760 leslie quit working was sounding good.

[tom hoffstatter]

My Leslie 760 bass amp circuit keeps burning up an R82 (15 ohm 1w resistor) any ideas or typical scenarios? I using a leslie preamp box, It works on other 760. Tom in Oklahoma

[David Anderson]

The most obvious reason for burning up R82 would be a short in the driver transistor it's attached to, Q12, but the driver transistor is also direct-coupled to the preceding transistor Q11. If Q11 opened up or stopped conducting, the base of Q12 would swing positive and conduct too much current, burning up the resistor.

Because the two transistors are direct-coupled, either one could be the problem. If you're lucky, running diode tests on them will reveal a problem.

[tom hoffstatter]

Alright cool, I'll check it out. :)

[enor]

I replaced all 4 power transistors on my "gig" Leslie 760 (which is actually 760 guts in a 142 cabinet). Made a big difference in "punch" and headroom, even though it wasn't malfunctioning before. Small cost, big gain!

[tom hoffstatter]

Yeah probably a good idea thanks for the tip...

[Sweet Pete]

Not all amp boards are the same. And the power supplies differ over the run as well.

This power supply has a failed attempt by a previous owner to recap a bypass ...
that 63V 1000uf someone threw in there barfed all over....
and yet the amp/x-over board,a 1980, works perfect off of a 1973 power supply!
Notice both amp sockets/plugs are white? And both amp boards are identical? 1980.
PS also has the later motor plug/socket. And of course,the orange wire to nowhere.

The earlier type treble amp saturates more like tubes than the later style to my ears.
The '71 and '73 760's I have are the early type treble amp.
Later amp boards in general have matching bass/treble amps.I have a 430 board with two orange plugs.
And the 11 pin HL822 has four amps that appear identical but show different in the schematics......

[David Anderson]

While it's possible that the output transistors are involved in this problem somehow, this particular resistor burning up is not a typical symptom of an output transistor failure. Of course, it can't hurt to test the output transistors, too.

Pete is right about the different versions of the 760, but the Bass driver modules tend to stay the same. It's the Treble board that was tweaked. In late production, you do find two Bass driver circuit boards instead of a Treble board. This was likely due to production winding down and Leslie using up the parts they had in stock, especially once production was moved to Illinois.

Interesting history, but, again, it doesn't really bear on the problem described.

[Sweet Pete]

What exactly is causing the resistor to burn up? David offers a diagnoses that would eliminate other possibilities and is worth a try!
Current and heat ; could be both too much + not enough - and too much heat caused by bad insulation on old semiconductors?

I lucked out so far with my 760 finds,and will start by rebuilding the '71 which sounds best of the four.
To be fair,second best goes to the newest;1980.
The '73 and '76 are different.They appear to be second runs on redesigned circuit boards.
Maybe Leslie had better first runs on these;correcting the issue later?

Having a look at other brand circuit boards of the same era shows Leslie had good to great quality control on the products.
Look at a Sunn or Dynaco or Macintosh and note the differences. Not all boards are created equal is the theme of my theory!

[Jyvoipabo]

My Leslie 760 bass amp circuit keeps burning up an R82 (15 ohm 1w resistor) any ideas or typical scenarios? I using a leslie preamp box, It works on other 760. Tom in Oklahoma

The capacitor C53 500μF / 6V is not a model of reliability. If it has never been replaced.
Another scenario is : C53 leaks, Current in Q12 increase, Q12 short and R82 smoke.

JP

This amplifier is very sensitive at the value of resistors near output transistors.
Check it carrefully if output voltage is not close to 0 Volts (No signal- No load).

Don't replace good transistors by new transistor. The quality of old (silicon) transistor is certainly better than new ones.

[Drawbar Dave]

Not all amp boards are the same. And the power supplies differ over the run as well.

This power supply has a failed attempt by a previous owner to recap a bypass ...
that 63V 1000uf someone threw in there barfed all over....
and yet the amp/x-over board,a 1980, works perfect off of a 1973 power supply!
Notice both amp sockets/plugs are white? And both amp boards are identical? 1980.
PS also has the later motor plug/socket. And of course,the orange wire to nowhere.

The earlier type treble amp saturates more like tubes than the later style to my ears.
The '71 and '73 760's I have are the early type treble amp.
Later amp boards in general have matching bass/treble amps.I have a 430 board with two orange plugs.
And the 11 pin HL822 has four amps that appear identical but show different in the schematics......

Thanks for the history lesson. I never knew some of that and I'm a bit of a geek on the finer details. The 760 (preferrably wood cabinet version) is my favourite Leslie. I have had 122's and 147's and while nice never cut through with the power that the solid state version had. I'm glad mine is an earlier model with the differing bass and treble amps.

Why does yours have the orange flying lead from the PSU?

[enor]

One of my 760 amps has green boards like the one in the pic, the other has the circuit boards made from an orange-yellow material that appear more or less handmade. Is this orange one the earlier version that you're talking about, Pete?

[David Anderson]

The capacitor C53 500μF / 6V is not a model of reliability. If it has never been replaced.
Another scenario is : C53 leaks, Current in Q12 increase, Q12 short and R82 smoke.

That's possible, but if C53 shorts, then it bypasses R82.

Also, having worked on many SS Leslies, I kind of have to disagree with you about the old silicon transistors Leslie used in these being better than new ones. The SS Leslies were built during the CBS era, and they suffered from some bargain component sourcing issues, especially where the smaller transistors are concerned. You do have to make sure you source new transistors from a reputable dealer, but if you do, results are usually good.

Actually, I've found the electrolytic capacitors used in SS Leslies to be better quality, in many cases, than other components. You find many resistor problems on the boards.

But with a transistor's emitter resistor burning up, the first thing to look for is a bad transistor. If you don't find that, then you start looking elsewhere.

- - - Updated - - -

One of my 760 amps has green boards like the one in the pic, the other has the circuit boards made from an orange-yellow material that appear more or less handmade. Is this orange one the earlier version that you're talking about, Pete?

I've never seen an orange-yellow board like you're talking about. Prototype? Something cobbled together for late production?

The 760s went through some relatively minor version changes. The power supply went from using four SCRs to two triacs for motor switching. Some tweaks were made to the Treble driver circuit, apparently to limit output power and prevent blowing horn drivers. Then, in very late production, you sometimes find two identical Bass driver boards instead of a Bass PCB and Treble PCB.

The 860 had always been made with two identical driver boards, and reducing build complexity could reduce cost.

[Jyvoipabo]

That's possible, but if C53 shorts, then it bypasses R82.

It was another scenario.
To know why and how, you have to check all the components.
After you can start replacing deads.

JP

[Sweet Pete]

One of my 760 amps has green boards like the one in the pic, the other has the circuit boards made from an orange-yellow material that appear more or less handmade. Is this orange one the earlier version that you're talking about, Pete?

All of my 760 amp boards are green.I do have a 430 amp board that has no x-over; with two orange plug (bass?) amps.The boards are also orange-yellow.
These amps fit a 760 as well,signal and x-over plugs are the same as the green ones.
I'll have a look at the dates on that puppy! It had a 'space expander' tremelo scanner and two mono cabs.Not many made or much info available.
Power supply has a 5 pin socket and an AC cable like an 11 pin.The 5 pin cable might have powered the second cab.

[Wes]

"orange wire to nowhere" - I suspect this power supply once lived in a 710.