Help on cornet needed

farsch · « **on:** 5 Oct 2003, 12:10 pm »

I have finished the cornet now. I have powered it up, the Y3 is lightened,
the X7, X7, U7 not.
Any idea?

Caps and diodes are checked, 7vac at the green wire is there.

Best René

mgalusha · « **Reply #1 on:** 5 Oct 2003, 06:14 pm »

René,

You indicate you have 7 volts for the 12A*7 filament circuit from the transformer. What is the voltage coming off of the diodes before it goes through R21? I don't remember the exact voltage on this side of the diode but it I think it's around 7.5 or 8. R21 drops it to about 6.3v, depending on the transformer and your mains voltage.

If the voltage is OK on the input side of the resistor and not of the downstream side it's possible you have a bad resistor. If it's not OK on the input side I would suspect the diodes. You said they were checked. Did you test each diode or do you mean the installation direction was checked?

If all the diodes are OK try pulling all the 12A*7 tubes and check the voltages at pins 4-9 and 5-9 on each tube. It should be about 6.3vdc. It's possible you have a shorted heater but unlikely. Use caution when checking the heater voltages at the tube socket. Pins 1 & 6 have over 300 volts on them.

Actually, it would be best to pull the rectifier tube as well when checking the filament voltages. Better safe than sorry.

Hope this helps.

Mike G.

farsch · « **Reply #2 on:** 5 Oct 2003, 06:25 pm »

Mike,

thank you for the advice. I will check these tomorrow.
diodes weren`t measured I just checked correct installation.

What means: shorted heater?

Best

René

pretzel_logic · « **Reply #3 on:** 5 Oct 2003, 07:22 pm »

It figures we're having problems while Jims at VSAC. I am having problems with my Cornet blowing fuses. It's immediate upon power up so am having a tough time here. The pre worked great for over a week so something obviously blew. I'm going to take the diodes out and check them but also want to check the trannie, just not sure how yet.

If we didn't have problems we wouldn't learn, hopefully we figure it out and learn another lesson.

Brian

mgalusha · « **Reply #4 on:** 5 Oct 2003, 09:49 pm »

René,

A shorted heater means that possibly the filament leads inside the tube have shorted. It's not very likely this could occur but if it did there is a fair chance it would prevent the other tubes from lighting up. My guess is you have a bad resistor or some bad diodes.

Brian,

I read your other posts about your Cornet dying. Does your fuse blow if there are no tubes installed? If so you may be correct and have shorted some diodes. Yes, the RadioShack diodes will work.

To test the transformer, you might consider un-soldering the output leads from the PCB and measuring the output voltage. Obviously use extreme care since the high voltage leads are 550V!

If it were me, I would un-solder just the green wires (filament circuit) and see if the fuse still blows (all tubes removed). If it doesn't blow, then measure the voltages out of the transformers where the are soldered to the PCB. Much safer than having the wires disconnected.

It's also possible the rectifier tube somehow shorted and is causing the fuse to blow.

Another thing to check is C11 and C8. These caps are across the secondaries of the transformer and a short would certainly cause blown fuses.

Speaking of VSAC, I haven't read a single report of the show.

Good luck on repairing your Cornet. Jim will certainly be able to offer better advice than I but the above items are likely the prime suspects.

Mike G.

pretzel_logic · « **Reply #5 on:** 6 Oct 2003, 04:16 pm »

Thanks for the tips Mike. I have already pulled the diodes to check them and have readings of 3.5k anode to cathode and 37k cathode to anode. This seems low according to another post I read though am not sure. I also checked C11 and C8 and they are not shorted according to my meter.

To check the trannie should I go ahead and put the diodes back in and solder up the trannie or can I check it totally unconnected? Also where do I put the negative lead of the meter? Dumb question probably but still new to this but learning.

Brian

farsch · « **Reply #6 on:** 9 Oct 2003, 01:38 pm »

My cornet works now it sounds great!]

hagtech · « **Reply #7 on:** 9 Oct 2003, 05:46 pm »

Seems you did fine debugging this one. Per some emails I believe the fault was the 2.4 ohm resistor in the heater circuit. Mouser has sent a 2.4k ohm. That would make heater voltage REALLY small.

Anyway, glad you are back up and running!

jh

farsch · « **Reply #8 on:** 9 Oct 2003, 08:59 pm »

hi jim

thanks. It was a good to get such a support.
As a newbie in building a tube amp its was very helpfull and I learned a lot.

On one hand I'm happy that it was just such an "easy" prob, on the other hand a stupid mistake that I believed the delivery of Mouser and didn't checked what they send.

Anyway I'm up and running!