ZX475 Hi Idle Current / Heat

[ObjectReality]

Hi Phorum,

I'm new here. I've recently unpacked my old ZX475Ti and ZX600Ti amps that have been in storage for quite a while now. I've replaced the leaky power supply capacitors and the rail capacitors in both. The cases are suffering from the usual Ti spiders leg rust marks, so for now I've wrapped the 600 in carbon fibre look vinyl wrap just to see how it looks.

I'd forgotten just how good these amplifiers sound, the 475 is connected to a pair of bookshelf speakers in my office and it sounds fantastic!

One thing I have noticed is that idle current on the 475 rises as the amp warms up to what seems to me to be too high a level. With no music playing the amp will start off idling at around 1.9 amps, but over the course of an hour or two, the idle current rises to 7.8 amps where tops out. The left side of the case is uncomfortable to touch at this point, reaching about 55 degrees C. It seems to have no effect on the sound quality at all.

I should mention that I also replaced the fans in both amps as they had both failed - they would twitch rather than spin up - even when removed from the amp and connected directly to a PSU. There seems to be plenty of air flow from the new fans, so I'm confident that its not an issue of in adequate cooling.

My question is, does this seem normal, what can I check? I don't remember the 475 getting so hot before (though it must be over 20yrs old )

[Silverhorse]

Hi and welcome to the phorum.

It should be less than 2A on idle any time.
after many years it is possible that the bias of your amp is out of balance.
with the older models, ZPA, ZX it is possible to adjust the bias but I'm not sure if it's possible to do it with the Zxti.

Jason will help you further with this one but it can take awhile before he sees the new post.

[ObjectReality]

Thanks for the response Silverhorse

Looking inside there is a potentiometer on each of the 4 channels, and the all appear to be pointing in roughly the same direction.

I think you're correct that the pots have aged. I may start by taking readings of the current value of each and giving them a few sweeps end to end (power off!) to clean them.

[Silverhorse]

If you don't know how to adjust the bias I should not touch them!
These are very sensitive and while you are turning these pots you have to measure the voltage drop over the output emitter resistors of each channel + you have to know the amount of voltage drop over these resistors. I think Jason or somebody else who know more of these amps can help you further.

[Jacampb2]

Yes, don't touch the bias pots. It literally takes only seconds to short an output of you don't know what you are doing. That said, it does sound like something is going wrong with the bias control. Am I reading correctly in that all channels sound ok?

To measure bias current you need an accurate millivolt meter. The TI amps have test points for bias between the last row of transistors and the fan baffle. You need to measure from the test point to the driven output for that channel. The amp should be allowed to idle and stabilize for a few minutes. The TI amiss should be around 1-1.5mV on each channel. You must have an accurate millivolt meter.

If one or more channels continue to drift upward with temperature, there is a fault in the bias control. It is very unlikely to be the bias pot. I would start with new thermal compound for all the heat sinked parts. There will be a smaller to-126 cased transistor under the heat sinks for each channel. This transitor is responsible for thermally tracking the outputs and adjusting bias automatically. If either that transitor, or really any of them, lose a good thermal interface to the heat sinks, then you can get thermal runaway in the outputs. These amps are going on 20+ years old depending on model. The heat sink compound does dry out and lose effectiveness.

If you take them apart and reapply, first, DON'T USE ARCTIC SILVER type compound. It does not belong in an audio amplifier. Use a quality silicone based ceramic compound. My favorite is dowsil 340. Second, pay attention to screw length. Some of the screws are a few millimeters longer than the others. In the wrong places, they will bite you.

If that doesn't fix it, the problem is more in depth. Bias really shouldn't drift with time. It routinely needs adjusted if any transistors in the poweramp are changed, but it would be ready for something to happen while it's in storage that would necessitate adjustment. I have far more detailed instructions on adjusting bias in a thread on my FB group. I'll post a link if you are interested.

Good luck,
Jason

[ObjectReality]

Thank you for your response Jason I really appreciate it.

To answer your point, yes the amplifier sounds great, my test set up is a pair of bookshelf speakers on the front L/R channels and a 12" subwoofer bridged on the rear channels. That said, I have tested both the front and rear outputs full range.

I have a reasonably good multimeter, but honestly I can't say if its accurate to 1-1.5mV, I'll see if I can pick one up.

Thank you for the advice regarding the heat sink compound (and screw lengths!), I've ordered the type you mentioned. I'll replace that as my first step. It would be great to get this old amp running well again!

If you get time I'd appreciate the link to your FB group.

[Jacampb2]

These are the test points for each channel. You don't have to have the amp out of the case. You can measure directly across the emitter resistors like you did, but the amp should be at thermal equilibrium when testing, and that's hard to do with it upside down. 5mv is way higher than they should be. It's fairly of that all 4 channels are that high.

My FB group is here: https://www.facebook.com/groups/1497695 ... ?ref=share

Good luck,
Jason

[lilviper]

These are the test points for each channel. You don't have to have the amp out of the case. You can measure directly across the emitter resistors like you did, but the amp should be at thermal equilibrium when testing, and that's hard to do with it upside down. 5mv is way higher than they should be. It's fairly of that all 4 channels are that high.

My FB group is here: https://www.facebook.com/groups/1497695 ... ?ref=share

Good luck,
Jason

Dangit.. is it mirrored off fb anywhere?

Sent from my SM-G988U using Tapatalk

[Jacampb2]

No but I can copy it here later. I'll try to remember to do so over the next few days.