Help/tips on - ITT 8033 refurbish job - channel cutting

[trbst]

Hello all,

Been reading around this forum for a while as I've recently gotten myself into a challenge, trying to fix my ITT 8033 amp. All great tips and lots of knowledge around, so I hope I can get some help with my project.

The amp's relay used to cut in an out or amp would not turn on at all so I changed all capacitors on the power supply board. I checked the output transistors and they look good, so eventually it started working, with none of the popping or hissing sounds from before.

It worked like that for a few days but since last night I have started experiencing problems with one channel - it would either cut out about 50%, meaning that I would not hear the mids properly for example and overall volume would be lower on it. If I turn the amp off and back on it may start playing properly without problems.

Issue is narrowed down to the amp because flipping the cables would cause the other speaker to exhibit these issues.

So in short, it's intermittent, it's not an all or nothing issue (only some frequencies for the most part), and it's on one channel only.

My question is, where would you suggest I start with my checks?

Thanks in advance!
T.

 

[trbst]

One thing I forgot to mention is that I have not yet recapped the pre-amp board which I should probably get on asap. Still if any tips on what other things to look out for, I'd be much obliged.

 

[indianajo]

One sometimes injects bad solder joints into a project when replacing electrolytic caps. Finish the first job, the amp channel, before starting another job.
You're going to have to check the channel with the power on. Warning to newbies, >24 v across your heart can stop it. Use one hand at a time, other in your pocket if you can't control it. Use an alligator clip lead to speaker ground for meter/scope ground. 1 v at 50 A can burn your flesh to charcoal through metal. Wear no jewelry on hands, fingers, neck. Touch no metal even with power off until measured at <1v to ground. Bleed down big capacitors with a resistor & clip leads as required. Don't work distracted by media or conversation, and don't work alone.
One energizes the bad channel with a signal generator or FM radio. I like the second because rock stations have a beat, which makes the pointer move x times/second, which looks nothing like ultrasonic oscillation.
Then look for the first stage where the problem occurs. Possible probes are oscilloscope, analog VOM on 20 vac or 2 vac scale with .047 uf cap in the negative lead to prevent response to DC voltage. 3rd choice is a sound probe, which is another amp and speaker with a voltage protector in front of it. Voltage protector has 2 k resistors hot & ground. Then .047 uf >200 v cap hot and ground. Then cross hot & ground with 2 LED plus to minus, so voltage is clamped to 1.5 or 2 vac in either direction. from there hot and ground go to input of the other amp.
BTW when re capping, I only do electrolytic caps, the ones with a plus on one end, minuses in balls on a stripe, or the letters NP after the voltage rating. Those are the bottles of water sealed with rubber, good or cheap. The plastic film caps are forever, except some tiny brands made 1960-1970 like SCR in france. Disk caps usually are forever unless the silver plate is showing. Low voltage (50v) caps can be opened or shorted by rail voltage rampaging through the system from shorted output transistors. Doesn't sound like the problem here.
When you find the stage where the problem isn't yet, you look for stupid results in the next stage. diodes or transistor be junctions not at 0.6 v. Solder joints that make the problem go away when you push on them with a wood stick or meter probe. Capacitors with the same DC voltage on both sides. PCB Lands with different DC voltages on the leads on each end. Resistors with more DC voltage across them than the wattage rating would allow. VAS or input transistors with less than 1/3 supply voltage C-E. CCS transistors are different. Op amps where silent the 2 input voltages are not the same as the output voltage. Connectors that work when you push on them but not not. Especially punch down connectors, these oxidize in 10-20 years and block low volt signals. Ribbon connectors are punch down connectors. Pot wipers can do the same thing, need wiggling and/or spraying out.
I keep a trash speaker on the amp while testing, incase something I touch makes the problem go away. I put electrolytic caps minus to minus in series with the speaker to prevent DC damage to speaker. 1000 uf is enough, 10000 uf will allow more bass through.
Happy hunting.

 

[patrick101]

your case maybe a broken or bad contact relay, you can knock it to see if it will show you something.

 

[trbst]

Hello,

indianajo - thank you so much for taking the time on all this! I will try and see what I can do with that.
I don't have any other tools besides digital multimeter so I'll use that, but the problem I have is not fully understanding how to distinguish and check the different stages as you said. I'll Check the schematic and see if I can measure some of the voltages shown there and see if they match.

patrick101 - I tried, no reaction. I do have a spare relay so I'm contemplating changing it out anyway.

Thanks!