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LH1000 preamp board removal

ArtechnikA

ArtechnikA

I endorsed a check once...
Feb 24, 2013
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www.artechnika.com
#1
Outdoor gig with wicked winds. Somehow a gust caught the back of my rig and blew it right over onto its face. The towel bars offered a little protection but the straight-ended plug still punched out the back of the socket. Yes, I'm going to be using 90º plugs from now on...

The input sockets are circuit-board mounted and the solder connect9ions are on the bottom. The preamp board has gotta come out. I've removed every screw I could find and it's still stuck in there. Feels like it's something around the front panel.

I'm confident in my ability to solder in a new socket but I can't see what's holding the circuit board in place.
 
#2
There's almost 100% guaranteed to be additional damage, what you are seeing is the tip of the iceberg.

That’s why it’s not coming apart.

(Based on experience)
 
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#4
eniac said:
Especially because that thing weighs 31 pounds. So, that was a major impact. Wouldn't be surprised if there were even some metal (chassis) deformation due to the weight of that toroidal power transformer.
Click to expand...
Bingo, plus more ;)
 
#7
Just windy. The guitarist's stack blew over too, a few minutes later [actually during the performance] but kept on ticking.
So you're telling me I haven't missed any fasteners. Cool.
I will defer to AgedHorse' experience, but I'm not going to scrap the amp without digging further.
It's heavy, but it is also a very stiff chassis. Looked for bends or ripples with a study straightedge and observed nothing out of the ordinary.
Looks like the front panel comes off after the towel bars and a couple of other small screws are removed; I'll report back.
 
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#8
OK - here's The Real Story [pictures later...]

The preamp board is attached to the front panel with two more screws that can be seen/reached only after the front panel is removed from the chassis.
I don't know who convinced the manufacturing engineers to use those stupid Philips screws to hold the towel bars on. They are a major PITA to reach. I think I'll be using Allen-head fasteners to re-assemble... We will see...

So next is to pull the half-dozen Molex connectors - I have already photographed where they go for reassembly... For completeness, you also have to pull the push terminals on the power switch. Then unsolder the jacks, replace [probably not with enclosed PCB mount jacks] test, and final assembly. I did verify the tube lights up, but after a fall and 10 years in service, I'll probably replace it anyway.
 
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#9
The plastic bushing jack is necessary for maintaining ground current routing. Don’t change it to a different type.

I recommend leaving the tube alone unless there actually IS an identified problem with it. In general, most tubes that are replaced “just because” are perfectly good, with thousands of hours of life left.
 
#10
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#11
agedhorse said:
The plastic bushing jack is necessary for maintaining ground current routing. Don’t change it to a different type.
Click to expand...
Please elaborate; what's going on inside the socket that I cannot reproduce with external jumper wires. Just trying to understand
agedhorse said:
I recommend leaving the tube alone unless there actually IS an identified problem with it. In general, most tubes that are replaced “just because” are perfectly good, with thousands of hours of life left.
Click to expand...
I understand, but I also understand the additional effort involved with removing the amp from its rack and opening the top when it does fail, which Murphy asserts will occur at an inopportune time. Yes, I understand the statistical 'infant mortality' issue... I was saved at this gig because I happened to have a spare amp in the truck, which I do not always. And I could have plugged directly into the PA as a last resort. It's just failure probability tradeoffs...
 
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#12
eniac said:
I've had a ratcheting right-angle Phillips head screwdriver for about the past 45 years. It's saved my bacon many times. You can also get manual ones.

https://duckduckgo.com/?q=right+angle+phillips+head+screwdriver&ia=web
Click to expand...
I have one. There isn't much room, 'cause the bottom screws are right up against a corner and the preamp board is in the way.
I finally used a P1 screwdriver bit in a 1/4-inch socket on a 3-inch 1/4-inch drive ratchet handle. That worked. The top screws are not too bad, and the hot setup on the bottoms was to get them broken loose, then hold in place and turn the towel bar off...
 
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#13
ArtechnikA said:
Please elaborate; what's going on inside the socket that I cannot reproduce with external jumper wires. Just trying to understand

I understand, but I also understand the additional effort involved with removing the amp from its rack and opening the top when it does fail, which Murphy asserts will occur at an inopportune time. Yes, I understand the statistical 'infant mortality' issue... I was saved at this gig because I happened to have a spare amp in the truck, which I do not always. And I could have plugged directly into the PA as a last resort. It's just failure probability tradeoffs...
Click to expand...
The bushing needs to be the same material (ie. if it's a plastic isolating bushing then it needs to be replaced by the same). Just because the bushing may be metal on a PC mount jack doesn't mean that there isn't a separate sleeve contact for circuit ground, this can get tricky but if often done for noise (or discharge protection) reasons. I know that Hartke has consistently used isolated bushings on the back panel, but they have done it several ways on the input. Some companies use also the sleeve as EMC discharge ground to the chassis and the ring as circuit ground since the ring terminal is connected to the sleeve once the plug is fully inserted.

