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fixing a Rel Strata III subwoofer amp

geoturbo

geoturbo

tinkerer w/o highly evolved auditory stimulus cpu
I just got this Rel Strata III subwoofer with a non working 100W amp, year of built is around early 2000; I will be trying to share/get suggestions how to fix this amp or failing miserably :)

The amp board appears heated in multiple spots:





I found some very valuable information googling the internet about other similar issues:
https://www.diyaudio.com/forums/solid-state/198466-own-rel-sorm-iii-sub.html

Some components in the burnt areas appear to be those suggested to be swapped:

Change Q9 & Q10 (BF469, NPN) to MJE340
Change Q4 & Q5 (BF470, PNP) to MJE350
Change R8 (12k, 2 Watt) whilst measuring ok to 15K, 3 Watt or better, 5%
Change R6 (10k, smd, 1/8 Watt, drifted to 8400ohm!) to 22K, 1%

plus some other to be checked in values. Of these, smd R1 resistor appears to have drifted.
and will be substituting C2+C3 psu smoothing caps (6800uF 63V per cap to 10000uF 63V) and few electrolytics (C5, C14, C16 from 100uF 25V to 100uF 35V or better).

so I just ordered the parts mentioned and will try a repair in house.

I would also like to ask whether anyone has made some modifications to the output mosfets with good experience.
The amp is having a single set of BUZ900 + BUZ905 output mosfets which are rated at 125W 160V, however due to heating conditions in standard form I reckon it is not very safe to upgrade the mosfets to BUZ900D/901D + BUZ905D/906D rated at 250W or add a second pair of BUZ900 + BUZ905 in parallel..
 
Given the lack of ventilation in that application + signs of overheating I wouldn't tempt fate looking for more power. My Stratas are barely ticking over 90% of the time. If one were to fail I might consider substiuting a simple ClassD amp, as I think they run cooler? Someone will correct me if I'm wrong about that.
 
Assuming class ab as there isn't masses of heat sinking then yes class d will run cooler. Be careful with mosfets. Static sensitive.
 
Thanks for your interventions, Gents.

Yep might be an ab class amp but not sure here how to tell - nor finding it in the specs.
Surely it is not a class a, but in any case lack of ventilation looks like an issue here..
might try to measure the internal air temperature whenever at play.

If too hot I may decide to drill some holes at the bottom (for fresh air in) and some holes at the top (for hot air out) as a passive circulation solution.. or using an active solution..

not sure I want to invest too much time though..

but also forcing hot air out might be interesting too to have proper room set for a pair of BUZ900/905 mosfets added.. let's see..

As for the static thing, does this has to do with the handling/touching of mosfet components when off the board and risk of ruining them? thanks
 
I would be wary about changing the output devices, unless you have a scope and some design experience. Bigger devices, or multiple pairs, will have more capacitance, and could easily lead to instability.

If you want to put some thermal protection in, put a simple thermal trip on the heastsink, and wire it into the mains feed. If it gets too hot, it will pull the power off, until it cools down enough to reset. Only costs a few £/$.
 
Yes, not dealt with MOSFETs personally but because the gate is insulated, static can build up and puncture the insulation to the channel underneath. You can earth yourself to the case you are working on so you are at the same potential as the circuit. This is what I've done in the past when I made a circuit with a CMOS chip in it. Just make sure it isn't connected to the mains!
 
What a waste of lateral mosfets.
In my opinion a Sub could get away with a class B vertical mosfet or bipolar. Negative feed back can take care of crossover distortion at 100 Hz no problems

The high Rds of laterals must be a source of heat here.
 
davidsrsb said:
What a waste of lateral mosfets.
In my opinion a Sub could get away with a class B vertical mosfet or bipolar. Negative feed back can take care of crossover distortion at 100 Hz no problems

The high Rds of laterals must be a source of heat here.
Click to expand...

It does seem an odd choice. They are expensive for a start. Their performance is not needed, and yes they will get hotter than many other solutions.
 
Something like the SKA vertical common source design would be a good choice, with quiescent turned way down. Basically rail to rail and very efficient.
 
Parts have arrived and I have found some time to give a shoot to try to fix it.. but it has not been a success.

I had some difficulties as the original pcb was severely damaged by heat and I needed to reroute some resistors.

I took a deep breath and fired it up with no signal, the sub is actually misbehaving as the cone moves very slowly to the outside and remains still.
I measured and found about +0,2VDC on the output, letting it on for some time the woofer's coil heats up.
I tried to put some signal in and it sings, I just think the dc on the output is not right..
I will double check resistor values as time will allow (I am on vacation right now), but it will be very hard to do it with no schematic.

Any hint or idea where to look for? Thanks
 
Looking at the comments on the Diyaudio thread, although the photos are all missing now, the gist is that the REL designer did not realize that SMD medium power transistors need substantial copper area to reach their power rating. The original Hitachi circuit, as used by Maplin etc, used THT transistors
 
You can just solder thick capacitor legs to the transistor pads, vertically. 10mm on each pin does the job. Fiddly though.
 
Thank you @davidsrsb.

Googling for Hitachi Ldmos it seems like the circuit is very similar to the Strata iii one.. it seems that one potentiometer is responsible for the output bias (or should it be better to call it quiescent current?).

I think it shall be possible to spot which one it is with this info..
Will keep you all updated, thanks.

The original smd driver transistors are now substituted with through hole MJE340/350 plus some heatsinks added.. will post a picture soon.
 
I am interested to know the dc resistance of the REL driver, the Hitachi circuit was really optimized for 8 Ohms. Sub woofers are often 4 Ohms or less
 
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