Problems with an old Samson Servo200 amp - question about transformer

[RionZ]

Hey folks... I recently put an old amp back into service - in combination with some speakers I've had lying around for some time.

First off, the amp seemed to work as expected, powering the speakers OK. Then I noticed one of the tweeters seemed to be defective.

In order to rule out the crossover in the speaker, i used the direct speaker output from the amp to test the tweeter directly (amp was on low and only touched to tweeter briefly).

When I did this, the mains input fuse blew on the amp (2.5A glass fuse, just above "kettle" lead input").

Now, even after disconnecting amp from all audio inputs and speaker outputs, every time I switch it on, the fuse blows.

I've partially dismantled the amp, checked everything I can visually and generally poked around as far as my limited technical knowledge will allow...

I'm finally at the point where I suspect the transformer isn't right...

After getting through a whole pack of fuses, I now have it on a protected circuit so can test.

What I observed at first was that the protection kept kicking in accompanied by a rhythmic buzzing from the transformer. Disconnecting the power from the output transistors and heatsink didn't make any difference so began to suspect problem with the main power board. So, I disconnected the transformer's AC outputs (there are two sets of two: 2x31V and 2x20v) from the main power board.

Now the transformer buzzes continuously and the voltages are way over what they are supposed to be (about 100v for the 30v winding and 60v for the 20v winding).

Does this sound like a bad transformer?

Is it likely that touching the speaker wires to a blown tweeter would cause this kind of problem? Turns out it was blown already as i tested it with a multimeter - maybe i should have done that first! - and got infinite resistance, i.e. no coil resistence. Maybe I inadvertently shorted the positive and negative speaker wires somehow (although not via the tweeter itself if the coil was already blown).

Finally... if I do need to replace the transformer, how close do the specs need to be to original? The board states 34vac and 18vac is required. The closest dual output transformer I can find so far is 2x32v + 2x15v @300W. Plate on back of amp states 340W - so hopefully 300W is close enough?

Would really appreciate any advice... I'm loathe to assign the amp to the spare parts pile since it's actually had very little use, but if it's going to be fixed, I need to do it myself or not at all since professional repairs are likely to cost way more than an amp of this age and "quality" is worth.

Cheers

[Folderol]

Sounds like partially short circuit primary winding on the transformer. The tweeter issue is pure coincidence - besides are you sure it's a moving coil type? Did you check on the terminals themselves?

You might get away with the lower rated one, especially as the voltages will be a little lower as well. it will depend on what regulated voltages need to be obtained. Try and get hold of a schematic for it.

P.S.

Windings would be:
20-0-20 = 40 overall
30-0-30 = 60 overall

so if you were measuring across the ends, while still way out, it's not quite as bad as it seems

[RionZ]

I found a schematic: https://www.audioservicemanuals.com/s/s ... schematics

It's a bit fuzzy, but seems to be talking about AC48v and AC16V - just to confuse matters further! on the pcb board itself it says 36VAC and 18VAC where the two sets of wires come in from the transformer.

As for the tweeter - it's a titanium dome tweeter for JBL control 5: https://www.ebay.co.uk/itm/284196549714

Moving coil? I think so.... yes, took a reading across the terminals. The other "good" tweeter gave a reading, forget exactly but probably around 4 ohms.

The 240v primary winding on the transformer gives 3.4ohms, which sounds roughly normal from what little I know, although I don't have any specs for the transformer specifically.

[Folderol]

OK, that's a usable drawing. The only thing I can't seem to trace is a +24V line from the PSU, I can't see what it connects to! Everything else looks like you'd get away with it. Transformer DC resistance doesn't mean much.

You need to check the transformer wasn't a victim! So look for shorts, especially around the output transistors.

[Wonks]

The only thing I can't seem to trace is a +24V line from the PSU, I can't see what it connects to!

It powers the two fans. Page 2, Grid 6A.

[Folderol]

The only thing I can't seem to trace is a +24V line from the PSU, I can't see what it connects to!

It powers the two fans. Page 2, Grid 6A.

Ah! Thanks. Shouldn't really be a problem then, they'd get about 20V instead, so would run a trifle slower.

[Wonks]

That's one supply voltage for them. The voltage on the other side of the fan seems to be modulated to provide 2-stage speed control.

[forumuser931182]

I would also suggest you measure across the output transistors to make sure they haven’t blown - normally the thing that goes with power amps. It would be very sad if your new transformer fried itself straight away.

[RionZ]

I would also suggest you measure across the output transistors to make sure they haven’t blown - normally the thing that goes with power amps. It would be very sad if your new transformer fried itself straight away.

Yes - I don't want to go buying a new transformer without knowing what caused it to blow (if it is indeed blown) in the first place!

