Recent content by okwestern

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...

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...

Yes, I'm an electrical engineer, so I'm quite confident working with this. I've done quite a lot of work with various Ampegs (SVT3/7/8 PRO and BA115HP/HPT). Whether it was proper analytical work or just pure luck I got them running again, I don't know.

The amp was bought with blown mosfets, so I can't trust that the pot is in a reasonable position. Since I'm replacing the mosfets, I might as well save me a lot of headache and replace the stock pot with a multiturn (much easier to adjust biasing). Yes, I have a service manual, and have worked...

I want to replace the bias potentiometer with a multiturn potentiometer on my BA115HP. I want to make sure the pot isn't set too low when powering the amp(driving the mosfets too hard - I've heard about fireworks). But I read some time ago about ringing/oscillation if the potmeter is too high...

I have an Ampeg SVT 3 Pro with CBB (metallized polypropylene film) capacitors for C2/C3/C10. The cap goes to the plate of the triode. Not sure if it affects the sound? The original is WIMA MKS4 100nF 400V (metallized PET film).

No problem. Thanks for everything so far :)

15-20mV average on each bank. But I figured I would take care of the TP10 problem, before upping the bias. I was afraid that bias could increase when replacing other components, and to be on the safe side keep it a little low. I've heard about fireworks when the bias gets too high.

Yes, it's the voltage over the resistors (mV, not mW - I was sleepy when posting yesterday). Which points (and dc/ac?) would you recommend measuring to get closer to the problem?

Forgot the sign. Yes, Q7 is the top (npn), and is 2.48Vdc, and Q6 (pnp) is -3.031Vdc. I guess slightly unmatched MOSFETs doesn't influence this? I have 3.7/17.9mW (IRFP-140R) and 8.3/9.6mW (IRFP-9140R) for the two banks.

I measured the emitters of Q6 and Q7 at 2.48Vdc and 3.03Vdc. Is that enough unbalance for the servo?

How will the low reading at TP10 affect the amp (sound, amp life, etc)? And where do I start troubleshooting this?

Good to know square vs. round. What about TP14/15, they are square but 0.4Vp-p with a sawtooth symbol, so they should be AC as the DC on this rail is +/-50V? Or how do I interpret the ones with sawtooth waveform? Not much experience, but as with everything, everyone have to start somewhere :) I...

Thanks. If I assume all DC measurements should be without signal, which of them should actually be with signal (if any)? And for TP10, do you mean U3B (as HPT doesn't have a U5 - perhaps you looked at BA115HP)? I hooked up to TP10 again and have the same measurements. How do I troubleshoot this?

My fathers Ampeg suddenly start humming like crazy after playing it for a while, but it he usually turned if off and on, and it would work for a few hours again. Opened it and cap C52 was cracked in the top. Replaced it and then measured bias voltage. Left bank was not a good match (though both...