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
Thanks Peter......it looks good from a few feet away!
I'm home early most Fridays as you know, just email me early in whichever week you choose to drop in......
Craig
Main power supply capacitors........
I don't like changing these out as it requires a lot of heat to flow the solder, I'm always afraid of damaging the new capacitors. And there's very little material on the lugs of the new capacitors for soldering to.
Whilst pottering about with the power supply capacitors I decided to pull out the two glass fuses and test them, pulled the 2A and all was well, pulled the 315mA and unfortunately left one metal end cap in the fuse holder as I removed the glass enclosure.....this resulted in a long thread of fuse wire stretched between my hand and the fuse holder! I have bundled the wire back into the enclosure for now, I'm guessing I will need two replacement fuses...one 2A quick blow and one 315mA quick blow, I cant locate these on the circuit diagram?
The two large filter capacitors are not original, they look newer than the Beomaster.As you also mentioned, you cannot leave the fuse like that. It'll act as a short.
Martin
Your correct, the picture is of the new capacitors I have just installed....this is a shot of the original capacitors ;¬)
Moving right to left changing pots and caps
Well, the ones you changed are the ones I didn't touch. I just replaced the idle current ones... that you let alone!
Jacques
Jacques......don't worry, I am awaiting delivery of 25 turn Bourne pots for the idle current trimmers, according to Rudy they are less prone to drift with temperature
Look what turned up in the post this morning ;¬)
Good! I'm sure you'll love this Beomaster, it just sounds great on about any speaker.
Finished the recap and now reassembling the components, and the 315mA fuse I pulled apart.....
Done as much as I can on the front panel, it looks like the ding in the front will be a permanent feature sadly
Powered up and adjusted the quiet current trimmers to the prescribed 5.5mv without problems, which was encouraging.....however the optimism didn't last!
Despite both quiet current mV setting up fine when I connected a pair of speakers to the outputs and an Ipad playing Spotify to the Tape2 input the sound coming out of the left channel is horrendous!....the right channel plays really sweetly, the left sounds muffled and reverby and quite horrible, it responds to the volume control but clearly something is adrift, swapping the speakers round made no difference, connecting the Ipad into Tape 1 makes no difference.........I will need to look beyond the capacitors to solve this new development, anyone who has experienced this is welcome to pitch in of course........
Update on this......the volume control actually has no effect on the left chanel, seems like the signal from the Ipad is passing straight through to the speakers, it also appears that the sound quality improves as the Ipad volume is lowered to the point of almost vanishing...so its hard to say for sure.
This could be a left side amp power transistor replacement? Martin......if so I know the drill ;¬)
Is FM clean in both channels?
Martin.....no the left Chanel sounds like a bag full of hammers on FM too, the volume slider has no effect on the left Chanel on FM either
and thanks for the reply ;-)
Following further investigations into this problem with my scope, comparing readings at both left and right channels with 4khz injected, I have placed an order for all the power amp transistors previously recommended by Martin for a previous restoration........this was not so much as a result of any fault I discovered but driven by a slip with the scope probe which caused a brief flash followed by the overload relay cutting in............such is life!
started the replacement of the power amplifier transistors, changed out the following so far:
TR116,TR117,TR119,TR220
0IC100,101,102 & 103.....i'm awaiting delivery of OTR100 and TR115.
I've changed out R177 trimmer and both R178/R179 wirewound resistors, both 330ohm R171 & R172 resistors
And also done D105...D104 IS Todays exercise along with R276...then I'm waiting on the postman, however in the meantime I have treated myself to a magnifying lamp.......and with this I can see exactly where I created the short with my scope probe....I hit the top of R162 which is connected to the base of TR114.......I suspect I should be changing out some other componants as a result of this, or at least testing them, will start with R162.
If you grounded the base of TR114 in the DC offset circuit, the following components would be in questionfor replacement or at least a good check:D100D101D102D103TR112TR113TR114TR115 (... and further outwards)Also OTR100 and TR119Depending on the condition of TR112 and TR113 perhaps TR11 in the overload circuitcould also be of interest.I suppose it was only a very brief ground contact, else resistors such as R156R157R159R166R168would also need checking.Complimentary DC-coupled amplifiers are wonderful.It takes no more than one tiny component at one end to start an avalance running all the way to the other endand even the tiniest and briefest short results in hours of work.Martin
Thank you for the shopping list above, I appreciate your time and advice, I will replace all of the above.....I will check them as I do so as I have checked the power amp transistors I have already replaced......none of which have proven to have failed, the darlingtons are difficult to check but they measured up very similarly to the new ones, all the other regular transistors have so far measured fine, which has been disappointing
Some of the components are hard to find, can anyone confirm the suitability of the following in parenthesis ()
D102 ZPD 2.7 (BZX79-C2V7)
D103 ZPD 12V (BZX79-C12)
D101 1N916B (1N914)
TR11 BC212B (BC556BTA)
I've managed to locate all the other components.....
