1973 50W JMP - 2563E

OdgeUK · Post by **OdgeUK** » Thu Sep 20, 2007 7:12 am

Well, I decided I had had enough of measuring voltages, so I just plugged in a guitar and went for it.

Both channels worked. Sounded as I remember (flippin lovely). No squealing feedback (must've been that microphonic XF4). Still not possible to get the amp overdriving though as vol even on 3 is still WAY too loud for home.

Still had a background hiss problem. So I resoldered input jacks and their respective grounds. Plugged it all in again......RESULT: Volume I, affecting both channels and Volume II doing nothing.....look over the amp for ages....then see that one of the V1 connections has broken from the solder joint! Resoldered. Channels working ok again

So, now. I have a working amp, but still troubleshooting the hiss. The hiss exists with both volumes on zero, but gets much worse as I turn up Vol I. Even with no guitar plugged in. Sounds like someone is frying eggs when Vol I is dimed.

Played around with a chopstick and couldn't get the hiss to decrease or increase at all, even by disturbing all the grounds and moving the AC carrying wires around. Did notice that tapping the NFB purple wire with the chop stick resulted in a knocking sound comin through the speakers. Whereas other wires did not do this.

Not been able to work on the amp since. But the next plan is to resolder V2 and Impendance Selector switch. Then see if this hiss goes. If this works, I'll need to test that the amp is ok with a guitar and volumes maxed (might need to book a nice soundproofed room for that!), then onto the LarMar PPIMV!!!

EDIT********

I changed all the pre-amp and power tubes too (with known working ones from my bass head). No change to the hiss. I did think that maybe a pre-amp was dodgy, 'cos V1 doesn't glow as brightly as V2 and V3....

neikeel · Post by **neikeel** » Thu Sep 20, 2007 7:32 am

Failing or open series dropping resistor in the bypass networks leading to preamp stages; these are the 8K2 and 10K next to the bias caps and the two 10ks ain the middle of the board, usually 2W or so. They form a series string from the highest voltages at the screens of the power tubes to the plate supply for the input preamp tubes.

Check these are in spec and also both the 100k plate resistors on V1 and the 100k plate resistor on top of V2 (did it get a bit hot too?)

These resistors can get hot and drift upwards in value, dropping the plate supply voltage to the earliest stages too low for proper operation. They can also be cracked or broken sometimes.

Bypass capacitors can get leaky, shorted or open just like power filter caps- test them the same way.

Suggest you replace those grey thingies you have with some new MO 2watt resistors and at the same time put in a new 330uF low voltage V1 bypass electrolytic

OdgeUK · Post by **OdgeUK** » Thu Sep 20, 2007 10:38 am

How do you test capacitance with a DMM? Will I get accurate readings when caps are in situ?

neikeel · Post by **neikeel** » Thu Sep 20, 2007 1:34 pm

OdgeUK wrote:How do you test capacitance with a DMM? Will I get accurate readings when caps are in situ?

Afraid you can't test those with a DMM you need an LCR meter!

You can test the resistors in circuit tho'.

It is possible to get a digital LCR meter for less than

OdgeUK · Post by **OdgeUK** » Tue Sep 25, 2007 5:14 am

Erm.....had a bit of a disaster last night. I found a dodgy solder connection on one of the filtercap earths, so I resoldered that, resoldered the impendance selector and resoldered a suspect connection on V2.

Fired her up, and the background hiss was a little better, but once I started turning all the EQ controls to 10, it was really very noticable. Prescence control crackling too when I turn it. Channel 1 also still sounds like someone frying eggs when turned past 7-8. Channel II not as bad, but still noisy on 10.

So....I played it for a bit anyway, as it still sounds cool if you ignore the hum. Then I thought I'd try the ol' chopstick thing again. So I flicked standby on, lifted up the amp onto it's side (so I had access to the underside of the chassis) and flipped the standby switch..........Lots' of smoke and bright white flash from V4!! Flipped standby back on and turned amp of at the mains. Inspected V4. Insulation on two wires nicely baked, but no real indication as to where the source of the heat was from. So I went to bed and cried myself to sleep

What bothers me is that i didn't rewire v4 at all. And it was ok till I tipped the amp up! And I really double checked the V4 connections for stray solder or touching wires.

Bit scared to turn it on again now.

neikeel · Post by **neikeel** » Tue Sep 25, 2007 5:25 am

Well at least you know it works and you can get it to run!

