DIY Marantz 7 phono pre

[Hocdm]

Hello all,

I've browsed the forums here before, but this is my first time posting. The main reason I've never posted before is due to my lack of experience with building electronics. I've built speakers, worked on turntables, and modified amps in as much as changing capacitors and resistors, but this is the first time I've built anything from the ground up. I was hoping for some advice..

I've just finished building a Marantz 7 phono pre with a pcb I found on ebay. I'm using a Glassware Audio PS-3 power supply with an Antek AS-05T240 transformer (the 220v tap). I'm running 6.3v heaters with Russian 6N2P tubes. After finishing it and plugging it in, the right channel works fine but I'm not getting any sound from the left. Also, the transformer gets uncommonly hot. Not just warm, but too hot to even touch really. I've explored around the circuit with my multimeter in as much as I know how and can't find any shorts. Everything is, as far as I can tell, connected and grounded properly.

I'd really appreciate it if any of you could give me a bit of advice on what might be happening or what to do.

Thanks for listening..

 

[Stefanoo]

If transformer is getting extremely hot you should not connect it.
Is the built a dual mono? Does the transformer have separate windings for each channel or even better one core per channel?
If so, just try to swap windings to rule out whether you have a faulty transformer or not.
If transformer is ok, then proceed to measure impedance across input of PSU and compare it to the other channel.
If you find it to be particularly low then we can take it from there.

If you post a picture of the boards/built and schematic I/we will be able to help you better with your journey.

 

[Hocdm]

Thank you for the response. I appreciate the help.

The PSU isn't wired dual mono, at least not as I understand it. The transformer has dual primaries and I'm using both of them. The secondaries in use are one 220v tap and two 6.3v taps. I've got the dual 6.3v taps wired together to make a CT with +6.3v and -6.3v.

I'm afraid I have no way to measure impedance. My multimeter is a very basic one.

 

[SY]

Post a schematic with DC voltage to ground readings at each tube pin, and it should be pretty straightforward to diagnose.

 

[Stefanoo]

Thank you for the response. I appreciate the help.

The PSU isn't wired dual mono, at least not as I understand it. The transformer has dual primaries and I'm using both of them. The secondaries in use are one 220v tap and two 6.3v taps. I've got the dual 6.3v taps wired together to make a CT with +6.3v and -6.3v.

I'm afraid I have no way to measure impedance. My multimeter is a very basic one.

so you have two secondaries. I reinforce SY's request for schematic and voltage measurements.

One more thing: disconnect transformer off the board and power it up and measure AC voltages at each secondary.

Also post a picture of the built, that will give us better understanding.

 

[Hocdm]

Hello again,

Got the schematic here. I think I found out why the left channel wasn't working. After checking the wiring 3 times previously, I can't believe I didn't catch it. The B1 connection on the last valve (closest to output) had come loose, but the wire is so stiff it still 'looked' attached. I fixed it, but I didn't try the unit in my system because the transformer is still getting very very hot. I took SY's advice and measured the DC voltage on the tube pins with reference to the ground of the B1. Here's what I found..

Starting with the tube closest to the output
Pin 1 and 6 are reading 266v
Pin 4 and 5 are reading 61v
No voltage shows up on the other pins

Tube 2:
On pins 1 and 6 I'm getting 146v
On pins 4 and 5 I get 60v
No voltage shows up on the other pins

Tube 3:
I get 129v on pins 1 and 6
I get 61v on pins 4 and 5
Again, no voltage showing up on the other pins..

The schematic shows two B+ voltages, but when putting it together I decided to go ahead and use the 266v coming out of the psu for all three tubes. I wanted to basically get it running to get an idea of what it sounded like before going to the trouble of trying to get two B+ voltages from by psu pcb. I'm obviously no tube expert, but I wouldn't think that that is what is causing the transformer to get so hot. Would it?

Should I still disconnect the transformer and test it unloaded?

Thanks SY and Stefanoo. It's cool of you to help out.

 

[Hocdm]

Oh yeah. I forgot to mention that I'm currently looking for the usb cable that connects my camera to the pc. If I find it (hopefully), I'll be happy to post a pic of the build.