Yorkville 200KB

[scurvydan]

Hi everyone! I'm new here and new to electronics but learning fast. My friend gave me a 200KB to troubleshoot. It turns on, and I can hear static on the speaker but adjusting the levels on any of the channels does nothing. If I plug in a keyboard, I get no sound but as I turn it up the clip light comes on at very low levels. It does this on all channels but no sound. I tested the speaker on a different amp and it is fine. So my question is.. where do I start? I want to put as little cash into this as possible. I have resistors, caps, diodes of all sorts already and a couple of the op amps on hand. Should I start with the op amps and transistors first or is there something else more obvious that I might be missing. Thanks for any help that you can offer!

[Enzo]

On the rear there are two jacks marked send and return. Those are your effects loop. Plug a spare cord from send to return. Now does the amp play?

If not, then plug your keyboard/guitar/instrument into the return jack. Can you hear that?

[scurvydan]

I found the master volume pot had a bad joint on one leg. I have sound on all channels now but still clip lights at low volumes and everything sounds like the "cutoff" knob got turned down... I don't know how else to explain it... hollow sound I guess. That should be a capacitor issue right? Thanks for the suggestions Enzo

[Enzo]

Capacitor? It could be anything. Did you try my suggestions anyway? They are designed to point out which part of the amp is faulty.

[scurvydan]

Yes I did. I got sound from going into the return. Jumpering the loop made no difference. But that was after I fixed the pot.

[Enzo]

That means the power amp is OK, and your issues are in the preamp.

ANy chance you have the schematics? Yorkville is usually helpful with those. At least in my working years.

If all four input channels sound the same in terms of your problem, then I'd start with the master section. I might make sure the low voltage supplies are OK. I assume 15v pos and neg? I would also check any op amps in the master section for DC offset on output pins.

[scurvydan]

That is the kind of wisdom I am seeking.. Yes I have the schematics. Yes +/-15V, I will start with your suggestions. Thank you so much.

[Enzo]

Can you post the schematic? Or link us to it? You have only four posts so far, so you may be limited in upload file size, I don't know for sure. It has been a long time since I had four posts.

[scurvydan]

sure

[Enzo]

Cool, thanks.

I am a little foggy tonight, but it appears to me the FX return signal comes in before the master volume? When you plugged into the FX return and it sounded OK, did the master volume work?

My aging eyes are trying to fathom this schematic all crammed onto one page.

[scurvydan]

Yes the master volume controls the level when plugged into fx return but like all channels I get a clip light if I turn it past 2. Just got off work. Gonna poke around a bit and test the voltages like you suggested. I went to bed after I posted the schematic last night.

[Enzo]

Oh, well that is interesting. If you run a cable from FX send to some other amp, do the input channels come through over there OK? Ah I see channel clip and also master clip LEDs. Is it the master that blinks? Or the channels? Or all of them?

I'd be looking for DC offfset on op amp output. Also I might apply a sine wave to a channel and advance it while watching on a scope. For example pin 1 of U201 is the output of the master control stage op amp. Scope that. Is it really clipping? or going off-center?

[scurvydan]

You are such a great help, I appreciate the knowledge you have. I have tested voltages at the bus connections on CONN1 & CONN2 and got 14.5+ and 14.6-
I get the master clip light from the fx return and when any of the channels go up to 2 or more on the volume. Also the channel clip lights come on at very low volumes.
While I was looking it over I found this....
https://drive.google.com/file/d/152e...ew?usp=sharing
MJF6388 has a chip out of it. The only other time I've seen that is when an IC gets too much voltage and explodes. I think I will replace that first but will have to order one.

The very first thing in the datasheet says "Designed for general−purpose amplifiers and switching applications, where the mounting surface of the device is required to be electrically isolated from the heatsink or chassis."
Yet it is mounted to the chassis with thermal paste and a screw.

[Enzo]

That missing hunk is not a good sign, but if they are giving you a solid 15v, it likely is not the problem. Yes, it is mounted right on the chassis because it is plastic, no exposed metal. That is what the data sheet means. It is electrically isolated, because it is uninsulated. So we don't need the mica wafer or the shoulder washer for the screw.

Really, apply a signal and scope what is going on. We need to know what is triggering the clip light. Odd they all go on.

How hot is the signal you are feeding it?

[scurvydan]

To clarify, the clip lights don't all come on. If I put a signal on CH1 and turn it up to more than 2 the clip light starts to come on for CH1 only. If the main volume is turned up past 2 then its clip light will come on as well.
I put the scope on pin 1 of U201 and put a synth through channel one. (I don't have a signal generator for my bench yet) When I keep the volume low and play a sine, the signal stays at 0 but as I turn up the volume on the amp and the clip lights start to come on then there is a big negative bias to the signal.
So is that it then, just change that op amp? can't be that easy.