Decouple this preamp?

[CJ]

no, when that upper transistor shuts off, emitter base voltage of Q13 gets all weird.

right now Q13 Veb is 0.497 v

good news is that the lady who borrowed an amp because of my ineptitude loved the loaner amp so much that she bought it for 500 if we gave here 50 for the Frontman 212 which was approved by the big kahuna so i am off the hook for demerit points,

i am wondering about Q14 and Q 15, are they also overload protection?

and i wonder why diodes D28 and D29 do not mess up the waveform?

i replaced a 470 ohm fusable resistor R87 that was also suspect, but no change.

 

[CJ]

ok this thing might be fixed,

looks like it was those Fu resistors,

"Disadvantages​

The disadvantages of fusible resistors are discussed below.

• Fusible resistors have a limited lifespan and will eventually burn out under normal operating conditions, requiring replacement.
• The cost of replacing fusible resistors can become significant in applications where they are frequently subjected to overload conditions.
• Fusible resistors’ power ratings are low, which means they can’t take on high-voltage applications.
• Another disadvantage of fusible resistors is that they can only protect against overcurrents; they cannot protect against undercurrents.
• They also cannot protect against short circuits, which limits their use to just protection from overloads.
• They are also more apt to fail prematurely than other types of resistors because they are susceptible to thermal shock and vibration, which can cause them to crack or shatter."

so R87,88,89 and 90 were being intermittent, only seen this a few times with normal resistors,

 

[CJ]

Q12 and Q13 were pulled and hfe measured at 208 and 313 so no transistors on order,,


there is a thermistor that we checked out on the heatsink but it was working ok, it is buried in the mute circuit,

one thing i can not figure out is how the signal gets from the diff pair to the driver transistors,

all i see is pwr supply noise on the collectors of the diff pair,

but there is clearly a nice single driving the drivers and outputs as seen by these waveforms,

good signal on the downstream of D28 and D29, but just power supply ripple on the upstream side,>

 

[JohnRoberts]

Q12 and Q13 were pulled and hfe measured at 208 and 313 so no transistors on order,

one thing i can not figure out is how the signal gets from the diff pair to the driver transistors,

the collector loads of the first LTP is a second differential pair, one side pulls up and the other side pulls down. the funny diode something 28, is a little extra trick to cut off the pull up, when the pull down is pulling down.

all i see is pwr supply noise on the collectors of the diff pair,

but there is clearly a nice single driving the drivers and outputs as seen by these waveforms,

good signal on the downstream of D28 and D29, but just power supply ripple on the upstream side,>

d29 is similarly cutting off the pull down when amp is pulling up.

If its working crack a beer...

JR

 

[CJ]

ok so that second pair cancels all the pwr supp noise and pulls out the signal,

Q14 and Q 15 be a bias spreader?

btw there is an error on the schemo, C55 s/b 47uf like it's cousin C56, at least that is what is stuffed on the pc board,

Thanks for all the help, the beer's on me!

edit: and after three hours of working i came back from a coffee break and it is doing the same thing,

start over on tuesday,

 

[ccaudle]

looks like it was those Fu resistors

I guess maybe not after your last post, but just for my benefit, is there any way to tell fusible resistors by sight? I don't recall seeing that notation on a schematic before, but I don't have schematics for every piece of gear I've touched, so possibly I have seen some and not noticed what I was looking at.

 

[CJ]

Fusable resistors have kind of a non glossy finish with a slight bit of texture, and a lot are light blue in color,

I ordered a new Q12 and 13.

Evilbay has Toshiba replacements but that is always sketch,

Very glad to find out that digikey has some modern replacements which can be had for pennys on the dollar compared to evilbay

All you have to do is sub a K for the 2 on the part numbers

ie
2SA1013 becomes KSA1013
2SC2383 becomes KSC2383

Onsemi

Hope the freq response is sane enough to provide stability.

Still thinking of cutting some traces and putting in an RC network on the rails to get rid of all that per supply ripple.

There are different hfe groups but as long as they match for NPN and PNP then the circuit will be ok.

I will take some scope shots soon.

 

[CJ]

here are some scope shots of Q12

the signal is superimposed on the pwr rail ripple which gets canceled out,

what if one rail has more ripple than the other due to filter cap variance over time?

this is why i wanted to put in a filter section between the main rails and the pre driver circuit.

i could not wait for the transistors to come in so i found a set of

KSA1220

and

KSA2690

bigger package but same pinout, 160 V 1.2 A , hfe about the same,

so far the amp is running right, we will see for how long,

 

[CJ]

amp seems to be ok after replacing q12 and q13

did a test on Q13, ran 10 ma thru it just like the circuit only with 4 volts on the collector instead of 38,

then we put a soldering iron on it,

current went up to about 20 ma then started to fall, gradually went down to about 5 ma,

most of the time current will run away til the junction pops, not this time.

so an intermittent transistor of the weirdest kind was to blame for this headache


next Frontman 212R that comes in will take about ten minutes to fix such is the life,

thanks for all the help!