Fuse Blows Immediately on my Fender 85.

[Wooji]

Hi, I am new here. Also, I just purchased this amp knowing it had this problem. I checked the power transformer between the 2 black wires is reading 177 Volts. But I don't think my volt meter is exactly accurate. I hooked up a light bulb load tester, I built, to see if I could isolate the problem area. But it just gets so bright immediately I shut the power down . So do you guys have any idea what could be the problem with this fender amp ? Or any thing that I could check ? I would appreciate any help .

[J M Fahey]

IF you are using a series lamp fixture, the lamp glowing brightly means you have a problem, most probably shorted (burnt) output transistors, not excluding some other nasty possibilities. Anyway, the series lamp is protecting it from getting worse, and letting you measure something. Try to post the schematic or a link pointing to it, so we all speak about the same thing. Try to read what the output transistors are, just to begin with something.
Bye.

[Wooji]

Here is the Schematic .

[J M Fahey]

Hi Wooji, thanks for the schematic.
Start by replacing the two main suspects, Q1 and Q2, Tip 142 and Tip 147. Do not mix them up, be very careful when desoldering and pulling them from the board as to protect it.
Rough handling will mean tearing the donuts/solder pads, and that will make life miserable for you.
Use a very good solder sucker, solder wick won´t hurt either, and in an extreme cut the old transistor pins before desoldering to easen their removal.
Place the new ones carefully, using new insulation hardware (micas, grease, screw insulators) and make sure their cases (the metallic part or the central pin) make no contact with the chassis or heat sink.
Also check and if necessary replace R84 and R85 which might be burnt or severely overheated.
After that, hook it again to the series lamp and check that everything works.
If not, post again.
Bye.
J M

[teemuk]

...And while you're at it, check out these service tech notes from Fender: (taken from http://members.tripod.com/NatCade/index_nf.html)

Overheating Solid State Guitar Amps

In most cases the thermal switch will shut down the unit, and after cooling, it will turn on again. The problem is caused by poor heat transfer from the output transistors through the mounting bar to the diodes, leading to poor tracking of the bias circuit, and excessive bias current flow through the output transistors. In most cases the output transistors are not seated onto the mounting bar.

Inspect the insulator used between the transsitors and the mounting bar. The overheating units all appear to be using Silicone insulators (Sill Pads). these are soft and flexible.

A simple test will verify the overheating condition. Using a DC Volt Meter, attach the leads across one of the emitter resistors (.47 ohm/5watt). Turn the power on and let the amp idle with no input signal or output load. Obsrve the DC voltage drop across the emitter resistor. The measurement will climb up to a certain level and stabalize. This usually takes 3 to 5 minutes. An overheating amp will measure 80mVDC to 250mVDC.

Modification procedure:

Desolder and remove the transistors (TIP142/TIP147). Remove the Sill Pads (insulators).

Install Mica Pads using a liberal amount of thermal grease to both side.
Remount the transistors and tighten securely (6 to 8 lbs.), and solder the leads.

Repeat the test. A good measurement is in the area of 40mVDC to 80mVDC

The TIP142 and TIP147 Darlingtons have a wide gain range. In some cases the transistors may need to be replaced to obtain a lower idle (bias) current (editor note: It's been my experince that 50% of these amps will eat the replacement parts. Although more expensive, non-generic parts have worked well for me in keeping the call back ratio way down).

Fender 85 and 185 Channel Switching

during early production of the 85 and 185 series amplifiers, some 1N4148 diodes were accidently mixed with diodes in the bin holding type 1N4448 diodes. When the P.C. boards were stuffed, some of the 1N4148 diiodes were put into circuits requiring the 1N4448. the result of this mix-up causes the channel switching feature to malfunction after 15 minutes or so od operation from a cold state.

On the Deluxe 85, CR21 through CR26 must be 1N4448 diodes. Channel switching on the Deluxe 85 may be further improved by changing a zener diode and two resistors. Change R90 from 1.5K ohms to 1.8K ohms, amd R92 from 10K ohms to 2.2K ohms. Check R89 to make certain it is 10K ohms.

On the 185 series, CR25 through CR29 must be type 1N4448 diodes. Be sure R113 is 2.2K ohms. CR24 must be a 1N5236B 7.5V zener diode. Resistor R108 must be 10K ohms, and R109 1.8K ohms. The R108/R109 ratio is important. R109 was originally 1.5K ohms and was changed to 1.8K ohms.

[Wooji]

OK, thx . I ordered the parts, and should get them by Wed. 18th. I will let you know when I get them installed.

[Wooji]

I finally received the parts on the 2nd (yesterday) . And while removing the old transistors, I noticed there wasn't any insulating material between them and the heat sink. So will that cause more problems to the rest of the components or not ?

[J M Fahey]

You found a big one there!! Probably somebody tried to repair it and didn´t bother to use micas there.
As Teemu points, some units used silipads insulating rubber.
I never trusted it, mica/grease works much better.
Go on slowly and carefully with your servicing.
Good luck.

[Wooji]

Well I replace the 2 Transistors - 142 & 147 , and the 2 WW resistors - R84 & R85 . I plugged it back into the test light  before turning it on .  It worked without the light coming on at first , then all of a sudden the light came on when I turned up the volume. Not bright , dimly lit and gets brighter when the volume gets louder , especially on the bass notes.
Reverb doesn't work but everything else seems too. Sounds really good on clean . I played it for about 20 minutes or longer. Thx again for your help Guys.

[Wooji]

Also the light gets brighter the longer it is on. But it is still working, and reverb is working just not real well.

[J M Fahey]

Hi Wooji.
So far so good: you can play, clean and dirty, only not terribly loud because of the lamp limiter. Good!
The lamp glowing brighter on higher power, specially on bass notes, is what´s expected.
The only thing that stil worries me a little is the "getting brighter after 20 minutes" which might indicate some thermal riunaway.
As Teemu pointed, quoting Fender literature, there seem to exist some cases of bad thermal tracking (the diodes not making good contact with the heat sink); check that.
You might also play (still with the lamp) for 20/30 minutes, until you feel heatsinks at least warm, and then turn down the volume but not powering it off. If after one hour of not playing it´s still hot (or, worse still, hotter) and/or the lamp keeps getting brighter, then you have thermal runaway.
If everything appears normal but you still aren´t sure, replace the series lamp by a 100 Watt one, which will let you play quite loud, and still offer some protection. Bye.

[Wooji]

Its kinda wierd, the amp is working great now, everything is working as it should. My series light starts out as hardly noticeable glow then slowly gets a little brighter, the longer the amp has been on. Then stays at a certain point of brightness. Which looks about like 15 or 20 watts in brightness.  Which isn't that bright for a 75 watt bulb. So all the parts must have settled in.

[J M Fahey]

Hi Wooji, I think that as a final safety test, you can use a 100W series lamp, play quite lous for 1/2 or 1 hour, and if everything still looks OK, you're done!
Power it straight without the lamp, play sometime while checking it now and then, and that's it!
Congratulations on your servicing skills.
Bye

[joecool85]

What a great story to read along with, it's great you were able to fix you amp with a little help from the guys here.  This is one of the main reasons I started SSG, it's really cool to see my project come to fruition and actually helping people out.   :tu:

[Wooji]

Ok , here is the Amp I repaired , and my to Rock & Roll Butcher Knives , lol.  Also I am going to go ahead and replace the 2 - WW 5W 330 k ohm resistors also since they are showing signs of overheating.