Acoustic 450 Inrush Instability Issues

[Lazzerhed]

I’m new to this forum and this is my first post. I have an Acoustic 450 that has been awesome until I moved 6 years ago and put it in storage for 4 years. I got it out of storage and I’m having trouble with it now. I previously replaced all capacitors including the massive 7800uF power cap about 10-15 years ago. The problem started when I turned it on after I got it out of storage. The output popped at a continuous rate of about one or two pops a second. It concerned me so I turned it off to repair it. I thought that it was the capacitors in the power section so I replaced them all except for the big 7800uF one as it is expensive. I haven’t passed signal through it yet but I noticed that the voltages were fluctuating at the same rate as the pops I heard before. The voltage at the 7800uF cap is showing 86 volts but it fluctuates between 86.7V and 86.4V. At the Q407, Q409 and Q411 power transistors, the voltage should be about 47 volts at the base but my Fluke multimeter is showing these voltage points spike to O.L. The weird part is when I left it running a little longer, it stabilized after about 2-3 minutes. Once it stabilizes, all of the voltage points check out as they should. When I turn it off and back on it’s fine. It seems to take about 10 minutes before it does the voltage fluctuation. I’m really baffled by this and am hoping others might understand what could be causing this unstable startup. How can I fix this inrush instability?

Here’s the link to the schematic I’ve been using.
https://musicstudio.bigredroo.com.au...cs-Rev_1.5.pdf

The power section circuit is board 170045 btw.

[Jon Snell]

Electrolytic capacitors, like car batteries, do not fair well when not used.
It may recover in time or replace the faulty coupling capacitors with the correct value components.
Insert an empty jack plug into J505. If the popping is still there, it is an graphic/amplifier fault, if not, a preamp fault.
I wonder what value you used for C505; 7800uF is not a standard value, 8200uF is as near as you'll get. https://uk.rs-online.com/web/p/alumi...citors/1443386 unlikely to cause your issue though.

[Lazzerhed]

Hi thanks for the response. I replaced the C505 with the same value capacitor that was in the schematic. It calls for a 7800uF/100V capacitor. The new one is a 7800uF/150V. The voltage on it shows the same voltage throughout the voltage instability. I’ll try the plug trick to see if I can isolate the issue. However it does seem to be a power amp issue as that’s where the main voltage fluctuations occur

[Jon Snell]

I would, if it is the amplifier, suspect the following (circled in green) capacitors.

[Lazzerhed]

I had already replaced all capacitors on the power amp circuit board, and it was still doing this odd voltage swing. I went ahead and ordered the 8200uF/100V cap you recommended to see if it could be the main power cap. I put my analog Simpson multimeter on the main power cap and on the base of the Q407, Q409 & Q411 power transistors. the main power cap only seems to fluctuate in millimeters while the 46V of the power transistors fluctuates by ~20V. Specifically, switch on and it spikes to 86V then drops to 46V and then begins to fluctuate down to 23V and back up to 46V at a rate of twice a second. Originally, I thought the large fluctuation on the power transistors (and what's feeding them) was causing the millivolt fluctuations at the power cap but now I'm wondering if a minor fluctuation on the power cap is translating to the major fluctuations at the power transistors. More to be revealed when it arrives!

[Jon Snell]

I had already replaced all capacitors on the power amp circuit board, and it was still doing this odd voltage swing. I went ahead and ordered the 8200uF/100V cap you recommended to see if it could be the main power cap. I put my analog Simpson multimeter on the main power cap and on the base of the Q407, Q409 & Q411 power transistors. the main power cap only seems to fluctuate in millimeters while the 46V of the power transistors fluctuates by ~20V. Specifically, switch on and it spikes to 86V then drops to 46V and then begins to fluctuate down to 23V and back up to 46V at a rate of twice a second. Originally, I thought the large fluctuation on the power transistors (and what's feeding them) was causing the millivolt fluctuations at the power cap but now I'm wondering if a minor fluctuation on the power cap is translating to the major fluctuations at the power transistors. More to be revealed when it arrives!

Unfortunately, in higher impedance circuits like these, it is not a good thing to connect any analogue meter to the base of any transistor as with a 20k/volt scale, the meters loading with skew the results.
Bad enough with the capacitance of a quality Fluke.

These amplifiers are 'single ended' with a single power source and have no centre point or zero voltage rail to measure from.
With a single rail power supply, as the amplifier powers up, the biasing changes and the output voltage can swing almost from 0v to rail voltage for a second or less. This is normal.
A balances amplifier uses two rails; +ve and -ve with respect to a zero point, usually halfway between the rails. They tend to swing violently from rail to rail, (similar to the single rail types) at power up, unless there is a mute circuit in operation that cuts off the supply current to the drive transistors for a second or so.

[x-pro]

Check diode bridge BR501. Check C408. Replace C407! If it is ceramic, there is no need to check it. It may fail briefly at voltages over 50V. Or temporarily remove it. Unsolder one lead of R438, which is on the collector of Q414. In this case the preamplifier will not work. Check the stability of the power supply.
If it is not stable, temporarily unsolder Q408, 409, 410, 411.
Do not unsolder R430 and R431 instead of unsoldering Q408-411.
Check the stability of the power supply.

Translated with DeepL.com (free version)

[Lazzerhed]

I replaced C407 and C408 and it definitely helped a lot! There was only a small voltage fluctuation but it was still there. I bought a new 8700uF/100V cap just as Jon suggested and it works great now! I was reticent about replacing it because the last time I did it was about $80-90 for the large can power cap. The one you suggested, Jon, was only $17! Now all voltages are smooth and ripple free. Thanks for all your help!