Ampeg SVT-7 Pro No Power On

[ca7922303]

Brought up voltage slowly with Variac. No fan or lights coming on at any point. Was able to get some DC voltage readings on main board of about (125vdc) on right side of main board facing front of amp. Left side no DC voltage.

[glebert]

Do you have a schematic that is correct to your board? IMHO these things are dang near unserviceable (unless this is different than the one I had). I think most people just replace the main board if they can be purchased. There are cascaded SMPS power supplies, multiple protect modes that aren't really documented, and probing is dang near impossible because of the tight layout of the FETS, heatsinks, and caps.

I guess you could check for shorted output devices

[Jon Snell]

It is basically a Mackie 450Mk2 without the power factor corrector for more efficient use of power.
If you are not experienced with Switch Mode and Class D, take it to someone more qualified or you may kill it completely.

[Jon Snell]

Just a simple test, if the Mos Fet output transistors, Q10 - 13 are OK, check the secondary diodes on the power supply board. If faulty, replace ONLY with the original type and specification.

[ca7922303]

Do you have a schematic that is correct to your board? IMHO these things are dang near unserviceable (unless this is different than the one I had). I think most people just replace the main board if they can be purchased. There are cascaded SMPS power supplies, multiple protect modes that aren't really documented, and probing is dang near impossible because of the tight layout of the FETS, heatsinks, and caps.

I guess you could check for shorted output devices

I recently was able to resurrect a 6 pro. Is there alot of differences between those two?

[glebert]

I recently was able to resurrect a 6 pro. Is there alot of differences between those two?

Taking a look at the 6 schematic and some other threads on it, they look completely different to me. The 6 looks like a "conventional" amp (although lots of preamp and driver tubes). The 7 is all about SMPS and class D amps, so way more "digital."

[glebert]

It is basically a Mackie 450Mk2 without the power factor corrector for more efficient use of power.
If you are not experienced with Switch Mode and Class D, take it to someone more qualified or you may kill it completely.

Jon, I can't find any info about Mackie 450Mk2, is that the name of a module from a powered speaker or something? I guess Mackie and Ampeg were both part of LOUD Audio back in the day?

[Jon Snell]

Jon, I can't find any info about Mackie 450Mk2, is that the name of a module from a powered speaker or something? I guess Mackie and Ampeg were both part of LOUD Audio back in the day?

PSU attached and you will see they are very similar.

[The Dude]

Here's the SMPS schematic (attached). There is a known issue with U2, but you'll have to troubleshoot to see if that is your problem.

0032543-xx.pdf

[ca7922303]

Just a simple test, if the Mos Fet output transistors, Q10 - 13 are OK, check the secondary diodes on the power supply board. If faulty, replace ONLY with the original type and specification.

Q10/11/12 are switching on and off with diode test, negative probe to source, positive probe to gate(.57v), then positive probe to drain(0.00v).
D39/40/53/54(0.005v) diode test
D26/29/43/44/61(0.48v) diode test
D14(0.012v) diode test
D15(0.2v) diode test D16/17/23(0.4v) diode test
D12/13(0.5v) diode test
Z1/3/4/6(0.59v) diode test
D1/60(0.4v) diode test
D7/18/19/20(0.55v) diode test
D2/63/64(0.5v) diode test Q13 according to schematic is a 3904 and I'm unable to find it on board to test yet.

[ca7922303]

Here's the SMPS schematic (attached). There is a known issue with U2, but you'll have to troubleshoot to see if that is your problem.

[ATTACH]n982936[/ATTACH]

This is a Revision E board with a 2a265 in a socket w/heat sink. D9 is reading (0.5v) diode test

[Jon Snell]

The issue with U2 will not cause one rail to fail.
It is caused by either a damaged Shottky rectifier, a short circuit on the Power supply output lines or a short circuit in the power amplifier.
Where did you measure 125volts? The rail voltage is + - 55volts!

Edit; You cannot measure the output Fets with a DVM successfully, except for a short circuit.

[ca7922303]

I recently was able to resurrect a 6 pro. Is there alot of differences between those two?

Correction: I should have said, I was involved in resurrecting a 6 pro, the expertise came from members of this forum(G1, Nevetslab, others).

[ca7922303]

C177

The issue with U2 will not cause one rail to fail.
It is caused by either a damaged Shottky rectifier, a short circuit on the Power supply output lines or a short circuit in the power amplifier.
Where did you measure 125volts? The rail voltage is + - 55volts!

Edit; You cannot measure the output Fets with a DVM successfully, except for a short circuit.

U8/11(-156vdc)
D12/60(-156vdc)
R46(-156vdc)
R49(+156vdc)
Q2(-156vdc)
Q20(0.00vdc)
Q10/11/12(0.00vdc)

Looks like anything northwest and west of T3 is (0.0vdc)

What can I use to measure the output FETs? Photo of positive probe on R46, black probe on chassis ground. Variac @ 115vac. Looks like R46/U2(2)/C177 or C26 failed and we're replaced?

[Jon Snell]

So ... the psu is faulty yes or no.
U8 U11 ... schematic of your board would be useful, save us guessing.
You state 156volts. From where to where?