Fender FM 65

[Valvehead]

Working on a Frontman 65 ..it came to me with a loud hum . I found some burnt resistors inside = R76 / 80 / 82 / 83 . I replaced those and the diodes that were right next to them since i have a boatload of 1N4448 . So D23 / 26 are new and ive tested every diode on the entire right side of the pcb .

I found that Q9 was shorted . I replaced that along with Q 7 / 10 / 11 since the board was out and i had some in the bin.

The 4 transistors on the big heatsink test good in circuit / no shorts . Q15 / 17 /18 /19

Powered up with the bulb limiter and shes glowing bright.

Any thoughts on what to look at next ?? I will pull Q6 / Q8 out and test since im getting funky numbers on meter in diode test mode (they test ok as far as ohms and no shorts there.

Frntman65.pdf

[g1]

Do you have a load connected? Disconnect it. Does bulb still light? Is there DC on the output?

[Valvehead]

Do you have a load connected? Disconnect it. Does bulb still light? Is there DC on the output?

yes , had a dummy load on. Took that off and bulb is dark , but ive got 32 VDC at the speaker out (with the bulb limiter )

not showing a short...but could one of these be the culprit = Q15 / 17 /18 /19 ?

[Jazz P Bass]

" but ive got 32 VDC at the speaker out"
Is that + or - 32Vdc?

You obviously had a catastrophic blow out.

Everything is suspect at this point as both rails shorted out.
You need to get the bias set up.
Q15 base: +0.6Vdc
Q17 base: -0.6Vdc
Remove the outputs for now.

At this point if the limiter stays dim, remove it.

[Valvehead]

" but ive got 32 VDC at the speaker out"
Is that + or - 32Vdc?

You obviously had a catastrophic blow out.

Everything is suspect at this point as both rails shorted out.
You need to get the bias set up.
Q15 base: +0.6Vdc
Q17 base: -0.6Vdc
Remove the outputs for now.

At this point if the limiter stays dim, remove it.

its +32vdc at speaker terminals

so as long as theres no load i can apply full power ? Im just nervous about burning up the new parts !

[Enzo]

Yes, just don't apply a load to it .

[Valvehead]

Yes, just don't apply a load to it .

thanks--i will look at it today

[Valvehead]

I fired it up and within 10 sec i have some parts getting real hot. Verifying with a laser temp gun . Some are right next to each other on the board . Those would be
D18 / 20 /21/22 , R75 ,R81 ,C51 , these all seem to test ok in circuit.

Also getting real hot = Q6 /10 / 11 , Q10/11 are new .

the 4 trans on the heatsink are staying cool.= Q15 /17 /18/19

ill pull Q6 out of circuit and test it ..............does the above info help? any parts to focus on?

[Jazz P Bass]

You never checked the bias voltage at the driver transistors?
If you did, please specify what you measured.

[Valvehead]

You never checked the bias voltage at the driver transistors?
If you did, please specify what you measured.

That would be on the base correct ?

Q15 /17 /18/19 all have +42v or -42v at center leg

[J M Fahey]

Center leg is Collector so + or - rail respectively.

So we know we have rails but nothing on transistors themselves.

To measure biasing measure base to emitter.

Check datasheet for pinout.

[Valvehead]

Center leg is Collector so + or - rail respectively.

So we know we have rails but nothing on transistors themselves.

To measure biasing measure base to emitter.

Check datasheet for pinout.

i could only find data that said EBC left to right , i guess that was wrong

so B to E i get =

Q15 .58V
Q17 .93V
Q18 .58V
Q19 .58 V

[The Dude]

Voltage drop on Q17 is too high. It is likely bad.

Edit: To be sure, the basing diagram on that transistor is BCE and it is a PNP. Polarity matters. You didn't state polarity in your measurements. A transistor wouldn't have a .93V voltage drop B-E forward biased. It might if reverse biased, indicating a different problem. So, remeasure and tell us the polarity base to emitter and/or remove the transistor and check it out of circuit.

[Valvehead]

sorry --- the above measurements were with transistor in circuit , amp power on , voltage mode on meter .
so red lead on base , black on emitter , +.93 vdc . Reverse the leads and its - .93vdc

i took the trans out of the board and in diode mode on meter i have normal readings of 0.6v b /c ( and 0 leads reversed) , but 0vdc both ways on C and E

[g1]

i took the trans out of the board and in diode mode on meter i have normal readings of 0.6v b /c ( and 0 leads reversed) , but 0vdc both ways on C and E

On diode range 0V indicates a short. So the transistor is shorted between collector and emitter. Replace it.