Akai AA-1020 burning resistor woe!

[johnm]

Hi folks!

Hoping somebody can help me to fix up my Akai AA-1020 which has been (hopefully temporarily) put out of action due to my own stupidity!

The amp had been recapped recently, adjusted, and was working beautifully. Sounded lovely. The only problem was the old headphone socket was cracked and resulted in intermittent connection with the jack inserted. Fortunately I sourced another that was an extremely close match. Did a test fit of the new socket to make sure… and forgot I'd left it there... loose! Yep (slaps forehead!).

Had just turned off the power (thankfully, or it might have been worse) after a listening session, and the socket decided to fall into the internals at that point, creating a spark and there was a loud pop. As I said the power was off, so it was either residual charge left in the reservoir cap, or from touching the live terminals of the on/off switch (probably the most likely). Obviously there must have been some short. The spark was large enough to burn off a corner of one of the solder tags on the new headphone socket as it fell to the chassis base!

Whenever the amp is turned on now, R16b (150R) starts smoking almost immediately. Replaced it with a new one and the same result, so obviously I've really screwed something up. This is alas out of my league really, so was hoping somebody might be able to look at the circuit diagram's below, and the PCB layout (smoking resistor circled in green) and spot a potential component to check and change. I'm hoping the transistors haven't blown from that short, but sod's law being what it is I'm guessing that's what's happened.

I can’t take any voltage readings on the amp board at the moment as that resistor starts heating up immediately. Have checked for any obvious burn marks on all PCBs (there aren’t) and there’s no sign of any blown components, at least not visually.

Main PSU and Amp PCB circuit (rest of the circuit not shown is for tone control and RIAA boards):

Amplifier Board:

What it looks like for real:

Where to start?

Any help would be very gratefully received

Thanks.

- John

 

[JonSnell Electronic]

Check TR4, TR5 and TR6 plus surrounding resistors.

 

[johnm]

P.S. What looks like burned power resistors - the rectangular 'white' ones at the rear - are merely a stubborn coating of nicotine! The previous owner must have been a human chimney!

 

[johnm]

Check TR4, TR5 and TR6 plus surrounding resistors.

Jon, thanks for the very quick reply

How do I go about doing that if the resistor starts smoking right away? Do I need to desolder those transistors to test them? My testing knowledge is regrettably rather basic I'm afraid :/

 

[sgrossklass]

You can do some basic checks in-circuit using the trusty diode test function.
pn junction with >> 0.7 V? Probably open.
Looks like a dead short? Probably is.

Note that these VD1222 bias diodes (D1) can go intermittent (open) at times. A chain of three small-signal diodes in parallel may be used to troubleshoot such issues.

The output stage looks like it may benefit from a Baxandall diode... would require redoing bias though.

This amp probably is slower than it would have had to be since they skimped on an output inductor.

 

[johnm]

Thanks for your reply sgrossklass
Yes I'm going to check all the transistors. Have looked at a few youtube videos so think I know what to do now. I'm assuming they're all blown but fingers crossed anyway.
Have been Googling the VD1222 diode replacement, and it appears the common advise is to replace with a series pair of 1N4148s instead, so have ordered a batch of those. The original is deliberately angled very close to the heatsink, so will do the same with the 1N4148s but will insulate with a thin-walled heatshrink sleeving to prevent any shorts.

 

[johnm]

Ok I've been testing the transistors and all but one of them I'm sure need replacing. I'm going to desolder them all tomorrow and check again just incase being in-circuit is affecting readings with my Amprobe 240 meter.

So planning ahead (if need be) I'm unsure whether to try and track down new old stock transistors for the following, or get modern equivalents. I'm not looking to 'improve' anything as I loved the sound of this amp as-is, so any advise on substitutions (or whether to stick with originals perhaps from a donor amp) would be great, thanks.

Transistors:

TR3 / 3b = 2SD438E (currently installed on PCB) (2SC1166O or Y is indicated on schematic).

TR4 / 4b = 2SB560D (currently installed on PCB) (2SA661O or Y is indicated on schematic).

TR5 / 5b / 6 / 6b = 2SD371Y (120-240 hFE) or 2SD371O (70-140hFE).

