Strange and Lazy repair !!!

[bsco]

If you have the manual you can identify what screws went where by part number.

Thanks guy...I don't have the manual but pretty sure I can get it......
Cheers

[km6xz]

I don't understand the point of isolation transformer for the scope. A current probe is not connecting to the DUT at all. I have one that is good for 120mhz, and opens to clamp on wires up to 14 ga and small enough to get in tight to dense point to point wiring. I also use a cheap Chinese clamp ammeter/multimeter that is really useful around the shop.
A good current amp/probe is pretty expensive for a scope so for its frequency of use most shops would not waste the money if it sits on the shelf most of the time. If you are diagnosing SMPS, it is a definite plus.

I never charged estimates since I figured it was a waste of time to diagnose, put the unit up waiting for an estimate approval. I would post a pre-approval amount for a given type of unit. Say a 100 w tube guitar amp might be $80 average labor plus parts.Since we never shotgunned, parts were usually low. If they were not ready to pre-approve a $80 labor fee, I had to question myself if I wanted to get involved with someone who might not be that interested in getting it repair, so when it was done, it might be 2 years before they picked it up. By establishing a higher than expected cost of repair, the customer was always happy when their total bill turned out to be lower than they anticipated. Calling for an increase in an estimate is not good for a number of reasons, and it turns off customers to pay more than they expected.
When we started doing a high volume of shipped in items, mostly digital decks we set a fixed price including parts to streamline the operation, with no need to contact the owner unless we needed additional information. It was always tricky to do something so personal and individual like a guitar amp when shipped in. We do not know what they expect it to sound like, since slight differences between their expectations and their memory and the technical condition of the amp that we find on the bench can be far apart.
If we did an amp and verified that is performed to spec and sounded just like the 300 last repairs of the same model, there was no telling whether the sound was what the customer wished or hoped for. We shied away from any mods except for reliability or safety, unless done at time and materials with the customer sitting next to the tech the whole time to approve any tone or operational changes. We could voice one fairly easily with decade boxes clipped into a function amp and try different values in critical areas for caps for example while the owner played it. Much more frequently we were de-modding amps that had been butchered by home hobbyists/tone gurus/rant amateurs.
Just selling a mod job is a recipe for conflicts because amateur players have very high expectations for mods turning them into the next Hendrix.
I hated units that were disassembled by the user and unless they were a good customer, would usually not accept them since we had no idea what was missing in the pile of parts they managed to scrape up from the garage floor.
We were customer oriented and wanted them to be happy and confident that they were going to be taken care of but we were not desperate for work since every day pallets arrived loaded with manufacturer owned or customer transfers through the manufacturers.
I would much rather decline a job than have to charge a ton of money for a reconstruction of a pile of parts. It was too disruptive of work flow and often required way too much hand holding.
The most effective tool in the whole shop was the counter girls. They did not take in a unit until the customer demonstrated the problem and could reproduce it. That alone, meant 30% more effective efficiency in the shop. We only charges $65/hour for years until it was finally raised to $75 when I gave the shop to the employees. Other shops had to charge 30-50% more just to cover the waste of time and energy trying to reproduce intermittent problems. Setting up a little demo stand in the lobby allowed the girls to guide the customer through reproducing the problem and verifying the problem was with the unit he brought in(often it was something else connected to the system). Put it on the customer to demo it instead of accepting any unit with "just check it out thoroughly" as a request when they really mean is "when playing flat out in the sun in a kegger in the dirt, the amp would just shut down for 30 minutes". The girls almost always knew the gear and how it was used better than the customer so they could ask leading questions to get them to pinpoint the actual complaint, or spot an obvious problem such as a dirty insert where they would just clean it and hand it back to the customer, with a pretty smile. bench time was best use of benches than running a unit for hours trying to reproduce an intermittent problem that might never have been in that unit in the first place.

[nickb]

I don't understand the point of isolation transformer for the scope. A current probe is not connecting to the DUT at all. I have one that is good for 120mhz, and opens to clamp on wires up to 14 ga and small enough to get in tight to dense point to point wiring. I also use a cheap Chinese clamp ammeter/multimeter that is really useful around the shop.
A good current amp/probe is pretty expensive for a scope so for its frequency of use most shops would not waste the money if it sits on the shelf most of the time. If you are diagnosing SMPS, it is a definite plus.

The transformer was for the case where a current surge measurement was done using a series resistor, not a current probe. Like you, I too have a current probe that goes out to 50MHz but I expect we are the exception since they are very expensive. Many folk reading don't even have a 'scope.

