Paramour 1 - 27 mv hum problem

[Paul Birkeland]

There is something connected on the primary of the OT or on the output of the OT (including the binding posts) that shouldn't be there  or that is touching both terminals.  The signal is sitting there appropriately on the input side of the transformer but isn't making it out.  There should be a little less than 1V of signal at the binding posts under your testing conditions.

[patrickamory]

Hi Paul,

I'm stumped. I can't see any short circuits or connections in the area. Is it possible there's something wrong with the OPT?

I've attached some pictures.

Thanks,
Patrick

[2wo]

Sorry to say but a lot of the solder joints don't look good at all, especially the strip adjacent to the outputs. If it were me I would cut/unsolder all of them and start over...John

[patrickamory]

Sorry to say but a lot of the solder joints don't look good at all, especially the strip adjacent to the outputs. If it were me I would cut/unsolder all of them and start over...John

No worries about criticizing the joints - I made this amp 17 years ago and it was my first project... they are really ugly, I agree.

I'll give that a try and see if it solves the problem. Thanks!

[patrickamory]

Well, I reheated and cleaned up the solder joints on the terminal strip by the outputs.

Reconnected the amp, and now it doesn't power up at all! Which is odd because I didn't touch the PS.

Checked the fuse - intact.

Power cord wiring - intact.

Power supply wiring and joints - look fine.

Checked resistances - all check out except for T25 which should be 200 ohms but is coming up at 1200 ohms - I don't know if this is significant? T25 is where the positive side of the parafeed cap connects with the blue lead on the OPT, which the documentation says connects the primary to the plate.

Checked voltages - 120 VAC from the power cord, 0 VAC at T1 and T2, other side of the PT.

Any further advice?

[Paul Birkeland]

Based on your descriptions during this thread, every time you are going in to do work on the amp, the problems it has change.  This does indicate lots and lots of flaky solder joints.  Starting over isn't a bad idea, and if you sent it to me for legacy kit repair work, that is likely what I would do.

-PB

[galyons]

Based on your descriptions during this thread, every time you are going in to do work on the amp, the problems it has change.  This does indicate lots and lots of flaky solder joints.  Starting over isn't a bad idea, and if you sent it to me for legacy kit repair work, that is likely what I would do.

-PB

I think the PB's advice is spot on!  It is a simple build with relatively few solder joints.  Disassemble, lose the stranded wire and re-solder all of the connections with good solid core wire.  You will likely spend more time chasing problems around the unit than a rebuild would take.

Cheers,
Geary

[patrickamory]

Thanks guys! I'm going to do it.

[patrickamory]

I have not replaced the wiring but I have resoldered every component and am still seeing the same problem and getting the same voltage readings.

Before I send this off to Paul (if he'll accept it), I wonder whether there's an obvious problem that anyone can see from the input voltages - possibly a problem with the power transformer? The PS voltages are at 60-80% of what they should be. The C4S diodes don't light up (I thought one was bad but it's fine), and zero voltage reaches the 2A3.

I've even disassembled the power cord and checked that 120 VAC is coming through from the wall.

HV secondaries: 262 VAC on each (315)
First filter cap: 250 VDC (395)
Second filter cap: 176 VDC (390)
2A3 filament: 0 VDC (60)
2A3 plate: 0 VDC (365)
C4S input: 122 VDC (295)
12AT7 second section plate: 120 VDC (155)
12AT7 second section cathode: 1.3 VDC (2)

The figures in parentheses are the proper voltages according to the manual & C4S manual.

All resistances check out.
Both tubes glow.

It seems to me that insufficient power is making its way through the PS to the C4S, which in turn means the 12AT7 is not driving the 2A3.

I have checked and resoldered EVERY connection throughout the PS, paying special attention to the PT leads.

Sorry - I promise this is the last post I'll make about this amp if the answer is still bad soldering joints - but I thought that maybe the consistency of the voltage readings might be a clue.

[Edit: attached original schematic, pre-C4S.]

[Paul Birkeland]

First filter cap: 250 VDC (395)
Second filter cap: 176 VDC (390)

You are on the verge of destroying this amplifier.   That is 275mA of DC current being drawn from a transformer designed to provide 75.  You will blow these up if you keep running them.

If the 2A3 plate is 0V, then this is extremely concerning.  Perhaps the 2A3 isn't inserted in the socket properly? 

Something is drawing way more current than it should.  The power transformer voltages seem low because you're slamming it with several times what it's supposed to provide. 

You are welcome to send these to me, but I will strip everything out any only reuse the transformers. 

[Paul Joppa]

This made me look up my old manual - sure enough, there is no resistance test at the 2A3 plate. We must have thought there was no possible way the plate could be grounded. We were so innocent back then!

Anyhow, the plate IS grounded. No point in checking anything else until that's resolved. Only two components connect to the plate; that should make it easier, right?  :^)

[patrickamory]

This made me look up my old manual - sure enough, there is no resistance test at the 2A3 plate. We must have thought there was no possible way the plate could be grounded. We were so innocent back then!

Anyhow, the plate IS grounded. No point in checking anything else until that's resolved. Only two components connect to the plate; that should make it easier, right?  :^)

PJ you were absolutely right! For some reason I had a lead connecting T21 back to T12, thus grounding the plate. I must have mistakenly soldered that in when redoing the wiring around the cathode bypass cap which I replaced at the beginning of this project.

The diodes light up and all voltages read correctly now.

On to the hum adustment and then putting this back in the system. PB, I will probably send both amps to you for a rebuild anyway when my regular amp returns from the shop. Thank you so much for your help, gentlemen.

[Paul Birkeland]

We must have thought there was no possible way the plate could be grounded. We were so innocent back then!

I did see this once in a SEX kit also.  It's imperative to check the DCR of the plate choke on that channel to be sure it survived the incident.

[patrickamory]

I did see this once in a SEX kit also.  It's imperative to check the DCR of the plate choke on that channel to be sure it survived the incident.

Check it out of circuit right? And it should be 270 ohms?

[Paul Birkeland]

Yes, you should be able to measure it in circuit.  It may be off by 10% or so; that would be normal.