Hacker MC600 Music Center

[lawrenceenglish]

Hello, I signed up to post in this forum because I'm having trouble sourcing some transistors that I fear are getting a bit too old. I am having to listen to the center in mono as one side of the stereo makes the sound seem like it's underwater. It's not from the pre amp, it's coming from the power amp output stage, as I swapped the boards and then that channel was good again.

The transistors are on the A213 amp board and the numbers are TE 727 and TE 728 both are M528. I have attached pictures (correctly, I hope!). Unfortunately, I have been unable to find any schematics or any lists that state alternatives.

Are there any Hacker enthusiasts that can help please?

[G6ONEDave]

Hello and welcome to the forum. I did a quick check on the service data link above and it seems that the Hacker MC600 is not listed, perhaps other members could confirm this. I do have the service sheets, which are 2 sheets of A2 double sided papers, which I could scan and email to you. You will need to PM me your details.

With regard to the TE727 and TE728, I likewise after a quick check did not find data for them, however you might get away using BD131A and BD132A type transistors. The suffix 'A' is a slightly higher max voltage. But first I would confirm that they are actually faulty, as usually outputs either go very leaky to short and occassionally open circuit. It's more likely that there is trouble with the front end/driver stage lockfit transistors creating the noise.

Dave

[vidjoman]

I think the TE numbers are date codes.

[paulsherwin]

Motorola were notorious for using obscure house codes on their transistors.

[G6ONEDave]

The 'TE' numbers are as referenced in the service data and not date codes in this instance. The service data lists the following: TR207=TE727 (PL101) and TR208=TE728 (PL102), TR205=MTJ373 (BD515) and TR206=MTO320 (BD516), D201=MZ2360. These transistors are all Motorola devices.

Dave

[lawrenceenglish]

Thank you all very much for your help. I'll look into PL101 & PL102. Thank you.

[lawrenceenglish]

Hi everyone,

This is a little bit of an update from my last post about my Hacker MC600 and a new appeal for help from the hive mind.

Original fault - bad transistor. One channel works perfectly, however the other channel seems as though there's a dodgy transistor.

Following on from advice from the hive mind here I have replaced all the transistors with the same numbers apart from the two main output transistors.
TE727PNP replaced with BD442 which tested as a PNP and the TE728NPN with BD441 which tested as an NPN.

But the amp is still not working despite having changed all the transistors. It still seems as though there is a bad transistor despite replacing and testing them all. The preamp has been tested and this js perfect on both channels. All the voltages are the same on both channels.

Does anyone have any idea as to what to try next? I'm at a total loss so any advice will be gratefully received.

All the best,
Lawrence.

[Radio Wrangler]

Lawrence, I've merged your new thread into the old one. In general, it's best to continue with the same set in the same thread, then you don't have those reading your exploits jumping all over the place and perhaps missing instalments. It gets messy if one set is spread across multiple threads.

Threads are closed automatically by a timer from the last post, but they do have to be inactive for some time for this to happen. Moderators close threads that have gone stale with the same material starting to repeat, or if they've wandered far off of the original purpose.

If you have more to add and find a thread closed, for whatever reason, just click on the little warning triangle at bottom left and say why you'd like it reopening. We're happy to do so.

David (several Hacker portables, two working, the rest to do,)

[lawrenceenglish]

I've attached some photos for attention.

[SiriusHardware]

The amplifier looks like a DC coupled design which, if so, means that failure of any component in any part of the power amplifier circuit can cause a knock on effect and damage the output transistors.

In other words, your failed output transistor may not have been the original fault, it may have been caused by the failure of a transistor earlier on in the power amplifier board.

When working on an amplifier like this is is best to test and if necessary replace ALL of the semiconductor devices in the power amplifier of the faulty channel before applying power again.

Those plastic packaged 'lockfit' transistors are notoriously unreliable. Try testing all other transistors, along with any low value resistors in the faulty amplifier channel, and be prepared for the possibility that one of the output transistors may have blown again due to an as yet undetected fault earlier in the power amplifier circuit.

[lawrenceenglish]

Hi.

Thank you for the info. I will order up a complete set of transistors and test all the old ones to see if any are bad. One thing the BC149 have a small cap across the Base Collector. Why is this? I am thinking I do not need this as the main outputs have changed.

BFN Lawrence.

[Michael Maurice]

Originally Posted by lawrenceenglish

Hi everyone,


Following on from advice from the hive mind here I have replaced all the transistors with the same numbers apart from the two main output transistors.

