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Old 23rd September 2012, 10:06 PM   #1
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Arrow TSE Post Mortem

I'm so bummed. My OPT seems to be fried (of course the single most expensive item had to be the part to go...) I'm trying to find what went wrong so that I don't have the same problem again.

Here is a little background information:
This is mostly a stock build with 300b's, an Edcore XPWR131, and a 370VAC motor run cap. I'm using Transcendar 3K 20Watt SE OPT's and their 10H inductor. The 300b sockets are remote mounted with an extra 100ohm resistor at the grid pin of the socket and a ferrite bead on the extension. Total wire length from the PCB to the sockets is about 3". More importantly, this amp WORKED as configured for at least 10 hours. My problems started when I tried a musical test after I had moved everything into a steel Hammond enclosure.

The problems started when I moved everything into an enclosure. My first thought was that there was a short somewhere. I've tested everything I can think of however, and just cannot find any shorts. I've also triple checked my wiring, and haven't seen any issues there either. Before the final test, I did a voltage test with just the 5AR4 installed and verified that B+, B-, and all filament voltages were good.

On power up with all tubes installed and speakers hooked up, there was initially no music in either channel. I heard some scratchy sort of sounds from the left speaker just as I noticed the unmistakable smell of magic smoke. I killed power to the amp immediately, but the damage was done. Total powered on time was approx. 45 seconds to a minute. Upon inspection I found that two resistors had fried. R29 was dead and smoky, as was the extra 100ohm grid stopper at the pin of the left channel 300b socket (after R33). The R33 resistor is fine. To begin troubleshooting, I removed all tubes and replaced both damaged resistors. While I was testing for any shorts and other damaged components I discovered that my left channel OPT tested open on the secondaries (the good right channel tested at approx. 120Ohms). I disconnected all wires from both OPT's (input and output) and tested resistance again (right channel still tested open).

Does anyone have any theories on what happened? Did the OPT fail, and did that cause the other damage? Could this be a tube issue? I'm at a loss. I know for sure that everything was previously operational, because of the previous run time.

There are three things that were different (I know: bad, bad, bad...) This time I connected a CD player directly to the RCA inputs. I think George mentions doing this in the checkout section of his manual. However, I am not using a volume control (inputs are wired directly to R2, L2); could this be an issue? I'm also using different speakers (less efficient, but known to work). I also did not have a meter on any of the test points at power on (due to the chassis being in the way). So I've no idea what the B+, B- were when the event happened.

As for the OPT, is there any way to salvage it? I'm really hating the idea of having to get a new one

TLDR: Fried on startup = R29, Left OPT, 300b Grid Stopper resistor. 2 questions... What could have caused an error like this? Is my OPT salvageable/repairable?

Thanks!

Last edited by acceler8; 24th September 2012 at 04:53 PM. Reason: Added TLDR: http://en.wikipedia.org/wiki/Wikipedia:Too_long;_didn't_read
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Old 24th September 2012, 10:40 PM   #2
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The plot thickens... For some strange reason, I decided to test the OPT one last time. Hooked it up, and BAM 125ohms. This is very strange. I kNOW I tested it like 30 times last night, and it always tested open. So now I'm thinking it's an intermittent problem. I start wiggling wires and turning over the transformer. Wiggling had no effect. But... Turning the OPT over in mid air did affect the reading. I put my meter on Min/Max mode and saw 99.7ohms to 157.5ohms. Is this normal? Is it working like a compass and picking up stray magnetic fields? I'm very confused.

As always, thanks for any help!
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Old 25th September 2012, 02:44 PM   #3
GanzerD is offline GanzerD  United States
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Someone could correct me,but....Shouldn't the primary on those read something like 3000 ohm?...... and the secondary something like 8 ohms?
DaveG
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Old 25th September 2012, 07:35 PM   #4
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Quote:
Originally Posted by GanzerD View Post
Someone could correct me,but....Shouldn't the primary on those read something like 3000 ohm?...... and the secondary something like 8 ohms?
DaveG
Those are impedance values, where the secondary impedance of 8 ohms looks like 3K ohms to the output tubes. The DC resistance values are not the same as the impedance values..........

Acceler8: Since you got no output from either channel when you did the chassis transfer, it sounds like your problem is something other than your output transformer. I would go through the check out procedure again slowly, tubes out check for filament voltage, rectifier tube in check for B+, driver tubes in, etc.

Last edited by boywonder; 25th September 2012 at 07:39 PM.
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Old 27th September 2012, 01:04 AM   #5
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Originally Posted by boywonder View Post
Acceler8: Since you got no output from either channel when you did the chassis transfer, it sounds like your problem is something other than your output transformer. I would go through the check out procedure again slowly, tubes out check for filament voltage, rectifier tube in check for B+, driver tubes in, etc.
Thank you so much! That was perfect advice, and exactly what I needed to do. It took two evenings to get the OPT wired back in and go through each step of the checkout procedure from the top. I never did find a fault, but everything is working perfectly now. I guess I can blame it on using ratty beater speakers in my initial test.

Thanks again!

p.s. Even with el cheapo 300b tubes, the sound is sublime! Thanks George!
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