Fuse blowing in 300b

[justin brown]

Hi all,
I contacted John about this a few weeks ago, but the problem seems to have developed further. There is a slight issue with my esl 63s, so One thing are checking them over for me ready for the show at Scalford hall. In the meantime I retrieved my old ATC SCM20s from the loft. These are a fair bit harder to drive than the quads, and I was just accepting the lack of power (in the sound) as it is just a temporary measure. Anyway, by driving these speakers I am not sure if I have damaged the 300b. Now, when I replace the main fuse, it blows upon switch on. John had suggested that the rectifier may be at fault. I have ordered a new one, but I was wondering how this affects the fuse, especially when the amp is tested without the valves being fitted. Is it possible that I have put a strain on the other components if my rectifier was at fault? And how do I go about solving this?

Thank you.

[John Caswell]

Hi Justin,
The main HT rectifier is a solid state device(s) and as such if one or the other of them goes faulty it will draw excessive current which is reflected back through the power transformer to blow the input fuse.
You need to check the HT rectifier diodes (D1-4) using the multimeter on the diode range to see t which is short circuit. Usually black lead to cathode (Bar on the diode) a good diode should read approx 400-600R on this range. You cannot use the low ohms range as it will not have enough current to "turn the diode on". Obviously replace the faulty ones(s) then report back here.

John

[justin brown]

Hi John,
Ok, I will check this later on this evening when I get in and let you know.
Thanks.

Justin.

[justin brown]

John,
Would you be able to send me a schematic of the 300bpp please?
(Or anyone else?)

Thanks.

[justin brown]

Hi John,
I think I have found the rectifier diodes. I can see two grey wires which connect from the diodes to the main power transformer. There are four diodes in a line. The two in the centre (next to each other) are facing the same way. The two at either end also face the same way, but face the opposite direction to the two in the centre. With the multimeter on the diode setting I am getting 134 from the two at either end, and only 88 from the two in the centre.
Please can you confirm that I am testing correctly (although I cannot see any other diodes in the circuit), and what I may be doing wrong. There are no obvious signs of component failure, either.

Actually, maybe there are signs of ageing. I have noticed that there is a bit of a brown syrup type residue around some of the joints of the diodes. I take it this what is referred to as 'leaky?' If this is the case, could I buy some replacements from you? At this stage in the game, I am unsure what else I should buy as spares, and do not want to buy unnecessary items to build up my £20 minimum order...

Justin.

[Richard]

Originally Posted by justin brown

Hi all,
I contacted John about this a few weeks ago, but the problem seems to have developed further. There is a slight issue with my esl 63s, so One thing are checking them over for me ready for the show at Scalford hall. In the meantime I retrieved my old ATC SCM20s from the loft. These are a fair bit harder to drive than the quads, and I was just accepting the lack of power (in the sound) as it is just a temporary measure. Anyway, by driving these speakers I am not sure if I have damaged the 300b. Now, when I replace the main fuse, it blows upon switch on. John had suggested that the rectifier may be at fault. I have ordered a new one, but I was wondering how this affects the fuse, especially when the amp is tested without the valves being fitted. Is it possible that I have put a strain on the other components if my rectifier was at fault? And how do I go about solving this?

Thank you.

Hi Justin, just seen on the pie thread you've not been able to sort this yet.

I wouldn't expect your other speakers to have damaged it. Sometimes the fuse will blow through fatigue as it is closely rated as standard. What did you replace it with? Was it a slow blow? Did you uprate it slightly to say a 2.5A slow blow? Does it blow with no valves at all fitted inc GZ37? I'm not sure if you're referring to the GZ37 as rectifier when you say you've ordered another.

If it blows with no valves at all inc GZ37 check the mains TX and wiring for shorting, and the solid state diodes as John says, but checking the diodes in situe can sometimes be tricky with capacitors still connected. The brown sounds like flux residue, there's nothing to leak in the diodes.

[justin brown]

Hi Richard,
Thank you very much for your reply! Time is running out for the show, and I really want to take part in it!
I contacted Matthew yesterday, as John hasn't replied as of yet, and his comments concur with yours (about the flux). I have just received my new gz37 nos Mullard, so I will try again with the valve rectifier in.
When this problem started to occur a few weeks ago, I replaced the fuse with the same rating, and everything was ok. But then, I had this slight issue with my Quads, and dug out my old ATCs. Within a few hours of listening, my 300b went again. New fuse. Blown. As I have a packet of them, I tried again, just to try and understand the problem a little better. This is when I posted to John. He seems to think it is the solid state rectifier, rather than the valve, but, looking at it (and I sent Matthew a photo of it), the diodes look absolutely fine.
One thing that does sway me towards Johns' thinking is that the amp should measure without having its valves fitted (of course), and it is not. Would it help If I posted some photos? The whole circuit looks to be in very fine fettle, with no signs of component failure anywhere, so this really has me stumped (especially with me being at such an elementary stage)!

Justin.

[justin brown]

Just tried again, and the fuse blows immediately upon switch on. I don't know how relevant this is, but quite recently I refitted my covers for the TXs as I had them gold plated. Some of the bolts unscrewed from the top (as in, the nut unscrewed with the bolt in place), and with others the whole bolt came away and I could hear the nut fall and hit a component/the chassis. This meant that I took the bottom plate off and found the odd nuts which were missing. Unfortunately, I have not been able to fit some of the nylon sleeves which help hold the nuts/bolts in place, as they are impossible to tighten up because of the position of the other (fitted) components. I am pretty sure I found all of the nuts and washers, but thought I better declare this now, as it does seem to be a shorting problem. But, I have been using the amp for several months like this, so if it has shorted, why now, and not when I initially re-fitted my covers to the TXs?

[Richard]

Hi, meters and values for diodes vary and often the value doesn't mean a lot just whether it works one way around but not the other. I wouldn't put much store by any reading with the diodes in circuit.

Do be sure to discharge C9 and 10 first and if there are balancing resistors across them this will affect any result.

If the diodes seem to be working one way and blocking ok the other that may be enough.

The first caps C9 and 10 take a lot of hammer. If one has gone short circuit it may be causing the fault.

Carefully discharge those caps and check they are showing 0V with a meter. Fit a new fuse and power up the amp again with all valves out. Then, if the fuse blows, power off and check voltage in each of those caps again.

Thoughts are that you might catch one cap with voltage and the other with 0V if it is shorted. Otherwise check or replace them anyway as they must be the most likely components after a TX or wiring short or the diodes themselves.

[Richard]

Re the dropped hardware try shaking the amp again! It could be a washer dropped across junctions and lodged there I suppose