Pye Black Box howls when switched on

[milannoir]

I bought this recently from auction. It makes a brief loud howling noise when it's switched on, rising in pitch. Obviously it needs fixing. Anyone know what causes this? What new parts are put on these usually? Also the green wire is snapped off the solder joint at the head shell spade connector. What is the usual fix for this, try and solder it or a complete new set of wiring?

Pete

[HamishBoxer]

Certainly try and solder carefully back on.The howling sounds like unwanted feedback though may be more than the wire off.

[Ed_Dinning]

Hi Pete, try removing the bottom cover from the chassis and see if there are any obvious modifications.

Standard work on these would be to replace about 8 caps. They sound nice when operating.

Ed

[leslie5555]

It looks as though someone has been inside this before judging by the way the mains lead has been cut and a choc block connector used to join the cables together.
Not a good sign.

[julie_m]

If someone joined the new mains lead to the old with terminal block as opposed to soldering the new lead to the mains switch and earthing tag, that could be a sign that they hadn't the confidence to go much deeper into the electronics. So the amplifier might not have been tampered with. Some pictures of the underside of the amplifier chassis would help.

Probably the first thing would be to test the speaker transformer primary windings for continuity. A few hundred ohms from each anode to the centre tap is good, more than a few kΩ or more than 10% difference between the two halves is bad.

The cartridge wire will need re-soldering to the push-on terminal, but remove it from the cartridge before soldering. Crystal cartridges are sensitive to heat, and you don't want to risk damaging it. If it's dead anyway, you should be able to bodge one of the generic Chinese red stylus cartridges into the tonearm somehow.

If you get a cheap pair of headphones from a pound store, cut off one of the 'phones and make a temporary connection from the wire ends to the amplifier input instead of the pickup cartridge, you can feed it from anything with a headphone socket (CD/MP3 player, Walkman, mobile phone, radio .....) You should hear whatever is coming from the other headphone, through the Black Box loudspeaker if all is working.

Otherwise, you will need to take some DC voltage readings at the valves, and compare with the service data (available "up top", and helps keep this place advert-free).

[PJL]

Don't turn it on again until you have serviced it and changed the capacitors. Running it without servicing is very likely to result in damage to the output transformer which is an expensive repair.

This is a BB/A version using PCL83 valves. The service data is not available up top for this model.

[Station X]

The circuit can be found in post #11 in this thread:-

https://www.vintage-radio.net/forum/...d.php?t=136602

[Ed_Dinning]

Hi Pete, this unit with the PCL83's is known for being less stable than its predecessor with EL42's. It has no less than 3 feedback loops, one of which is a well hidden length of resistance wire!!

Ed

[milannoir]

Hi everyone, pictures of this PCL83 version are on this closed thread https://www.vintage-radio.net/forum/...d.php?t=137393
It often takes me a while to start working on things.
I thought it would be wise to take the amplifier out to look at it before I started on servicing. It looks like you remove the amp by removing the knobs first, correct me if I'm wrong, and the knob that's on it (one missing) doesn't want to pull off. any advice on this much appreciated, I don't want to break the knob.

Thanks, Peter

[wd40addict]

The knobs on mine were very reluctant to come off. First I squirted WD40 on the shafts and left them for a day to soak in. I then used two ordinary kitchen knives to very gently, but firmly lever them off, one each side. I used some cloth to protect the wood from the pressure.

Note I actually did this whilst away in a holiday cottage so items involved were whatever I had to hand.

[Edward Huggins]

Assuming there is a sufficient gap between the knob and the cabinet, link a cloth around the knob from behind, and pull off. This will protect the woodwork from leverage.

[Herald1360]

Originally Posted by wd40addict

Note I actually did this whilst away in a holiday cottage so items involved were whatever I had to hand.

What self respecting bodger would go on holiday without a supply of WD40?

[Peter88gate]

.....especially when your forum name is WD40addict

Peter

[leslie5555]

First thing to do is carefully remove the valves, then remove the four screws on the under side of the cabinet, this will allow you to push the amp sideways so as more of the spindles are sticking out ,
This will give you much more room to make a decent even purchase on the knobs and carefully pull them off.
I just had a look at you're other thread and seeing the pics makes me think these knobs have small screws that need loosening before the knobs can be removed.

[dseymo1]

That was my first thought - I don't have a BB to hand, but I'm sure that at least some had knobs with grubscrews.

[wd40addict]

Mine on my mkii were definitely only held on by unbelievably strong D Spring. See picture.

[leslie5555]

I just had a look at the one in my garage its a BB/A with the four speakers and the knobs are held on by small screws, these are usually much easier to remove than the mk 1 and 2 are.

[PaulR]

If you can get access, prising the spring off the knob whilst the knob is still on the shaft works well.

[milannoir]

Hi everyone, I was advised to test the amp on my Black Box with a cut off headphone

So I assume people generally put the wire to the cut off phone in whatever connector it is for the input jack before plugging it in, it looks like an RCA on the Black Box.

Thanks, Pete

['LIVEWIRE?']

Not the input jack, Pete. My first thoughts on reading this, was that a headphone or earpiece can be used as a signal tracer, but it would have to be one of the old High Impedance types (IIRC around 2000 Ohms) not a modern low-Z earphone. I stand corrected! I've just read Julie's post. Never tried to feed a signal into an amplifier that way, but it would no doubt work