Session Rockette 30 Repair Questions

[Rontone]

Hi Guys,

I am currently checking over an old Session solid state amp for someone, I normally do some simple amp repair/restoration for myself on older amps and some guitar effects, never been great with theory and the solid state IC's etc

This old 1980's Session is in pretty much original condition it seems, it was working fine for the guy but has started blowing the internal fuses, I had a check inside and there was a pair of 3.15A fuses, but the PCB is marked 2.5A so I will be getting some 2.5's to put back in,

I've seen this before in Hi-fi amps and its normally the output transistors going down and associated parts most of the time, I have checked the mains transformer, seems fine, giving out an exact 28v up to the fuses,

Next I was going to check the rectifier just in case, can't imagine its popped, just need to lift the board to have a test, but I think its unlikely, but they do appear to be rated for 2A....

The main filter caps will be changed as usual, I have some nice Panasonic 2200uf's to put in,

Otherwise all I can imagine is that the output transistor pair is shorting? They are BDV64B + BDV65B so are still available,

The other transistors are all MPSA92 and BC560's, also some TL072 IC's, but not sure if they would/could make fuses blow if faulty?

I have a transistor tester but not the tiny clamping leads to test them on the board, so could rig up some wires to test the output trans, or waste some time removing them and sticking them in the tester...

Which would be the most likely wrong here? I'm thinking order some output transistors and stick em in with fresh caps first,

[PRR]

Welcome.

Some of us have never seen a Session. It is good to link a description and a schematic

If it is melting 3A fuses, something must get HOT. Like 100 Watts in a box that should not shed 30 Watts. Is that a clue?

And yes, the small parts DO matter. If the outputs go short, the BD139 will try to pull the load through the base junction and bust a gut. Replace the big transistor and the now-sick small transistor will fry it.

[shooter]

while you're waiting on parts
disconnect the HT from PA section,
disconnect the signal from Pre to PA ,
use an "OPEN" dummy jack in FX return
 jack in some music,
 connect a small cheap speaker to FX send n listen

[Rontone]

Thank you for the replies,

Firstly yes, forgetting the schematic was a classic mistake! Also with this particular amp, page 3 of the pdf posted above is not applicable to this amp, I was mistaken at first when I said MPSA92, there seems to be a MOSFET version of the output stage as well as the version that I have here with BDV type transistors, and BC types

Secondly I'm not great with theory at all, I have worked and trained at an amplifier workshop and built many complete amps but it was just component insertion, following plans like an amp kit and PCB building really, so I got a lot of practise soldering and rectifying my mistakes, but none on theory at all (only put one power cap backwards while I was there in the first month or so, made the quality control guy jump when testing! since then I triple check every bit of soldering)

If it is melting 3A fuses, something must get HOT. Like 100 Watts in a box that should not shed 30 Watts. Is that a clue?

And yes, the small parts DO matter. If the outputs go short, the BD139 will try to pull the load through the base junction and bust a gut. Replace the big transistor and the now-sick small transistor will fry it.

Do you mean the large resistors?

Its built well inside, I have lifted the PCB to check some components, I have attached pictures and I noticed straight away its very scratched around the transistor area and the solder has perhaps already been worked on as the flux is very messy and the solder joints quite 'blobby' compared to the rest, incidentally on all the transistors so perhaps they all got very hot,

Maybe because R33 which on the schematic is a 1R5 resistor paired with R29 coming out of the transistors to the output...

It seems to be replaced with a different brand of resistor, soldered perhaps onto the old leg, and its marked R33 so I think someone may have read the R33 on the board literally and put a 33R in there??? It measures half on ohm so is perhaps burnt inside...

while you're waiting on parts
disconnect the HT from PA section,
disconnect the signal from Pre to PA ,
use an "OPEN" dummy jack in FX return
 jack in some music,
 connect a small cheap speaker to FX send n listen

That is a great idea to check the preamp section, I am having a look to trace where the power attaches and see if I can lift a few components to stop it,

The signal coming back from the FX Send/Return board is wired separately so can detach that easily, I have a few small radio and tape recorder speakers I could hook up,

Open Dummy jack, is that just to engage the switching jack? Do I just use a spare jack with unsoldered connections?


Luckily all transistors in total would cost less than £20 with a pair of filter caps too, so should be cheap to replace everything, any other parts that test bad, I have a good stock of smaller resistors and caps.

I will investigate further tonight, there is one odd looking bit of soldering around the BD139 which may be bridging a gap

[PRR]

When you get tired of replacing parts time after time, following in the footsteps of those who went before you, get an LM3886 power amp kit with heatsink and replace that sad power amp.

[Rontone]

I like the way you put that, it is a shame as it seemed at first it might be a simple fix but this will be time consuming to restore, very strange with that odd resistor in there

Those kits are cheap on eBay, tempting to jam one in there, I will see what he wants to do about it, it was a decent sounding amp before it went down

[Rontone]

I had a bit of time free to have another closer look at this, make some small leads for my tester and try to learn more

Its a cheapy multi tester from eBay, but has been ok so far for simple tasks with fx pedals

The BC548 is hFE= 141, Vf= 712mV
The BC560 pair are hFE= 189/198, Vf= 672/668mV
The BD139  is hFE=53, Vf= 653mV,

So after looking at the datasheets, those figures for hFE seem in the right range

But the BDV-64B and -65B power transistors are both measuring as diodes?

Uf= 552mV and C=404pf, I have repeated the tests on both and both give a similar reading every time, I will try test another way as this tester has been strange in the past with a few parts



First am I right in thinking this is a push pull pair on the output, and that R29 and R33 on the schematic act as a kind of voltage divider before the output?

