Philips AH578 destroying resistors on driver boards, please help!

RRLegend

Active Member
This thread may sound familiar. Last week I met a fellow AK'er through a CL ad. We got to talking and, while I was interested in his Philips 578 knowing it had some problems, he was interested in trading for a Kenwood receiver I had. Ended up we are within a mile of each other and we have a lot in common and it was great meeting another hifi fanatic!

I'm still learning troubleshooting and was wondering if i could get some assistance. I haven't powered the amp up but was told when it's turned on, it goes into protection mode and/or burns up R232 on the driver board.

I managed to remove one of the driver boards and took some pictures of it and where it's located in the schematic.

So, I've located the burnt resistor on the schematic. In my mind the logical thing to remove and check next would be transistor Q209 which appears to be just upstream from R232. Any other ideas??

One more important note. I was told that after replacing R232 on one board, R232 on the other driver board fried. Logically I would tend to think that a component is screwed up that is common to both boards (or supplying both boards. The protection board is the last circut before g the signal goes to the speakers, correct?

This is where I'm at and with your help would like to learn how to do this on my own.
 

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Q209 is a driver, which could certainly cause issues, but also check the output transistors.
 
Boy that is one sh*tload of power resistors on a single board... guess they're cheaper than transformer windings...
 
Out of the 16 output transistors, one tested bad.
I am going to replace the burnt resistor and the bad transistor. I checked the three relays on the protection board and none seem to be welded closed. Next I'm going to check Q232 on the opposite driver board.

Assuming it's good, do you think it's safe to fire it up...ahh poor choice of words. Ok to touch the switch ot on???
 
I've never taken a test to officially prove it, but I have been called a dim bulb a time or two.

Thanks for all of the help and I will post an update when I get the parts installed.
 
Do you have a pic of the transistor? I looked around, didn't see anything promising.
 
From the voltages I can read on the schematic you need a transistor with a Vceo rating of 160V, at least.

Original part numbers for Philips are hard to trace
 
Check with Echowars as he has first hand experience repairing them and loves working on them (hehe). He also can advise on removing he input filter he does not care for and rerouting the ground system that he also did not care for.
 
The filter, IIRC, was on the matching preamp. The grounding is a bleedin' mess on this amp, made worse by cheap-ass film bypass caps on the driver boards. I could not get the amp stable until all were replaced with new Panasonic polypropylene. Hopefully, you won't have the oscillation issues that I saw from the power supply & rectifiers.

As far as power transistors, the On-Semi MJ21194's worked fine...once all the other burnt resistors & transistors were replaced.

No, I can't say I care much for this amp.
 
The filter, IIRC, was on the matching preamp. The grounding is a bleedin' mess on this amp, made worse by cheap-ass film bypass caps on the driver boards. I could not get the amp stable until all were replaced with new Panasonic polypropylene. Hopefully, you won't have the oscillation issues that I saw from the power supply & rectifiers.

As far as power transistors, the On-Semi MJ21194's worked fine...once all the other burnt resistors & transistors were replaced.

No, I can't say I care much for this amp.

This thing is beginning to sound like a total headache.

I still enjoy learning electronics and troubleshooting though and being able to pull two plugs and remove 4 screws to remove the driver boards is kind of nice. Apparently one of the few good things about this beast.

Anyhow, will the MJ21194's replace all of the 16 power transistors? The reason I ask is the unit I have has 3 or 4 different transistor id numbers on them and I would like to replace them all, along with the pre drivers, burnt resistors, and now, the crap caps on the driver board.

Has anyone listened to one of these things?
 
This thing is beginning to sound like a total headache.
It is.
I still enjoy learning electronics and troubleshooting though and being able to pull two plugs and remove 4 screws to remove the driver boards is kind of nice. Apparently one of the few good things about this beast.
The problem is that you cannot troubleshoot the board very well with it removed (at least not with power on, which would be nice). And the amp loses all mechanical integrity with the heatsinks & drivers removed, so moving it results in a swearing session that causes sailors to blush.
Anyhow, will the MJ21194's replace all of the 16 power transistors?
12 output transistors, four drivers. In any event, I've replaced them all with MJ21194G's. If you can find four devices that you trust, then use them for drivers (Q210, Q211) and then use the On-Semi devices for the output transistors.

Looking at my notes, I now recall that a major problem with this amp (besides the cheap caps) is that the Zobel filter (in this case, properly a Boucherot cell, R245, 33 ohm 5W, and C220, the 0.1µf 100V cap) is located at the output side of the parallel resistor/inductor combination. I do not recall the specifics on how I modified the driver board, but the cell was essentially moved from the output side of the inductor to the input side. The cell is there to provide a default load to the amp at high frequency, which it cannot do when on the far side of the coil. At any rate, the 0.1µf 100V cap was replaced with a high-quality Panasonic 0.1µf 400V job, and the 33 ohm 5W wire-wound resistor replaced with a 5.1 ohm 3W Panasonic metal film resistor. This cured more than a few issues with the amp.

Before and after shown below.
 

