Sometimes You Just Get Lucky

[bob p]

I put a Carver M-1.5t on the bench yesterday. The chief complaint was that it was a fuse popper. I thought that was pretty odd, as this Carver amp has a protection circuit in it to shut it down under excessive current conditions.

Based on my extensive experience with Carver amps <ha!>, I figured the problem had to be on the primary side of the PT, perhaps in the diac/triac on the magnetic field whatzit regulation board. The triac had a telltale brown spot at the M1 lead, so I did some power-off testing, thinking that this had to be the problem. It wasn't.

So I ran through the amp wtih my diode checker and found that all of the transistors in the amp tested OK. All of them. things were getting to the point that I had to test the amp powered-on. I called it a night and put off the rest of the testing until this afternoon.

Much to my surprise, today I powered up the amp on a variac and it didn't pull current. So I powered it up with full voltage and no load, and it just sat there with the running lights on. No protection circuits triggered. No fuses popped. What?

I calibrated the high voltage rail on the PSU board and dialed it up from 108 to 124VDC. Then I calibrated the bias on both channels, which were reading 0.1 mV. I set it to 3.4 mV and traced some small signals into no load. So far so good. Then I gradually increased the signal to full power at no load. 72 VAC when the clipping indicator came on.

Eventually, I worked my way up to 72 VAC RMS into 8R on each channel. That's the rated output of 650W RMS. The amp is now doing a burn-in test with real music as I type this.

Sometimes you just get lucky.

[Enzo]

See? It is just a matter of having the right experience....

[Steve Conner]

It'll be back :P

[bob p]

See? It is just a matter of having the right experience....

well, i guess i have plenty of experience, i just haven't had the right experience.

Enzo, thanks again for helping me through that other Carver amp. I never would have navigated my way through the undocumented protection circuit without your help.

I don't profess to be an experienced Carver guy by any stretch of the imagination. On this one I got really lucky. I never would have guessed that the owner would give up an amp for dead and that the problem would have been that the amp was biased to zero.

The bias problem kinda surprised me. The RP1 trim pots on the channel cards looked like they were set in a proper position. What amazed me was that as I gave the thumb-wheel type bias pots the smallest perceptible rotary movement, the bias instantaneously JUMPED from 0 up to 5 mV. I had to try to dial it back down with practically no motion to get the bias back down to 3.4 mV.

I had not expected that the trim pots would be so sensitive to such a small movement over their entire range. I'm thinking that there HAS to be a problem here. Maybe the pots just had a worn spot and they need to be replaced. Maybe there's a problem in the bias supply.

Does it make sense to you that the amp would have been popping fuses with the bias set to zero?

[bob p]

It'll be back :P

If you think that I'm missing something Steve, please let me know!

[Steve Conner]

Well, you didn't find any problem that fits the symptoms. Being biased too cold doesn't cause an amp to blow fuses. Sound lousy maybe, but blow fuses, no.

So that says to me the fuse blowing is caused by another intermittent fault that you haven't found. Maybe the bias pots actually are bad like you thought, and they go open circuit now and again and slam the bias way too hot?

The way I like to find intermittent faults is just to try and reproduce them, or better still, make it blow out permanently :-) So I'd load the thing fully on a dummy load at both extremes of line voltage while banging on it with a fist, or something.

[Enzo]

I keep a rubber mallet next to the bench for exactly that purpose. Though I have to admit, God's own rubber mallet - your fist - is even handier most of the time.

If the bias control were dirty electrically, it could potentially cause trouble, and turning it back and forth might have cleared the dirt.

[bob p]

Well, you didn't find any problem that fits the symptoms. Being biased too cold doesn't cause an amp to blow fuses. Sound lousy maybe, but blow fuses, no.

So that says to me the fuse blowing is caused by another intermittent fault that you haven't found. Maybe the bias pots actually are bad like you thought, and they go open circuit now and again and slam the bias way too hot?

The way I like to find intermittent faults is just to try and reproduce them, or better still, make it blow out permanently :-) So I'd load the thing fully on a dummy load at both extremes of line voltage while banging on it with a fist, or something.

It didn't make sense to me that fuses would be popping if the bias were the only problem in the amp. But I've never actually seen the amp pop fuses. That's just what the seller told me, and he didn't seem to really know what was wrong. Who knows if his description was accurate.

I've had the amp playing music non-stop for 3 days now. I freely admit that I am too chicken to drive a carver amp with sine waves into a resistive load for an extended period of time. If I don't incinerate the amp with a sine wave (carvers don't like sine drive), I might incinerate the house. Sine waves and 1KW of power make resistive loads too hot to leave them unattended. So I've just been using music.

I haven't been beating on it with a mallet. I'm chicken about that too! I can see how jostling a pair of defective trimpots might reproduce the problem. I've also been thinking that those 20 year old caps in the low/mid voltage rails need to be replaced, based on age alone. They are known problem points in this model of amp, as they've been known to make amps blow up when they go bad.

So rather than starting off by beating the amp to death, I'm thinking I'll replace the PSU caps (something anticipated to fail) and the trimpots (something prone to mechancial movement). Are there any other prime suspects that I should consider before shock testing?

[Alex R]

Sine waves and 1KW of power make resistive loads too hot to leave them unattended.

So iron your shirts with them.