ML2 Overhaul Questions

[thermionic]

Hi,

A friend who runs an art gallery / restaurant has a pair of ML2s driving Klipschorns. One of the ML2s appears to be somewhere around 6dB down on the other (suggesting a blown device?). I will be staying for a few days with this friend and I’m taking some tools over for the job, i.e. a digital ‘scope, iron, Hakko 808 + DVM.

Obviously, a full restoration job will be out of the question during one week (the amps may not need it?), but I’m trying to get as much background info as I can prior to going out to see the amps. Ideally, I'd like to leave the amps in a state that will enable them to give good service for a few years without maintenance...

There have been a few threads that I’ve searched and it seems that there’s a debate as to which components should be replaced in an ML2 that’s in original condition. This is what I’ve picked up so far:

Definite procedures to carry out:

Resolder all connections

Replace all small electrolytic caps

Re-bias as per procedure in manual.

Clean all connector contacts

Bring back up slowly on variac and check DC offset



Debatable procedures:

Replace all Output Transistors…???

Replace large filter caps (only replace them if ripple is getting through?)

I presume that it’s best to leave sleeping dogs to lie with regards to the circuit breakers – if they ain’t broke, leave them alone? Unless anyone knows of maintenance procedures?

I’d appreciate it if the members here could give it to me straight as to whether I’m out of my depth going near these amps without access to a full workshop… I’ve been building amps for 20 years and am not fazed by the ML2s in my own workshop. What does intimidate me is trying to tackle them ‘in the field’ so-to-speak... If you know of any unusual tools I might need, please let me know. Likewise, if this is the kind of job you wouldn’t go near without access to a well-appointed lab, please say so…

Thanks,

Justin

 

[thermionic]

Anyone? Would you replace all O/P devices as a matter of course?

Would you classify replacing the main filter electrolytics as excessive if they're not passing mains ripple?

If they were my own amps I would replace the above. As they're owned by a friend, I'm trying to be careful. Having said that, if they're in service in his venue, then the costs can be offset.

Thanks in advance,
Justin

 

[Jan Dupont]

I see no reason to replace the output devices unless they are faulty...

However I would replace the large filter caps. The ML-2s are old and the big filter caps must have exceed their normal working life time of 2000 hours.

 

[thermionic]

Thanks, Jan - that makes perfect sense,

I was trying to weigh up the wear of 30-odd years of thermal cycling. However, matching new pairs will be awkward, so although the amps need to be in daily use, I think it makes sense to replace them only if needed.

The 6dB drop on one amp is more likely to be a fault in the VAS stage, wouldn't you say? If an output device were blown, wouldn't there be a DC offset, thus causing the protection to kick in?

Justin

 

[Jan Dupont]

I don't think the output devices causes this 6 dB drop.
There can be two kind of faults with the output devices;
1. One or more devices or short circuited = DC on the output
2. One or more devices broken but not short circuited = Sound would be normal, but there is a big risk than the remaining working output devices will blow up if the load passes their limit.

I would also guess that the problem may be in the VAS stage....
Is the sound clear and natural on the faulty channel ?
I have a copy of a "short" service manual if you need it.

Happy hunting

 

[Roar Malmin]

What is the signal source? If balanced, maybe one of the phases are gone? This is commensurate with a 6dB loss. If unbalanced, is the other phase short-circuited?

Regards
Roar