Finished Cornet Octal - transformer failed after 1 hour

TChung · « **on:** 25 Jan 2004, 10:00 am »

Finally finished assembling Cornet Octal with CCS around midnight. Really excited! Checked all the voltages and there were all within a couple volts per specs. Listened for an hour. Sounded great even without any burn in but there was a slight hum so I decided to isolate the RCA jacks. When I was done I turned it on again but silence and smelled something burnt and it was from the transformer. The rectifier tube was on but the other 3 tubes were stone cold. Checked and only there was no 6.3V filament voltage, the rectifier and the high voltages and were fine. Finally, the fuse blew. Put in another fuse and it blew immediately.

I saw 3 or 4 other Cornet builders also reported problem with bad transformer. I got mine from Angela. I wonder if that is the common link. I am going to replace the transformer from another source in case Angela has a bad batch from Hammond. I don't know if there are other components that were also damaged. Should I also replace the diodes and the 10,000uF/10V caps since they are in the 6.3V filament section even though there is no visible damage?

Tom

RogueAngel · « **Reply #1 on:** 25 Jan 2004, 12:51 pm »

Tom,

When my transformer went, it took R20 and C9 with it. R20 was obvious, because it went up in flames. C9 looked fine, but I removed and tested it and found it had also failed.

I would recommend removing and testing of all power supply components to be sure nothing else failed.

Good luck,
Bobbi

gurevise · « **Reply #2 on:** 25 Jan 2004, 03:43 pm »

Do you guys use 270BX Hammond or 272?
I got 272 in it and mine runs a bit noisy and very warm, almost hot.
My Cornet is the standard version, not octal.
I'm concern about longevity of this transformer. Thinking about possible replacements.
Any ideas what other transformer could be used for Cornet?
Other manufacturers or different transformer series from Hammond?

Sergey
Dayton, OH

RogueAngel · « **Reply #3 on:** 25 Jan 2004, 04:50 pm »

I use a 370BX. A search on the web found that the 270BX is more prone to inducing hum. Can't verify that myself, but that was the word in the asylum.

Grover · « **Reply #4 on:** 25 Jan 2004, 07:47 pm »

I've been following the various issues with the failing Hammond transformers. I'm not sure if it's consistent with a particular model or particular supplier. Anyway, not wanting to take a chance I ordered a custom transformer from Electraprint. I sent the data page of the Hammond 270BX from the Hammond website to Electraprint and they're making one that matches the specifications. Cost for this is $75 plus shipping. Jack at Electraprint says their transformer will be bigger than the Hammond, and I don't have it yet. I imagine it will fit atop the Hammond chassis and give enough room for the board, but I haven't verified this yet. I just thought I'd pass this along to other potential Cornet builders. I've read enough problems with the Hammonds that I felt more comfortable going elsewhere.

mgalusha · « **Reply #5 on:** 25 Jan 2004, 08:13 pm »

Tom,

Did you put a capacitor across the power switch? Jim sent these out to all the folks who had purchased boards as a possible fix for the transformer problems. I used the 270BX and it was built with a cap across the power switch and I've had zero problems. It has been used a lot since being completed last June and I have forgotten and left it powered on for days sometimes.

I believe Jim was theorizing that a current spike from turn on/trun off could be causing the problem. I don't know if that's why mine has been trouble free but it certainly can't hurt.

mike g

TChung · « **Reply #6 on:** 25 Jan 2004, 09:57 pm »

I am going to order the Hammond 370BX given that it is supposed to be of a better quality design. I am tempted to go with Electra Print but my layout is very tight -- the transformer is surrounded by big caps. I am concerned that the Electra Print might be too big.

No, I did not have cap that goes across the switch. It was not on the Octal list. I will certain add one now.

I will also replace all the parts in the power supply area since I cannot tell which are bad. Anyway, they are not all that expensive.

This is a great support group!

Tom

hagtech · « **Reply #7 on:** 26 Jan 2004, 05:37 am »

Yes, the 370BX has more iron and should run cooler. The 272 has much higher output voltages and you need to change R2 to compensate. I built one this way but forget the exact value.

jh

pretzel_logic · « **Reply #8 on:** 30 Jan 2004, 01:57 pm »

I had a tranny go sometime back, posted about it here. It's also the 270BX but since replacing it I haven't had a problem. I did put in the cap Jim sent out but that was just recently. I think there was just a bad batch of trannies out there.

Brian