Fender Deluxe 85 Red Knob problems

[teledude61]

My band mate was having problems with his Deluxe 85 and asked me to check it out. I told him it might take a while to figure out the problem. He said to just forget it then and gave me the amp. (Yay for free amps!)

I determined the speaker was fried and the speaker connector lugs needed re-soldering on the circuit board. At some point the amp had been modified with an effects loop but that was removed and I assumed the amp had been returned to factory specs. However, the fuse keeps blowing as soon as I switch on the power.

The problem I have is that I don't know which transformer cables go to which connectors. I have the schematic and layout diagrams but it doesn't accurately describe the color of the wires.

Does anyone have a photo of a fully functioning circuit board with all wires connected? I would sure appreciate seeing one.
Thanks.

[DrGonz78]

Your gonna need one of these by the time you figure out where to plug all the transformer wires onto the pcb board. Light bulb limiter!! Very important...
http://www.ssguitar.com/index.php?topic=2093.msg22315;topicseen#new

This links to a thread on here that has the schematic and some similar troubleshooting scenarios.
http://www.ssguitar.com/index.php?topic=1040.0

Got to look at that schematic for some clues where to place the transformer wires. We can see on the schematic that CP3 and CP5 are for the secondary wires from the PT. CP4 would be a center tap if there is one, my guess there is one. CP10 looks to have the hot side of the mains as it is fused and then travels back out after a thermistors(think that is what that is) through CP9 to the primary of the transformer. Wait for others to confirm this as I am not 100% on this...  To figure out those wires we need more info as I made an assumption we are running on 120v here. Where are you located, which country? What are the colors of the wiring on the power transformer? I am guessing there are 5 wires or are there any other ones? Let us know or take some pics for references, good luck.

[teledude61]

I do live in the US and the amp is a domestic 120v model. The transformer has 5 wires. On one side there is a black and a red wire. On the other is a black, red and yellow, and a red wire. I will attach a pic that shows how I have it connected. I choose this configuration based on the attached PC board layout diagram that I received from Fender. I still need to re-solder the speaker terminals so I have not tried to power the amp on yet. Any suggestions would be appreciated.

[teledude61]

Here is the layout and schematic from Fender.

[g1]

  Your transformer wiring looks correct.
Now you need to find out why it blows the fuse.  That is where the limiter lamp comes in.
Do not connect any speaker to the output until you determine there is no DC voltage on the output wires.
  The speaker may have been fried by a blown power amp and you don't want to ruin another speaker.
  The output devices (Q2 and Q3) should be checked for shorts, and their emitter resistors should be checked for open condition.

[teledude61]

Thanks. I was hoping it was something simple. I don't have a proper meter or ability to check the output devices. I think it's time to send it off to a trusted amp tech. After all, the amp was free so a reasonable bench charge still makes the amp a great value  :dbtu:

[DrGonz78]

Got to respect someone when they know their limits to working on electronics. However, if you were ever going to learn about amp repair then this is a great starting point to learn. To be honest this type of repair can be very straight forward. A meter does not cost very much for a basic Digital Multimeter and a solder iron can be had for around $10-20. Building a light bulb limiter is very easy. Mainly testing the output transistors and some of the associated circuit for damaged parts is very easy. Taking the whole thing apart and the careful soldering of replacement parts is really the hard part.

Learning these skills have really helped me fix a lot of amps and I am really a primitive repair guy. If you have the drive to learn then just realize that there are always great people here to help too(as you must already know). The techs on this site & a few other websites are really great teachers of amp repair. Then again you really have to WANT to learn or have the time to do so. Good luck  :tu:

[Roly]

As the good Doctor Gonz says.

You can go an awfully long way with amp repair just on commonsense and careful observation.

It's not a quick process; read instructions/questions, post answers, digest responses and go around the loop again, but we have hand-held quite a number of people who made no claims to technical skill through to the point of making loud noises again, including some strange ones and real cows, and if you are unlucky enough to get really stuck there is still plan B of your local tech or tech friend (but that's been pretty rare here).

[teledude61]

Thanks for the encouragement. I have done quite a bit of work on tube amps. I took electronics way back in the day when we studied tubes (transistors were still unfamiliar to my teacher). I have a good soldering iron but I have a very basic (ie. cheap) Radio Shack analog meter that I bought almost 30 years ago. I should probably upgrade my meter regardless of what I decide to do with this project. Since this is not a critical project, I may just try to tackle it my self. I have already had it sitting in my "to do" pile for a few years now along with a 1954 Fender Deluxe, a 1955 White Higher Fidelity amp, and an early 1970's Sound City 120 head. I just need to decide if I want to spend the time and effort to fix this amp as opposed to spending the same time and effort on creating my music and art (I paint as well). Thanks again for your support. I will keep in touch.

[DrGonz78]

The analog meters are worth keeping for certain things but nothing like a very accurate digital meter. I am very jealous of your other amps you are working on!! Sounds like fun!  :dbtu:

[Roly]

Well %$^&!  With a background like that you shouldn't have any real problems at all.  Transistor amps require a slightly different approach because they are current mode and rather less forgiving of the "oops" factor, but I started building amps when valves were the only game in town, and at the bottom line it's all just the same electronics with much the same stupid faults whatever the precise technology.  Kirchhoff and Thevenin still rule, Ohm (and Murphy).   

I've done a bit of a mud map to servicing solid state amps more or less aimed at older bods who have experience of valves but not transistors;

http://www.ozvalveamps.org/repairs/solidstateamprepair.htm

As for meters, nuthin' wrong with a moving coil meter, or a DVM, and I have several of each which are pulled out for specific jobs where I want to clearly see trending or rapid fluctuations, or where I want high accuracy, or need high meter resistance.  Knowing their limitations and how to allow/compensate for them is the most important aspect (in fact of all test gear).


Okay, for this one you need to check the rectifier/power supply for damage, then have a close look at the output transistors because, given your symptoms, it's a fair bet one or more are shorted.

Servicing solid state output stage can be a bit tedious because they are DC coupled and you have to be sure that you have found all the dead transistors (&c) before you power up again (e.g. failed o/p transistors can also take their associated drivers with them).

Bringing the power stage back up is where the limiting lamp really comes into its own because unlike valve stages that warm up and will give you some warning of distress before any smoke happens, but a faulty solid state output stages will go blammo instantly if the initial power isn't limited.

On the up side, even with such limiting and reduced supply voltage, if the output rail biases to half whatever the supply is you can be pretty sure that the stage is healthy and step up the limiting lamp wattage until you are getting full voltage on the stage.

Given that you have a background in electronics you are way ahead of many who come here and I would urge you to have a go at this one.

HTH

[J M Fahey]

Maybe it's worth trying to give it a chance.

Get a cheap digital multimeter, specially because it will have a "diode check" function which you will need, and build the bulb limiter.

Maybe you were lucky and it won't go much further than replacing the output transistors and a couple extra parts.

The big unknown here is:

a) if the fuse blew, it was replaced by the original one and blew again, we are still dealing with the original, usually repairable damage.

b) at some point somebody tried to "repair" it by using a nail/aluminum paper/30A fuse pulled from a truck .

In this later case, a $20 in parts repair becomes a nuked PCB nightmare.