Aleph A30 - troubleshooting tips

I have successfully built Aleph 2 in the past but I did not have much luck with the Aleph A30 amp that I have built recently. I'm soliciting for some help from this forum - troubleshooting tips.

Board used:
BrianGT (the lastest GP which I'm sure quite a few folks here have already built successfully).

Parts used:
Except for the output transistors and the front end transistors, all parts are based entirely on Brian's part list from his web site. Matched transistors was obtained from a forum member.

Issues:
1) On one channel, starting up using a variac, 10mv on both output, heatsink heated up nicely, sound is 'muffled", and after about 5 to 10 minutes, smoke comes of one of the output transistors (connected to + power supply side).
2) On the other channel, I see smoke coming out of one of the output transistors (also connected to the + power supply side) as the variac voltage reaches 90VAC.
3) No smoke coming out form either signal boards. Thanks for that.
4) Power supply boards measure +24.5V and -24.5V. So, I know power supply is okay.

Questions:
Before I dive in and tear everything apart to start troubleshooting, is there something obvious that I have overlooked? Some troubleshooting tips?
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
At least you have a variac. I would start by measuring output
DC offset, and the DC values across the Source resistors on the
output transistors to find out who's drawing the current. Then
I would look to see what the drive voltage is on that transistor
and/or the voltage across its Gate stopping resistor (221 ohm).
 
I built an A-30 using Brians boards...Haven't really hooked them into my system yet...Bias looks good across the Mosfets...they get nice and hot on large heat sinks...

without anything soldered on to the input nodes on the boards, but with a speaker on the outputs...circuit is dead quiet....I Soldered some twisted pair on the inputs and noticed some hum on the speakers, and hooking up to my system, a lot of hum ...I am trouble shooting this...Brian's boards are pretty nice..and I am looking forward to a successful project.
 
Did some troubleshooting today .....

For both channel:
Apparently, the insulator pads for the output transistors got too hot and it started burning and thus the smoke. I'll need to order bigger pads.

For one of the channel:
I replace the burnt insulator pad with a new one (I only got one good pad left from previous project), took the measurement as shown in the Pass Schematic diagram, DC output is about 10mV, other reading looks fine too, turn the amp on, sound comes out, no hum. So, this channel is okay.

For the other channel (the nightmare channel):
I temporary replaced the bad pad with a new one (not so good pad), took the same measurement as the other channel, the reading are almost identical, but the sound coming out is "muffled". Spend a lot of time troubleshooting it, eventually I found the problem .... the speaker + and - terminal are shorted to the chassis (I'm really pissed for making this error), meaning essentially the output is shorted and thus the "muffled" sound. After rebuilding the speakers + and - terminals, turn the channel on, sound comes on, it's listenable except for the huge hum in the background.

Going forward:
I suppose having the output shorted probably have cause some damage to the signal board/output board even though it measures almost identical to the other channel. I'll need to do some more troubleshooting. Looking at the schematic, it looks like if the output is shorted, both + and - rail volatge will go right through to ground. Does it looks like Q1, Q2, Q4 and Q5 will go "leaky" rather than the output transistors? What other components could go bad other than the transistors? If you guys have some ideas, please post.
 
Sil-Pad choices

I think I should get some Sil-Pad first before going any further. What do you guy/gal use for your A30 or class A amps? Is Sil-Pad K-10 good enough? Digikey part # BER120-ND.

Spec says Sil-Pad K-10 is a insulator pad, for TO-247AC use, Silicon/Film construction, 0.006" thick, electrical breakdown voltage of 6000 volts a-c Min., and can withstand continuous heat up to 180 degrees C.

I know some of you are using even thicker pads. Got Digikey part #?
 
Update ...

To get rid of the bad hum on one channel, I have replace Z1 to Z5 and Q1 to Q5 on the signal board. That seems to work. Hum is gone.

I have cook the amp by playing music for about two hours this afternoon and in 100 degree F. weather. Both channel are working now. It's hot.
 
Glad you found your problem FCEL.

This is not a threadjack attempt and have a quick question that probably doesn't warrant a new thread. I am building an 'a30' with a 'a3' output section using brians boards. In other words, The a30 will be short 2 OP fets per channel:smash::att'n: :hot: . Do I need to mod anything in the front end or can I run the front end per the a30 schematic? I am thinking I can, as long as I am willing to play with r13brian(r19pass).

Thanks in advance,
Marc