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Author Topic: HH IC100 overheating BD711 transistor...help !  (Read 2039 times)

phatt

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #45 on: October 13, 2018, 08:45:13 AM »

I am talking about the lower of the two 2N3773 output transistors. If it was myself working on the amp I would first remove all the connections from this tranny and do a resistance check across the B/E terminals. If it reads zero on resistance range that is your problem (or one of them!) and the device must be replaced. Hope this helps anyway.

If that is dead, I'd be replacing all the transistors, as R10  around Q2 (the VAS one)was overheating so likely others are stressed by now. The 741 is likely fine.  IME not worth fiddling around.
Phil.
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #46 on: October 14, 2018, 10:58:02 AM »

hi Kiki and Phil
tried the guitar through 'power amp in' and it remains very distorted so looks like power amp right enough.

I have replacements here for the tip29 and tip30 robbed from a dead HH pa amp and that read Hfe of 53 and 130 on my meter.

So would the plan be -
Order up a new 2N5415 , test the 2N3773's as described above and if no reading from B to E replace and see if we get a result ?

Anything els you could advise that needs done ?
Would be interested in your assessment of my updated voltage readings in post number #39 above....

Will be a few days till i get round to this as have a newborn in here plus the other monsters but will get back to all soon.
thanks again for all the help so far

cheers
rob
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sj-castle

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #47 on: October 14, 2018, 02:02:53 PM »

Hi all thanks for all this great information and to every one contributing here. I also have almost the same exact problem.
The amp was not in heavy use but was working fine until last week when turned on it was fuzzing away and the back plate heating up hot. I have replace the 4700 caps as one of the originals had blow a hole in its top. The amp now is (working) you get a clean sound but still heating up, but not all the time so a bit tricky to pinpoint whats gone.

Out of interest in my version R15 is also missing AND C4 also never installed, no solder on pads.
Using my DVM on diode test the two driver Trs look good showing a diode drop to base from collector and emitter. But the upper TIP29 is running hot, the main concern is the heating of the back panel. ie the 2N3773 should I just replace them.

I was in the process of measuring +/- rails etc, when it started getting hot again and the + had dropped to 40 at that point.
I will continue testing. I have access to a scope if need be but would not know what to check with it.

Thanks again to all.
... Simon
« Last Edit: October 14, 2018, 02:06:34 PM by sj-castle »
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vintagekiki

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #48 on: October 14, 2018, 06:26:19 PM »

Do not replace ad hoc any transistors, if not previously tested them in what condition.
Test each transistor with the multimeter in the circuit or removed from the circuit.

Testing a transistor with an analogue multimeter

https://www.radio-electronics.com/info/t_and_m/analogue-multimeter-voa-vom/testing-diode-transistor-with-multimeter.php
https://www.instructables.com/id/How-to-test-Bipolar-Transistors-if-you-have-an-Ana/
https://www.scribd.com/doc/34162981/Transistor-Test-Using-an-Analogue-Multi-Meter
https://www.mikroe.com/ebooks/components-of-electronic-devices/diodes-and-transistors
http://120studio.com/tech/transistors.htm


Simple transistor testers
https://www.elprocus.com/simple-transistor-tester-circuit/
http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/images/TransistorTester.gif
https://electronicsforu.com/electronics-projects/hardware-diy/easy-transistor-tester


From the 30 year practice, the most common failures in power amp are the output transistors 2N3773, the driver transistors TIP29C/30C, the emitters resistance 0.33Ω/6W.
After each power amp repair, necessarily set the idle current 2N3773, and check the DC offset on speaker output jacks.

Reminder for advanced users
If you have tone generator, oscilloscope, audio voltmeter and 8 Ω dummy load, by measuring voltage at audio voltmeter, check at the oscilloscope at which power, power amp is symmetrically limited.
P = U2 / R
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #49 on: October 15, 2018, 07:18:43 PM »

thanks Kiki ,
i've desoldered the power and driver transistors from a donor pa head and will test them all using the info from your links to see which ones may be feasible to use....

i'll also take readings from my VS musician head which is working fine and compare them to the corresponding parts in the IC100 i'm trying to repair and see if anything else looks weird.

I'm enjoying trying to learn here but it's certainly quite complex compared to my dinosaur valve rigs ;)
cheers
rob
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vintagekiki

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #50 on: October 16, 2018, 03:09:00 AM »

I'm not old-fashioned, but the ohmmeter in the analog multimeter 20 kohm/V (with pointers) best proved for a semiconductor test in the circuit.
Measurement with ohmmeter (measurement range x10 or x100) can detect a faulty semiconductor in a circuit without desoldering.
If you have a 2 amplifier with the same part number PCB (for example AS 00014/2), simply compare the same semiconductor in the same conditions in the correct and in the faulty amplifier.

Warning.
Disconnect amplifier from the AC outlet.

When any measuring with ohmmetar, amplifier must be disconnect from the AC outlet.
When desolder any component, the amplifier must be disconnect from the AC outlet.
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #51 on: October 16, 2018, 01:28:26 PM »

ok Kiki , appreciate the safety warning...

i have a fluke meter only so hopefully it will help make sense of things when i get round to these tests soon...

cheers
rob
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #52 on: October 19, 2018, 01:18:53 PM »

hi guys
just on to ask for some more advice before i attempt any more of this repair with the IC100.....

i have taken readings off the driver and power transistors in circuit in the IC100 and my working VS-Musician head with power on ..

all transistors read similar except for the lower of the 2N3773's which reads - 47.2v on its emitter while the 2N3773 in the working VS Musician amp reads 0v...

