G9 weirdness

[Nele]

Hi,

I finished my G9 unit a while ago and on first powerup it worked without any problems. I decided to leave it on for a night and everything seemd fine. However, after taking it to a recording session I discovered it didn't work anymore.

I opened it up yesterday and discovered the earthconnection was loose. After restoring it it seemed ok but now I still have problems on one channel.
I built the G9 using OEP transformers on the first and Lundahl on the second channel. The Lundahl channel is ok but the OEP channel is causing problems.

First of all there is a lot of hum. I tried to find any earth problems but couldn't detect any. The hum gets worse when touching the input transformer.
Then today I noticed a second problem. On power-up the channel produces a high-pitch sound which increases in frequency until inaudible and after a few moments the unit shuts down. 78S12 problem? It is connected to the bottom of the housing.
I also think the input gain switch is broken because it generates a lot of noise.

I tried switching tubes and switchboard but that didn't help.

Any ideas or pointers? The oeps are shielded though there is only one connection between bottom and top of the shield.

Thanks,
corneel

 

[MikoKensington]

I'm assuming the OEP is PC mount. How much spacing do you have between the transformer and the board?

 

[Nele]

they are mounted on the board so there is virtually no space..

 

[gyraf]

..78S12 problem? It is connected to the bottom of the housing.

Hopefully isolated from the chassis? Otherwise you'll get a serious hum loop there.

Touching transformer generating hum points towards the transformer can not being grounded. Check and recheck.

If all else fails, try switching transformers between channels to see if problem follows transformers or amplifier side.

Jakob E.

 

[Nele]

touching the pcb-board I mean.. I'll check the transformers..

 

[gyraf]

Make sure that your G9 is fully enclosed in shielding metal before judging hum levels. With the lid off (or if there's bad metallic contact to some chassis parts) external hum can be picked up very loud.

 

[Nele]

Checked my faulty G9-unit a while ago and the screening can of the OEP-input transformer was loose.. I fixed it and.. still no success.

But I decided to give it another go tonight. After opening up the unit I discovered the big connector from the control-pcb of the problem-channel wasn't pushed all the way down on it's header. I also noticed the output-connector was shifted one pin to the right, so there was no output connection!

After solving these errors the unit worked but continued to shut down after a few minutes. As suspected this was caused by insufficient cooling of the 78S12. It got extremely hot so I placed a piece of aluminium between the regulator and the bottom of the chassis and now it's been on for over an hour without any problems and making my 'Duke Ellington Live at the Blue Note' test-cd sound very nice :wink: .

Thank you Jakob for the design!! Very nice!!! :guinness: :guinness: :guinness: :guinness: :guinness: :guinness:

Tak!

 

[gyraf]

Congrats Nele, on getting that project off the ground...!

:thumb:

Jakob E.