Troubleshooting Yamaha T50 by Soldano .. smelly and noisey

[chopstuck]

I don't have too many PC board amps - this one has got me worried.

It has a not so nice smell coming out of it after it warms up.

It's louder than my deluxe but not a 50 watt loud.
My Bedrock lead 50 just kills this thing. (500v on the plates)

Nothing obviously burnt. Looks to have new-ish PS caps and bias supply caps.

It has had the PS choke mod by Soldano with Mike's Signature.

I have tried a few things. The voltages check out. both 6L6's are biased at about 39mv at idle with plates at 450v. The heaters are a little low at 6.15v.
I pulled each tube one at a time and watch the heater V change slightly by +0.04v.
I replaced an intermittent impedance switch. Sounds better, still smells. The noise I get sounds like low freq boiling water and is changed by the tone controls on the High gain side.Looks like the tone controls are at the end so the noise could be from anywhere. These boards are pretty cramped. Traces look fine. Amp sounds bad. Not enough clean power in clean modes. Nasty clipping in lead mode.

Were there any bulletins for this ?

[selloutrr]

As far as PCB design go Soldano are some of the best. I could show you 100 that are worse without having to leave Guitar Center.

has it been sitting for some time? Often dust burning off from the tubes heating up will cause an odor.

If you are worried about it being transformer related test them. that will quickly rule out if they are being slowly cooked.

can you see any wires that have become brittle and started making contact with ground that shouldn't?

In most cases volume is directly related to tube life in a known working design. If it feels quiet test the tubes. preamp and power.

[chopstuck]

Thanks for the reply,

I have been able to get rid of the nasty distortion with tube swapping.
I was able to get more drive by swapping out the PI tube (7) and the one previous to it (5). I have more power and headroom. The amps now sounds really good but still smells hot. I hate to say it but maybe I should wait until the failure makes itself known. Probably not a good idea in pcb instruments.

I have read about heater traces getting hot on many pcb amps and folks jumpering them with heater wire. I just don't see where the trouble lies.

[selloutrr]

heater trace issues usually = cold solder joints followed by pops or clicks casued by arching which can be heard while the amp is on, and in some cases burn marks are noticeable around the Heater pins.

If you smell burning can you describe the smell.

[chopstuck]

Yes, it's the hot plastic smell. The kind that under ventilated electronics frequently make. I have no sputtering or loud pops etc. I may have to flip the board over. This is tricky- Lots of unsoldering to do.

[selloutrr]

put a piece of tap on each wire you have to remove and write where it goes. if the board is labeled use that as the marking. if not I use A,B,C,D,etc. if i run out i switch to lower case of numbers.

an ispection mirror may help. make sure the caps are discharged and it's not plugged in before poking around... SHOCK!!!

[DonMoose]

I've had a T50 and T100, both with low-quality rework around the reverb mod - neither of which had had Mike's hands on them. The T100 had a fried plate resistor on the mixer stage (V4) (in this version of the reverb mod, the plates had been separated and the reverb was being fed in after the MVs).

The T50 had some bad joints and not-quite-cut traces in that area.

Mike wouldn't let that kind of thing slide if his shop had been responsible for that mod, but an amp that's close to 20 years old may have been through some less-qualified hands.

Hope this helps!

[dynaman]

Mikey designed it...he didn't make it. Any reference to Soldano build quality is moot.

Silly question, but does the board have any obvious heat/burn spots??

[DonMoose]

Mikey designed it...he didn't make it. Any reference to Soldano build quality is moot.

Apart from the signed Choke Mod and any other work done/checked at that time.

[chopstuck]

I flipped the circuit boards and it wasn't too hard. Just unscrewing some jacks and pots from the chassis. Turns out the board is single sided so nothing to be found there. I did check for burns discolored areas etc and it looked new. Nothing wave soldered though. Everything by hand. I can see the flux rings around the joints. I did find that someone had recapped the amp and hot glued the caps to the board. I found some hot glue on a near by resistor that looked browned. Replaced that one. Still smells but maybe not as bad. My nose tells me the the PS board smells a little worse so I'm pulling the big caps and cleaning under there. I did find an unsoldered wire that fell out when I flipped the board. That was easy to fix.

Seems the schematic is much different from this amp. My amp has much different filtering. More on the B+ and only 33 vs. a published 47 mfd on the bias supply.

Well just kind of sharing my strategy. Next... PS board.