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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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1st Mar 2017, 10:10 pm | #1 |
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Dynatron SRX30D: LH quieter than RH output
Hi Everyone,
My trouble-shooting on this one has drawn a blank so I need some help Here's my problem: Audio output of LH speaker is much quieter than RH speaker. Its not the speakers as swapping them round makes no difference. The same problem happens on radio and gram so its not the audio input. I have found a partial circuit diagram from the forum & followed the HT volts All the spec voltages are correct in both L and R main amplifier circuits. All the spec voltages are correct in both L and R circuits on tone control PCB. I was expecting to have found a difference between L and R by now but nothing and I'm not sure whats left to check! Any ideas? Also, does anyone have the FULL circuit diagram for SRX29/30? Its not on my service data CDROM but someone had a copy of it in a previous post https://www.vintage-radio.net/forum/...ad.php?t=76045. I've only got half of it as it spans two pages. sx30 p23 (1).pdf I have spotted one peculiar thing. When listen to the quiet LH channel output only (by having the balance control fully down) and then switch off at the power button the audio takes a few seconds to go silent but before it does it gets momentarily louder. When I listen just to the RH channel and switch off this side does not get louder before it goes silent. So far this is the only difference I've been able to find. Might be nothing but worth mentioning. Cheers, Nicola |
2nd Mar 2017, 2:19 am | #2 |
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Re: Dynatron SRX30D: LH quieter than RH output
Its all on plugs, can you swap the main amps over? Establish whether its the amp or preamp.
These silicon triodes don't tell you what's wrong. |
2nd Mar 2017, 1:52 pm | #3 |
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Re: Dynatron SRX30D: LH quieter than RH output
Good Tip Sam - its a problem in the main amplifier. If I swap over the leads connecting the two mains amps to the main chassis/tone board the quiet audio output swaps sides.
So because all the voltages match the specs can I say that all the resistors and capacitors must be OK. So this narrows it down to being a transistor issue? |
2nd Mar 2017, 2:21 pm | #4 |
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Re: Dynatron SRX30D: LH quieter than RH output
Examined the circuit and noticed that two transistors are absent from the board (same for both L and R amplifier so not the cause of the fault). I've included two photos labelling the TRs so you can see whats what. So that means there's a total of 7 potential TRs causing the fault.
I've tried the flick test to see if any particular TR cracks into life - but nothing. So I suppose the best way forward would be to replace all 7... |
2nd Mar 2017, 2:24 pm | #5 |
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Re: Dynatron SRX30D: LH quieter than RH output
unlikely to be transistors
more likely caps excluding the output coupler . |
2nd Mar 2017, 2:31 pm | #6 |
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Re: Dynatron SRX30D: LH quieter than RH output
These are the codes for the 7 TRs in the circuit:
BO130Y x2 MA8003 x2 C1131 x1 C1115 x1 MEO412 x1 Are there any modern equivalents that could be used instead? Thanks, Nicola |
2nd Mar 2017, 2:34 pm | #7 | |
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Re: Dynatron SRX30D: LH quieter than RH output
Quote:
If one of the main amps is faulty it will always be that side that fails. Frank |
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2nd Mar 2017, 2:55 pm | #8 |
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Re: Dynatron SRX30D: LH quieter than RH output
Ah, have I got this the wrong way round. I swapped the leads connected to the main amps at sockets SKT602 and SKT603...
Just drawn it out - yes, faults in the pre-amp bit! Duh |
2nd Mar 2017, 2:55 pm | #9 |
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Re: Dynatron SRX30D: LH quieter than RH output
So do I Frank, both main amps must be OK, its the signal going in that is faulty.
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2nd Mar 2017, 3:12 pm | #10 |
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Re: Dynatron SRX30D: LH quieter than RH output
So the problems in this bit - the tuner PCB- the bit I don't have a full circuit diagram for :-( But I can see two TRs in the board TR401 (BF194) and TR402 (MAB003)
(the tone control PCB doesn't have transistors so that rules out the board at the front) Last edited by indigo.girl; 2nd Mar 2017 at 3:24 pm. |
2nd Mar 2017, 3:20 pm | #11 |
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Re: Dynatron SRX30D: LH quieter than RH output
That snippet is power, the pre amps are the MC483 integrated chips!