If one side of those resistors is the wrong value and/or burnt and cuts out, the balance will change between the pair and make much more work for one side of output section,

Sorry my terminology and explanations are not quite there yet

[PRR]

...am I right in thinking this is a push pull pair

Yes.

...that R29 and R33 on the schematic act as a kind of voltage divider before the output?

No.

power transistors are both measuring as diodes?

Any BJT transistor is diodes.

These particular parts are Darlington transistors which "have more diodes" which may be confusing your tester.

Transistor amps are typically very synergistic. You can't diagnose them part-by-part, because each part depends on other parts. If we were all younger, we'd try to learn how to measure "all" the voltages and deduce the problem. This is very good brain-training. But it is too hot, muggy, and smoky here to begin to think hard.

[Rontone]

Judging by the amount of posts you have, you do spend a huge amount of your own time basically doing the work/maths for me and countless others

I see you are in Maine, we havn't been getting much TV news on those wildfires here in the UK and it mostly shows the damage in California only, I had no idea the smoke travels that far east and causes so much disruption, I imagined maybe the next state eastwards a few hundred miles but 3000miles, and then drifting from Canada fires also....

Hope you guys are alright out there

[PRR]

California is worse this year than 1959. Plumes are out to Greenland some days. It's not like that here, and we do heat with wood in winter, but this is real unusual atmosphere.

[shooter]

I much prefer the sunsets created by a sandstorm than a forest fire


if you haven't ventured down the SS forum page, here's a couple links to my dive down the rabbit hole.


Sans Tubes (el34world.com)
 Upgrade Dcavers (el34world.com)

[Rontone]

Hi Guys,

Been a while, had to put this away and get on with a queue of other work,

Now cleaned it all up, nearly got parts all here, just got to choose the right resistor power rating, they can be seen in the picture above next to the output transistors,

They are 18mm long so I imagine at least 2W, I was going to buy some 3W to make sure, but could they be higher wattage in area of the circuit?

[Rontone]

Hello all,

I got some 3.5w resistors for this, they are a bit smaller than the originals as seen in the picture,

Do you think a 3.5w is suitable for R29 and R33?



Also, the BC183 was touching the heatsink with a little blob of compound, it looked deliberate, [can be seen in the last picture]

Should I put it back that way?

[shooter]

3.5w is suitable for R29 and R33?

doing a little reading looks like the transistor can handle 10A max, 5A typical.  The resistors will "feel" all that current
hard to determine from the original one the watts.  If it's mine, i'd try the 3.5w, put a meter across the R then power up, no signal, measure V drop and do the math.


as long as the driver is plastic, i'd dab some heat sink compound and make it close

[Rontone]

Thanks for that, nearly everything back together now to test it! Woohoo its been sat around here ages

[Rontone]

Its turning on, fuses not blowing so that's great, thanks for all the help

I can hear the amp working in the background but some very loud hum that sounds a bit like motorboating over the top so these caps are going I think, its been sat around years before it was last used and then years before that

Will change those next and see what happens

[Rontone]

Would there be any advantage to upping the 220uf caps value in the Main PSU?

I have some 330uf caps here in the right voltage, saves me ordering more 220's

[shooter]

no real advantage, no real down-side either, solder away

[Rontone]

Capacitors turned up, fitted,

It turned on, nice and quiet now,appeared fine, but the transistor area got warm after about a minute, then the 500mA Primary/Mains In side fuse went... 

So, guess its still a part going down in the circuit, or I've done a booboo somewhere

If 500mA was reached on the 240v side, shouldn't the 24v secondary side have more than 2.5A at that moment, if it was a problem on the circuit side and a part shorted wouldn't the 2.5A fuses on the board go first?

Maybe it is a transformer problem, I only checked the secondary output, not the primary for problems

I will check the transistors yet again [I need a nice tester to make this quicker!!!]

[shooter]

 
can't tell you how many fuses I've found saved by parts that fail!
IF!!! you understand electronics repair pretty well you can "simulate" a load on the PS by using an appropriate sized resistor as a
dummy load.
if there's a thermal shorting issue, that should kill a fuse rather than brand new parts if the problem is with the PS.

[PRR]

...the BC183 was touching the heatsink with a little blob of compound, it looked deliberate, ...Should I put it back that way?

YES!!!

...the transistor area got warm after about a minute, then the 500mA Primary/Mains In side fuse went... 

The bias transistor MUST sense the heat in the power devices!!!!! Otherwise they warm-up, and warm-up, and warm-up, until they melt.

But I doubt it is that simple. Put a jumper to SHORT C43 (actually to short the bias transistor, but C43 is easier to short). If all else is well, this will work "fine", stone-cold, only a little (maybe a lot) "hoarse" on small sounds.

If that is still sick I fear the next step is to replace those 1.5 Ohm resistors with 1K resistors. That won't pull a speaker, but will lame the beast so it can't burn itself up and you can take some measurements.

That's not a stellar bit of audio gear, and even in a Supply Chain Crisis you are at the point where you should just go buy a chip. Do all this better and with protection.

[Rontone]

So, thanks for helping with this, its been a journey, a small circular journey, but I have learnt a lot,

PRR, your suggestion of a LM3886 module, is it a case of simply attaching it to the preamp outputs, removing the power amp input area and power connections,

Looking online some of the premade modules or boards run on 20-28v so that should be fine, I have some nice quality heatsinks removed from various hifi units and PCs to fit in there

https://www.ebay.co.uk/itm/275029054129 seen this and just asking the seller if he has any more info, otherwise there are some PCB kits available

Think its time to get this poor thing going properly and release it back into the wild