Valve amp - Output transformer bust?

[Bradwell]

Looking at repairing a valve amplifier that's suffering from a low output volume. Marshall JCM 2000 DSL 50 - from reading various forums the transformers used in this model are cheap and prone to failure.

I've run a few quick checks as follows:
Swapped all the valves for a set tested as fully working in another amp - no difference.
Run the pre-amp signal out of the FX loop into the power section of another amp - this works fine and would suggest to me that the problem is in the power stage in the Marshall amp.

Finally, when mains power to the amp is switched on there is a lot of hum and vibration from one of the transformers (I know a bit of audible hum is normal).

Before I get time to fully investigate this over the weekend does anybody have experience of similar issues and know of a decent supplier for amplifier transformers in the UK?

Any help on this one would be most appreciated.

[mikeydee]

Hi Bradwell,

I repair amps all the time.

So pre-amp is fine. Do any of the buttons on the front make any difference - esp mute if fitted.

Next, do the valves light up (sorry) just asking.

You've changed the valves already - Did you check the HT fuse ok? Check on a meter.

Next: you can confirm power amp stage problem by seeing whether you get output by plugging guitar into return jack.
If you get a strong signal, then the issue is probably send/return jacks - It's very common for low output to be caused by the send and return jacks oxidising.
If output still low check the phase inverter (PI) valve. First is there any sign that the silver top has gone white? If so, then the glass has leaked. Better tho to replace the PI for another. If you don't have one, then take the one from V1 (nearest input jack) and then use your guitar via the return jack again. If it works then the PI was bad. If not, then you have not proven anything.

At this point, you have to open the amp up to go any furthere and you need to know what you are doing in working with high voltage.

There is a common problem of bias drift on TSL/DSL. This only effects certain serial numbers but can cause output TX to fail. Check the valve bases for signs of discolouration. They should be reddy brown but sometimes head black when this illness strikes.

BTW I know everyone likes to rubbish cheap chinese parts but seriously Marshall will probably sell you the exact right TX for £30-50. They happily last 10-15 years. That's not too bad.

Mike

[Subthumper]

[quote name='mikeydee' timestamp='1427479500' post='2730988']
Hi Bradwell,

I repair amps all the time.

So pre-amp is fine. Do any of the buttons on the front make any difference - esp mute if fitted.

Next, do the valves light up (sorry) just asking.

You've changed the valves already - Did you check the HT fuse ok? Check on a meter.

Next: you can confirm power amp stage problem by seeing whether you get output by plugging guitar into return jack.
If you get a strong signal, then the issue is probably send/return jacks - It's very common for low output to be caused by the send and return jacks oxidising.
If output still low check the phase inverter (PI) valve. First is there any sign that the silver top has gone white? If so, then the glass has leaked. Better tho to replace the PI for another. If you don't have one, then take the one from V1 (nearest input jack) and then use your guitar via the return jack again. If it works then the PI was bad. If not, then you have not proven anything.

At this point, you have to open the amp up to go any furthere and you need to know what you are doing in working with high voltage.

There is a common problem of bias drift on TSL/DSL. This only effects certain serial numbers but can cause output TX to fail. Check the valve bases for signs of discolouration. They should be reddy brown but sometimes head black when this illness strikes.

BTW I know everyone likes to rubbish cheap chinese parts but seriously Marshall will probably sell you the exact right TX for £30-50. They happily last 10-15 years. That's not too bad.

Mike
[/quote]


+1 . The mains transformers all ways hum on the jcm 2000s. Worth also checking the 16 ohm output Jack. They use the switching earth tag to isolate the 4/8ohm outputs when the 16 ohm is used to prevent overloading the output transformer. The connector in the Jack sometimes gets dirty and makes a bad contact which can sound like a bad output valve or transformer. Give it a blast of switch cleaner and exercise it by pushing a Jack plug in and out a few times.
As mentioned, going inside the amp involves high voltages, if your not sure what your doing take the amp to a tech. It really does hurt I promise!!
One last thing. These amps are full of multi connectors which can all easily be mixed up. Photograph from all angles and Mark every single one before taking anything to bits .
Good luck cheers Just

[Bradwell]

Thanks for the replies, pretty sure I've found the problem (I'm ok working with high voltages and know to discharge the capacitors before touching the circuit boards).

All the controls on the front panel work fine and there was no signs that the valves were failing - when I said I tested the amp with another set I meant full set of pre, PI & power.

One of the spade connectors between the power transformer and circuit boards was beginning to oxidise (yellow/brown colour rather than a shiny metallic finish) and melt through the plastic insulation - This is the connection for the 6.3V supply to the heating elements in all the valves. Like I said the TX has a lot of hum and vibration to the point I'm pretty sure it's on it's way out (definitely the power TX and not the output TX). I figure that would explain the problems of low volume from the power stage but everything otherwise seems to be working normally. Heater supply failing -> weak thermionic emission from cathode.

I read about the common problems with 16 ohm jacks on other forums and checked the switching connections - no problems. There was a number of other known issues that I read about but no sign of those occurring in this amp.

I had a look at replacement TXs online and there seems to be a lack of suppliers in the UK. Personally I would go for uprated components from USA/Canada but the amp belongs to a friend who wants to get the replacement from Marshall - fair enough.

[mikeydee]

Yep it's about 4 amps on that heater connection, so needs to be A1.

All transformers hum to a degree but you can reduce it by tightening up the four screws. That will help - up to a point.

So is it working now?

[Bradwell]

Nah, changing the connector hasn't helped anything & the TX has always been bolted up pretty solidly. Sounds like a swarm of bees, I've got a few big valve amps and they're all much quieter on idle.

I'm being paid to change the valves & bias the amp which is easy enough. Changing the TX would be a simple enough task but my friend wants it sent back to Marshall. Got the new set of valves but they can wait for Marshall to replace the TX first.

Good learning process and I'm pretty pleased with myself for figuring out the fault.

[dmccombe7]

Does it have a 110V power option which would go to different tapping on TX. Could try it with a portable 110V tx to see if that works altho think you have ID'd the fault anyways.
I recently had my AmpegSVT4 Pro upgraded from 110V to 230V tx. Its an older model and no TX tapping point. All soldered directly under insulation.
Cost me £265 but its an original Ampeg tx.

Dave