Here are the transistors on the heatsink - presumably the ones in question?:

https://drive.google.com/file/d/1uS-RY_TuVMOXnAQDOHehlxdhHA0e3JIB/view?usp=sharing

Here are my readings with multimeter in diode testing mode, hope the diagram makes sense - I forgot to label the direction of reading on the right-hand set, but they're the same as the left-hand ones. also, i mistakenly labeled the right-hand transistor in each set as "tip35c", when in fact on closer inspection they are tip 36C's

https://drive.google.com/file/d/1fzkaPgjjRNWMk2LQ0Z34mXHYquTU5Z1I/view?usp=sharing

Odd that I'm getting readings in both directions on the two smaller centre transistors, although as you can see the readings are all fairly consistent.

[Folderol]

That's pretty normal, and no obvious problems there. To a diode test transistors look like back-to-back diodes. However, what is the reading right across the outside legs?

[Folderol]

Hmm, something screwy here. What you've got doesn't match the drawings, and while most pages say for a range of models, the power stage is specifically marked Servo 600.

I've just done a trawl around the 'net and can't find one for the 200. Yours has a pair of output transistors for each channel, the 600 has six!

[forumuser931182]

http://www.hifiengine.com appears to have a schematic for Samson Servo 200 but not sure if it’s the same as you already have.

[RionZ]

http://www.hifiengine.com appears to have a schematic for Samson Servo 200 but not sure if it’s the same as you already have.

Unfortunately same schematics, nice site though!

[RionZ]

That's pretty normal, and no obvious problems there. To a diode test transistors look like back-to-back diodes. However, what is the reading right across the outside legs?

LEFT group, Left to right -
Tip35C - (left leg negative): 886, (reversed): 554
smaller unmarked - 546 / 610
Tip36C - 600 / 836

RIGHT group:
Tip 35c: 885/582
smaller: 548 / 612
Tip36C: 598 / 867

Sound OK?

[forumuser931182]

Sounds good. Would probably be dead short or complete open circuit.
Can just compare both channels throughout or just a visual for burnt/damaged components.
If you do replace the transformer just turn on with no load briefly and feel the output transistors to make sure one side isn’t getting hot. If all good you can leave on longer and test voltage for no volts on the output.

[Folderol]

Also Check electrolytics - especially the big smoothing ones. They should appear short (or nearly so) at first but with a climbing resistance.

[ef37a]

You have had some bad luck there RionZ but also some excellent advice.

Warning to all! NEVER connect a moving coil tweeter directly to an amplifier to test it! Always have a capacitor in the path, 1 or 2.2uF electrolytic will do for test, must be non-polar or foil of course for normal operation.

Dave.

[RionZ]

Not been easy trying to source a transformer with the right specs.... this is the closest I can find: https://www.aliexpress.com/item/4001175590204.html?gps-id=pcStoreJustForYou&scm=1007.23125.137358.0&scm_id=1007.23125.137358.0&scm-url=1007.23125.137358.0&pvid=32932b47-1324-452e-b1ce-4a63e0ee3400&spm=a2g0o.store_pc_home.smartJustForYou_925926001.0

Description is a bit vague about ampage, in fact quite a fun read with all the dodgy translation!

Even so, I'm unconvinced I wouldn't be wasting my money... Something must have caused the transformer to go wrong in the first place, and like I said the amp has had very little use, plus surely the fuse should have blown BEFORE any damage was caused... so the only other conclusion is that at a rubbish transformer in the first place - and so replacing it with another rubbish transformer seems a bit mad - especially since the transformer will cost me about 70 EUROS including delivery and I could easily pick up a used working amp of the same spec on ebay for that money.

Looks like it will end up a box of spare parts after-all. And maybe I'll save my pennies and get a Crown, Mackie, or similar.

[James Perrett]

If you find an IMG Stageline STA400D or STA800D for a sensible price then snap it up. I've just discovered that they're based on the Hypex Ucd modules which are extremely high quality and you can buy a whole amp for less than the modules cost for the DIY'er. Unfortunately they appear to have been recently discontinued.

Edited to correct the model numbers - JP

[RionZ]

If you find an IMG Stageline ST400 or ST800 for a sensible price then snap it up. I've just discovered that they're based on the Hypex Ucd modules which are extremely high quality and you can buy a whole amp for less than the modules cost for the DIY'er. Unfortunately they appear to have been recently discontinued.

Sounds good - I'll keep an eye out, cheers

[ajay_m]

It's normally quite hard to blow a transformer, other than overheating it. This is a scenario where normally an 'old school' repair-person would connect the unit to a variac and cautiously ramp up the input voltage while monitoring the secondary windings.

One way to test for serious problems is to connect the transformer primary in series with a high wattage incandescent light bulb, such as 100W, which will limit the primary current to a safe value. If the bulb lights up with the secondaries disconnected then, yes, definitely you have a problem such as a shorted turn on the primary.