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HH VX-150 resistors getting hot

O

okwestern

Jun 1, 2020
63
10
#1
Not really a bass amp, but a general purpose PA, I've just brought back to life, a VX-150. Some caps had up to 100 ohms ESR, while others tested OK. Replaced all electrolytics anyway (except the 4700uF on the power supply).
Sine wave looks OK on scope, but R46 / R47 (620R 3W) gets very hot. After 1 minute, 135'C. Then I shut it down to prevent damage.
Is it normal, or could point to a specific problem?

Also, the driver board has a few potentiometers, but the schematics doesn't give me any voltages.
Any idea how to adjust bias, etc.?
 

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#2
I'm not familiar with the amp discussed although I've owned a ton of H/H stuff in my younger years, but there seems to be several threads about them here...

HH VX-150 Lower Bias, Less Disortion

I have a HH VX-150 that I've finally finished cleaning up and refurbishing. A little while back I removed the VGA daughter boards as that was causing significant distortion. I have been experimenting... I removed the input DC caps C1, C2, C7, and C8 and added links in their place. I also...
www.diyaudio.com www.diyaudio.com

Good luck.
 
#3
ajkula66 said:
I'm not familiar with the amp discussed although I've owned a ton of H/H stuff in my younger years, but there seems to be several threads about them here...

HH VX-150 Lower Bias, Less Disortion

I have a HH VX-150 that I've finally finished cleaning up and refurbishing. A little while back I removed the VGA daughter boards as that was causing significant distortion. I have been experimenting... I removed the input DC caps C1, C2, C7, and C8 and added links in their place. I also...
www.diyaudio.com www.diyaudio.com

Good luck
Click to expand...
Just a note... there's some REALLY bad advice contained within that link.

It's typical DIY hobbiest recommendations without understanding what the negative side effects are.

Those resistors are going to get hot since they are the series portion of the series-shunt regulator. How hot can be calculated from the load current from IC 1 and IC2 plus the current through the shunt regulators plus the voltage drop. This should equal the voltage squared across the resistors divided by the resistance.
 
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#4
Thanks for the feedback. Yeah, I won't start ripping out capacitors, when I don't know the extent of the effects. They're probably there for a reason.

On both resistors I'm measuring a drop from +/- 41.8V to +/- 14.8V, so it's dissipating ~1.2W.
Should be within the 3W spec, but they get awfully hot. I could order a few 5W metal film or cement, but I need to get it operational before I get the parts. Not sure if it's any point, as it will dissipate same, but stay more stable maybe(?).

What can happen if I leave the original 620 resistors in place, and one or both pops? Will things go haywire, damage other components (mostly concerned for the power amp section, availability for MOSFETs and then matching them), or will it go into protection mode, or similar?
(They now measure 649 and 640. Schematics doesn't specify tolerance, but they're at least within 5%.)

This has been in a commercial environment (TV station), probably non-stop powered since around '85 when it was new.
When I received it, the zener and capacitor after the series-shunt regulator was burned (for both channels), so I replaced them, along with all eletrolytics.
 
#6
I wouldn’t worry about the resistors, they will run hot.

ESR values aren’t all that critical, and older style parts typically had higher ESR so you have to be careful in some circuits when replacing with newer parts. These circuits were developed around those earlier specs.
 

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