HSU Research VTF-2 MK1 subwoofer amplifier repair

[taligent]

Symptoms: Sub powers on and has good power rail voltages and +/- 15v rails, but does not output sound.

Troubleshooting: The area around IC104 (a 7815 voltage regulator) on the amp board shows signs of heating (burnt PCB) but the regulator itself seems to check out. This was a good opportunity to test out a newly delivered AVR transistor tester - the capacitance and ESR testing showed that nearly all of the Jamicon electrolytic caps on the amplifier board had issues with capacitance value or ESR.

The capacitors near the 7815 regulator showed nearly zero capacitance (likely dried out), while the remaining capacitors were off by 50-75% and some had ESR over 50 ohms. I replaced all but the large power supply caps:

• C132: 47uF 16v
• C133: 10uF 50v
• C139: 100uF 16v
• C141: 100uF 16v
• C501: 2.2uF 50v
• C502: 1uF 50v
• C503: 47uF 16v
• C506 10uF 50v
• C508: 10uF 50v

Result: Success! The amp fired up and showed good output - all that remains is a scratchy gain pot. This was a free sub so for now the replacement capacitors are from the scavenged parts pile of older Nichicons and Elna caps.

Hope this helps anyone running into a similar issue!

[CapLeaker]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

Nice repair. Usually those little caps always take a beating, specially those near a heat sink.

[budm]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

the -15V power supply has the 1K power resistor is close to the bridge rectifier but still close to a small cap, the 1K for the +15 is right next to the regulator and caps, when you dropping +/-40V or so down to +/-15V, the heat will have to go some where.
Those two resistors do get quite hot and since the board is basically sit in and air tight enclosure so it has poor thermal management which is bad for caps, but that is typical design for these subwoofer, they made them to last for a little while.

[petehall347]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

i dont know who decides where the hot parts go in relation to the ones that want to stay cool is but they should be looking at getting a different job if they care about the product rather than sales .

[momaka]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

The capacitors near the 7815 regulator showed nearly zero capacitance (likely dried out), while the remaining capacitors were off by 50-75% and some had ESR over 50 ohms. I replaced all but the large power supply caps:
...
Result: Success! The amp fired up and showed good output

Awesome, great to hear you got it going!

From my limited experience with amps, it seems that a lot of late 90's and 2000's stuff has cheap caps that just go bad after 5-10 years in service. So I am not surprised you found so many bad ones, especially given the poor thermal design.

i dont know who decides where the hot parts go in relation to the ones that want to stay cool is but they should be looking at getting a different job if they care about the product rather than sales .

If I had to guess, probably no one at the company gave two sh**s about the product lasting. They just wanted it out the door and selling to bring in $$. Just looking at the whole PCB layout makes me cringe. A lot of 90's and 2000's (and onwards) amps are made like this - at least the cheaper models anyways.

As NewEgg products often say about cheap stuff:

"Pros: Product works great!
Cons: Didn't last long"

[mabrow]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

Symptoms: Sub powers on and has good power rail voltages and +/- 15v rails, but does not output sound.

Troubleshooting: The area around IC104 (a 7815 voltage regulator) on the amp board shows signs of heating (burnt PCB) but the regulator itself seems to check out. This was a good opportunity to test out a newly delivered AVR transistor tester - the capacitance and ESR testing showed that nearly all of the Jamicon electrolytic caps on the amplifier board had issues with capacitance value or ESR.

The capacitors near the 7815 regulator showed nearly zero capacitance (likely dried out), while the remaining capacitors were off by 50-75% and some had ESR over 50 ohms. I replaced all but the large power supply caps:

• C132: 47uF 16v
• C133: 10uF 50v
• C139: 100uF 16v
• C141: 100uF 16v
• C501: 2.2uF 50v
• C502: 1uF 50v
• C503: 47uF 16v
• C506 10uF 50v
• C508: 10uF 50v

Result: Success! The amp fired up and showed good output - all that remains is a scratchy gain pot. This was a free sub so for now the replacement capacitors are from the scavenged parts pile of older Nichicons and Elna caps.

Hope this helps anyone running into a similar issue!

Hi All,

I recently purchased a used HSU VTF-3 MK2 for cheap. Upfront I knew there was no audio output. Power off/on lights work and good fuse, just no sound.
Is this sounding like cap issues to those out there that have dealt with this?
I saw Taligent's post on cap replacement and I'm trying to source new caps. Anyone know a good source for caps?
Thanks!

[vestaviascott]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

Ditto. I picked up a thrift store find HSU VTF-3 MK2. No audio output, but power light front (logo) and back green auto power LED light up. I hooked high line input left and right and got no sound. Immediately felt heat from the bottom right section of the plate. Took the plate off and found that the heat appears to be coming from one of the 15v voltage regulators.

Not sure how to proceed. Just replace these two or should I test them and move to capacitors like OP? Don't want to risk further damage due to how much heat I'm getting when the unit is powered on.

[momaka]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

I saw Taligent's post on cap replacement and I'm trying to source new caps. Anyone know a good source for caps?

In the USA:
Digikey.com
Mouser.com
https://www.badcaps.net/store/

Not sure how to proceed. Just replace these two or should I test them and move to capacitors like OP? Don't want to risk further damage due to how much heat I'm getting when the unit is powered on.

Try replace the caps near those regulators first. Chances are they may be dried up and making the regulators not able to regulate properly anymore.

Depending on your expertise and test equipment, you can also check the regulators out of circuit by building a test circuit, or you can just simply replace them with new parts (these are standard +/-15V regulators, so shouldn't be hard to find.)

If new regulators still overheat, there may be something else wrong with the amp downstream that is pulling too much current and causing the regulator(s) to overheat.

Generally, the 78xx and 79xx linear regulators are thermally protected, so they shouldn't burn out even if overloaded.

[vestaviascott]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

In the USA:
Digikey.com
Mouser.com
https://www.badcaps.net/store/


Try replace the caps near those regulators first. Chances are they may be dried up and making the regulators not able to regulate properly anymore.

Depending on your expertise and test equipment, you can also check the regulators out of circuit by building a test circuit, or you can just simply replace them with new parts (these are standard +/-15V regulators, so shouldn't be hard to find.)

If new regulators still overheat, there may be something else wrong with the amp downstream that is pulling too much current and causing the regulator(s) to overheat.

Generally, the 78xx and 79xx linear regulators are thermally protected, so they shouldn't burn out even if overloaded.

OK, thanks very much for the information. The 15v regulators actually just arrived from mouser, I ordered both positive and negative. I don't have any replacement caps or an ESR to test them. I suppose I'll go ahead and replace the regulators and see what she does.

[momaka]

Re: HSU Research VTF-2 MK1 subwoofer amplifier repair

Sounds good. Keep us posted how it goes.
Given how close those small caps are by the regulators, I wouldn't be surprised if they are dry already (and part of the problem.)

Instead of moving the caps or the regulators, one can also just use better caps. From the pictures above, it seems like those are regular 85C GP caps. 105C entry-level low ESR from the good Japanese brands will probably last much longer. Something like Panasonic FC, Nichicon PW / PM / PS / PJ, Rubycon YXJ, or Chemicon LXZ / KY should all work well.