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I might add that most of my complaints are with the parts choices and the construction, where Philips bought the cheapest crap they could buy to keep construction costs down. When the damn thing is working properly it sounds very good. So it's not the design (in most cases), but the execution.
 
I have the complete set, tuner, preamp and power amp bu mine are in black. Mine have not been repaired, updated or anything per the prior owner. Soundwise, very close to the best I've heard and good enough that when I subbed them in for my Mc system, it became very hard to justify the Mc system price wise. The tuner is excellent, the preamp very good but a bit pricey when new but an absolute bargain today as the power amp was very fairly priced when new but today again, a bargain. Not much is known about or of them.

While the circuit design and layout may no be to everyones liking, the sound is extremely good when in spec or at least in the same spec as my set. Shortcomings are also some of their strengths. Such as the boards are easy to get to as they are plug in but, it also means every few years opening them up to clean the contacts or they can start to get a might noisy. Much of the switching is non-mechanical capacitive switches enploying discrete components principally a dual darlington and cap circuit. With their age, rather than mechanical switch problems the caps can be far enough out of spec that some of the switches do not work; I am having this issue with the preamp power switch, it turns on but not off.

The amp is also modular and designed for being worked on through the use of a set of extension cables to connect the boards back to the plugs. Soundwise, it is very good and mine has none of the stability issues mentioned by EW. I've had 2 ohm systems on it and driven it hard, though not for long periods. Heat buildup was not much more than at long term idle. With 8 ohm speakers, the unit is warm but not so much so as to be uncomfortable to the touch. Detail is very well balanced, not too detailed to appear thin nor smeared to be noticable. Bass is very well presented with almost no coloration but not a thumper. High end does add very slight metal sound to string instruments. I like piano better through it than with either my McIntosh MC250 or 2105. Not just a dynamic difference but better definition and detail. I am not generally a high power amp person, 20 watts is fine for me but with 200 watts per channel, I can appreciate power.

It is one of the better amps I've had on my Bozaks and if I owned it before getting the MC2105 which I got just for them, I'd not have bought the 2105. On run of the mill speakers such as original Large Advents, they actually sound good to me, I'm not a lover of the Advents from the date they were introduced, sorry Advent lovers.

IMHO, the amp and preamp are so very underpriced for their value these are a pair where the investment in a going over by a competent tech that in the end cost them less than many competing sets and still have a good bit of change left over.

There is some cost cutting in the series but more where it will not effect the sound. One thing to be careful of is when picking the amp up, it is not well balanced AND those massive heat sinks will remove mucho skin. It is as within spitting distance weightwise as the MC2105 so not a lightweight. Remember it has one power transformer vs the power transformer and to output autoformers in the Mc. Something like 69lbs vs 67. I also like the meters better. When I run the 2105 the meters just move while at the same output the meters on the Philips are bouncing and dancing to the music. I'm not a meter lover but the Philips is the more impressive of the two.

The amp on the Bozaks vs the 2105 that I love is an improvement. The Bozaks lighten upin presentation, the highs especially seem to open up proving Rudy knew how to desigh an excellent tweeter. The twin 12" woofers work with less effort and present a more convincing bass. The pair, Bozaks and Philips are a pipe organ lover's dream. The 2015 with the Bozaks is no slouch and I'd be prefectly happy with it if not for the Philips.

Yes, I am up on these pieces. Hope it helps to know you have an excellent piece worth getting back up and running.

I will add I respect EW's assessment and I am not an under the hood person so defer to him. Like a car lover, I do not car whether the car has a fixed or independent rear end, leaf or coil springs but, only when I get into it at the end of the driving I like it. One reason why one of the finest cars I owned was a Checker and another a MG 1100.
 
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Hello everybody

I'm working now since 2 days on a 22AH578.
I had it in a very bad condition and I'm rebuilding it. Half of the 355N where dead, same for the drivers, the 148N3 & 158P3, toasted resistors ....

I checked and replaced all the bad transistors, diodes, resistors and when it put it back with only 4 x 355N1 on the left channel ( Q210 + Q211 + Q212 + Q213) the famous R232 located on the driver board burned after 2 seconds and 3 x 355N1 died also.

So I removed the heatsink and I tried to check the DC voltage on the driver board but
every time I powered on the board the R232 burned ...
I suddenly realised that if only R232 burns (and some time also R231 + Q209) and never R230 (+ Q208) that means that the excessive current who destroys R232 is not comming from the +78V but from the ground.
And bingo :banana: the output of the board was shorted to ground by K303 (protection board). One contact was soldered ! Why ? I don't know.

Now I can check the DC voltages on the board and R232 is still alive.

Next step : put the power transistors (MJ15024) back on the heatsink.

To be continued...
 

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Thanks for the great information everyone.

Pat66, I look forward to more of your posts as you progress restoring your AH578. This will be a long term project for me as I will be away from home for a month or two. I'm bringing the service manual with me to study and learn so when I get home I can hopefully start making progress again.

I'm hoping by replacing the cheap caps, output/power transistors, burnt resistor, and pre drivers, I will have an amp that sounds as good as it looks.
 
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