Fender pro 185 repair trouble!

[jasonguitar]

I'm having trouble trying to repair a Fender Pro 185 for a friend. The filter caps had dried up and the 2 resistors R175 & R176 the 2-270 ohm 7watt had been charred. I replaced the caps and put in 2-270ohm 10 watters and the amp sounds terrific but the resistors start smoking after 10 mins with around 90v+ and 90v- on the front legs where there is supposed 50v+ and 50v-. Upon further inspection of the schematic it appears that someone had labeled the PT secondaries and the their terminals on the pc board as 1, 2 & 3 but had the red/white wire which the schematic indicates as cp11 at cp12. Thinking this may have been my problem with too much voltage on the front end of R175 & R176 I put the red/white wire back where the schematic indicated at cp11 and right away the mains fuse and thermistor blew. I replaced them and now when I fire up the amp there is a super loud hum and the thermistor is heating up so fast its sure to blow a fuse again. When I check the secondary voltages I get 48vac(white) 17 vac(red/white) 14.5vac(white) Could it be that the red/white wire was wrong from the factory? It appears I have shorted out something when I switched the wires. That's what I get for trusting my eyes just following the schematic without checking secondary voltages first. Help! Any suggestions? Thanks.

[Enzo]

Ah geez, and here I was scolding you in the other thread so you'd start a new thread, and you already did. Sorry.


I don't know from wire colors, but if the transformer has two whites and a red/white, then the red/white is the center tap and belongs on CP11. That is what the schematic says. Make sure the whites are on CP10,12. Before doing that though, leave the three white wires in the air and measure them fior AC voltage. There ought to be more or less 35VAC between the red/white wire and either white one, with about 70v between the two whites.

I don't get your voltage readings. Transformers make voltage between pairs of wires. You list a voltage for each wire, which I have to assume is made referenced to ground? Just check them flying loose. If the center tqap is on CP 11 and yopu get some AC voltage to ground from there, then you have burnt open a ground trace on the board.

35VAC rectified and filtered makes about 50VDC. 70VAC rectified and filtered will make over 90v. SO yes, your wires were mixed up. SOunds to me like your transformer may have been damaged. Go to RG's New Page 1 site and find his very simple transformer tester. Make one and check out this transformer.

The loud hum? I'd say that was a blown output putting DC on your speaker. Disconnect all speaker loads until that is sorted out. The amp doesn't have to be loaded. Of the outputsw are intact, then we may have blown a filter cap to oblivion.

[jasonguitar]

Excellent advice man. I really appreciate the help. So yeah, I was measuring from ground to each secondary wire. So when I measure between center tap(red/white) and either white wire I get the same reading of 36vac. So the tranny should be ok then? Looks like I may have fried an output transistor? Like I said before, whoever had been in this amp before me had mislabeled the secondaries so they had one of the white wires on cp11 and had the centertap on cp12 though they may not have actually been plugged in that way and when i reconnected them after replacing the caps/resistors I assumed they were labeled correctly. Guess I'll have to pull the output transistor board and use diode checker on the output devices.

[Enzo]

For now just measure resistance between the legs on the transistors where they sit. SHorted is shorted. Nothing in the circuit could mask a shorted transistor. Something else could make them look shorted when not, but nothing will make shorted ones look OK. You can pull the parts later if needed.

Without the speaker, DC on the output won't blow fuses.

[jasonguitar]

Sorry, I had this ready to post before I got your reply: So I pulled the output transistors and Q18 and Q19 are definitely damaged. Interesting that the schematic calls for MJ15003 transistors for Q17-Q20 but there were 2N3773 types in Q19 and Q20. The specs are slightly different. I'm gonna check locally in the morning to see if I can get replacements before I go ordering them.The filter caps seem ok when I do the basic analog meter check. The R175 and R176 resistors are ok. Is there anything else you would suggest I check before putting in new devices? Thanks.

[Enzo]

IN transistor power amps like this - which is to say most solid state guitar amps - the transistor type is not critical. The specs may be slightly different, but they are essentially the same thing. What you don;t want to do is mix them. Make them all 2N xxxx or all MJ15xxx. You are probably looking at a repair.

If you can get the real parts - either type - locally, fine, but please don't add NTE stuff to this situation. Unless you plan to replace ALL of them with the NTE.

Whenever output transistors fail, always check the ballast resistors to see if any went open - those 5w 0.22 ohm ones. And always check the drivers, those MJE15030/31. And there are looks like three 47 ohm resistors associated with them, check for opens.

Those 3300uf main filte3rs were not probably happy about 90v, so look them over, they might be fine, but if any are bulging. If the plastic end is bulging, tear it off to see if the metal beneath is bent. The plastic ends warp and look bulged when the cap is not.

Beyond that, leave the speaker off until we know it is OK. And we'll just have to fire it up and see.

Look up "light bulb limiter," make one, and use it.

[jasonguitar]

All the resistors and devices you mentioned check out ok so I guess we're ready to get the new output devices. Local store has the right parts but the guy wants over $10 each! I'll order em from Digikey at $3.92 each.

I had rigged up a light bulb limiter years ago when I first started tinkering with tube amps but put it away once I got a variac and I can't seem to locate it now. I see that they are indeed a useful tool to have on hand. I went to the hardware store and got the stuff to put a proper one together.

Thanks for your time and expertise Enzo! It is greatly appreciated!

[jasonguitar]

Enzo, So with new output transistors installed the amp fired up and sounds great! Voltages are right on the money now! Only thing that's bugging me is the little surge when you first power up. I know these things are notorious for burning out the inrush limiting thermistor and popping fuses which have already been replaced in this amp twice. Can I upgrade the thermistor to fix this problem? Would installing a surge protector fix this? Thanks so much for your help! Jason

[Enzo]

That is not a rare problem WHEN AMPS HAVE A PROBLEM, but reallky, thousands of those amps live their entire lives without burning one up. And that turn-on surge? That is what the thing is for in the first place. EVERY electronic system will have a turn-on surge of some amount, empty caps have to charge after all. 2 seconds of half again what the idle current is will not stress the limiter.

[jasonguitar]

Ok. So there really isn't any way to limit the audible "surge sound" and soften up the power on? The loud brief hum.

[Enzo]

Unless it is making noises for several seconds or longer, no. It is quite common for solid state amps to thump,pop, or squawk briefly at power up or down.

[jasonguitar]

Very good. Working with solid state is obviously new to me. I've become so accustomed to the soft start up of tube amps. Thanks again for your generous advice Enzo.

AND HAVE A HAPPY 4TH OF JULY!!!

[J M Fahey]

So there really isn't any way to limit the audible "surge sound" and soften up the power on? The loud brief hum.

Well, there is something you can do, if it drives you crazy.
You can make your amp more "tubelike" by installing a "standby" switch , which interrupts the hot wire of the speaker output jack.
By instructing musicians/operators to turn power on, wait 10 seconds and only then turn "standby" on, you will get perfect results; of course when turning it off reverse the procedure.
Works flawlessly every time.

[Enzo]

Yes, there are ways around it like JM suggests. But be aware that the noises it makes won;t hurt it. The loudest noise it can make will be no louder than the maximum audio output of any other signal. It can;t get louder than its clipping point.