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1152DC problem child

Jailtime

Jailtime

Standin' on a corner
Got an 1152DC on the bench now, it's giving me the run around. With the preamp engaged, the right channel has unstable DC offset (about 50mV fluctuation). The right channel also has a low level pop/fizz noise in the speaker, not affected by volume or input. Left is quiet.

With the pre isolated from the main amp, the right channel DC is stable. Both channels are quiet with the pre isolated.

I tested it at 30Hz, 1KHz, and 15KHz on the scope, output on both channels was clean and even. Made good power at 92 WPC.

So the power amp is good, that right preamp channel is driving me nuts though. It is recapped, and has all new small transistors in the amplifer board, power supply and preamp section. The heatsinked drivers in the amp are original. I also re-soldered the pin connectors for the preamp board. I'm at a loss where to look next. :dunno:
 
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Are you sure you don't have a capacitor in backwards or a solder bridge somewhere in the pre amp section? It sounds to me like DC is leaking out of the pre amp into the power amp somehow.

Tom
 
To give this thread some closure - there was some really low level noise in the left speaker as well, which confirmed my suspicion of a power supply problem. Q857 and 858 7V zener diodes were the culprits.
 
Zeners are always a bit noisy anyway! That's why there's usually a pretty sizable electrolytic across them. Good catch!!
 
This amplifier is still broken, low level fizz in both channels still, not affected by volume. Power amp is dead quiet. I've re-soldered the pin connections for the preamp and filter boards, checked grounds, rebuilt the entire power supply down to the resistors. Also installed a new power switch and snubber. The noise doesn't show up on the 'scope, so I have no idea where to look next. I'm about to part the damn thing out, since it apparently can't be fixed.
 
Jailtime said:
This amplifier is still broken, low level fizz in both channels still, not affected by volume. Power amp is dead quiet. I've re-soldered the pin connections for the preamp and filter boards, checked grounds, rebuilt the entire power supply down to the resistors. Also installed a new power switch and snubber. The noise doesn't show up on the 'scope, so I have no idea where to look next. I'm about to part the damn thing out, since it apparently can't be fixed.
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Fizz in all inputs?

On both AC and DC coupling? I'm sure that switch is clean by now also.

Do the turnovers and filters have any effect on it?

It's been years since I worked on an 1152dc for some reason I think the microphone and headphone contacts needing cleaning.

Couldn't hurt to clean em up and see if it helps. Hope you get it sorted out. I remember mine being a sweet sounding integrated!
 
Yup, fizz on all inputs, both AC and DC coupling. That switch is clean. Filters and turnovers don't affect it.

Haven't tried those mic and phone contacts, gonna have to give that a shot. The power amp really is great sounding using the main in, I'd really like to get it sorted.
 
Might have a lead - was switching the tape monitor, seems like there might be a bad joint in it. Can get a pretty good hum by just tapping the extension shaft, and the fizzy noise goes away. I'll re-solder that tomorrow.
 
Cleaned the mic/phone jacks, and re-soldered the tape monitor switch. Still no fix. I took a video to try to catch the noise, left channel has a sort of constant creaking noise you can hear, right channel pop/fizz didn't pick up well. Camera has a terrible noise floor, the tapping sound is from it's mic.

I also checked over all my work for solder bridges or incorrectly installed parts, didn't find a thing wrong.

Shotgun the resistors in the preamp? All the transistors in this thing are new, except the phono section.

 
I don't have any shorting plugs. Strange discovery today though - I can control the volume of the fizz/pop in both channels with the tone controls. All three, all the way up makes the noise fairly loud. All three, all the way down quiets it down pretty good. Engaging the tone defeat switch makes it so the tone controls have no effect on the noise.

Bad joints or bad ground on that front panel board possible?
 
Sounds like you are getting close. I assume you checked for the transistors that are likely failures? SC458 is one of those and there's a thread here on AK with the more complete list. I can send it if you can't find that list.

Since it's both channels equally, it's got to be a part that is in common to both.
 
Jailtime said:
I don't have any shorting plugs. Strange discovery today though - I can control the volume of the fizz/pop in both channels with the tone controls. All three, all the way up makes the noise fairly loud. All three, all the way down quiets it down pretty good. Engaging the tone defeat switch makes it so the tone controls have no effect on the noise.

Bad joints or bad ground on that front panel board possible?
Click to expand...


RE: shorting plugs.
Cut the plugs off a cheap RCA cable. Strip and solder the inner and outer wires together.
 
I'd be focussing my eyes on the 800 section of the preamp board; the preamp power supply. It directly feeds all the gain devices in the preamp section off of one rail.
 
All the transistors, electrolytic capacitors, and resistors in the preamp and power supply are new. Only original parts left are the film and ceramic caps, so they're getting shotgunned next.

New resistors in the preamp seemed to make the most improvement. Cold power on gives the most trouble now, couple good DC pops from the left channel, quiets down to an occasional fizz after about 10 seconds. Right channel seems to be quiet. It's gotta be a failed component somewhere, since it is a warm-up sensitive problem.
 
Jailtime said:
All the transistors, electrolytic capacitors, and resistors in the preamp and power supply are new. Only original parts left are the film and ceramic caps, so they're getting shotgunned next.

New resistors in the preamp seemed to make the most improvement. Cold power on gives the most trouble now, couple good DC pops from the left channel, quiets down to an occasional fizz after about 10 seconds. Right channel seems to be quiet. It's gotta be a failed component somewhere, since it is a warm-up sensitive problem.
Click to expand...
If it's warm-up related, maybe a cooling spray would help isolate? Even compressed air in a can works for this.

It sounds like you're homing in on the problem. Good luck
 
I don't want the amp to hear this, or it might throw another tantrum. But it looks like shotgunning the ceramic and film caps in the power supply and preamp got it fixed.
 
The PCB traces on the 1152DC face upwards and dust, dirt and environmental crap land on the high gain RIAA active stage causing all sorts of noise coupled through the preamp front end. Often transistors (2SA-872A) will latch on (and the outputs 2SA-912/2SC1885) and cook the series resistors (R440/439) to the RIAA stage. They are a special ultra low noise transistor and should only be replaced with NOS originals if you find leaky ones. I would definitely check the two 47uF/50V (C435/436) capacitors on the phono preamp. They are virtually 100% sure to be vented or dead. They can randomly pull down the preamp rails causing this type of trouble.
 
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Well, it was dead quiet for about 15 hours of play time after shotgunning the ceramic and film caps in the power supply and preamp. Noise is back in the left channel. Tried the R439/440 and A872/C1775 in the phono stage, no fix.

I'm going to part this piece of **** out, before it ends up in the dumpster.
 
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