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
As I with stupidity shorted my 901, not only one but 2 times now, I decided to change all that could be bad.
Diodes 2D19 to 26, D27
Darlingtons: 2TR21/22, 2TR30/31, now TIP 121, TIP 126
TR 33= BD236, TR32= BC547B
Trimmers: 2R137, 2R186, 2R173
All electrolytic caps are new, also the big 3000 uF, now 3300 uF.
Connected over Variac and Dim Bulb tester, and as described in the manual, tape on.
Slow increased the current to 120V, DBT no light, adjusted 32V supply, easy adjustable.
Idle current trimmers was turned all way down, tried adjusting R/L channels to 10 mV, but increasing from "0" going slowly up, passing + - 4 mV the current accelerated by itself, first slowly up to 6-7 mV and then faster up to and beyond 10 mV, the same in both channels. Keeping the adjustment below 4 mV it stayed stable.
I left the adjustment at 3.8 mV, selected FM, tuning in a station no problem, tuning light works, stereo light on, connected a speaker to the right channel, had to turn up the volume to "8" (out of 12) to hear something, and muffled sound, normally I could listen very clearly at volume level 3-4.
After 10 min playing the DBT started flickering, 2TR30/ 31 and 2TR33 got hot, turned off.
As I have the same failure in both channels, I suppose the the failure must be before the end stages, but where do I start to look ????
Help appreciated
Collecting Vintage B&O is not a hobby, its a lifestyle.
Maybe a stupid thought, but the tip's replaces the original BD697/BD698?
Do they have the same pin out? Could it be that the Tip's has BCE where the BD has ECB? Just a thought.
But readjusting after 10 minutes isn't possible? What idle current do you have?
No contact between the tip's and the heatsink?
Beobuddy: But readjusting after 10 minutes isn't possible? What idle current do you have? No contact between the tip's and the heatsink?
No heat sink mounted, without speaker connected and Tape selected, volume on "0", the Idle current cannot be adjusted, if adjusting to over 4 mV, the current goes up to over 10 mV, by itself, and I have to turn back the trimmer to below 4 mv to keep it stable, as soon as I adjust to over 4 mV it starts going up again.
The TIP 121/126 are BCE as the BD 697/698, and that is clearly marked on the PCB.
How about the Service Tips on page 7-1 in the service manual about the measurements of the build transistors? Do you get comparable values?
Ralph-Marcus
Without heatsink, the transistors will cause a thermal runaway.A transistors amplification increases with its temperature. Therefore, a smaller transistor sitting earlier in the circuitmonitors the temperature on the cooling fin and regulates the bias.The BM901 has it sitting in a blob of white heat paste with a thermal connection directly to thecollector pin (base material) of one of the output stage transistors.
Martin
Dillen:The BM901 has it sitting in a blob of white heat paste with a thermal connection directly to thecollector pin (base material) of one of the output stage transistors.
I hope that is the reason, will put everything properly in place with the heat paste,
It is indeed more wisely to mount the Tip's on their right place on the heatsink. Make sure that you use proper mica sheets. You have to isolate the Tip's from the heatsink.
As Dillen mentiones, the casing of the Tip's are connected with its collector. So if you don't isolate properly, it will be one big shortcircuit, which will damage more then you ever wished for...
FYI and others.
Darlingtons are special transistors. Where normal (power)transistors have a gain of about 30-70, darlingtons have a gain (hfe in the datasheet) of at least several hundreds as Darlingtons can be seen as/are in fact 2 transistors in series. This means that with the slightest change of current to its base, it will result in a hugh change of current in the collector with the accompying power dissipation. To achieve a stable condition, transistor are mounted to a heatsink.
The only reason for setting the idle current in a cold or warm state/environment, is to prevent disstortion. Part of the audiosignal, the upper/positive part is done by the positive section around TR 22/30, where the lower/negative part is handled by the negative part around TR23/31.
So, the interaction between the upper and the lower half is set by the idle current, They have to work flawless together.
You can compare it with a relay race. Each runner already runs slowly before taking the stick and runs its part.
I learn everyday here on the forum, did not realize the importance of the cooling system, will come back soon with the result, after placing everything correctly.
I learnt from a similar situation IIRC on a BM5000. I had to replace the whole output section, including the transistors. To cut a long story short, I used driver transistors with too high a hFe (C-grade instead of B-grade), and that caused a thermal runaway.
As soon as I put in driver transistors with lower hFe, everything ran OK.
I mention this, because I see that you also replaced the transistors. Try using new B-grade or even A-grade BC547's.
Did you replace the bias poti's?
Do not substitute SPS5418 with MPSA14, hFe is too high. MPSA13 works fine.