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2nd Mar 2017, 3:23 pm | #12 |
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Re: Dynatron SRX30D: LH quieter than RH output
The snippet of the circuit is the Stereo indicator circuit, no point looking there at the moment.
I am presuming that both Radio AM/FM and gram share the same fault. The circuit you posted earlier had an IC in each channel, being careful not to short out pins on the IC's have you checked the voltages are the same on each chip? Frank Edit and has Sam states Power. |
2nd Mar 2017, 3:28 pm | #13 |
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Re: Dynatron SRX30D: LH quieter than RH output
OK - so it is in the tone board - but in the MC483 chips. That makes more sense...
I have already checked the voltages at each pin and they match the specs and match each other. The only difference was in pin 6 which is suppose to be 0V. I picked up 78mV on RHS and 1 mV on LHS channel but I didn't see this as significant. Mentioning it now just in case. |
2nd Mar 2017, 3:40 pm | #14 |
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Re: Dynatron SRX30D: LH quieter than RH output
I've had this myself before, the cause are the capacitors on the tone and volume control board. Replace the lot.
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2nd Mar 2017, 3:41 pm | #15 | |
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Re: Dynatron SRX30D: LH quieter than RH output
Quote:
The DC voltages may be correct -- in the steady state with no AC signal. But as far as DC is concerned, the capacitors might as well not be there -- they charge up to some voltage and then no more current flows in or out, barring any change in voltage. And if a transistor was conducting more or less than it should, then that would upset the DC voltages around it. So my first inclination would be to suspect a capacitor. And if there is a capacitor across a transistor's emitter resistor, that's the first one I'd pounce on; since the purpose of this capacitor is to increase the gain. If that capacitor was not there, then any additional collector current due to an increase in base current would flow through the emitter resistor and increase the voltage across it; thus reducing the base-emitter voltage, and hence the base current. The transistor would be fighting against itself! With a sufficiently-large capacitor wired across this resistor, the emitter voltage will be unable to change suddenly; so the emitter voltage stays almost unchanged, and the self-opposing tendency is reduced. If an emitter decoupling capacitor had gone low-value, that would certainly cause reduced gain.
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2nd Mar 2017, 3:59 pm | #16 |
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Re: Dynatron SRX30D: LH quieter than RH output
Thanks Michael and Julie So looks like a cap issue then. Just for my clarity this means focusing on the tone control board. There are no transistors on this so no emitter decoupling caps to pounce on.... is there any way to test the cap in situ? Or just remove one at a time and replace if its short circuit.
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2nd Mar 2017, 4:07 pm | #17 |
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Re: Dynatron SRX30D: LH quieter than RH output
This is some text from the service manual describing the MC483 modules. I hoping from the datasheet I might be able to work out which caps are linked to the emitter functions in the chip.
http://pdf1.alldatasheet.com/datashe...83MS8G_10.html |
2nd Mar 2017, 4:32 pm | #18 |
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Re: Dynatron SRX30D: LH quieter than RH output
That datasheet seems to be referring to a different part altogether
An emitter decoupling capacitor would most probably be an electrolytic type, between 10 and 100 µF; and one as old as that would never fit on a thick-film module. So it will be somewhere nearby, and connected directly between two pins on the module, or from one pin to ground (or maybe the positive power rail, in the case of a PNP transistor).
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2nd Mar 2017, 5:50 pm | #19 |
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Re: Dynatron SRX30D: LH quieter than RH output
The only way you can test a capacitor in situ is to use an ESR meter,
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2nd Mar 2017, 9:06 pm | #20 |
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Re: Dynatron SRX30D: LH quieter than RH output
Based on Julie's suggestions I have replaced capacitors C302 (150uF to a 220uF), C306 (4.7uF) and C332uF (4.7 uF). These are the electrolytics near to the MC483 chips that are in the uF range. C303 and C305 are also nearby but are nFs so I have not replaced these.
Once removed none of the old cap had short circuited. Replacing them one at a time made no difference to my problem. LHS audio is still very quiet. |