Crate amp hum

[monsteranus]

I have a low hum in my amp with all volumes controls all the way down. When its first powered up there is no hum. within a few minutes the hum appears. Its a CR160 thats old. I do plan to replace the filter caps and did swap out the existing caps with a couple I had on hand. Both sets of filter caps still have the delayed start of the hum.

Ive read several other existing threads here and have seen some suggestions I will check. Any direct response to this problem will be greatly appreciated.

[Enzo]

I assume you have at least four power supplies. Two for the powr amp, and two for the preamp. Powr amp is likely +/-35-40 volts or something. And the preamp probably runs on +/-15v. See what each of those does as the amp warms up. See if one of those 15v rails is wandering for example.

[monsteranus]

29.5 v +/- 16 v +/- and 15 v +/- Thats what I see on the schematic. The 29.5 v comes to the preamp board with some electrolytics and diodes to drop it to the 15 and 16 volt supply for the preamp. I considered replacing all those electrolytics where the power transition happens.

[Enzo]

It's not about what you see on the schematic, it's about whAt you see on the circuit board. Don't just throw parts at it, take the measurements I suggested and see what is happeneing.

Scope them too. If you have no scope, then set your meter to AC volts and re-check each suply. measuring a DC voltag on an AC meter will show you how much ripple there might be on the supply. See if the ripple increases after the warm up period.

[monsteranus]

I metered some power points a bit ago. I got 60 VDC across where the power feed to the preamp board enters, my drawing is marked as 29.5 VDC. I got 33 VDC where my drawing shows a 16 VDC test point.

On AC scale I got about 77 mV on the 60VDC and the 33 VDC on the AC scale started high then quickly dropped to 1.6 mV. The 77 mV reading was just slightly higher before settling out at 77mV.

I cant say I saw any difference between measuring at initial power up and when hum developed.

[monsteranus]

Its been a long day and while sitting here think I just realized I did not meter this correctly. I will re-do it tomorrow.

What do I look for as a reaction for using the AC scale as you suggested?

[Enzo]

If your 30v rails have 77mv of ripple or less, they are fine. I'd expect the 15v rails to be more or less free of ripple, a few mvs maybe.

What I was looking for was maybe a 15v that dropped to 10 or 12 or something on the DC, or maybe went from almost no ripple up to a couple volts or more of it. SO a pretty observable thing, if present. If you don;t get that, then the power supply is probably OK.

When you stick the AC voltmeter on a test point, do give it a moment to settle. Going from zero to some DC voltage looks to the meter like a big AC spike, so give it a second to get over that and settle into a reading.

Sounds like you didn;t have your meter connected to ground for those readings.

[monsteranus]

I metered it properly today and the voltage reading are proper ecept I cant locate a connection point for the 15v supply. Using the AC scale has the reading to settle out steady but one thing I noticed was some difference in the readings between the 29.5 v supply where the 16v supply had equal values showing on the AC scale.

Its rather chilly in my shop today and the hum never started as it has been doing. Last night I cleaned the input jacks. I read a post somewhere to try that since the jacks connect the input to ground with no plug in it.

Thanks for the assistance Enzo!