I tried another two 2N3773's from a scrap donor PA amp but got the same results with distorted output so they were both likely to be dead....

At this stage the 2N5415 is still overheating but the previously overheating tip29c is now not getting overly warm but they give very similar DCV results to the working VS amp so might still be serviceable

in your experience , does this look like i just need a new 2N3773 [ or pair of ] ?
thanks rob
« Last Edit: October 19, 2018, 06:34:20 PM by bobster »
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phatt

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #53 on: October 20, 2018, 09:34:47 AM »

Although both may use the same transistors they could be completely different designs.

Q/ Do both amps use split supply rails?
If the VS head reads 0Volts on the emitter it maybe single supply in which case it would read 0Volts.
Just look for your main filter caps,, does the other VS head only have one big electro cap?
 if so it's a single supply circuit.
Chum,,,
You are chasing your tail using second hand parts,, Just go buy some NEW parts,,, build a light bulb limiter  :trouble
(so you don't burn out new parts before you have a chance to check for faults),,
with no speaker,,fire up the amp and check voltages.
If the output is close to zero VDC then read the voltage drop across R28 & R34 do some maths and if all is well then  connect the speaker.
*ALL that is done with limiter in circuit*
When you are confident that the circuit is stable then you remove the limiter and recheck voltages.

Of course with limiter in place voltages will be lower so you have to recheck the voltages and the bias.

I recall you replaced C5 and it distorted badly,, I hope you removed it and reinserted the old one?
The original looks like it could be a mica cap ,, I'm not sure but maybe ceramic won't work there.
Also the ceramic may be a low voltage type and hence it has failed.
Others here may know a more professional way to fix this but that is how I do it.
I run power amp repairs full bore into a load with lamp limiter on and read the voltages while it's working to weed out any lurking problems. ONLY then do I remove the limiter.

As I'm not a teck by trade I don't try to fix exotic circuits that are way beyond my ability to fix.
As I tend to get a fair amount of 60's 80's Amps they are often small fairly simple circuits and hence easy to fix.
I did pm you about this a week back but maybe you have not checked your messages.
Phil.
« Last Edit: October 20, 2018, 09:48:22 AM by phatt »
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #54 on: October 20, 2018, 11:48:50 AM »

Hi Phil , thanks again for your help.
I never got an alert for a PM but will check the messages here...
i have also ordered two new 2N3773's and a 2N5415.....

the other amp i made the comparisons with is from the same stable and maker and it's power board is very similar in design and interchangeable as far as i am aware and also has the same two big electro caps and general topology   .
i did suspect that little 150p cap [ threw the old one out unfortunately ] and will have a look in my stash for a non ceramic replacement and try it there to see if anything gives..
i only tried the scrap amps 2N3773's as an experiment as i have to wait for parts to arrive but did think they would be likely to be dead ..
i am reading very low mv on speaker outs well within limits according to Kiki...

happy to admit i'm floundering here but am determined to get this amp working again and am not in a major hurry plus i'm learning [a little! ] along the way ;)
thanks
rob
« Last Edit: October 20, 2018, 01:16:25 PM by bobster »
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g1

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #55 on: October 20, 2018, 01:50:45 PM »

The VS musician can not work if it has 0V at the lower 3773 emitter.
So you may be mixing up the lower and upper 3773's, or mixing up emitter and collector.
The 'lower' will have -48V on one of it's terminals.

How about posting your E,B, & C idle DC voltages for the 2N5415, Tip29, Tip30, upper 3773, lower 3773.
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bobster

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Re: HH IC100 overheating BD711 transistor...help !
« Reply #56 on: October 20, 2018, 08:55:00 PM »

hi G1 ,
i took these voltage readings a few days ago and have copied from another post earlier in thread...

TIP29c [ BD711 on my amp ]    C = 46.5v  B = 1v      E = 0.5v
TIP30c [ BD712 on my amp ]    C =    - 46.1v  B = - 0.6v   E = 0v

DC mv across  R28    = +0.6mv     [ on 200mv scale ]
DC mv across  R34    =  +0.6mv

Power Q's
Q1    2N3773      C = 46.5v    B = 0.5v     E = 0v
Q2    2N3773      C = 0v         B =  - 46.8v   E =   - 46.8v


2N3440     C = 46.9v   B = 0.6v   E = 0.1v            [ all positive voltage readings ]
2N5415     C = 1.2v     B = 46.8v     E = 47.4v      [ all pos voltage readings ]

*2N5145 is now heating up fast and the Tip29c is no longer heating up since i replaced C5 of 150pf as suggested in case of high freq osc.... [ it's possible my replacement C5 is faulty so will will try another one there soon ]

R10 of 100 ohms has +47 volts at either end of it
C5  of 150pf [ replaced ]  has +47 at one end and + 1.2v at the other end which is collector of 2N5415 i think

the junction between R35 of 100 ohms and D3   has + 109mv which then after a short time drifted down to around +100mv

According to the power amp sim Phil put up this reading between R35 and D3 should be negative and around -155mv?

hope all this gives a clue as to what parts may need replaced or are causing issues...

thanks rob
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