[bsco]

kn6xz...thank you for that reply....I have had people come to me as well with a problem with their amp and I could not reproduce the problem...would have to call them back and get them to either check all their cables, effects units, etc or have them drop everything off to me...there have been quite a few times when the problem was not the amp at all, but a bad cable or some other piece of equipment connected to it...


on another note.....I work by myself and from home...(as I am now retired).....so I pretty much take in whatever comes my way.....I do charge for estimates with the condition that if they go ahead with it, that price is not exactly carved in stone as it can vary somewhat but still has to be reasonable.....as I am the only tech in these parts who is actively doing electronic repairs , and these people have more that one unit, I will get all their business sooner or later.....and what I lose on this repair I will make up elsewhere......within reason of course...

but you are right.....if I had a shop space that I was renting. and a ton of equipment coming in from manufacturers, then I would not be able to tie up time doing estimates or taking equipment that has already been dis-assembled by the owner or some other tech.....

and thanks for the tip on the clamp on meter.....I don't presently have one yet but I will soon....I don't think I am doing enough work to justify the purchase of a scope current probe.....but the clamp on ammeter seems like a good piece of equipment for me to have.....
Cheers

[g1]

Attached is an example of an Ampeg SVT that uses an external 10A Slo-Blo mains fuse with an internal 15A Slo-Blo mains fuse in series. Note the 50% larger internal fuse. This design technique is mostly found in high powered and generally more "modern" equipment.
Tom

[ATTACH]30115[/ATTACH]

Thanks Tom....I had a look at the schematic...I see the mains fuse 10A user accessable.....the second fuse I found was a.5A slo blo....didn't see the 15A fast blo there..
Cheers.....

Look at the pdf Tom attached, it shows the back up fuse after the power switch. It's actually a typo and says 1.5A but it should read 15A slow. So again we see 50% over for the internal.

[bsco]

got it g-one.....that was the fuse that I saw but the .5A threw me off.....i thought it was a very low rating for such a high power amp....so this fuse is a 15A fast blow.....a 50% increase....I tried to look at some Fender schematics but they only have one main fuse user accessable...I was going to check out some Marshall schematics when I get the chance...thanks for pointing out the typo to me...
Cheers,
Bernie

[g1]

so this fuse is a 15A fast blow.....a 50% increase....

In this case it's slow blow, same as the external user fuse.

[The Dude]

This little gem showed up in my shop today and I immediately thought of this thread. It's a Vox VT100. The symptom? Distorted output. Several of the wires had come off because of poor soldering. The previous "tech" also melted the part number off of the IC while soldering the wires to the leads. So, off to research which STK goes into the thing......

[The Dude]

It's an STK404-130S. Found one for less than 8 bucks. It's on the way........ BTW, cleaner, more reliable, and less work than soldering a bunch of wires to an IC.

[Audiotexan]

LOL, I was close! I had actually posted STK404-120S...but then I deleted the post, when it smacked me in the face that whoever did that hack job, could have easily just grabbed whatever was handy, and then put it in place.

[MarkusBass]

Look at the pdf Tom attached, it shows the back up fuse after the power switch. It's actually a typo and says 1.5A but it should read 15A slow. So again we see 50% over for the internal.

Attached is an example of an Ampeg SVT that uses an external 10A Slo-Blo mains fuse with an internal 15A Slo-Blo mains fuse in series. Note the 50% larger internal fuse. This design technique is mostly found in high powered and generally more "modern" equipment.
Tom

[ATTACH]30115[/ATTACH]

I hate to disappoint, but this is not the case (double fuses) in SVT-2 Pro. The first fuse is for the whole amp (including power transformer) and the second is for the filament transformer only. The second fuse is 1.5A and the value on the schematic is correct.
Double protections are rarely used because users can do all kind of stupid things (that the manufacturer cannot foresee), for example: plug in a shorted speaker cable, plug in an amp for 120V mains power supply to 230V mains power, pour a beer on hot valves. This would require a triple protection and in extreme case a quadruple protection . But it is much cheaper to allow the amp to fail and fix it.

Mark

[bsco]

[ATTACH=CONFIG]30137[/ATTACH]

This little gem showed up in my shop today and I immediately thought of this thread. It's a Vox VT100. The symptom? Distorted output. Several of the wires had come off because of poor soldering. The previous "tech" also melted the part number off of the IC while soldering the wires to the leads. So, off to research which STK goes into the thing......

What a lovely soldering job.....This guy should get an award.......an idiot award to be exact....

[tedmich]

[ATTACH=CONFIG]30137[/ATTACH]

This little gem showed up in my shop today and I immediately thought of this thread. It's a Vox VT100. The symptom? Distorted output. Several of the wires had come off because of poor soldering. The previous "tech" also melted the part number off of the IC while soldering the wires to the leads. So, off to research which STK goes into the thing......

kinda looks like this


Sanyo 100w mono module, datasheet here:

http://pdf.datasheetcatalog.com/data...wlpx9ha3wy.pdf

[Jazz P Bass]

So what exactly happened to the IC pins themselves?

Must have laid the IC & heatsink over & snapped them.

"Here's your amp back. Fresh as a daisy."

[Chuck H]

I've never seen fuses replaced with aluminum rod. I've seen plenty that were wrapped in aluminum foil (sometimes from a cigarette pack).

What about lazy repairs that WE do. C'mon!?! I know there are some shortcuts we've all taken that are less than ideal but known to be "good enough". Is anyone here willing to fess up? I'll go first...

On a few occasions where board removal was a real PITA I've replaced "through hole" PCB components by snipping the leads and soldering the new component/s to the old leads. Likewise, when making value changes on such boards I've also just piggybacked components on existing ones rather than pull the board to change the existing value. I'm not proud of this, but I don't expect it to cause any real problems either.