Hey guys, I picked up a crown ic150 preamp that was extremely dirty on the outside , but very clean inside. I hooked it up and heard nothing at all until I turned the tuner/amp and the preamp to nearly max volume, but it's still very quiet. There is no visible issues with any of the caps (though I know they could still be bad). All wires and solder joints seem to be ok. Muting has a wire bridging it and is tightened down. Cleaned pots with deoxit, and removed corrosion/oxidization from rca Jacks. If one of the caps failing would cause this issue, which one/s would that be?? I'd rather not remove every single one of them to test them, just to put them back in if it can be narrowed down to a select few. Thank you very much
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century tek
Addicted Member
What are you using as a source? Tape monitor in / out?
Both channels would indicate a power supply issue but check the simplest things first as most of these (I say most) types of symptoms can be attributed to such things as buttons or switches in unknown positions that cut off the audio, improper connections, or simply operator error.
Both channels would indicate a power supply issue but check the simplest things first as most of these (I say most) types of symptoms can be attributed to such things as buttons or switches in unknown positions that cut off the audio, improper connections, or simply operator error.
Ok so I'm pretty new to this, obviously , but manual tells you what to test, and I am having a hard time identifying what they are referring to or even what to set my multimeter to. Most searches I've done say I need to set it to read resistance, but the manual says to read voltage as far as transistors go.....I feel like a drunk girl on prom night right now!! I've attached pics of both, the instructions and my main board...
Alright guys...decided to go for testing less anticipated problems. I fear it may be the transformer. Meter jumps a tiny bit and falls on all three leads, which would explain why I have enough power for the "on" light to be on, but have now power. Fml. Anyone know how much those are, or where to find one?
Ylli
Super Member
When testing on the operating circuitry, you would look for specific voltages are certain points. When checking transistors (best out of circuit) then you would be using the resistance or 'diode' range on your DMM.
"Meter jumps a tiny bit and falls on all three leads" What three leads, and are you trying to measure voltage or resistance?
With power on and your DMM set to measure DC volts, check the voltage (reference to the chassis) at the emitter of Q8*. Should be +18 volts. Now check the emitter of Q9*, should be -18 volts. Now check the voltage on both sides of D3 - one side should be near 0, the other should be 20 volts or a bit more.
*schematic I have is low res. Not sure of these designations.
"Meter jumps a tiny bit and falls on all three leads" What three leads, and are you trying to measure voltage or resistance?
With power on and your DMM set to measure DC volts, check the voltage (reference to the chassis) at the emitter of Q8*. Should be +18 volts. Now check the emitter of Q9*, should be -18 volts. Now check the voltage on both sides of D3 - one side should be near 0, the other should be 20 volts or a bit more.
*schematic I have is low res. Not sure of these designations.
I have alot to learn on schematics , but there are 3 wires coming from the transformer to a small board with 4 connection points. I placed my negative to the only one that has only a wire bridge screwed to the chassis, indicating it's a ground, and touched the possitive to each of the other leads, and got that little bounce in the needle.
Ylli
Super Member
Sounds like you are measuring the transformer secondary. Power on or power off? If power off, you would want to measure resistance. You should see very near zero ohms from each of the hot secondary leads and chassis ground. If you are measuring with power on then be sure your DMM is set to measure *AC* volts.
Update! I think I've got it narrowed down. After checking the fuse in the back, I saw that it has no resistance what so ever...so it's blown, indicating a possible short in one of the transistors. I checked all three on the board, and sure enough, when touching the collector and emitter leads on the one pictured (located at Q9 on schematic) I have 0 resistance, while the other 2 do have a few ohms. I'm just need to find one to replace it with, as well as a fuse to see if that fixes it.
Thought I'd just add to this thread.... got one of these preamps on the bench, bad T2-81 (crown part #2962) transistor in one channel of the phono preamp. Before I just pick a generic low noise phono preamp transistor, it's an NPN, does anyone have a "preferred NTE cross for this one? Thanks! Or even a modern cross to a non NTE part.