Powerboard question

[martin manning]

From one of those three ground points? If it's wired correctly that's ok, the whole circuit is floating and likely getting that voltage from capacitive coupling in the transformer. They should all read the same if they are connected together, though.

[Oddvar R]

It's the same-

[talbany]

1st thing I always do before I attach the power cord and fire it up is throw one side of my meter to the chassis and ohm out both sides on each filter. No dead shorts on the + sides and dead shorts on the - side along with the bias supply filter (+ to gnd) last are grounds to the B+ main, either center tap or gnd for a FWB.. This has saved me a TON of fuses

BTW. I hope you tested and reformed those old (100uF) TVA's since they are now 40 years old. (1981's)..If not you should!

Tony

Good point, I will. I have just had these lying around, and so they came in handy. But at the moment the voltage seems to be fairly adaquate.

It's really not all about the adequate voltage on the mains, it's for prolonging the life of the caps and or having them fail catastrophically possibly taking out your power transformer as well.You can do them right in the amp if you have a variac. If you don't want too at the very least make sure they don't get hot on you. I've had several of those old TVA's go bad and get warm while reforming them.

Tony

[martin manning]

An even better way to reform power supply caps: Insert a 100k resistor as shown, turn power on, and leave the standby on standby (since your other caps are new). Monitor the voltage across the 100k, and wait until it stabilizes, which may take an hour or so. You can try waiting a day and repeating the procedure to see if the leakage current can be reduced further. Don't forget that some of the current flow is through the 2x 220k balance resistors; subtract that from what you see going through the 100k.

[Oddvar R]

I will try that, but I bought a set of two F&T to possibly go there instead. I used to be a "fan" of NOS stuff, but see there are some bad sides to it. The TVA's wasn't used, but time takes it's toll I guess. If we want to or not.

[Oddvar R]

So now I am without sound, just low noise when I touch the jack tip, so I have to go over it all a bit more investigative.

On the layout. in the upper blue circle, there seems to be to groundings in parallel?

And the middle blue circle, that's the point to send the missing fet board to ground?

The low blue circle it says "vol", is that for the volume pot wiper?

Sorry if this is a bit to easy for some of you.

[martin manning]

On the layout. in the upper blue circle, there seems to be to groundings in parallel?

The heavy black line is an error. There should be only two ground wires from the front panel, one from the bus and one from the input jacks.

And the middle blue circle, that's the point to send the missing fet board to ground?

Yes, but not directly to ground. That point should be grounded through a 150k-180k resistor to simulate the FET current draw.

The low blue circle it says "vol", is that for the volume pot wiper?

Yes.

[Oddvar R]

So now the amp is making sound, but has some issues I don't understand. Hopefully I can get some advise here:

The audio is very weak through the input jack, but when I touch the parts circled in blue, the speaker almost jumps out of the cabinet. When I hold my hand over the same parts, there is increasing noise volume, and when I touch the cap with the arrow, much noise is introduced. Seems like there is an grounding issue?

Secondly I get no reaction from the relay, except that the level knob works, and gets the in signal signal from the relay board? If I understand it right, the board gets it's voltage from the "manual" OD button on the front and the direct to board wire from the DC source. On a normal 9v battery you measure both poles to get a reading, but I can't see this here? Only reading is 23.5 volt to ground?

After a while one gets blinded?

[pompeiisneaks]

So that means wherever you're touching the circuit is working fine, but somewhere before that, the signal is being over attenuated. Either a tube isn't conducting very well due to a solder joint needing to be a bit better, or some other similar issue. I'd chopstick the components in the signal path between the input and that area to see if one area under pressure suddenly either cuts audio or makes it stronger etc. that may need reflowing of the solder, or the component might be bad.

As for the relays, I also had issues on my first build. Ensure a few thigns:

1. when you flip the switch on the panel or on the footswitch that you see the 12V on the other side of the relay coil. Basically you should see 12V on one side and nothing on the other, but when engaged you should see it across the other side. Or at least some voltage
2. make sure that if you have a polarized relay that you've connected the +12V to the rigth side of it.

Pictures of the relay hookup will help. The 12V relay power should be on it's own circuit so you should be able to measure 12V coming out of the regulator with reference to the other side of the regulator circuit etc.

~Phil

[Oddvar R]

I have tried every connection on the board, and soldered and re-soldered, and changed some parts. No obvious loose ends at this time.

The relays doesn't show 12v anywhere, I get 8.6V AC at the enterpoint on the board. The only reading I get is 23.5v from one point to ground. I have enclosed the schematics of the 12v transformer.

[pompeiisneaks]

Okay that relay says per it's datasheet that it's a 5v relay.

https://panda-bg.com/datasheet/2206-270 ... A-DPDT.pdf

I was just saying it was 12V because the dumbles I'd built used a 6.3v transformer, through a voltage doubler, into a 12v regulator, which then used 12V relays.

8.6V seems a bit high for the 5V relay, but not sure if that's going to cook it. It says the max voltage of the relays on the datasheet is 130% of the stated, so 5 * 130% = 6.5V so at 8.6 it might have cooked the relay. They dont' seem to be polarized so that part seems fine. How are you doing the power supply for the relays? Can you show the schematic you're using? Is this just a case of you doing a 12V supply and using the wrong relay and it's gone bad? 8.6V seems low for a 12V but the relay may have shorted to being like a resistor instead of open/closed and is causing a voltage drop?

~Phil

[Oddvar R]

That might be it. It didn't occur to me it might be 5v. Never knew I bought any other than 12v. But I bought a couple of 12v's tonight so I'll see. Thanks so much for spotting that.

[martin manning]

I get 8.6V AC at the enterpoint on the board. The only reading I get is 23.5v from one point to ground. I have enclosed the schematics of the 12v transformer.

This is a voltage doubler circuit, and it should produce about 17VDC from 6.3VAC.

Is the 8.6VAC measured from one green lead to the other? If so, I would expect to see about 23VDC at the output like you are measuring.

If the transformer secondary is 12VAC, the 8.6 VAC doesn't make sense unless there is a large current draw somewhere. I would disconnect the leads from the output and remeasure. If the transformer is 12VAC, you'll see ~34V, and then I would just use half of the secondary (one green lead and the green/yellow lead).

[Oddvar R]

Only trouble is there are only two secondaries, the two green ones. I also get 8.6Ac voltage when they are disconnected from the board, the input is 237v.

[martin manning]

Ok, I would add a regulator like this. You only need one more tie point, and I believe it will be fine with a small U-shaped heat sink.