Avalon vt-737sp

[Jdm5149]

Hi. Got this unit in non working order. With help from Greg I found out I was not getting the 6v from the power supply. All other voltages were correct. I began tinkering and eventually got the 6v but in the process my B+ went from 180 to 70. I basically redid the whole power supply with the exception of resistors and now stuck. Any ideas on how to get the 180 back

 

[NoisyIndividual]

If you replaced capacitors, you may have damaged the pads or vias around the cap leads that connect the top and bottom traces together.

 

[Jdm5149]

Any idea of which cap. There is only one schematic around and that’s for the +-34v. I did trace one resistor that has 256v and 70 on other side. But when I lift the resistor tests ok.

 

[gyraf]

..do you know what your B+ is supposed to be, measured where..?

 

[NoisyIndividual]

Probably easiest to trace the connections between transformer HT winding to diode rectifier(s) to cap pin, and the caps ground back to the other end of the HT winding.

 

[NoisyIndividual]

You may find the HT caps ground is supposed to be connected to the other grounds, and this is now floating.

Just go round with a meter with everything switched off and the HT cap discharged, you should be able to determine the circuit easily enough and what is missing.

 

[Jdm5149]

B+ is 180. I originally had it. There are test points on motherboard and also on the molex connector. I could have possibly messed up trace when redoing bridge rectifiers. Can’t really trace power supply while connected since bottom is covered. Kinda a newbie to troubleshooting but I’m learning. Greg from Avalon has really been gracious with help. Also gyraf. I’ve built ur ssl clone. Have to go back and redo because I messed up the upgrade board to get it to act like a real ssl.

 

[kags]

Have you checked the molex connector for heat damage? That’s pretty common, though it prob doesn’t explain your issue.

 

[Jdm5149]

Yes. Replaced that also. I’m thinking it’s something with the board. I guess I’ll reflow some connections. At one point I had no voltages and suddenly the 6v and +-34 and 12 v appeared where needed. Just would like to know how the 180 makes it Thanks so far for your help !

 

[pucho812]

You done it. You done called the Avalon whisperer.
On a working unit you have 5 x dc voltages needed to have a working unit off the power supply. This does not include the regulation to give you +/-15vdc for the opamps that drive the meter and do the dc servos. And does not include phantom which is regulated from the b+ line.
B+ of 180-185vdc
+/-34vdc for the class A discrete
+12 for relays
+6 for for heaters.

If the heaters went out. It’s usually capacitor c23 in the psu and the kbu6d rectifier. I have replaced more of those than I can count. As always cut the kbu6d out before desoldering it as it absorbes too much heat and will damage the pcb.

If your B+voltage dropped like that, make sure nothing is shorted on the transistors.

 

[Jdm5149]

Hi there. I have seen your posts while working on it and they were very helpful. So was Greg from Avalon repair. I had the all the voltages except for 6v but now I have all except the 180. I have 70 at t1 when connected. I have 256v at r2 on power supply board so I’m assuming it’s making past the rectifiers. On the other side it passes 70 thru. Is that correct ?

 

[pucho812]

Hi there. I have seen your posts while working on it and they were very helpful. So was Greg from Avalon repair. I had the all the voltages except for 6v but now I have all except the 180. I have 70 at t1 when connected. I have 256v at r2 on power supply board so I’m assuming it’s making past the rectifiers. On the other side it passes 70 thru. Is that correct ?

No that is not correct.
256v does not sound correct either.
Check for shorted caps and transistors.

 

[Jdm5149]

K will do. Thanks

 

[Jdm5149]

Seems like q1 and q4 were blowing out but I replaced and still not getting the 180. All caps tested fine. Getting the 12v and +-34 at all locations. The 6.3 went down to around 5. Still just getting around 60 at t1. Even t4 has diminished. I use to get between 50-70 volts. Really frustrating. After replacing those transistors they are not getting damaged but still no 180. I am still testing 250 and above on a lot of resistors near the on board heat shrink. Can I take the power supply out of the unit and flip over the supply to test better. I wish I could just flip over the unit but the power supply has it own separate enclosure

 

[Jdm5149]

K. Found burnt resistor R10. Do u know the value. Seems like orange black brown brown

 

[chrispbass]

Be interesting to see the outcome to all of this. My friend has one with that has just blown the fuse.

 

[Jdm5149]

Yeah. It’s a bit frustrating but I’m learning how to test things and troubleshoot. I’ve gotten a lot of things cheap and fixed them rather quickly and cheaper. Some have just corroded parts or missing fuses. Recently got a 5150 block letter amp head for 200$ that just needed and clean connector and turned right on. I’ve gotten 201 space echos for same 200$ that both had just been missing one fuse. I got this 737 for 200$ and have put another 300$ in parts. Once I get it going a few pots need to be swapped also. Seems this unit is an older one and has been worked on a lot.

 

[pucho812]

K. Found burnt resistor R10. Do u know the value. Seems like orange black brown brown

Sadly I do not. I do not have those schematics.

 

[pucho812]

Be interesting to see the outcome to all of this. My friend has one with that has just blown the fuse.

Test with power transformer not connected in circuit.
Test with PSU not connected. In circuit but connected to power transformer.

Check rectifiers.

 

[a soBer Newt]

R10 is 100R it should connect between B of Q4 and C of Q3