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Project/One Mark IV protection

U

usdharris

New Member
I've come across a strange one.
When the unit came in for repairs, one channel had shorted outputs. I replaced the outputs and found a weak coupling capacitor and the unit fired up. I had it on a Dim Bulb Tester, thankfully, since 30 secs after firing up, it thumped, the bulb lit up and the protection circuit kicked in. All did their jobs. After the protection time out, the unit came back on and worked again while repeating the same process over and over.
My first test was to pull the pre-amp-main jumpers. Doing that the unit stays on and I can get output from the pre amp out and it never shuts down. That was telling me that the outputs are not running away on their own, only with signal. My next test led to reconnect the jumpers and found that if I flip the lo filter on to either the 15hz or 30hz position that the unit keeps running and produces output. Fairly significant output for a long time. Since it was still on the DBT I didn't spec it out but at least it trip the relay.
So now I'm back to looking at the preamp drive and get the scope out. I notice high freq oscillations on one channel during peaks of audio. But, if I remove the pre-main jumpers, the oscillations stop. There must be some interaction with the output and tone board that is not filtered out. Unfortunately since no one has been able to produce any schematic for this beast, I'm not sure where to look.
I'd appreciate any insight that you guys might be able to come with.

Thanks, Don
 
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This is really baffling. With the pre-main jumpers out, the output of the preamp seems rock solid. I can look at all of the tone connections with the scope and see signal on each stage. With either of the pre-main jumpers in place, if I place my scope on any of the tone transistors, I get the oscillations. The oscillation is a rock solid 1Mhz sine wave. Almost like a signal generator.

Then when I place the lo filter switch in the neutral/off position, with any amount of audio going out, the DC offset on the speaker output will start going up and down until it gets to a point that trips the protection relay. While it is fluctuating, I can flip the low filter on and get it to stop bouncing the DC offset. Also, the audio waveform on the speaker output will show the 1Mhz waveform and it will turn on and off with the audio level peaks.

I have replaced all coupling capacitors and a couple resistors that showed signs of over temp but still measured ok.

I'm now wondering if my replacement output transistors are not the correct replacements. The originals are 2SB655/2SD675 and I've used NTE284/285 in their place. Is it possible that these have a weird characteristic that might cause this?

I feel I'm grasping at straws but it's come to that.
 
usdharris said:
This is really baffling. With the pre-main jumpers out, the output of the preamp seems rock solid. I can look at all of the tone connections with the scope and see signal on each stage. With either of the pre-main jumpers in place, if I place my scope on any of the tone transistors, I get the oscillations. The oscillation is a rock solid 1Mhz sine wave. Almost like a signal generator.

Then when I place the lo filter switch in the neutral/off position, with any amount of audio going out, the DC offset on the speaker output will start going up and down until it gets to a point that trips the protection relay. While it is fluctuating, I can flip the low filter on and get it to stop bouncing the DC offset. Also, the audio waveform on the speaker output will show the 1Mhz waveform and it will turn on and off with the audio level peaks.

I have replaced all coupling capacitors and a couple resistors that showed signs of over temp but still measured ok.

I'm now wondering if my replacement output transistors are not the correct replacements. The originals are 2SB655/2SD675 and I've used NTE284/285 in their place. Is it possible that these have a weird characteristic that might cause this?

I feel I'm grasping at straws but it's come to that.
Click to expand...
You used what now? NTE???
Should be MJ21193/4G or MJ21195/6G.

Maybe to the point of replacing the small signal transistors too.
 
I had them on hand and they cross to that so I thought what the heck. I'll order your suggestions and see from there.
 
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