Valve Amplifier - WEM Dominator Problem - Trouble-shooting!

[rodabod]

Hello lads and ladettes,

I was on my way to work this week when I saw this in the corner of my eye sitting next to a bin in the street. It is a "W.E.M Dominator Bass MK1" which is the same as the guitar amp version, but with a 15" celestion. Yum. Valve line-up 3X ECC83, 2X EL84 - 15W.

Here's where the problems are though... At first it didn't power up. After removing a five pence piece, a one pence piece and a five cent coin from the chassis plus a bit of messing about, I got it to power. It looks as if this amp has had a few beers and a few grammes of coke spilled down into the chassis over the years, but it is not too bad.

Channel 2 works fine, in fact, it sounds brilliant. It's probably better than my Marshall DSL, but, back to my problem.... Channel 1 is giving me no output, apart from a faint sound when the amp is on full volume. :sad:

The hard-wiring, jacks, etc. look good, and I have swapped a few preamp valves in case there were any extremely duff ones (valves and caps are original from 1971!).

So, where would you suggest I look for any problems? I can see a few electrolytics in the circuit and some "mustard" caps (not sure what types these are...) I have cleaned the valve sockets, etc.

I have not turned the board over yet to look for dry joints, as this requires removing a lot of parts... Should this be my first task?

Here is a schematic for anyone who might be interested:

http://britamps.mysite.wanadoo-members.co.uk/watkins/schematics/dommk3.html

 

[AnalogPackrat]

Hey, nice find! Looks like both channels are identical except for the bright switch (one switch and one cap). Since the power section works, you know which section contains the problem. You should be able to compare one channel to the other and locate the fault. Given that you found some junk in the chassis, I'd try to pull the board out enough to have a look for more stuff that might shorting something from underneath. You've probably just got a single fault somewhere--bad joint (wait, you said the previous owner was a coke head), open resistor, bad cap, etc. Good luck!

A P

 

[AudioJohn]

I'd almost put money on the 100K anode load resistor having failed open circuit.......

Before powering up - check the value of this resistor with a meter - compare it with Channel 2, which you have said works! As the 2 channels share the two halves of the valve (twin triode) - this is almost certain to be OK.

If you have a scope - apply a test signal to the input - and see how far it gets!


JG

 

[rodabod]

Thanks for your help, guys.

[quote author="AudioJohn"]I'd almost put money on the 100K anode load resistor having failed open circuit.......
[/quote]

Err, John, it's my round, what are you having? :razz:

The second 100K resistor I checked is completely open-circuit. I've scratched each lead and rubbed contact cleaner on each end, but there is still no continuity.

So.... I'll get some 100K resistors tomorrow. Can someone suggest a suitable Wattage rating for these? Valve electronics is not my strong point!

Also, should I replace the power filtering caps? They are over 30 years old.

Thanks again,

Roddy

 

[ward]

What a great find !!!!


I believe the dominator is very similar to the marshall 18watt.
www.18watt.com
The marshall 1974 circuit was derived from the dominator.

The 18watt cloners use 1/2Watt for the anode resistors.

But better wait for a second opinion,
I wouldn't believe myself !

 

[ward]

Here's a marshall 18watt schematic for comparing.
http://18watt.com/Storage/18watt_Schematic_08-28-03.pdf

 

[AudioJohn]

0.5 watt should be fine - but get 1 watt versions if you can.

If you PM me and let me have your address - I will put one in the post!

As you state - with the amp having been made over 30 years ago, it might be a very good idea to replace the electrolytic capacitors. Certainly wont cost much to do this!

JG

 

[rodabod]

Thanks John and Ward for the info!

John, I'll pick up some resistors at my local shop tomorrow.

Interesting to see that Marshall 18W too.

I'll get the electros done some time - when I tapped them hard I got a big crackling noise through the speaker - not sure if this is a sign of a bad cap. However, it doesn't hum, so I may be ok.

I will let you know how I get on.

Cheers!

Roddy

 

[rodabod]

Fixed!

Sounds excellent. Really.

A light crackle can be heard at full volume - I assume this is the old filter caps...

Cheers!


Roddy

 

[Marik]

Roddy,


That pink cable... :shock:













Can you change it?...













Or is it a part of sound coloration? :razz:

 

[rodabod]

[quote author="Marik"]Roddy,
That pink cable... :shock: [/quote]

:razz: No, it's actually red.... I took the photo in poor lighting and my digital camera was confused....

Or is it a part of sound coloration? :razz:

Ok, I admit it..... I use it to get that camp, 70's Glam-Rock sound!