Help Repairing Vintage Philips 544 MFB Speaker

[CTNovice]

Hi guys,

I'm going to start off by saying that I am complete novice, and do not really have much experience in electronics repair. I am handy with a meter, have a decent soldering iron, and most importantly a can-do attitude.

Right, so I inherited a pair of 544's from my late father. Unfortunately one of them recently died on me, so I am wanting to have a go at repairing it. One of the output transistors for the mid/high section of the amp blew (TS444B) and I have discovered that the voltages in this section of the board are way off. I have attached a schematic, with the problem area outlined in red, and measured voltage written in, in red, as well.

While inputting my voltages on to the schematic, I did realise that the +5 supply is reading about 6v too high, so I will investigate that in the meantime. (hopefully I just wrote it down incorrectly)

For clarity, I have desoldered both output transistors (TS444A & B) so that I can test without continuously blowing the fuse. The measurements for those transistors are thus taken from their respective pads.

If anyone could shed some light, and/or point me in the right direction, it would be very much appreciated.

Happy to provide more info an measurements as required.

Thanks in advance and greetings from a not so sunny South Africa.

 

[natasha27]

I was working on this Philips 544 MFB speaker a few days ago, and I came across your problem. Then I noticed that if the power supply voltage is not correct, the TS444A and TS444B transistors do not work properly. Have you checked the power section properly? It will be clearer if you tell me what voltage you are getting on the transistor pads. If you share the schematic and voltage readings, I can help you with the details. The whole thing was a kind of “Fix My Speaker” experience for me!

 

[rjenkinsgb]

One of the output transistors for the mid/high section of the amp blew (TS444B) and I have discovered that the voltages in this section of the board are way off.

TS441 looks suspect.

Most of the power rails are unregulated, so can read high while there is a fault and less current is being drawn. (Plus AC supply variations can affect them).

(First time I've seen the post for some reason).

 

[ahsrabrifat]

Hi guys,

I'm going to start off by saying that I am complete novice, and do not really have much experience in electronics repair. I am handy with a meter, have a decent soldering iron, and most importantly a can-do attitude.

Right, so I inherited a pair of 544's from my late father. Unfortunately one of them recently died on me, so I am wanting to have a go at repairing it. One of the output transistors for the mid/high section of the amp blew (TS444B) and I have discovered that the voltages in this section of the board are way off. I have attached a schematic, with the problem area outlined in red, and measured voltage written in, in red, as well.

While inputting my voltages on to the schematic, I did realise that the +5 supply is reading about 6v too high, so I will investigate that in the meantime. (hopefully I just wrote it down incorrectly)

For clarity, I have desoldered both output transistors (TS444A & B) so that I can test without continuously blowing the fuse. The measurements for those transistors are thus taken from their respective pads.

If anyone could shed some light, and/or point me in the right direction, it would be very much appreciated.

Happy to provide more info an measurements as required.

Thanks in advance and greetings from a not so sunny South Africa.

You mentioned +5V reads 6V, which is too high. That's a red flag. Is the regulator or supply circuit damaged?

 

[rjenkinsgb]

You mentioned +5V reads 6V

There is no +5V; the + numbers are IDs for PSU outputs; The +5 one is supposed to be 31V when loaded, but fed through two stages of R-C filtering, each with 24K series resistance.

 

[CTNovice]

Hi guys, apologies for the delay in replying, for some reason I didn't get the notifications! Since my post, I have managed to fix the speaker. There were several transistors that had died, including TS441, TS456 and TS424. Frustratingly, during the repair process, I noticed that there are several errors on the schematic. Like R707, which is labeled as 33,2k, but is in actual fact a 3,3k resistor. Even more frustrating, was that once the speaker was up and running again, the bass/woofer was much softer than the working speaker I have. I traced this to TS424 (BC548A), which I had replaced with a BC546B. It turns out that this was another error on the schematic, and that it is in actual fact a BC558A! (NPN vs PNP) Once that was sorted, everything sprang to life again.

Edit: just remembered, these Philips do not like being run through a dim bulb tester! This also affected voltages measurements quite drastically.