Looks fine.
Martin.....thanks for that confirmation, I have now gone ahead and ordered all the parts recommended above. This done I decided to potter around cleaning up a bit more with the help of cotton buds and isopropyl alcohol and discovered this nasty crack in the PCB.....
The crack runs two ways one is harmless enough but the other runs right across the emitter track of TR110, the solder side of the PCB confirms the break in the track as can be just about seen in this pic, top right where the TR110 print can be seen, not easy to get a good shot of this
Consulting the circuit diagram gives me a degree of optimism that I have found the root cause of the original problem with this chanel, shame I'm such a clumsy oaf when troubleshooting........
As I was on a roll I decided to remove and check the diodes and found D102 was destroyed.....D100,101 & 103 all measured ok cold. Its frustrating having to wait for parts to be delivered ;¬)
Have you tried RS components they deliver very quickly, I use them for almost everything
Thanks Beo3000 I will give them a go next time I need to acquire more components, having taken a closer look at the damage to my PCB i think that I have marked the break in the wrong place on the circuit diagram above....tracing back the orange wire connected to the cracked track just before the emitter or TR110 I discover it goes back to the emitter of TR4 and is actually supplying the 24vdc for the whole left chanel rail and not just to the emitter of TR110. More optimism of the crack being responsible for the original problem.
All parts now received and the replacement is moving on....all power amp transistors have now been replaced and I'm on with Martins recommendations following the discovery of the point of contact for the earthing blunder. I have still only discovered the one clearly destroyed component (D102) though I'm not finished yet, is it possible that this diode alone could cause the overload relay to energise? I cant understand how this would cause DC at the output.......hence energising R1. Also I see that TR112 & TR113 are both BC547C transistors when TR113 should be BC182B?...cant see much difference on the published data sheets for these two so I'm going to replace both with BC547C devices.
The differential amplifier sets the DC offset.Its emitter ballast is TR114, on the base of which you find the zenerdiode D102, setting its stabile working point.A problem in this circuit not only can - it WILL cause DC offset problems, but it's not likely to cause more damage downstream (apartfrom the speakers and they are protected by the safety circuit and relay).
Thanks for that....so D102 maintains a steady 2.7v at the base of TR114 holding it ON, loss of TR114 would mean loss of functionality of both TR112 and TR113 and therefore TR115 &TR116...is that right?
However its good to know that I'm on the right track with this........thanks for your help
Sadly...having completed all the replacements the overload relay persists in energising on power-up, looks like more investigation will be required......not good news
How much DC is on the output rail?
Dont know.....I will connect my DVM to the center rail and power up this afternoon, the DVM has a hold feature and should get a reading before the fault relay opens, I will also take a look at C138 as there has clearly been DC on the rail. When powering up the overload lamp briefly illuminates and the the fault relay kicks in...there is no accompanying flash or crack followed by a burning smell when power is applied....which is encouraging.
Ok...did some measuring, its difficult to get an accurate measurement before the fault relay kicks in but I repeatedly measured more than 30vdc briefly at R163, the right channel R263 was measuring around 12vdc again briefly, the pic shows the components I have replaced so far....not much left ;¬)
Well, repairing these beauties is often a headache.If the darlingtons and their emitter resistors are all OK, the problem is most likely around TR112/113/115.Could also be a cracked solder joint or another PCB break.
When you replaced zenerdiode D102 - was it shorted?
Unfortunately D102 came out in two pieces! as it turns out I have plenty of spare transistors....even a set of darlingtons, the emitter resistors are new so should be fine, I will remove and check everything again.........I wonder is it possible that the right channel got damaged by the short I created ? It's not likely I know.....
The output darlingtons are fitted using short leads.The leads for some of the darlingtons are to cross on the way to the PCB - not all go parallel and direct.In other words - not all PCB pads are in the order B-C-E, some are in a different order.I have seen cases where repairers weren't aware of this - or simply got it mixed up.From the photo I cannot tell if yours are fitted correctly.