Fender M-80 Chorus Bias/Avalanche

[ilyaa]

hey!

got one of these fuzzy carpet amps on the bench -

R155 and Q12 were blown - had -40 VDC on the left output (yowza!)

i replaced R155, Q12 and Q13, and then did the bias mod for good measure (swapped out R127, 129,154 and 156 with 2K resistors) -

the left side had some funky looking electrolytics so i replaced those, too, as well as Q10 and Q11 (just being thorough!)

now: amp turns on without blowing up, BUT if i measure bias across R165 or R166 i see it starts out okay (around 25 mV) and then starts to creep up, with no sign of stopping. i turn the amp off when it gets above 60 mV because i can tell itll keep going until kaboom.

any thoughts what might be the issue? the so-called avalanche diodes (the BYV 26Ds) measure OK but i have a feeling they are not, in fact, OK. however, i am not that familiar with this bias circuit so i am not sure best way to proceed -

thoughts/advice? thank you!

ilya

[Jazz P Bass]

Well you do have a working amp to compare voltage readings to.
First off, is R 165 cement resistor still good?

[ilyaa]

yeah the power resistors are all good -

all voltages read pretty much exactly the same both sides until the bias starts to crawl up

[ilyaa]

anyone have any thoughts??

[phatt]

Well you mentioned  some obscure bias mod,, and now the bias has a problem.
Hum,,, logic says return bias to stock and see if it remains stable.

I have no idea of this bias mod but be I'd careful with mod claims unless it has been verified by many.
Phil.

[ilyaa]

this bias mod is in a fender service bulletin -

its in the schematic i attached!

and i have reason to believe the original issue (a blown output) was due to the overheating/runaway problems associated with the unmodded bias circuit

[phatt]

Thanks, now I see what you mean.
Sorry I can't be of more help but my guess is that if the voltage divider of R127 & R128 are heating that is a sign that **Something else is causing the issue.** In my limited experience my guess is Q8 is being allowed to pull too much current and although it can be resolved a little by that Mod it may not be enough.

Now As I'm not the expert proceed with caution.

Having sent many transistors to silicon heaven trying to build and understand SS power Amps I believe that issue is not uncommon.
A while back I rebuilt my SS Laney Amp so as to achieve more Clean head room and it took me quite a while to resolve that very problem in the power stage.

Now I don't know if it would help on The M80 amp as my amp design was very basic but I finally resolved thermal run away issues by simply inserting 47R series resistors at the base of the output devices. (mine has 4)
That amp has now been running for ~6 years with no sign of any over heating. It's super clean up to (Ed removed the !) 80Watts @ 8-Ohm. I gig twice a month and sometimes more. After 2 hours at gigs the heat sink is only room temp. Yes I play LOUD

While researching options I tested the final design flatout into a load box for hours over several weeks and was never able to get any part hot enough to burn a finger. Without those series 47R resistors it would run a lot hotter and like you say that is concerning.
My circuit runs 45Volt rails and those resistors are 3k9 and 2k2 on my amp. (the boot strap section)
On your Schematic that would make R27=3k9 and R28=2k2.

**Now of course Your amp is a little more complex so it may not work. Others here would likely know far more than myself,, if it were my Amp I'd be setting up a lightbulb limiter and trying it out.

I have no doubt my simple design could be improved but cricky it's a guitar amp and simple is all you need.  There are a lot of designs that might perform far better than my simple circuit but I've noticed a lot of these over engineered designs seem to have issues while the basic designs don't.
Evidenced by the sheer lack of breakdowns of simple designs. It seems to be always the complex units.
Phil.

[ilyaa]

thanks, phatt!

ill give that a shot and see what happens!

[g1]

These amps with darlington type power transistors are especially picky about proper heatsinking and go into thermal runaway if it's not right.
Is the heatsink compound old and dried out?  Are the power transistors mating well to the heatsink?  Also check that the diodes in the heatsink are placed the same as on the good amp, and whether there is heatsink compound there.
Also possible the transistors are fakes depending where they were sourced from.

[Enzo]

And please never power up with the transistors not screwed to the heat sink...even for a few seconds.

[phatt]

These amps with darlington type power transistors are especially picky about proper heatsinking and go into thermal runaway if it's not right.
Is the heatsink compound old and dried out?  Are the power transistors mating well to the heatsink?  Also check that the diodes in the heatsink are placed the same as on the good amp, and whether there is heatsink compound there.
Also possible the transistors are fakes depending where they were sourced from.

Hi *g1* regard proper heat sink,, If it's the Amp model I think it is it only has a bar of alloy for a heat sink. I once fixed a Fender bass amp of similar design, it had melted the back panel plastic face plate. I simply added a big chunk of 4mm alloy with fins and it never over heated again.
I don't know enough to make a judgement on my above suggestion but I've bread boarded many Power amp designs and that boot strap divider section is always a bit touchy on many designs.
Phil.

[Enzo]

Bar of alloy bolted to the chassis?  The chassis is part of the heat sink.

[ilyaa]

wow great thinking everyone -

the heatsink compound WAS old and dried out -

i gooped some more on there and we are back in business! both amps work BUT they are not quite matched. the good amp that never had any issues idles at a lower current - about 24 mA - and is able to deliver more power. the fixed amp idles at a higher current - about 30 mA - but seems to have a bit less gain/does not get up to full power - there is still some headroom but compared to the other amp there is a significant difference, for sure.

i wonder: is this likely a transistor problem? i got what looked to be legit transistors to fix the bad amp but i dont think they were a matched pair....everything else about the amps is identical. any thoughts?

[g1]

What was your source for the transistors?

[ilyaa]

ebay.....am i bad?