Crown IC 150 Preamp popping

rocknroller

Active Member
Have a Crown IC 150 preamp, 14,000 series serial number so last revision made, that has a odd popping sound under very specific circumstances. Initally thought it was just dirty controls but after cleaning convinced it's a part failure, likely a cap but not sure which one.

The problem occurs on the RIGHT channel only (no source audio connected to rule out source issues), and only when one of the two phono preamps are selected (either) and only under these 3 conditions:
1) Engaging or disengaging the loudness control and the volume is set to approx 75% or less (if volume is higher, the engage/disengage doe snot cause a problem)
2) If the volume control is moved in the 80-90% range approximately, no other controls touched (but one of the phono inputs has been selected previously)
3) If the volume is approx 75% or higher and the phono input is DE-selected over to another source input type. There is about a 1/2 sec delay for the pop in that instance, like a cap that is releasing

None of these seems to occur with any other input type selected, only the phonos. That doesn't necessarily mean the issue lies on the phono board, since a part/cap could be stressed on the main baord when the phono circuit is engaged.

Visually, C8 on the main board (100pf brown mica) appears darkened, almost like it has been burnt.Given it's place in the circuit (see pic - between the loudness and the volume control) does it make sense that this might be the culprit? Or should I be looking at a different spot for the problem? Thanks is advance for helpful suggestions to DIAGNOSE the problem (not just blindly replace every cap on the board)

Main board excerpt
crown150-main_board part.jpg

Phono board

crown150-phono board part.jpg
 
Popping is not caused by capacitors but rather noisy transistors or resistors in the particular channel mentioned.

A bit of in depth troubleshooting seems to be in order. It's not really going to be anything simple unless you get lucky and discover you have a bad or dirty input jack or patch cord.
 
Not sure if the OP found the solution to his issue but thought I'd post this here. I recently acquired an IC-150 that is the same serial number range as above. I had a very similar issue that only affected the phono inputs. Controls were scratchy and popping at certain levels. After upgrading the opamps ,replacing all the electrolytic caps on both boards and cleaning/re-soldering the pots the issue was still present. The culprit or should I say "culprits" were the four 4.7 uf tantalum capacitors on the phono board. They look like small coupling caps but are actually polarized with positive and negative ends. They read good with an ESR meter and their capacitance values out of circuit read in spec so they were ruled out initially as the problem. Apparently there is an old Crown service bulletin that says to replace these to correct a phono board DC issue. I replaced them with 4.7 uf @ 50v polarized electrolytic's and it corrected the problem. Pre is working great now. Also I wanted to point out in researching this on another forum thread it was recommended to replace these with "non-polarized" electrolytic's. As stated the original tantalum's ARE polarized. Thought I would pass this info on.
 
I would be very wary. I've no idea about it's serial number but an IC150 I bought brand new in the 1970's (along with a DC300) destroyed all drivers in double Advent speakers.

The system was on but not in use, just idling. Suddenly, what could have been the loudest sound I ever heard happened. Only lasted 2 or 3 seconds and then stopped. I was stunned and confused by the event and only realized it was the stereo when I saw smoke rising from the speakers.

Removing woofers showed the flexible wires going from frames to cones were gone as in vaporized and the insulation material inside the cabinets was scorched and melted in spots.

The IC150 & DC300 were only a week or two old and went back to the dealer. I can't imagine the DC300 was faulty, it was just amplifying. I blame the IC150.

I was able to get full value for the amp and pre towards something else but was out of luck for speaker compensation. Apparently there was a disclaimer about damage caused to ancillary equipment.

It was very expensive to buy all new speaker components so to afford them, I had to settle for less expensive amplification,

I continued frequenting that shop and was even offered a job so I worked there for a time. What went wrong with the IC150 was never explained. It went back to the distributor but they did not or would not disclose their findings.

That shop had a tech on premises and he speculated "runaway feedback" but had no idea where it came from.

I've been more than gun-shy about those things ever since.
 
That sure sounds like some sort of DC related issue. I work for Backstage Music in Mississippi, been here since '78. We had an IC-150 and D150 Crown running a pair of klipsch heresy's in one of our locations that ran every day, six days a week from 1980 to 1995. Some inline fuses to the speakers would be in order, even says so in the IC-150 owners manual IIRC.
 
, even says so in the IC-150 owners manual IIRC.
That suggests to me that they were aware of an issue where that preamp could dump significant DC on it's outputs. DC300 is one of the few vintage amps out there that will pass DC without harming the amp.
 
Back
Top Bottom