"Neil"
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this?



** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the
soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and
supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked, the amp will oscillate at supersonic
frequency - probably intermittently.



...... Phil

boardjunkie wrote:

You likely have leaky drivers.

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Graham

On Oct 17, 9:02*pm, Eeyore
wrote:
boardjunkie wrote:
You likely have leaky drivers.

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Graham

Umm....WTF are you talking about? You never heard of drivers taken out
as a result of output device shorting? Happens all the time.

On Oct 17, 8:07*pm, "Phil Allison" wrote:
** Just one more try:

Did *YOU *buy the BDVs from Farnell or off eBay *???????

I ask because there are so many **fake** devices offered on eBay.

Never buy semis from eBay.

..... * Phil


Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

boardjunkie wrote:

Eeyore wrote:
boardjunkie wrote:
You likely have leaky drivers.

Made of silicon ? !!!!!

Bwahahahahahhahahhaa !

Umm....WTF are you talking about? You never heard of drivers taken out
as a result of output device shorting? Happens all the time.

Taken out =/= leaky.

Graham

boardjunkie wrote:

On Oct 17, 8:07 pm, "Phil Allison" wrote:
** Just one more try:

Did YOU buy the BDVs from Farnell or off eBay ???????

I ask because there are so many **fake** devices offered on eBay.

Never buy semis from eBay.

..... Phil

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

No ! CROOKS at work.

Graham

boardjunkie wrote:

On Oct 17, 8:07 pm, "Phil Allison" wrote:
** Just one more try:

Did YOU buy the BDVs from Farnell or off eBay ???????

I ask because there are so many **fake** devices offered on eBay.

Never buy semis from eBay.

..... Phil

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

Report them to the genuine manufacturer.

Graham

On Fri, 17 Oct 2008 19:08:49 +0100, Eeyore
wrote:



JP wrote:

Does anybody know what the normal operating temperature of the
darlingtons is?

It's not that simple. It depends on the dissipation and thermal resistance.

The *junction* or die temperature may be as high as 150C in plastic devices or 200C
in metal can and still be reliable but that's NOT the heatsink temp.

Google Motorola AN1040 (that's twice this week now I've mentioned that) !

Graham


Not my post but okay....

** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the
soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and
supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked, *the amp will oscillate at supersonic
frequency - *probably intermittently.

..... * Phil

Sorry its been a while since my last message!

I've just popped R29 out of circuit to test. Allowing for the error in
my DVM (doesn't go to Zero) the resisitor value is fine (as are the
other 0.33 ohm resistors).

All solder joints seem oaky and theres no sign of cracks or burning.

An earlier message suggested the MJF122 darlington so I popped this
out too and there are no short circuits. With the dvm set to diode
function on the ohm scale and the neg probe on the centre pin and pos
probe on left pin the reading is 0.73 & 0.64 with the pos probe on the
right pin.

"Neil" wrote
....

An earlier message suggested the MJF122 darlington so I popped this
out too and there are no short circuits. With the dvm set to diode
function on the ohm scale and the neg probe on the centre pin and pos
probe on left pin the reading is 0.73 & 0.64 with the pos probe on the
right pin.


Measure the bias currents into the 'base' leads... somehow,
somewhere, there's too much, usually a leaky, perforated
driver.
__
Steve
..

Appologies for the delay getting back to the thread - I thought I
posted at the weekend but it doesnt appear to have uploaded.

Anyway, I removed transistor MJF122 as someone suggests it might be
faulty. With a DVM set to diode function on the ohm scale and the +ve
probe on pin 2 there's no reading on pins 1 & 3. With the -ve probe on
pin 2 pin 1 reads 0.74 & pin 3 reads 0.66. With the -ve probe on pin 1
and the +ve probe on pin 3 there is no reading, and reversed the
reading is 1.03

I popped out the 0.33 ohm resitor and the value is fine, as are the
rest of them and the solder joints too

I have an oscilloscope - can anybody advise where and what signals I
should be looking for please?

Thanks in anticipation.