D1 & D1b (VD1222) actually both test fine, but will replace each with a series-connected pair of 1N4148s to be safe.

I think I'll replace the 5 watt 0.47R resistors R17, 17b, 18 & 18b as they all look a little sorry for themselves. Do I need to use the same brand/type (MPC71s) or would any 5W 0.47R power resistor do?

Thanks,

John.

 

[Welcome]

Ok I've been testing the transistors and all but one of them I'm sure need replacing. I'm going to desolder them all tomorrow and check again just incase being in-circuit is affecting readings with my Amprobe 240 meter.

So planning ahead (if need be) I'm unsure whether to try and track down new old stock transistors for the following, or get modern equivalents. I'm not looking to 'improve' anything as I loved the sound of this amp as-is, so any advise on substitutions (or whether to stick with originals perhaps from a donor amp) would be great, thanks.

Transistors:

TR3 / 3b = 2SD438E (currently installed on PCB) (2SC1166O or Y is indicated on schematic).

TR4 / 4b = 2SB560D (currently installed on PCB) (2SA661O or Y is indicated on schematic).

TR5 / 5b / 6 / 6b = 2SD371Y (120-240 hFE) or 2SD371O (70-140hFE).

D1 & D1b (VD1222) actually both test fine, but will replace each with a series-connected pair of 1N4148s to be safe.

I think I'll replace the 5 watt 0.47R resistors R17, 17b, 18 & 18b as they all look a little sorry for themselves. Do I need to use the same brand/type (MPC71s) or would any 5W 0.47R power resistor do?

Thanks,

John.

Touching those heat sink mounted diodes is asking for trouble. I would really leave them alone. As for nos transistors - there are none, plenty of fakes, though.

 

[sgrossklass]

For TR3/4, it looks like the trusty BC639 + BC640 should sub in fine.

As for the outputs, read the suggestions here. Swap out both pnp + npn.

If you need to take the board out, that might be a good opportunity to give it a good scrub with soapy (diskwashing detergent) water, sparing the trimpots as well as possible. Getting water back out of pots and inductors and other stuff like that tends to be a royal pain.

As for the VD1222 replacement, yes it's a double-edged sword. On the one hand and intermittent failure of these can cause the output stage to blow up like it's done here, though an oxidized bias pot could do the same (a check is definitely warranted). On the other hand it seems temperature coefficient with ordinary small-signal diodes may be different. I guess if all fails you could just rig up a Vbe multiplier instead, more flexible anyway.

I take it you do already have a bulb tester...?

 

[Mooly]

TR6 B-E junction together with R18 should clamp the voltage across the 150 ohm to less than 1 volt. So we can for certain that one or both these components is faulty (open) and/or print has been zapped.

There will be more wrong beside that but the above is a definite.

And use a bulb tester while fixing this.

 

[johnm]

Thanks for your help guys.

I'll go for the BC639/640s for TR3/4 then. Any hFE testing needed or can I just fit & forget?

The four output transistors are all NPN 2SD371s (no PNP), so following the information in that link, I can replace with either MJ21194 or MJ15015G types then.

Will test the VD1222s again out of circuit, and if OK will put back then.

The bias pots are all brand new sealed Bourns types, and it was all working beautifully before my silly mistake with that short. But yes I do use a bulb tester.

Have taken the circuit board & heatsink assembley out and the pcb is fine, no burnt traces. All fune on top too. No signs of any trouble except that burnt resistor. But will get those transistors replaced.

Cheers,

John.

 

[johnm]

Have taken the four output transistors out as i didn't trust the fluctuating readings with them in circuit (with my particular multimeter), and only TR6b is blown. The three remaining output transistors test absolutely fine using the 5 diode tests between pins.
Surprisingly, TR4b also tests as fine too, so I *think* I was reasonably lucky with the damage.
I'm going to change all four outputs to MJ21194Gs anyway so they all match, and have ordered those.
Have also ordered 4 new MPC71 type 0.47R emitter resistors too as the existing ones look pretty grotty.
Unless anyone thinks it's a bad move, I'll leave TR4/4b and TR3/3b as they are. Will of course replace the burnt resistor R16b, and will test R21b as that one was also in the immediate 'firing line' (or trace!).

