CREST Catastrophe

[sajti]

You can use electrolytic, bipolar will fit. Or maybe try small 4.7 -10uF like this one: https://hu.mouser.com/ProductDetail/WIMA/MKS2B051001N00MSSD?qs=sJjjjplDs9s2p6kvX4xh/A==

 

[sajti]

Change C8 and C30 with same value and frequency response remains flat. Avoid too high values. So 4.7uF gives 3Hz/-3dB, 10uF gives 1.5Hz/-3dB.

 

[ulogon]

My goal is to modify the fan control circuit to run slower at 75 degrees F and faster at 95 F

May be of some interest
https://www.diyaudio.com/community/...-temperature-controlled-analog-no-pwm.336346/

 

[Peter S]

Thanks Ulogon, great link. I have a will have to plot the temp vs fan voltage characteristics with my simple mod ( shunting R11 and R13 with another 1k on page 2). 9 volts at 75 degrees F is great, I will have to see if there is sufficient voltage increase as things heat up.

Regarding LF response, I had shunted C8 in the HIGH PASS cct on page 4, with 20 ufd. This produced a rise in response at 5 Hz. Reducing the shunt to 10 ufd reduced the rise but there is still a rise. It's always a mistake to contradict the experts...but could I not simply find a value for C8 that is somewhere between 2.2ufd (too much roll-off) and 10 ufd (too much boost)?
Thanks again and pardon my stubbornness, I am always looking for the easy way out!
Peter

 

[sajti]

I wrote the solution above: Change C8 and C30 to same value, and there will be flat response.

 

[ulogon]

I am always looking for the easy way out

The same here , although before deciding I also like to explore the genius of others.
And then I always choose the simplest path, which is very often also the most ingenious.

 

[Peter S]

Yes, exploring the genius of others is why I'm here!;
Thanks Sajti, you were right again of course! I added 4 ufd to both C 8 and C 30. Response is now flat to 5 Hz and down about 20% at 3 Hz. Exactly what I wanted. I have ordered four 6.8 ufd 63 V film capacitors as well as 12 new caps for the power supply, over $500 CDN---ouch!
Hope to have this gentrified Crest back in the rack soon...the big 35 mm dia x 118 mm caps are on backorder...

 

[Peter S]

Slowly coming together!
I have spent way too much time on this project! So many modifications to this amp. Initially I dipped the transformer 20 times in electrical vanish to stop the buzzing noise. Fans were replaced with quieter models. I abandoned the idea of making it passively cooled, instead modifying the fan control circuits for a lower voltage at room temp. The high pass filter was modified to be about 3 db down at 3 Hz instead of 15 Hz. This was an excuse to buy my latest toy, a Leader Function generator.
One final modification, I have to cut a hole in the top cover to accommodate the taller caps. After months of searching, it was impossible to find caps that fit. Strange, new caps are usually smaller.
I really don't like the idea of cutting the cover, but it's not like this amp will ever be a vintage piece.
PS; Thanks for the help from you folks here on Diyaudio

 

[Peter S]

Even the Nichicon caps from Mouser are a bit too tall. Really hate to cut the top cover.

 

[full speed]

Hi Folks, a little update. It took 10 weeks for the "18,000 ufd" caps to arrive from China. Sure enough, they only measure 15,000 ufd but the big deal breaker was the height. The ad claimed 35*118 mm but they are actually 35*130 mm. I wouldn't be able to get the lid on.
At the moment I have temporarily installed four 9,500 ufd caps for the low rails and I hear no audible hum. I will purchase 15,000 ufd caps from North American supplier and be happy about it.
Thanks for all your help with the fan speed issue. As per wg_ski, I shunted R11 and R13 with another 1k resistor, and left R19 on the preamp board as the stock value. The fans run at about 8 or 9 volts at room temp and is quite tolerable. I will measure the voltage increase with temp asap.
Thanks everyone for your input

Nice clamp meter i have on my bench one ex623.

 

[Peter S]

That EXTECH 623 has been very useful for measuring capacitance and temperature. Great meter but goes through 9 V batteries pretty quickly. I would have preferred a Fluke but my second mortgage was not approved.