ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
This Beomaster suffered an output stage event at some point resulting in the loss of one channel.
I will replace any components that came near the line of fire, even though only two transistors (TR59 and TR63) test as failed. R577 (100 ohm) was toast and vaporised the trace where it connected. You can see the crater to one side of the preset.
My question (hello, Martin) is what to replace the complementary pair TR59 and TR62 with? The originals are BC310 and BC311. BC310 is available for a price, but BC311 is even more expensive.
Can I use BC141-16 and BC161-16 instead (cheap at Farnell)?
Finally, will any version of 2N3055 do?
Any other guidance happily received as this is my first experience of one of these fine units.
Krolroger: Can I use BC141-16 and BC161-16 instead (cheap at Farnell)? Finally, will any version of 2N3055 do?
BC141 / BC161 will be fine.And -16 versions will be fine here.2N3055 - I would look for a good pair of vintage Motorola or RCA - like the originals.Too many of the newer types are counterfeit and nothing but a genuine waste of money and time. I would prefer a nice pair of (even used) vintage transistors over newly produced ones any day.
The idle current trimmer will be bad.Check (or even better replace) all semiconductors and all resistors from the idle current trimmer forward - including thetrimmers series resistors etc. down to - and including TR61.
Martin
Hi!
A blown output if often the sign of failing components around the transistors, I would suggest you check them all.
- I would replace the BC310 & 311 with 2N5320 and 2N5322 which are still commonly available. Just be careful to meet the pinouts.
- Change the trimmers
- Change the electrolytics, I see one leaking on your picture.
- Match new transistors pairs within 10%
- Change the emitter resistors
- Basically, check everything in the output stage!
Good luck!
Thanks, Martin and Jean.
Should replacement resistors be rated at 1 watt or 2 watts?
Jean, when you say replace transistor pairs within 10%, are you referring to hFE? How critical is this?
Many thanks,
Replace like for like.1/4W resistors with 1/4W.2W resistors with 2W
Don't use metal oxide resistors in output stages. They don't handle spikes well and they can in some cases add to the risk of self-oscillation.Use what B&O used, carbon resistors for 1/4W and ceramic wirewound for the output stage emitter resistors.
The hFe is not particularly critical. As suggested, 10% will be fine - or just get two from the same production batch as theywill usually be closer than that already.The emitter resistors will compensate a great deal anyways, and there is a series capacitor in theoutput lead so even a slight DC offset won't ruin the pleasure.
Martin,
Thanks for this. No indication of resistor power rating is given in the parts list - I guess a trained person would just know - except for emitter resistors which are rated at 1W on the circuit diagram.
Looking at their size, am I safe to assume the vast majority of resistors in the output stage are 0.5W?
Finally, your advice about vintage RCA or Motorola 2N3055 is well taken; can I use the 'H' version as there seem to be more of these around?
Thanks again,
Simon
H-versions will be fine.
I now have a pair of RCA 2N3055H from 1984. They are showing hFE of 30 and 45 on my Peak tester.
Is this too far out to be comfortable?
Regards,
That's a bit far.Can you tell, if they are from the same production batch?
I'm on the trail of something a bit closer.
I did ask for the same production batch, not just from the same year!
I've now mounted the two (vintage) RCA 2N3055H (TR63 and TR64) with hFe within spitting distance of each other. The two I removed had both shorted.
TR64 has a thermocouple device attached to it. Its resistance drops as heat is applied.
My question is how to mount it to the transistor (with tape?) and how to test it - or don't they tend to fail? I assume these transistors get pretty hot.
You have to mount your transistors as they were originally. Heat transfer is critical don't mess with that part!
This is turning into something quite epic, but I'm getting there.
The TO-39 type transistors (BC310/311/312) have red fibre washers mounted underneath them seated around two of the three legs.
Do these serve any purpose? Just curious.
Presumably it's a good idea to demount TR45 and TR57 and renew the thermal grease.
The ring lifts the transistor up from the board to allow for both a little airflow under it and a little thermic insulation so that thetransistor, which runs quite warm, doesn't burn the board material, - while still supporting it.Without support, the quite heavy cooling fin on the transistor would cause its solder joints to crack and/or itssolder pads and copper traces to break away from the board.Fit them again.
Fresh thermal paste, yes.Use the good old white stuff.The modern grey stuff sold for CPUs etc. dries and hardens in just a couple of months. Perhaps that's justfine for a CPU (though I doubt it), but it's no good in amplifiers.Some people like to use the silicone "wafers". I don't .
Martin, thanks.
Just a side note: should I have used thermal paste on the 'new' 2N3055H transistors and the mica washer? The dead Motorolas I removed (original, I think) appear to have been mounted without.
I like to add some, even if B&O chose not to.To both sides of the Mica.
Every orange ROE cap I've removed so far has been well out of spec, so I need to replace those on the pre-amp board.
To save a wait for the postman, can I use 220uF at 35v rather than at 16v and 10v?
While it's out, is there anything else on this board that gives trouble?
I will put some deoxit on the user adjustable trimmers.