- John

 

[Mooly]

Sounds like a reasonable plan, and using a bulb tester will show any problems without blowing output stages.

 

[johnm]

Sounds like a reasonable plan, and using a bulb tester will show any problems without blowing output stages.

Good to hear from you Mooly - you were a huge help a few years back with getting a Sony amplifier back from the dead. Mammoth thread

Still trying to work out exactly what happened out of curiosity. The amp had just been turned off when the new headphone socket fell inside the amp and caused this damage. Right next to the headphone socket is also the power switch (uninsulated terminals in those days!) and the speaker A, B, A+B, Off (headphones working only) selector. I guess as the power had just been turned off the damage was either from residual charge left in the reservoir cap, or the socket briefly connected one of the power switches live terninals to the base of the amp/ground. Or shorted one of the speaker outputs (via the selector switch) to each other, or to ground.

 

[Mooly]

Hi John, yes I remember the Sony well with its self oscillating PSU.

It does sound a bit as though it could be the mains switch that the socket touched. Although normal fault-finding rules apply, you have to be more aware that there could be unexpected and seemingly unrelated problems in separate parts of the circuit. Its a little bit like working on lightning damaged circuitry in a way.

Hopefully though the damage is confined to just that area. A methodical approach using a bulb tester will soon have it fixed.

 

[johnm]

That was the one! I was very grateful for all your help

The new output transistors arrived yesterday, so I'll get those installed over the weekend. I think I'll replace the driver transistors too as the legs are all corroded on those. The others - TR1 & TR2 - are fine. Read somewhere that that type 2SC1166 & 2SA661 were prone to this and can go noisy. Will replace with BC639 & 640s just to be on the safe side.

 

[Mooly]

A lot of old Japanese 2SA/B/C/D types had peculiar failure modes, often they would go open circuit intermittently between B and E.

Watch the pinouts on the BC639/640 and use a bulb tester when powering up.

 

[johnm]

A lot of old Japanese 2SA/B/C/D types had peculiar failure modes, often they would go open circuit intermittently between B and E.

Watch the pinouts on the BC639/640 and use a bulb tester when powering up.

Taken me a while to do any further updates, and to construct a new bulb tester as the old one is MIA.

Anyways, work done over the past week:

All 4 output transistors replaced with MJ21194Gs - one of the original 2SD371s had indeed blown.

TR3/4 replaced with BC639 + BC640.

All 0.47R 5W emitter resistors changed.

VD1222s (D1 & D1b) replaced with series connected pairs of 1N4148s, with heatshrink insulated 'heads' resting against the heatsink as per the original diodes.

Powering on with the bulb tester went well. Powered up again and could set the idling current on each channel to around 40mA (a fair bit of fluctuation in readings here, but this was the case prior to the accident, and on a previous amp of the same model no. in the past). The DC balance adjustment via VR1/VR1b was successful on the left channel to 28.1V. However the left channel's DC balance would not change from 17.1V no matter how I turned the adjustment pot.

Nothing over-heating or any nasties I can see. Testing via headphones it sounds very nice indeed So I'm wondering if perhaps that initial surge might have taken out or shorted VR1b (30K) ? Would this seem likely?

Thanks,

- John

 

[johnm]

 

[Mooly]

Its getting there then

Knowing its history as we do... anything could be at fault. The fact it plays and the bias adjusts means that its basically OK, and yet this midpoint adjustment preset does nothing. That's a bit odd. If it was open then there would be no bias to TR2. If the preset was zapped, then the energy to do that must have flowed through TR2 or even via C4 and TR1

We have to start somewhere. What are the voltages on TR2 ? If you have 17 volts on the output then you should be seeing 17 or so on the collector. Check it. Also what is on the base and emitter. TR2 could be suspect I suppose although its rare to get a failure that leaves it partially working... but it happens.

Are the voltages on TR1 correct. Although its AC coupled and would normally have no impact on the DC conditions of the power amp stage because this is more like a 'lightning damage' type of repair all things are possible.