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Very Noisy "transformer" - Spectrum

AlanJ

AlanJ

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Joined
Dec 27, 2017
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Leeds, West Yorkshire
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Alan
Been working on my new acquisition, a bally Spectrum. Once it has fired up and all ready to go there is a terrible loud hum coming from either the main transformer, or something nearby in the bottom of the cab.

When you first switch on there is no hum noise at all, it's when the machine sparks into life, says a few words and is basically in attract mode, then the hum is there and it's loud.

At first I suspected the sound board and speaker, but I disconnected the speaker from the sound board and the hum is still there.

I disconnected the power from the playfield (by unplugging the plug on the power board in the bottom of the cab labelled "playfield" and tried that - machine all fires up ok - backbox and all boards in the backbox going, sound working, etc, and NO hum?

So, seems to be the power draw onto the playfield. No coils are locked on, just a load of incandescent lights flashing away nicely. I have a couple of thyristors out on the lamp driver boards, but this results in a lamp being out, so doubt that is the culprit, but......

Checked all other stuff, fuses all ok, non blowing or blown.

Anyone come across this before, or got any ideas please?

Many thanks,

Alan
 
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I'm not familiar with that era of pin but my TOM had a metal plate that the transformer was screwed to for EMI reasons I guess and that made the transformer hum.
You could try checking that it's screwed firmly to the chassis.
If that does not help then maybe use Led's to lighten the load on the transformer.
 
the popular advice on pin side is to whack the transformer with a hammer a few times

may have to resort to that as a final attempt




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You whack it to de-magnatise it. It can reduce the hum, use a rubber mallet if you have one.

As others have mentioned, check for anything lose.

Does it look like it's sitting in a dried out puddle, is there like a water stain around it on the base board?
Sometimes if the transformers go above load for to long they heat up and dump their wax/oil padding.
 
there is no leakage from the transformer. i tried whacking it with my rubber mallet. no good.

unscrewed it from the metal base plate and hung it in the air. no buzzing. or at least the transformer is still humming a bit but no sound board for it

i tightened the bolts on the transformer leaves to tighten but this had no effect

put some cardboard packing under the transformer and refixed to the metal base plate. the humming has reduced to a much more acceptable level.




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So the transformer has got a vibration hum that's being amplified through the mounting.

Sounds like a prime case for a good whacking.

Do some reading up on the process as it's surprising how hard you sometimes have to hit the thing.
 
the previous owner has reported that the humming started mid game and at the same time the pf lights went dim. and have stayed dim.

when i look at the gi bulbs they are pretty dim. these are switched lights unlike normal gi lights. i discovered that using the self test button. first press lights and dims all the feature lights and includes the gi lights.
there are three big thyristors bolted to the metal power supply board which i think (not sure yet) power these gi circuits. so perhaps there is an issue with that circuit. just noticed a dead bulb in the gi circuit. ill remove that first. as usual its the hardest one to get at [emoji23]. failing that. i wonder how i test each thyristor?


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Checked out ll the thyristors and the associated driver boards - 2 under playfield and one in the backbox. Looks like the 3 power thyristors are used for the back box GI and then one each for left and right sides of playfield. Also a 4th trial on a little board in backbox that drives 4 12v flasher bulbs in the backbox.

Anyhow - I disconnected all that and problem still there, so I know all that is ok.

Start to suspect the power board now. Tested out the voltages:
TP1 - 3.9V dc - seems LOW
TP2 - 223v dv
TP3 - 12v dc
TP4 - 6.9v ac
TP5 [- 43v dc

Looks like TP1 is the issue - should be 5.5-6.4 v dc

New bridge rectifier?
 
I finally figured the GI lights are ok, it was the switched feature lights on the playfield that were dim, once I fixed the 6.4v DC bridge rectifier on the power board, this restored all the games feature lights (of which there are very many) to full brightmess. The GI lights work of 6.9v AC and are also switched - using the power thyristors.

I noticed that the bunch of GI lights in the backbox insert switch off after a while, when the game is in attract mode, must be on a timer? They come back on again as soon as a new game is started. I thought I had an issue, but I think this is by design?
 
AlanJ said:
I finally figured the GI lights are ok, it was the switched feature lights on the playfield that were dim, once I fixed the 6.4v DC bridge rectifier on the power board, this restored all the games feature lights (of which there are very many) to full brightmess. The GI lights work of 6.9v AC and are also switched - using the power thyristors.

I noticed that the bunch of GI lights in the backbox insert switch off after a while, when the game is in attract mode, must be on a timer? They come back on again as soon as a new game is started. I thought I had an issue, but I think this is by design?
Click to expand...

if they are gi lamps then you’ll have an gi flasher driver board somewhere which is a small square board to cut /restore the power to a string of them. Driven by an scr off the main lamp board. They’ll also be a bulb next to it which is there so the scr has enough current draw to switch

The other option is they could just be controlled lamps that are switched on and off directly as a set
 
myPinballs said:
if they are gi lamps then you’ll have an gi flasher driver board somewhere which is a small square board to cut /restore the power to a string of them. Driven by an scr off the main lamp board. They’ll also be a bulb next to it which is there so the scr has enough current draw to switch

The other option is they could just be controlled lamps that are switched on and off directly as a set
Click to expand...

yes that’s right and there are also 3 big power thyristors bolted to the transformer/rectifier base plate in the bottom of the cabinet.

it’s a new one to me having gi lights that can be switched on and off, as my paragon doesn’t have that feature.


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AlanJ said:
yes that’s right and there are also 3 big power thyristors bolted to the transformer/rectifier base plate in the bottom of the cabinet.

it’s a new one to me having gi lights that can be switched on and off, as my paragon doesn’t have that feature.


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Click to expand...

Ah, yes there's those base plate ones to, i have one of them for use in pacman's lighting.
 
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