A 35V cap would work, but not for as long as a 16V or 25V, because it won't be excited.The trimmers will need replacing, deoxit will remove oxidation, it will not bring back material already eaten.How are the tantals doing?...A convenient servicekit exists:https://www.bonanza.com/listings/Beomaster-3000-3000-2-Beocenter-3500-Service-kit-capacitors-trimmers-lamps-/400100617?st_id=66280586
Thanks, Martin, noted.
I want to get my left channel functioning before I dive in and replace tants and trimmers. But a service kit is on my shopping list, all being well.
As a starting point, this unit was pretty untidy; looks like it might have done time in a damp shed.
These are the casualties so far:
Both TR63 and 64 were shorted as was TR59. Following Martin's advice, I've replaced all transistors and diodes down to TR61, using matched pairs as far as possible.
All electrolytic caps were way out of spec. Many resistors had drifted high but all the 1K8 were way off. I used 25 turn 250R presets for the idle current trimmers.
I needed to trim the lower mounting screw of TR63. I should have applied thermal paste to TR57 prior to mounting as it's almost impossible to do a proper job after the event.
Lamps replaced and sliders squirted with switch cleaner.
Nothing too scary on start up.
Adjusted both channels to 100mA.
Unit is now functional and sounding good but there is a problem with the left hand mono switch as the right channel cuts out unless the switch is depressed by 0.5mm.
So I guess that involves removal of the front fascia...
I'd be grateful if someone could post a picture of the dial cord/tuning slider connection as mine is broken.
Taking the front fascia off entirely is not for the faint hearted. Luckily it proved unnecessary.
Reflowing the mono switch connections to the board and a bit of switch cleaner seems to have restored normal service.
Would be grateful for a pic of the dial cord arrangement, anyone?
Not sure if this is what your looking for, the slider tab is held in place by the blob of glue you can see on the core where I have removed mine to take the slider and front bar off.
Another angle......
and one more....
Thanks, Craig, I assumed something was missing.
I have a blob of glue and a single prong on the back of the slider without even a slot to engage with the cord..
By the way, I did the tuning voltage and the two potis are pretty bad.
Has anyone changed out 129 and 132? I couldn't see an obvious (as in easy) way to do it.
Any brilliant fixes out there?
I had the same problem, added some epoxy to the prong to make it stronger, then after assembling epoxy the string to the prong, has worked for some 4-5 years now
Collecting Vintage B&O is not a hobby, its a lifestyle.
Thanks, Soren. I think I will use something a bit softer than epoxy so that it can be disassembled in future.
Has anyone made up new sliders for these? 4mm Plexiglass and a couple of hours with a razor saw should see to it.
I checked a few of the tants on the pre-amplifier board and they seem to be spot on so I've not bothered changing any of them.
The presets were poor so I changed them for 50K Piher.
Curious thing is that now the input from tape (CD) and phono 1 is quieter than it was previously. I had to turn the presets to their maximum position and even then I can't match the volume level from FM which is supposed to be the reference.
The difference with the FM level amounts to one division on the volume scale, I would say.
Just wondering if I should dive back in and change the tants?
I wouldn't change your tantal yet because they were there before the lower volume observation.
Are you sure you fitted 50K trimmers? Are the new Panasonic FC fitted onto the quieter channel?
What are the values?
If it was my amp,
I would have used film capacitors for both 4.7uF and 2.2uF (if they are 2.2 not 22)
Hi Jean,
The Pana FC's replaced 4.7, 22 and 220uF orange ROE's which were off by a mile.
The volume differential is only observed after installation of the Piher presets. I checked all of them and they are in the range 45K to 52K which is +/- 10%.
The differential is only between FM and Phono 1 and Tape, not between L and R. No idea about Phono 2 because I don't have a source with a pre-amp
Please check again that 22uF.
I see 2uF on the schematic which should be the 2.2uF original orange capacitor.
After the set warmed up, I was getting some distortion on the left channel with FM or any of the presets depressed.
Being an adherent of the poke-it-with-plastic school of diagnosis, and finding some connectivity issues, I thought that the problem probably originated either in the flexible pcb that connects the tuning presets with their corresponding buttons, or in the tuning voltage presets that I had previously thought of replacing but hadn't.
Having desoldered the flex pcb, I replaced the two presets with 50K (129) and 10K (132) Pihers, and remade all of the solder joints where the switch pins attach to their board in view of the effects over time of a spring loaded switch action.
The flex pcb went back on easily enough and I now have distortion free reception.
(Hi Jean, I rechecked caps and all were correct).
I haven't replaced the coupling caps or the main filter cap because they appear to be within spec, and there is no transformer hum, heat or strange noises. I suppose I should have checked mains ripple on my very basic oscilloscope, but that's for another month. I wouldn't replace them on spec because I don't like unnecessary electrical waste.
Setting the tuning voltage to 4.5v.
All done, and crowned with one of Martin's preset covers...
Nice job and great pictures!
So what was the problem with the volume differential between inputs after changing the pots?
Hi Jean, thanks.
To be honest, I'm not sure. The differential is slight and from what I can gather, volume equalisation between FM and Phono 1 inputs is not always perfect.
However, at some point in will be worth going back into the input/output board and changing out all the small value caps.