If you have replaced all the transistors with no effect then replacing them again probably wont cure the fault.

You need to also check higher value resistors which might have gone high or o/c. capacitors can also fail, possibly going short circuit.

Replace BC14x with BC54x and BC15x with BC55x etc.

Good luck.

[lawrenceenglish]

Thank you for the information, I will keep it in mind if changing all of them again doesn't work out.

I have changed the main output transistors to BD529 and BD530. The MT0320 has been replaced with a BD515 and the MTJ373 has been changed to a BD516. The original BC's are BC148B, BC157B, BC147C, BC159.

I have taken out all the capacitors and tested them and they all test over what they should be, which is a bit puzzling. I don't know what effect this would have, for instance the 63v 1000uf tests as 1200uf. I tested the same capacitor on the good channel and it reads 1033uf, this is the side that works.

Do you know of anyone that has one of these units for spares or repairs, if so I would be very interested, Hacker MC600 Music Center. I do believe the MC600DS uses the same amplifiers so that might also be a line of inquiry.

Thank you for all the help so far,
Lawrence

p.s. I have early onset dementia and my daughters help me write these posts, so if I post something that doesn't make much sense it's because I myself have posted without first getting it spell checked. Thank you.

[SiriusHardware]

You asked about low value capacitors from Collector-Base on some of the small signal / high gain transistors, they are usually intended to reduce the gain of the stage at RF / very high frequencies.

Regarding your ongoing problems, one useful technique you can try when you have a stereo circuit is to look for differences between the good and the bad channel.

These differences could be different voltages (with respect to 0V) at the same points in each amplifier when the power is on, or different resistances from the same points in each amplifier to 0V when the power is off.

If you are lucky you will find that any measurable differences between the good and bad amplifier are in a fairly localised area, circuit wise.

[lawrenceenglish]

hi thank you for some info about the hacker mc600 . i have put my scope on square wave
an it did show a a drop off on one transistor so i change it . the new one was the same. testing the transistor tested ok on a tester. going back over the amp the s/wave was all good. but still had adrop off on the 0ne transistor .. this left side has the same thing wrong all the time ..it did go right for about 6hr then went bad same thing again. it is working but the sound is breaking up . just like a bad transistor ..i have orded up a full set of transistor . i will post the result after fitting. bfn a very fed up lawrence.

[peter_sol]

You really need to feed in a 1khz sine wave and scope it and compare with the good board. There are also the current and mid point voltage pots to consider and check.

[lawrenceenglish]

hi peter thankyou for the info . i will be getting a new tester in a about 5 weeks. i will then try your repair/testing 1kh. please note i have to wait for a day when i am good as i have dementia . bfn and thankyou lawrence.

[lawrenceenglish]

UPDATE

It's working!

Firstly, I took off all the transistors and capacitors. I tested all the resistors, which were all good. Transistors and capacitors all tested good as well. So I put everything back, with high quality flux (commonly used with re-flows on laptops).

Secondly, I used a bench power supply and connected the negative to the earth pin, positive to supply pin (only 15V). I then shorted out the headphone pins to switch on the amp. I then placed my finger on the input pin on the amp, very good hum from the amp with no breakup.

Thirdly, I had a look at where the amp mounts on the back panel, which is metal, and put my meter onto diode mode and got a reading, on the first transistor of the power amp pair. Which shouldn't be there. The insulation on the heatsink to the back panel had broken down and was earthing out the transistor. I had a look at the other channel and it was insulated from the back panel not by plastic posts but by heatsink paste? I used CPU heat pads on the heatsink to back panel on the earthed out channel.

Lastly, I tested it with an ESR meter - all good. Hacker obviously needed the case to take the heat away. After 6 hours of use the amp only gets to 31C. The heatsink now has a clearance of 1mm to the back panel using the heat pads.

Thank you Roger for the Hacker parts, they were helpful in fault tracing my Hacker, and now it has spares for any future needed repairs.

And thank you to everyone who posted possible solutions, it was very interesting and I'm glad we eventually found the fault.

Now, my workshop (daughter calls it the shed!) is very small and I need the space for future projects, this Hacker and all it's spares (no speakers) is now up for rehoming. FREE TO COLLECTOR! It is fully working to my knowledge, the only thing I haven't tried is the record side of cassette deck, as I haven't been interested in recording on it. If you are interested please send me a message. I will also post a little note in the sale thread.

All the best,
Lawrence
(typed by daughter)

[RojDW48]

Good news, Lawrence - top job !