Beginner advice

[negcreep80]

Hi all, this is my first post, so please bear with me. I found a few older threads for beginners from awhile ago, but I figured rather than bump those, I might start a new one if that's ok.

First off, the reason for finding this excellent forum and starting a thread was my recent decision to open up an old Fender Pro 185 (my first real combo amp from years ago). Now I should disclose that the amp doesn't have any serious defects at this time...it's a little dirty cosmetically, some of the pots are scratchy, but the basics seem to be fine. My goal therefore is to use the amp as a good tool to learn how to diagnose the components, how to meter properly, what to look for physically, etc.

My first step is to obtain schematics, manuals, and whatnot...I emailed Fender since I haven't been able to find anything for a Pro 185 online. Once I get this (hopefully), what's the next step? I feel comfortable reading a schematic, but is it just a matter of testing every resistor, cap, etc.? If there isn't a noticeable issue to troubleshoot, is there a checklist of things I can test just for the practice? For example, is there a way to quantitatively diagnose a speaker as good/failing/bad, or is it all subjective, i.e. does it sound good?

In case you are wondering, I should say this is not an amp that I need or particularly care about, so I'm not too worried about doing any irreparable damage (though I don't plan on destroying it either).

What sites/books/resources would you suggest a beginner to utilize in this process? I certainly find plenty in my initial search, but sometimes it's hard to sort through to find good, concise resources that aren't either ridiculously easy (I do have some basics electronics skills) or overly complex. I'm also curious that since so many of the guides of them seem to focus on tube amps, how much can I gain from them since I can only really deal with my solid state Pro 185 right now? Is there any crossover worth in the info, or are tube and solid state amps separate beasts?

Sorry for the long-winded first post...I'm sure you all have more important issues to tackle, but if anybody can offer any sort of help/guidance in my quest, I'd greatly appreciate it. Feel free to ask follow up questions as well, since I'm sure I forgot some important details.

Cheers,
Chris

[booj]

That's a complicatd amp for a beginner to diagnose. You'll get test points that you'll see on the schematic that will show a specific voltage or waveform. Then you'll get a layout diagram that shows where those test points acttually are on the circuit board. You can then compare to the values given on the scat.
If your thinking about changing values to see what happens you'll soon see why some of us like "simple" and "eylets" or "turrets".

[negcreep80]

Thanks for the heads up, booj...I certainly don't want to bite off more than I can chew as a beginner. Like I said, my goal is just to learn a little more about testing and diagnosing, and I wouldn't be attempting any type of modification just yet.

I also have a little Kustom practice amp that might be a little more appropriate to my endeavours. But in the meantime, hopefully when I can get some schematics and diagrams, I'll have better questions to ask.

[clydey]

these are free jack darr 8 chapters search on net
google scribd for 2 books by morgan jones on tube amps
i hope he put the books on himself. on the scribd site if you want to print you have to download first or it just prints a selection of pages
also there is an earlier thread about books here with a free download pointer to servicing solid state amplifiers.
cheers

[JHow]

Chris:

I'm not trying to dissuade you from working on your current amp, but if your objective is to learn to learn about amps, it might be more effective to try your hand at building a small one. It sounded to me like your current amp mostly works fine - I'm not sure how much experience you will gain short of cleaning the pots and taking some measurements, and thinking about the circuit, which is all good, but maybe not as ambitious as it could be. You could wait a long time for something to go wrong so you could fix it, for example.

When building an amp, you get to think about the theory, and you get to put it into practice, learning what each part does to contribute to the whole.

Also, it's a lot of fun. Many of the posters are professionals, but many more (self included) are doing this as a hobby.

Just my thoughts. Regards, jhow

[KilgoreTrout]

As a rank beginner with a limited knowledge of electronics (though with a hard-earned respect for electricity after being electrocuted twice), I found Dave Hunter's The Guitar Amp Handbook: Understanding Tube Amplifiers and Getting Great Sounds extremely helpful and entertaining. It's written for guitarists rather than electrical engineers or technicians, so you may already be well beyond the level of knowledge presented, but it sure worked for me. He offers a step-by-step walk through the various components and stages of a simple tube amp. It also has lots of juicy photos of the innards of vintage and boutique tube amps, and a chapter on building your own (kit available online).

Oh, for the record, I've never been electrocuted by an amp. It was a power outlet and an old CRT monitor.

- Kilgore

[Bruce / Mission Amps]

As a rank beginner with a limited knowledge of electronics (though with a hard-earned respect for electricity after being electrocuted twice), ....
Oh, for the record, I've never been electrocuted by an amp. It was a power outlet and an old CRT monitor.

- Kilgore

ha ha... I don't think you've really been electrocuted, I think like many of us, you've had the piss shocked out of you.


e·lec·tro·cut·ed, e·lec·tro·cut·ing, e·lec·tro·cutes
1. To kill with electricity: a worker who was electrocuted by a high-tension wire.
2. To execute (a condemned prisoner) by means of electricity.

[KilgoreTrout]

ha ha... I don't think you've really been electrocuted, I think like many of us, you've had the piss shocked out of you.


e·lec·tro·cut·ed, e·lec·tro·cut·ing, e·lec·tro·cutes
1. To kill with electricity: a worker who was electrocuted by a high-tension wire.
2. To execute (a condemned prisoner) by means of electricity.

Thanks. Clearly the latest shock had an effect on my language ability.

- Kilgore

[scooperman]

There are simple amps which have simple schematics, so that lets you easily see the relationships. Here is the power supply, it sends power to this thing and that thing and that thing over there. Here is the input jack, the signal goes here, then it goes to this thing, then it goes over there. Simple amp usually means simple construction, few parts, easy to find your way around the circuitry by following the schematic.

The Fender London/Pro/Stage 185 schematic is pretty complicated looking, they stuffed a lot of schematic on a B size print. There are a lot of things in it which switch the signals around, controlled from buttons on the front panel or buttons on the footswitch. Schematics are not hard to find, copies of rev F and G are posted here and there on the web, and Fender should send you a nice copy with a user manual if you send them a money order.

I would say if you just want to measure stuff and follow the signal around, the 185 is not too difficult after you get a schematic, there are signal-tracing test points on it. Basically you replace the speaker with a dummy resistive load , plug in a sine wave generator set to the test level, turn it on and check the power supply voltages and then you follow the signal around. Quite a few dual opamps in DIP8 packages, so lots of the testing is probing pins 1 and 7 of those opamps. The pcb was made with a breakaway section, this section is mounted on the back panel and connects with a flat cable, it contains effects jacks and one opamp. Unfortunately the breakaway is not shown schematically, they do not identify where the signals leave the main pcb and come back to it, so that gets a bit tricky. You have to wander around the pcb with a magnifier to locate the components. The schematic shows all three product versions, which only differ in the number of output jacks and speakers. The Stage had one, the Pro had 2, the London was a head so it just has jacks.