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
Andreas: I see, of course i wouldnt want anything to blow. I would like to tell you a big thank you for going through this with me step by step. If possible i would like to send you a good austrian wine for your effort :)
I see, of course i wouldnt want anything to blow. I would like to tell you a big thank you for going through this with me step by step. If possible i would like to send you a good austrian wine for your effort :)
Well, I'd have no problems with that -- but let's get the Beomaster running first.
Andreas: I am thinking if there could maybe be a blown fuseable resistor around TP15.
I am thinking if there could maybe be a blown fuseable resistor around TP15.
A fusible resistor? In the 1970s? In a consumer product? I'd say the chances of finding one of those are rather slim...somewhere in the range of 0.00000x%...
Andreas: I also discovered a resistor soldered on the bottom board between TR31 Base and TR30 Emitter on the donor unit. I guess this is a newer updated BM. Maybe this has something to do with the different readings from both units ?
I also discovered a resistor soldered on the bottom board between TR31 Base and TR30 Emitter on the donor unit. I guess this is a newer updated BM. Maybe this has something to do with the different readings from both units ?
No. That would have no effect on the resistance between TP15 and GND. Forget about any updates you might find on the possibly newer donor board. Your board did work before recapping, so the basic design is sound. Don't even think of copying some of those "updates" in newer boards as long as you don't know exactly what they do.The only unknown now is the potential errors that may have occured during the recapping process. As long as you haven't applied full power to the boards we can assume that all components that are untouched are working normally. And we're trying to keep it that way by not doing too many of those "let's see what happens if I do this"-tests.
Since the stand by light never lit up, i hope the whole unit was never powered up in the first place.
Andreas: Since the stand by light never lit up, i hope the whole unit was never powered up in the first place.
Yes, exactly.
I looked at the circuit diagrams again, trying to figure out the safest way to test the boards. The design is rather confusing and there's no way to measure every node on the 15V rail and I fear we may have to do one of those "let's see what happens" tests and just hope for the best.
Here's what I would do if I had that unit on my bench:
1) First one final resistance test on TP15: unplug the tone control board 4 and board 7 and see how much the TP15 to GND resistance increases. I guess it won't change much, but if it does, we know where to look next.
2) If resistance did not change much, I'd be prepared to prep the unit for a first power-on -- but only for the 15V side because I don't want to see smoke from several corners at the same time.- first I'd disconnect the +/-31V supply to the power amp. My reasoning behind it: If there's no power supplied to that end then nothing can blow up if something is wrong. Unfortunately we're also losing the -12V supply, which is required for several functions on other boards but I think that shouldn't be a problem. All we want to see is that 15V on TP15 is available when we bring the unit out of standby and if the 2A fuse F1 doesn't blow there's a good chance that the circuitry on the 15V rail is fine.Looking at the diagram I'd say it's easiest to unplug or desolder pin 8a and 6a from board 8. That's the AC side of the +/-31V supply.Do not simply unplug the relay RL1 because it is needed to activate the 15V on TP15.- I'd keep the tone control board 4 connected to board 2, but the light bulb PCB 7 disconnected in order to get the smallest possible current on this rail. As a result we may be unable to put the unit back into standby with the OFF-sensor button, but that's not a problem because we only see this as a first "smoke"-test for the 15V supply.
3) Now I'd do the first test of the standby voltage on TP16.- clip your voltmeter to TP16 and GND- plug in the power cord- switch on the mains switch and keep you finger on the button. If you see smoke, switch it off quickly.I do not expect any smoke at this stage, but well, everything's possible.If the fuse didn't blow and if you read 15.7V +/-1.5V then the 15V regulator is fine and the standby LED should be on. So, TR27/28/29 and the zener diodes are fine (even though your diode test measurements above don't make much sense).If 15V at TP16 did not come on, we'll have to start measuring.
4) If the voltage on TP16 is fine, we can start the real smoke-test: bringing the unit out of standby.- when doing so, keep your finger on the mains switch, so that you can off the unit if necessary- you should be able to bring the unit out of standby by pressing any of the sensor or preset buttons- monitor the voltmeter to see if the load causes the 15V supply to drop. Normally it shouldn't.- if all looks good, keep the unit running for a minute, just to see whether something overheats- then switch off and unplug the unit
5) connect the illuminator PCB7- clip your multimeter to TP15 and GND (that's the switched 15V rail)- plug in the cord and switch on mains- bring the unit out of standby- now the lights should come on and if all ok, we have successfully finished the first power on test
Once this is done successfully, we talk about the next steps. Good luck!
PS: It goes without saying that the speakers are disconnected for all these initial tests! We don't want to potentially damage perfectly good speakers!
Eureka!
I found the reason for your seemingly low 26 ohms on TP15:
The volume indicator board 6 sits right on the 15V rail and provides a direct path to GND. Those 2 bulbs are spec'ed at 12V/1W, i.e. 144ohms. That is the resistance at operating point. Let's assume a cold resistance of approx 10%, i.e. 2x14 = 28ohms.Now your measurement of 26.x ohms at TP15 makes sense and you don't need to worry about too much smoke.
In light of this new discovery, I'd say you can skip the step of detaching the illumination PCB7.Nevertheless, I'd disconnect +/-31V for now as described above. It's just a matter of "better safe than sorry"!
And now you also know what's broken on your donor board: a bunch of light bulbs, including but not limited to 6IL1 and/or 6IL2.
TP15 without tone control and PCB7 - 26,4 ohm
TP 16 - 14,36v on stand by
TP 16 - 14,35 out of stand by
TP15 + PCB7 - 14,36v out of stand by, volume lights and PCB7 lights are on
the others on PCB5 are not lighting up
Everything seems stable and relay is switching
Wow, that was nerve racking... :D
haha ok i see, yea the bulbs on the donor unit are pretty much gone. With all the knowledge you gave me, i might take an attempt to repair that one aswell. Thats very good news that the readings of my BM finally make sense :))
No speakers were harmed in this process ;)
Cheers
Congratulations! That went better than I thought.It proves that you've done a good job with your recapping works and we can be reasonably safe to assume that all will be ok with the power amp too.
Here's the final steps:- unplug the unit and reconnect the wires to the 31V supply- keep the voltmeter connected to TP15 and GND because this will be our first adjustment- keep the speakers disconnected and turn the volume to minimum- plug in the cord and switch on the mains switch- bring the unit out of standby by selecting an FM preset- if there's any smoke from power amp side turn off the unit. I don't expect to see smoke but the change of capacitors may have changed the idle currents in the output stages -- it's unlikely that a massive change happened but it's possible- if everything looks good adjust TP15 to 15.0V with 2R151 (as per service manual)- now check and adjust the output transistor idle currents on both channels as per service manual- leave the unit on for 10 minutes and check or readjust these idle currents
All should be good now and you can connect your speakers to test all functions. If you're unsure or if you have very expensive speakers, you may want to try this with some low cost test speaker first. If you don't have test speakers, you don't need to worry too much. There is some circuitry in the ouput amp that should protect the speakers from fatal conditions.
YES! It's alive !! The BM is running like a charm. The 12mV at the trimmers are very stable and everything sounds like new. Thank you so much, youre amazing Manfy ! A big thank you also to Sonavor and Martin !
DM me your address :)
Yeah, nice job manfy. -sonavor