Bass Guitar amplifier problem

[sparkymike]

I have a Peavey TNT 150 bass guitar amplifier (1980's) here that has developed a mains hum. The hum is just about bearable, but I am sure this amp was quieter than it is now.
If anyone has worked on a similar rig and problem, I would be interested to know
what they found. It is a transistor amp, not valve. Mike.

[Craig Sawyers]

YouTube suggests that the three screws on the large preamp board connect the circuit ground to the chassis, can make a high resistance contact as the years pass and the copper tarnishes.

https://www.youtube.com/watch?v=EYHYd471siw

[GrimJosef]

I don't know this amp in particular but the question "What's causing the hum ?" is the one with the most possible answers in all of audio electronics. A quick way of eliminating a large number of possible options is to establish whether the hum is at 50Hz or 100Hz. If it's 100Hz then it's very often a power supply or power rail issue, which can include a fault somewhere else overloading the power supply. If it's 50Hz then it could be grounding, magnetic field leakage, raised circuit impedance somewhere, transformer vibration shaking a microphonic component ...

Cheers,

GJ

[Radio Wrangler]

Playing around with volume controls and tone controls to see which do and which don't affect the hum can be enlightening and reduce the area to be searched. Unplugging all inputs etc is another check.

David

[sparkymike]

The YouTube tip is certainly worth checking. All controls have no effect except volume control which increases the volume of the hum although I would not be using the amp at that higher volume. Guess it could be time to dig out the scope.
Mike.

[sparkymike]

Well checked out the YouTube tips but to no avail. The various earthing terminals/tags all read zero ohms to the main input lead earth. The neutral pin screw in the 13 amp plug was loose as is often the case. Why do they do that ?? Always the neutral and not the phase. So its probably a leaky cap and lucky for me, not many in the power section. The main electrolytics are 5000 ufd, 47 volt working. Almost impossible to check without removing the board. As this is a USA built amp, if needed, exact replacements may be hard to find. It looks like they are the type with lugs that are twisted after fitting in the board. Might be worth giving Peavey a call/email.They used to have a UK base in Northants, but I have an idea that got shut down.(Maybe not ?)
Mike.

[Craig Sawyers]

Called Twist-Lock caps. If they are Sprague they can be open circuit. Basically where the ground tape inside the cap is crimped to the can, it slowly electrolytically rots through over thirty years or so, and disconnects the innards.

They are real sods to desolder, particularly if the twist lugs are soldered to the board too.

Craig

[dragonser]

Hi, I seem to remember that the main smoothing caps can party fail. I seem to remember replacing them with some tag electrolytics, but using hot glue to hold them in and wires to connect to the printed circuit board....

[Goodmanguy]

It's the caps. 100% Many trans amps from this period fail like this after a number of years.

[crackle]

If the hum is effected by the volume setting, have you got a guitar plugged into the input.
If nothing is plugged into the input when the hum is present then try a jack plug which is shorted, if the hum goes then the hum is just a case of residual mains hum interference.
It should help with a guitar connected to the input, assuming the guitar cable and guitar are good quality and wired correctly.

Mike

[Valveoid]

I've had several of these amplifiers over the years and a couple of them had the hum issue. In my case the fix was simply the earth cable had come loose from the terminal inside the mains plug. Retightened and all good again...………..

[sparkymike]

I need to check the main electrolytics first with my cap tester. Only thing is the highest range is 2000 uf. The caps are 5000uf. I will have to try and and see. At least it should tell me if they are very low.
I will also check mains lead earth for continuity. Possible internal break. (I doubt it but you never can tell.)

Mike.

[ITAM805]

Mike, you say in #5 that the hum increases when the volume is turned up, this suggestS something in the pre-amp or EQ side perhaps? Have you checked the +/-16V rails? A scope on these should show if the filtering is amiss

[sparkymike]

In reply to previous post, sorry for that error on my part, that was with the guitar plugged in. With no input, hum level is constant.
Mike.

[sparkymike]

Apparently, the preamplifier can be unplugged and the power amp can be run on its own, so if the hum still persists, then I will know which section it is in.
Mike.

[John10b]

Hi Watford Valves do a wide range of amp components, worth taking a look.
Cheers
John

[sparkymike]

I found a good company in the US who stock all kinds of Mallory caps of the twist-lock type, typical price around $10-$12. Of course postage would be a deciding factor and probably cost more than the two items.
I did find out that a common fault on these amps is cracking of the solder on the main electrolytics lock tabs and connections, but on removing the board, all was perfect.A slight discolouration on the standoff earths but nothing else. Board removal is easy. Four screws that hold the heat sink part of the board,three screws on the other edge of the board, one jack socket to remove,complete with wires and a tie wrap to cut. That is enough to rotate the board to check soldered connections. I found a four page pdf with circuits etc.on line of the same model. I did see that the 5000uf caps were shown as 1000/35 on one drawing and 5000 on another, so perhaps they found that more smoothing was required.
Since removing the board and replacing it, the hum seems slightly quieter or am I going progressively deaf !! Hum is around 60hz., (or low C.)
Mike.

[Craig Sawyers]

60Hz (or 50Hz) is an odd frequency for the hum, unless it is being induced by stray field in the mains wiring. Most hum is second harmonic from full wave rectification artifacts and should be at 100Hz and harmonics thereof.

If it is indeed at 50Hz that indicates something very different from duff reservoir caps.

Craig