Randall RH200 Blown Transistors

[nick_watts90]

Hey Everybody,

I have a Randall RH200 Head that died a few years back and I’ve recently pulled it out and tried to fix it but I think I’ve gotten as far as I can with my experience. Initially it had a dead Main CAP and so it would hum and not produce the level of output it should (the hum was also unaffected by volume control) so I replaced the both the Main CAPs, the input Diodes and the transistor pairs (figuring something had to make the CAP fail). Plugged it in and it worked great! Until I turned it up to about half volume, at which point it blew the fuse and shut down, further investigation revealed it blew the transistors as well causing a short and hence the fuse. I’m not sure now where to look for the issue that’s causing this and hoping for some direction.

[nick_watts90]

G2-RH200-B.PDF

This is the circuit diagram I got from Randall and so far I’ve replaced

D9, D10, D11, D12 Diodes
C16, C17 Capacitors
Q8, Q9 Transistors
Q12, Q13, Q16, Q17 Trasistors (Q14 and Q15 aren’t in the particular Amp)

[J M Fahey]

Start by building a lamp bulb current limiter and plug amplifier into it, so you can measure *something* without blowing fuses all the time and further damaging amp.

By measuring voltages here and there inside the amp we´ll find what´s going on , instead of just replacing parts by guessing.

And welcome to the Forum

EDIT: I guess now we can trust the supply caps and diodes.

Driver and power transistors have blown so you´ll have to replace them, but then to avoid re-blowing once more we´ll use the lamp bulb limiter for safety , NOT connect speakers or any load for now until declared safe, NOT inject any signal, and just measure static voltages.
Just as a hunch, and since amp worked for some time, they might have been overbiased, but we´ll check that with actual measurements.

[Jazz P Bass]

Did you purchase the replacements from a reputable supplier?

That 'got to half power and they blew' sounds suspiciously like counterfeit transistors.

[nick_watts90]

Sorry it’s taken awhile to reply.

The Transistors came from EBay as I couldn’t find any of that type in aust.
The Caps where from mouser Australia
The Diodes where from Jaycar. (This may have been a case of nievity on my part but as jaycar didn’t have 6A2 [6amp 200 volts] diodes I went to 6A10 [6amp 1000V] Diodes as I thought the 1000v to be an upper handling limit and shouldnt effect the amount of current flowing through just the direction of flow, don’t know it this could be my issue).

I have ordered more transistors (which take a while to get here from Hong Kong) so when they arrive I’ll throw them in so I can test things, I’m not sure how to build a “Lamp Bulb Current Limiter” is there a guide on here somewhere to building one??

[J M Fahey]

The Transistors came from EBay as I couldn’t find any of that type in aust.
The Caps where from mouser Australia
I have ordered more transistors (which take a while to get here from Hong Kong) so when they arrive I’ll throw them in so I can test things

Sorry but I don´t trust the EBay/HK stuff.
Found 2SC5198 has been unavailable for years, the EBay ones don´t count in my book, so I checked Mouser (which I DO trust) with "parametric search" meaning I search others who "do and stand the same".

Looked for "140V" and "10A", from that list picked those with same casing and 100W or more dissipation.
https://www.mouser.com/Semiconductor...20VCEO%20Max|1

Those meeting specs are viable replacements, no matter the "name" printed on them.

Found this one, which afaik might be the same transistor , only made by another company:
https://www.fairchildsemi.com/datasheets/FJ/FJA4310.pdf

https://au.mouser.com/ProductDetail/...K1CgBSnQshGmQ=

Its complementary (which should replace 2SA1941) is
https://au.mouser.com/ProductDetail/...APkg0nRZNfjHw=

I much prefer those, straight from Mouser, than any "oriental promise".

As I trust Fairchild.

Toshiba? ..... of course ... but they haven´t made them for years, so ....

EDIT: by the way, what about drivers 2SC4793/2SA1837?

They should also be replaced.
Try to find them in Mouser; if "EBay only" we´ll again have to find current replacements which can be trusted.

[nick_watts90]

Thank you for all that info!
I initially replaced 2SC4793 and 2SA1837 also and tested them after the last test fried the transistors, they meter out fine but do you recommend replacing them too?

