checking for a bad diode

[EETStudent]

Not sure how this happened, but Im only getting a B+ of 10V. Obviously way too low. Did some troubleshooting and traced it all the way back to the rectifiers. Before the rectifiers Im getting 475VDC, after them I get 10VDC. I figured the diodes were bad so i checked them. Both had similar readings. They were both open one way, and both read about 1Meg the other way. Are these bad?

[milosch]

I'm surprised that you're getting dc before the rectifiers since it's their job to create it. Because they are likely attached to coil windings, the best way to check these may be by removing them first. Get a decent dvm with a diode range, imho. Then it's easy to tell. Better yet, replace them and remove all doubt, unless they fry again.

[EETStudent]

Well those readings I got, with the exception of the dc before the diode, were all taken with the diodes removed. But like you said, im gonna replace them to remove all doubt

[Jag]

It sounds to me like you had your DMM set to AC.

[EETStudent]

Well now Im really confused. I had a diode of the same type laying around and I assume it is good. I switched it in and out with the other two diodes and yielded the same results. Actually I should correct myself, that 475V before the diodes was actually about 47.5V. Anyway, I disconnected the wire that connects the diodes to the first filter cap and I took some measurements. I was getting about 330VDC after the diodes, which before the amp quit working I was getting nearly 500V. I then reconnected the wire to the filter cap and went back to getting 10V again.

The things is, the point at which I started having problems was when the cathode bypass cap on my EL34s blew, and i just figured that its 50V rating had been exceeded too much. I got bigger problems now though

[WholeToneMusic]

Check this

Try Checking the output transformer. With amp off and discharged check the resistance on primary using a multimeter, should be under 100 ohms, then check the resistance on the secondary, should be less than 10 ohms or less.

[Don Moose]

Also, what B+ do you have if you yank the output tubes?

[EETStudent]

Did what you guys suggested, and both yielded no problems. The OT primary resistances were 50 and 41 ohms. Secondary was 0.6ohms. I switched out both diodes with new ones and thought the problem was fixed. I yanked the power tubes and B+ was around 415 on Standby and 12V when turned on. I forgot I had the power tubes pulled out so I put them back in. With standby on, I got the 415 again and heard a little hiss so I thought i was good. Flipped standby off expecting sound, but rather I got 12v B+ again. Did the same procedure and now Im getting 12V on standby.

[EETStudent]

Ok another correction. I know for sure that just before the rectifiers, where the legs hit the high voltage AC, the DC voltage was -510V. I had the meter set correctly, and that is what I got. Just to make sure Im wiring them correctly, the high voltage hits the positive side(no white stripe), and the negative sides(sides with white stripes) meet together. Isnt that correct?

[EETStudent]

Oh thank the lord I figured it out. Before this problem occured, I was trying to rearrange my grounding schemes for better noise. Well I removed the local ground point that was connected to the HV center tap, and I forgot to put it back. Just put it back and its all good now. Good learning lesson

[WholeToneMusic]

Glad to hear!! Enjoy your amp.

Best Regards,
Brian

[Don Moose]

The basic ones are the hardest to find.

When I'm bringing up a new motherboard design, the mantra is Power, Reset, Clocks.

For amps, it has to be heaters, B+/GND, solder joints ... right?

[WholeToneMusic]

I hear you

I hear you Don, There are so many variables in an amplifier and the components have interrelate with each other. I usually start at the outside of the amp and work my way to the phase invertor, the phase invertor is about half way through the amp, from there you can tell which half of the amp the issue is coming from. I have know computer techs that use the same methods, working from outside to the motherboard. Best wishes

[mykey]

Not sure how this happened, but Im only getting a B+ of 10V. Obviously way too low. Did some troubleshooting and traced it all the way back to the rectifiers. Before the rectifiers Im getting 475VDC, after them I get 10VDC. I figured the diodes were bad so i checked them. Both had similar readings. They were both open one way, and both read about 1Meg the other way. Are these bad?

there is no DC before the rectifiers, time to get a good meter.the diodes must be removed from the circuit entirely to really check them. use an analog ohm meter and replace any that show the slightest reverse leakage. a digital meter is no good for checking diodes, especially if you are looking for very small reverse leakage. It would seem that you have at least one bad diode, or two.
sometimes you will find diodes that only open under power or load.

[EETStudent]

Yeh I got it all figured. Like i mentioned, I was trying to rearrange the grounding schemes and by the end of it all I had forgotten to assign a ground to the high voltage center tap from the PT. THAT was a problem I had never encountered in my young tech career. Like you guys said, I usually work from the outside in, checking the B+ and then grounding, bias and loose components, shorts to ground, etc...

As for my meter, Ive got a really good one my dad got from work. Its a Fluke 175 true rms multimeter. A fairly high end meter from what Ive researched. The ungrounded CT must have been causing the high dc voltage before the rectifiers. Maybe half of the AC flowed forwardly through one diode, then reversely through the other and the dc wound up at the ends(or beginnings) of each diode.