Fender Stage 112 se blowing fuses and transistors.

[gilmo789]

Hi,
I was wondering if anyone could help me try to repair a 90s fender solid state amp.

Schematic:
https://www.schematicsunlimited.com/...er-stage-112se

Symptoms:
Appears to work for a while but fuse pops after a short while usually accompanied by transistors Q5 and Q6. Replaced these, same thing happens.

Associated sand resistors get hotter than the others

Clean channel seems a bit quiet. Different volumes depending on input jacks 1 & 2
On power up with light bulb limiter, bulb briefly flashes brightly before settling down.

Things I've tried:
Tested every diode i can reach including rectifiers; seem good.
reflowed most or the solder joints
Big filter caps test correct capacitance
Tested all transistors and replaced bad ones

I'm pretty lost now? Any thoughts would be appreciated

[Jon Snell]

That sounds like the quiescent current is too high, also if only Q5 & Q6 blow, what is wrong with Q3, 4, 7 & 8?
Have you checked the voltage drop between the base of Q3 & Q8? You are looking for about 2.8volts as anymore than the the Darlington Pairs will draw too much current.
Is there fresh heatsink compound on the Darlingtons?

[gilmo789]

Thanks for the reply.
Will check voltages later. I'm not at the amp right now. Nothing is wrong with Q3, 4, 7 and 8

I haven't put any new compound on the transistors, but they're not getting hot and there is still a lot of the old stuff there. I'm running the amp at very low volume.

What would put up the quiescent current? The fuses blow while playing, it seems quite stable without any input.

[Jon Snell]

As Q3, 4, 7 & 8 is in parallel with the failing Darlingtons, seems a little fishy to me as they should all fail randomly, not just one pair.
Where did you purchase the new Tip142/147s from, hopefully not fleabay. Does it work OK with R105 and R106 out?
The quiescent current could increase if a diode was faulty but that might not be the case.

[gilmo789]

I did buy them from ebay, but I think they're decent parts: 142, 147 (looks like this, says morocco on it, made by Texas instruments).
Just to clarify, it is only Q5 and 6 that fail and the failure mode is the same every time: fuse blows accompanies by Q5 and 6.

I have a feeling the jack inputs may by dodgy. I re-soldered them and there's less hum and better volume. I've ordered new ones. Could bad inputs blow the transistors?

I have not tried it with the resistors removed. Are you suggesting removing the associated .47Ω sand-filled resistors? The numbers don't correspond to the Transistor numbers Q3 & 106, Q4 & 107, Q5 & 108 etc.

So if I get you right, you're suggesting removing resistors 108 and 109 in order to isolate Qs 5 and 6 from the circuit allowing me to run the amp with 4 transistors instead of 6?

[Jon Snell]

What we need to do is isolate the possibility that the new transistors are not as described.
By lifting their emitter resistors, they are out of circuit, current wise. Better still run the amp without the replacements to see what happens. four TIP transistors will be good for up to 60Watts without damage.
Not sure where you are but always purchase from a known supplier; RS or Mouser and you won't go wrong.
https://uk.rs-online.com/web/cp/1631...=TIP142G&sra=p https://uk.rs-online.com/web/cp/1452...147G&sra=p&r=t
https://eu.mouser.com/ProductDetail/...52Bc6OBOrzU%3D

[gilmo789]

Thanks Jon, I'm in Scotland btw.

Should I run the amp through the LB limiter while doing this?

[Jon Snell]

That would be safer through a lightbulb limiter, saves on mains fuses and catastrophic accidents.
Sometimes replacement components from fleabay can be the source of trouble. I have known replacement transistors look the real deal but are not what it says on the can.
TIP142/7 are manufactured by ST in China and have been for many years. Not sure where your Ti components came from.

[g1]

Did those ebay parts ship from China?

[gilmo789]

No they shipped from the UK and from Germany, I notice the supplier of the 147s now stocks the ST part rather than the old TI. I'm not really doubting the parts seeing as the failure mode is exactly the same as with the original OEM parts.
http://www.kessler-electronic.de/Hal...i28_7004_0.htm

TBH I'm beginning to suspect there might be a bow in the heat sink under the two offending parts, creating a gap. They are beside each other in the middle of the rail. I'm going to steal a IR thermometer from work and run the amp with and without Q's 5 and 6. See if I can find anything.

[Jon Snell]

That is exactly why we use fresh heatsink paste, it flattens the uneven surfaces.

[g1]

Did you try what Jon suggested in post #5, disconnecting the emitter resistors for Q5 & 6 (you don't need to remove the transistors)?
If the other parallel devices are not doing work, Q5 & Q6 will burn up like that.

[gilmo789]

I haven't tried it yet. It will be friday before I have another go. Are you suggesting that all the signal is being diverted to Q5 and 6? Why would this be? In that case would the amp be quiet withe the associated emitter resistors removed?

[g1]

Yes, if those other transistors aren't doing work, and Q5&6's emitter resistors are disconnected, the amp will be quiet (no sound).
Possible causes could be cracked or burnt traces, or bad solder, to those other output transistors.

[gilmo789]

Something different...
I removed r108 and 109, the emitter resistors associated with q5 and 6, and i bolted on the heat-sink tightly. I could run the amp quite happily through the LBL. All 6 PTs had DCV of about 38V and they all stayed around 26^oC. I played quietly through the amp like this for about 15mins. I then bypassed the LBL and tried to play through the amp on mains power. It worked for a short while before I heard a pop and if I run it through the LBL, the bulb glows brightly.

Blew Q7, short C-E. With the LBL off Q7 was much warmer than others ~46^oC.

taken all transistors out and tested as below with diode tester

TIP 142
Q3 C-B: 0.57, C-E: 0.47 (original part)
Q4 C-B: 0.57, C-E: 0.47 (original part)
Q5 C-B: 0.6, C-E: 0.47 (replaced part)

TIP 147
Q6 C-B: OL, C-E: 0.50 (original part)
Q7 C-B: 0.58, C-E: short (replaced part)
Q8 C-B: 0.58, C-E: 0.50 (replaced part

I've taken all the transistors out and measured voltages across the terminal sockets. TIP 147 voltages seem high...

Q3: 46V
Q4: 46V
Q5: 46V
Q6: 53V
Q7: 53V
Q8: 53V