Monster Amp Meltdown

[J!m]

For what it's worth: there are muffin fans that run quieter AND move more air than their grandparents did.

I have an upgrade fan (quieter and more volume) for my Alesis HD24 recorder I'll probably never get installed... It was a suggested upgrade for anyone using these recorders as they run HOT.

 

[grapplesaw]

Don't know. Maybe it had a hard life prior to you getting it.
I'd like to think it was designed correctly and just needs adequate air space around it.

I think it has a hard life with Peter LOL

 

[Skratch]

Beware on those NOS capacitors from a Chinese supplier. Nippon Chemicon is the most common knock off out there. They have the branded shrink wrap and will give you anything you want...

100% correct, do not buy, they are not what they say usually

 

[Peter S]

That's funny Glen! But I have to confess that the clip lights have flashed on a few occasions. Pink Loyd on the 150" screen can get me carried away.

Hi Jim, I bought the quietest fans I could find in that size about 7 years ago OK, further confession; I slowed the new fans a little more with some series resistors. The fan speed is governed by the amp but I would like to come up with a simple circuit to bypass my series resistors when temp increases...or ideally, modify the the fan control circuit in the amp. Crest's "slow" speed was not slow enough.

 

[Peter S]

100% correct, do not buy, they are not what they say usually

Too late, they're on their way...

 

[Peter S]

Further puzzlements, the manual says fans are governed by heat sink temperature but I have never heard the fans 'rev up' on start-up. Nor have I heard much change in fan speed, even after heavy use.

 

[Peter S]

J4 on the schematic is supposed to be connected to some sort of 3-pinned temp sensor presumably on the heat sink.

 

[George S.]

Probably very similar to a desktop computer.

 

[mlucitt]

Hi Jim, I bought the quietest fans I could find in that size about 7 years ago OK, further confession; I slowed the new fans a little more with some series resistors. The fan speed is governed by the amp but I would like to come up with a simple circuit to bypass my series resistors when temp increases...or ideally, modify the the fan control circuit in the amp. Crest's "slow" speed was not slow enough.

If you have two fans you can wire them in series so that each fan sees about 60VAC. This is enough to spin them at half speed, which is very quiet, but they still move enough air to keep things cool. You can bypass the fan speed control on the amp and let them run al the time.

 

[Peter S]

Hi Jim, they are 24 Volt fans but the series trick would still work. I have just discovered this morning that this amp does not have any temp sensing (see pics in entry #47 above). Maybe installing a sensor would solve the problem...No info yet on what sensor was called for.

 

[J!m]

Probably a simple thermistor that closes at a set temp.

I’m sure digi key etc has a billion to choose from. (Note temps are often in C)

 

[Peter S]

Getting in over my head---again. I have been using a Peavey GPS 3500 (about 600 WPC / 8 ohms) and it sounds great. Bass sounds deeper. I have discovered that the Crest has a non-switchable sub-sonic filter....This just won't do. I will have to reconnect the Crest with all the pieces out of the chassis, on a table so I can access things without having to wrestle everything in and out of the box.
I will measure the bottom end roll-off but I wonder if there is some way to modify the pre amp board to eliminate the high pass filter?

 

[Peter S]

So back to the fan issue. I have discovered the location of the temp sensors. The blank terminals must be for use with a different model. The LM35D temp sensor spec claims + 10 mV per 1 degree C. The sensor circuit seems to be working (to some degree, bad pun). Holding a soldering iron about half an inch away from the sensor caused the voltage at the fan to increase from 11 V (cold start) to 13 V after approx 1 minute.

The sensor signal is connected to IC 2 on the pre amp board. The output of IC 2A is then sent to the fan driver board (page 2). I have extended some connectors to the amp module can be accessed for testing while powered up.

Is there a simple way to mod the fan driver to have more 'gain' or variation of fan voltage for a given change in sensor voltage?

 

[Peter S]

What about removing / moving / changing the value of R 10 and R 12 on the fan driver board (page 2) The connection of Q 3 and Q 4 to the regulators via R 11 and R 16 (voltage divider) seems to reduce the effect of temperature change unless I misunderstand

 

[jbeckva]

Or a Mitsubishi amp on steroids! Keep those ideas coming Jim.

Well now, there IS a precedent for this LOL

https://forums.phxaudiotape.com/threads/i-gotta-pair.2973/#post-50112

 

[Peter S]

An update on the Crest catastrophe; The caps from China came in and of course they measure 15,000 ufd instead of 18,000. I could live with that but they are 130 mm tall and the ad claimed they were 118 mm. I was warned.....
At the moment, the amp seems to run quietly with four 9,500 ufd caps in place of the old 18,000's. My solution will be to buy 15,000 ufd units from Mouser.
The fan speed issue seems to be resolved by shunted the 1 k ohm R 11 and R 13 on page 2 with another 1 K ohm. The fan voltage at room temp is now just over 9 V at quite tolerable.

 

[Peter S]

The next dilemma is a built in high pass filter in the Crest. I have noticed a distinct improvement in the extreme low frequency response of a temporary replacement amp, a Peavey GPS3500. I measured the Crest CA-18 and the Peavey GPS3500 just to make sure I was not imagining things. I bought a function generator for this purpose---any excuse to buy more Leader test gear! Both amps were measured into an 8 ohm dummy load and the P to P voltage was measured on a scope (DC coupling)
The Peavey was flat down to 5 Hz. I would like to remove the high pass filter from the Crest but I am truly in over my head.

 

[Peter S]

On page 4 of the schematic, there is a SERVO/HIGH PASS section but I am unclear as to which cap (or resistor?) could be changed to remove or at least reduce the effect of the high pass filter.
Thanks again for all your help.

 

[Peter S]

With a little help from my friends, I was pointed to C8 on page 4. It is a 2.2 ufd film cap. I bypassed it with a 20 ufd film cap (sledge hammer approach). The response rose at 5 hz...this is probably dangerous! I will try 10 ufd.

Could I use a good quality electrolytic rather than a film cap? space is a factor for 10 ufd @ 63 V.

Here the monster breathing at 5 Hz

 

[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; J!m mentioned cutting holes in the top of the amp, I thought he was joking...