I suggest you take a couple of really clear pictures of the board, the PT and the burnt bit, in good light etc.

The flashing sounds like HT arcing, sometimes a bit of stray solder/wire floating around that created a short as it flicked onto exposed bits when you turned the amp over. Hint give the chassis a brush with a small paintbrush and hoover it at the same time once you have finished work each time.

Did your fuses stay intact? Had you set the bias yet?

As a restart check your connections again. Before you try it again properly pull all the valves and go through the initial fire up drill. If you want some new 10k (x2) and 8k2 (x2) MO resistors let me know and I will send them to you.

Best wishes

Neil

OdgeUK · Post by **OdgeUK** » Tue Sep 25, 2007 6:36 am

Ta Neil,

No, I haven't biased it yet. I need a Bias probe don't I? One of those socket shaped things that connects to a DMM? Where can I get a half-decent one in the UK?

Hopefully it was just some solder somewhere. Maybe it arced across some dust? The bottom of the EL34 did have a coat of dust, but it might've been the effect of the smoking socket!

I will check voltages again, with tubes out. Thanks for the parts offer. I'll let you know if I need them. I haven't installed the mustards and dropping resistor you sent me yet. I was going to get the amp working with original bits first, so that I could hear the differences.

I think the fuses are ok. But not certain yet. I certainly didn't hear them pop. And it only stopped arcing when I flipped the standby back on.

neikeel · Post by **neikeel** » Tue Sep 25, 2007 8:55 am

OdgeUK wrote:Ta Neil,
No, I haven't biased it yet. I need a Bias probe don't I? One of those socket shaped things that connects to a DMM? Where can I get a half-decent one in the UK?
Hopefully it was just some solder somewhere. Maybe it arced across some dust? The bottom of the EL34 did have a coat of dust, but it might've been the effect of the smoking socket!
I will check voltages again, with tubes out. Thanks for the parts offer. I'll let you know if I need them. I haven't installed the mustards and dropping resistor you sent me yet. I was going to get the amp working with original bits first, so that I could hear the differences.
I think the fuses are ok. But not certain yet. I certainly didn't hear them pop. And it only stopped arcing when I flipped the standby back on.

For general DIY purposes I would get a couple of 1ohm resistors and link pins 1 & 8 with one limb of the resistor and put the reistor between them and earth (instead of the exixting earth link). That way you can measure the bias with a DMM (1mV on your probe is 1mA). You will need at least a gold tolerance rated resistor (measured @ 1ohm) 1watt MO is what I use.

The chopstick method is good when looking for dodgy connections or wires causing interference with each other. Background hum would be your heater wires not twisted neatly or AC wires being bundled with DC wires.
Hiss will need your B+ line and plate resistors checking out.

OdgeUK · Post by **OdgeUK** » Tue Nov 20, 2007 6:23 am

Hi all!

Well......6 months later....and it looks like we are there with this restoration!

I handed the amp into a little independent vintage radio and electronics shop, as I'd had enough of troubleshooting in the dark! They called me yesterday to confirm that all the wiring seemed to be spot on, except for a slight difference on the speaker jacks, which they corrected. They also said they re-did a couple of suspect solder-joints, fired her up and the hum had gone! They are not 100% sure which action actually fixed the fault, but suffice to say, they only charged me

neikeel · Post by **neikeel** » Tue Nov 20, 2007 7:03 am

We will of course need clips.

Then you can return the favour as I have never made clips before, all I have are camcorder recordings of gigs.

OdgeUK · Post by **OdgeUK** » Tue Nov 20, 2007 7:07 am

neikeel wrote:We will of course need clips.

Then you can return the favour as I have never made clips before, all I have are camcorder recordings of gigs.

Do you know, I reckon I could do that no probs as I'm an IT manager with a fair few resources at my disposal. However, I don't have a recording Mic! I'll have to see if I can borrow my singers.....

thousandshirts · Post by **thousandshirts** » Tue Nov 20, 2007 3:54 pm

Odge, this has been a great thread to read. The difference inside the chassis from beginning to end is really just like day and night. Or, in the case of the amplifier, night before day! It has been given a whole new life. As Sir Neil pointed out, sound clips would just put the cherry on top for those of us who have been following along! Very cool job to undertake indeed, and, looks like a job well done.

Will

soldersucker · Post by **soldersucker** » Thu Feb 16, 2012 9:29 pm

Great post.

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1973 50W JMP - 2563E

Re: 1973 50W JMP - 2563E