I have found, in my long electronics career, that early or premature failure of new tubes is a MUCH higher risk than continuing to use a proven good operating tube. IME, the failure rate of a new tube is about 10:1 versus continuing to use an known good tube (provided it's not broken).

Regarding access to screws, what YOU think doesn't make sense may be because you don;t have the experience on the manufacturung and service side. With the proper tools, something that appears impossible is often amazingly simple.

You don't have to take my advice, you seem to be resistant to many of the suggestions that have been made.
 
#14
agedhorse said:
The bushing needs to be the same material (ie. if it's a plastic isolating bushing then it needs to be replaced by the same). Just because the bushing may be metal on a PC mount jack doesn't mean that there isn't a separate sleeve contact for circuit ground, this can get tricky but if often done for noise (or discharge protection) reasons. I know that Hartke has consistently used isolated bushings on the back panel, but they have done it several ways on the input. Some companies use also the sleeve as EMC discharge ground to the chassis and the ring as circuit ground since the ring terminal is connected to the sleeve once the plug is fully inserted.
Click to expand...
That makes sense; thank you.
agedhorse said:
I have found, in my long electronics career, that early or premature failure of new tubes is a MUCH higher risk than continuing to use a proven good operating tube. IME, the failure rate of a new tube is about 10:1 versus continuing to use an known good tube (provided it's not broken).
Click to expand...
We will see. It quit making sound after the fall. If the tube is still working, I'll keep it in service. And start carrying a spare...
agedhorse said:
Regarding access to screws, what YOU think doesn't make sense may be because you don;t have the experience on the manufacturung and service side. With the proper tools, something that appears impossible is often amazingly simple.
Click to expand...
I have no doubt the factory has specialised tooling that makes it easy. And that many repair depots may not have access to teeny Allen sockets, and I expect Allen screws are far more expensive. I also expect the power amps are installed after the preamp. Without them in the way, there's a lot more access.
 
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#15
ArtechnikA said:
I have no doubt the factory has specialised tooling that makes it easy. And that many repair depots may not have access to teeny Allen sockets, and I expect Allen screws are far more expensive. I also expect the power amps are installed after the preamp. Without them in the way, there's a lot more access.
Click to expand...

I am familiar with the manufacturing process at that site. Section 17.2.44.1 mandates the use of specific Mandarin swear words which makes things go more smoothly and is also a stress-reliever.
 
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#16
eniac said:
I am familiar with the manufacturing process at that site. Section 17.2.44.1 mandates the use of specific Mandarin swear words which makes things go more smoothly and is also a stress-reliever.
Click to expand...
Well there ya go. I've just been using standard American English mechanic language...
 
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#17
Updates...

I finally got the socket out of the PC board in the hopes of finding manufacturer markings. Nope. It's a Hartke Special...
Full Compass seems to have stocked them for a while but now they are NLA everywhere.
From looking at the schematic and the socket internals, the only thing 'special' appears to be an internal switch that grounds the tip terminal of the socket when a cable is not inserted. I think we used to call those 'shorting' or shunt connectors. This is a possible path forward, since those connectors are still available.
It was observed by someone else examining the schematic that the only difference between the 'Passive' and 'Active' inputs is the value of the padding resistor used before the 2 input paths converge. Since I have never used the Active input, the fast path to functionality looks to be to replace the padding resistor in that path with the value used in the Passive input side. Or more likely and potentially less destructively, adding a parallel resistor to produce the same net resistance...
 
#18
ArtechnikA said:
Updates...

I finally got the socket out of the PC board in the hopes of finding manufacturer markings. Nope. It's a Hartke Special...
Full Compass seems to have stocked them for a while but now they are NLA everywhere.
From looking at the schematic and the socket internals, the only thing 'special' appears to be an internal switch that grounds the tip terminal of the socket when a cable is not inserted. I think we used to call those 'shorting' or shunt connectors. This is a possible path forward, since those connectors are still available.
It was observed by someone else examining the schematic that the only difference between the 'Passive' and 'Active' inputs is the value of the padding resistor used before the 2 input paths converge. Since I have never used the Active input, the fast path to functionality looks to be to replace the padding resistor in that path with the value used in the Passive input side. Or more likely and potentially less destructively, adding a parallel resistor to produce the same net resistance...
Click to expand...
Long shot, but might be worth dropping Hartke a line. They might even send you a replacement jack for nothing. https://www.hartke.com/contact/

(Sorry in advance if someone's already suggested this. It's early and I'm not running on all eight yet.)
 
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