"Both traces are present, position works, sweep varies
etc. "

Excellent, now we can go from there. I am not an expert in scopes but I am, or at least was an expert troubleshooter.

Each trace is controlled by the proper position control without abnormal interaction between the two. It shows both traces simultaneously in chop or alternate. If this is all true that means the output and the channel switching circuits are working properly.

One question, relevant but not of prime importance, does you friend always use the X10 probe ? ALWAYS use the X10 probe unless you need the gain afforded by the X1 probe. this protects the front end of the scope and lowers circuit loading during testing. Since he cannot fix it himself he might not know this. Ask and tell when you get the chance. There is a chance that he actually caused the failure if the one channel in the past and then this channel now if he does not use the X10 probe(s).

Regardless we still have a direction. there is a slight difference though, if he used the X10 all the time we are looking for a fault that just happened, if he uses the X1 we might be looking at something else. But that does not stop us in our tracks.

You are going to need, at minimum a signal generator or a working scope to fix this. If DC voltages would reveal the problem most likely the trace would be off the screen on affected channel(s). Since it is not we are dealing with a dynamic situation, not a static one.

The attenuator is not likely to develop a fault that would cause no response in any range, before it is suspect as is after. There is nothing active before the attenuator except the AC/DC switch which is easily tested. Then we have the possibility of him causing a low value resistor there to open either by excessive slewing in the input signal or overvoltage slamming against clamping diodes. That is eliminated with a signal generator. It is all high impedance so take a generator trough a 2K resistor to the portions of the switch and see if you get deflection. If so it is right up front, if not, it is after the attenuator. A 1 KHz square wave is usually sufficient. About 10 volts P-P should get a rise out of it. If it is extremely distorted though it is still after the attenuator.

Then you need to find where the signal goes. If it is single sided PCB it is not all that hard. Point to point and multilayer PCBs bring in magnitudes of increased difficulty. But you can still look for identical components.

Being Hitachi and not HP or Tak is actually an advantage here. It is likely that the parts are not house markeds and thus can be researched. Like to get the pinouts of ICs and the specs on transistors. And being Japanese, or at least conforming to their standard rather than JEDEC or whatever, on transistors the collector is usually in the middle. It is much eeasier to determone of it is BCE or ECB than the US way. Actuially some European standards are like those of the US, but Hitachi is usually going to be BCE or ECB, unless it is a really high frequency transistor in which case it could be BEC or CEB. If it is like a 20 MHz scope or so you can almost count on it being BCE or ECB. Too bad don' t remember my login for Hitachi from my working days. If I find it I will get the print and somehow supply it, though I have no hosting right now. Maybe I could email it to someone who does or perhaps contribute it to BAMA or some other free file place.

But for now we must do without.

So now, look for the switching circuit as well, it will have at least 4 diodes per channel, high speed and it will be directly before the main vertical output amp. the generator with the resistor may help. Always use the resistor because otherwise you might cause further problems. And be on the lookout for a very small result on the screen because some of the stages might be mainly current driven. that means low impedance. just be attentive.

Get back here with a list of equipment you can use on this in the way of generators and working scopes and either their specs or model numbers. Don't reply via email. Put it out here so we can get input from others or perhaps help someone. I avoid doing this by email for those reasons. In fact I check this group more...

I will watch for your response.The 2K value for the resistor is not etched in stone, 1K would do but... and 5K would do but...

Just find out what you can.