EDIT: I found in Mouser MJE15032G and MJE15033G which seem to suitable replacements, can you confirm that??

[nick_watts90]

What wattage lamp should I use in the current limiter, the fuse in the amp is a 3A Slow Blow

[J M Fahey]

Thank you for all that info!
I initially replaced 2SC4793 and 2SA1837 also and tested them after the last test fried the transistors, they meter out fine but do you recommend replacing them too?

EDIT: I found in Mouser MJE15032G and MJE15033G which seem to suitable replacements, can you confirm that??

Excellent transistors, should work very well, just reheck and confirm pinout matches (compare datasheets).

What wattage lamp should I use in the current limiter,

60/75W is fine.
Beauty of SS amplifiers is that at idle they eat very little current, so through such a lamp there´s low voltage drop and measurements and tests (without speaker load) are possible.

Once given a clean bill of health, you can connect speaker load and if stable, plug straight in the wall and test LOUD.

[nick_watts90]

Excellent! Thank you!!

I’ve ordered those parts and they should be in tomorrow so I’ll post back when I have them installed and can start testing.

[J M Fahey]

You must always buy from the few trusted suppliers, because the worst about fakes is not that they do not work at all: they meter well (because meters only apply a couple volts to them, easy peasy) , and "work" ... sort of, specially at less than half power and on the bench, but then user plays them into clipping playing live or at rehearsal and *there* they blow, "burning" you as a Tech because you already charged for the repair and gave it a "passed" rating.

here is a comparison between fake and good TDA2050.
The counterfeit one *does* work , and by ear everything looks and sounds normal ... problem is it puts out 13W instead of 20 (hard to tell by ear) , and does not stand the tiniest short, while original does (at least for a few seconds):

[nick_watts90]

Ok guys, so I’ve replaced all the transistors with those above and have a lamp current limiter attached for testing, I can power it on however the lamp lights up to what looks like full brightness. I assume this means there’s a short in the amp module and it’s trying to draw more than it should. Where should I start testing to find the fault?

Update: I disconnected the Transformer from both the pre amp and the amp modules and tested the outputs, (There’s a ground and two actives going to each) and got 55V AC between ground and active on both going to the amp and 16V AC on both going the pre amp. I assume this means the transformer is good.

Update #2: I suspect the fault to be in the power section as when it did work for a short time the amp sounded fine and there was no hum or dissortion in the audio when it was run quiet and only when i turned it up (start adding load) did it fail again. Also when it did fail there was a moment when the hum of death returned and then pop went the fuse and all the transistors with it.

[nick_watts90]

So I’ve been poking around for bit with this amp plugged into the current limter and discovered that transistors Q12,13,16,17 have a thermal plastic between them and the heat sync however the other two don’t Q8 and Q9. However if I removed the screws holding them to the heat sync and push them off the sync slightly (air gap) the amp powers on and the lamp on the current limiter lights up for a second then goes almost completely out. Could it be that simple or is there something else I should test for?

[Enzo]

Yes... they MUST be insulated from a grounded heat sink. The collector of each connects directly to a main power rail, and so if bolted right to a grounded metal heat sink, they will short across the power supplies.

[J M Fahey]

Fully agree and add:

have a thermal plastic between them and the heat sync however the other two don’t Q8 and Q9. However if I removed the screws holding them to the heat sync and push them off the sync slightly (air gap) the amp powers on and the lamp on the current limiter lights up for a second then goes almost completely out. Could it be that simple or is there something else I should test for?

yes, it is that simple, you were shorting transistor case to heatsink so place mica+grease (best) or SilPads.
And before powering check that Collector (middle leg) is NOT in contact with heatsink or ground. No speaker attached.
If/whwn amp powers normally (lamp blinks and all but turns off) test power and speaker out rails: +V , -V , and Speaker out.
I expect +/- 40 to 50V DC and less than 100mV DC respectively.

Then measure voltage across wirewound ceramic emitter resistors R25/26/27/28/29/30 (one meter probe on each end) and scale 200mV DC.
Expect less than 50mV across each of them.
Closer to 5 or 10mV would be fine.

Still no speaker connected.

Once you confirm this, we proceed with some audio.