connections. The other wires are the voltages I mentioned previously.

Since you have the supply out, I hope you did the following. The PSU is mounted inside it's own chassis box that attaches inside to the main chassis but 4 screws on one side of the chassis, 2 screws on the back of the chassis, and 3 screws going through the heatsink.  If you don't  have the PSU chassis loose then it will be nearly impossible to get the PSU board back in without damaging it.  The chassis should slide forward just enough to give you the correct clearance.

the blue electrolytic, 10uF and 100UF are made by vishay. I forget the exact part or model number. Any high temp 10uF and 100uF will do as long as the voltage is right.  Without seeing one in front of me, I couldn't tell you what voltages they are, I think it's like 50V and 100V or 63V and 100V.  do read the caps before proceeding.

As far as testing the KBU6D even as a faulty component it might work, but it will still blow fuses. Do take good care when replacing it. It's a couple of dollars of part but very easy to screw up a trace when getting it out. I usually chop off the kbu6d to give me 4 individual pins. This allows for lower soldering temps to remove the pins as the kbu6d does a good job of absorbing heat.  This will help in preventing pulling a trace and causing damage.

 

[scott2000]

Also

Hey there!

That was super helpful advice and I'm sorry it's taken me so long to get back to you! I sourced some new fuses from RadioShack. I took off the cover, and disconnected the PSU cable from the mainboard. Once I powered it on, the new fuse blew. So, I'm safe to assume the issue is somewhere in the power supply.

Just double checking that the advice of the slow blo type fuse was taken?

 

[pucho812]

Also
Just double checking that the advice of the slow blo type fuse was taken?

I asm sure it was.  with a failed rectifier in these units, you get a few seconds before pop goes the fuse.