Intermittent noise in Rank Arena Receiver

[MooTheFish]

Hi all. New member here although havelurked the forums for quite some time. Usually I've been able to find whatever I was looking for but I'm pretty stumped with this one, so any help would be appreciated.

WALL OF TEXT INCOMING:

I recently acquired an old Rank Arena RA-402 reciever/amplifier for free, and while it might not be the best of old audio gear, I love it's looks and to me at least it has a nice punchy sound to it. It worked fine but had a fair amount of noise in the output and the right channel was giving me some issues.

Given the age of it I decided to go ahead and recap the power supply, amplifier and front panel, which I have done (with the exception of two caps on the left channel of the amp board, still waiting for the replacements to arrive). The recap certainly helped, but there's still a noticeable amount of noise across both channels regardless of where the volume and balance controls are set. Changing inputs makes no difference, nor does using the other set of speaker outputs.

If that was the only issues I could live with it, but the big problem is with the right channel. After switching the unit on and using it for a bit, a crackly, hissy noise comes through the right channel, gradually getting louder, until there's a loud POP through the speaker and it settles back down again. This process will repeat itself. But if I quickly power the unit off and back on again the noise clears up completely and it will continue to work fine for quite some time before the noise returns.

My current theory is that something is producing more noise as it heats up and then cuts out with the pop sound. What's strange to me is that the unit will still pass a signal throughout this process, and I have no idea why quickly powering it off and on would correct it. I've tried swapping the large resistors next to the main output transistors between each channel, as one of them had been replaced. But the issue remained on the right channel.

I've also tested all the other resistors and done a basic check of the transistors on the amp board and everything checks out ok, so I really don't know what to do next. Knowing me, I've probably missed something obvious

Any help is appreciated and sorry for the wall of text lol.

 

[mbz]

Do you have a schematic or service manual, I had no joy finding one, will be slow going without one.
There are a number of old transistors that are known to go noisy, there is an AK list, think it includes the following,
Type Replacement(s) Failure notes
--------------- ----------------------------------------------- ------------------------------
2SA720 KSA1013
2SA725 KSA992 or BC327 Shot noise
2SA726 KSA992F or KSA992E or BC327 Shot noise
2SA798 (2x) BC560B or (2x) KSA992
2SA809 KSA1381 Poor heat dissipation
2SA979 MAYBE faulty.. replace 2 hfe matched ksa992 crack, pops
2SC458 2SC2240 or KSC1845F or KSC1845E Leakage, noise, static
2SC631 KSC1845 (but test for hfe 350+) Prone to failure or noise
2SC632 KSC1845 (but test for hfe 350+) Prone to failure or noise
2SC871 KSC1845F or BC550C Leakage, noise, static
2SC1000 KSC1815Y Leakage, noise, static
2SC1312 2SC2240 or KSC1845 or BC337
2SC1313 KSC1815
2SC1318 KSC2383Y or KSC2690AY Thermal stress, thermal flex
2SC1344 2SC2240 or KSC1845 Static burst
2SC1451 KSC3503 Static burst, open circuit faults

The periodic popping might be caused by a dry solder joint, google for pixs, will need a magnifying
lamp to thoroughly inspect all joints.

I would doubt that the RA-402 has PRE-OUT/MAIN-IN RCA connections at the rear. If it has
then you can cross the connections (Left pre-out to right main in etc...) to help identify
if it's a pre or power amp problem.

Assume you don't have an oscilloscope, you might need to build an audio probe, but with no schematic...

 

[MooTheFish]

Do you have a schematic or service manual, I had no joy finding one, will be slow going without one.
There are a number of old transistors that are known to go noisy, there is an AK list, think it includes the following,
Type Replacement(s) Failure notes
--------------- ----------------------------------------------- ------------------------------
2SA720 KSA1013
2SA725 KSA992 or BC327 Shot noise
2SA726 KSA992F or KSA992E or BC327 Shot noise
2SA798 (2x) BC560B or (2x) KSA992
2SA809 KSA1381 Poor heat dissipation
2SA979 MAYBE faulty.. replace 2 hfe matched ksa992 crack, pops
2SC458 2SC2240 or KSC1845F or KSC1845E Leakage, noise, static
2SC631 KSC1845 (but test for hfe 350+) Prone to failure or noise
2SC632 KSC1845 (but test for hfe 350+) Prone to failure or noise
2SC871 KSC1845F or BC550C Leakage, noise, static
2SC1000 KSC1815Y Leakage, noise, static
2SC1312 2SC2240 or KSC1845 or BC337
2SC1313 KSC1815
2SC1318 KSC2383Y or KSC2690AY Thermal stress, thermal flex
2SC1344 2SC2240 or KSC1845 Static burst
2SC1451 KSC3503 Static burst, open circuit faults

The periodic popping might be caused by a dry solder joint, google for pixs, will need a magnifying
lamp to thoroughly inspect all joints.

I would doubt that the RA-402 has PRE-OUT/MAIN-IN RCA connections at the rear. If it has
then you can cross the connections (Left pre-out to right main in etc...) to help identify
if it's a pre or power amp problem.

Assume you don't have an oscilloscope, you might need to build an audio probe, but with no schematic...

Unfortunately, I don't have a service manual or a scope. Given that Rank Arena equipment is generally rehoused NEC gear, there might be a service manual for an NEC model which could be applied to this one..but I haven't found one.

There are a couple of solder pads that had lifted slightly, had to scrape off some of the solder mask and make a solder bridge to try and reinforce them and stop the trace from breaking in the future, so I might have another look and see if there are any more like that\dodgy joints in general. And i'll also make a note of what transistors it has while i'm there. Someone has clearly worked on this before, the headphone jack had been replaced at some point with one that didn't fit, nice twist and tape job on the wiring. I've just removed the jack entirely for now until I can find one that fits properly.

 

[mbz]

Sounds like a good plan, also use it as a learning exercise. While Rank Arena might not have a great name
I've been surprised time and again by such manufacturers/labels that make good sounding gear.

 

[MooTheFish]

Had another good look for bad solder joints, but couldn't see any (apart from one I had done earlier that was a bit sloppy, which i corrected). In terms of transistors, I didn't find any that were listed in that thread, the ones I could see in my receiver were 2SA640, 2SA661, 2SC945, and 2SC1166 on the amp board. Unless anyone has any better idea, i'm tempted to start swapping transistors between the two channels and seeing if the issue follows.

UPDATE: Swapped all the transistors from one channel to the other, no difference at all. I hope this isn't a sign that one of the main output transistors is failing, as they're the only ones left I haven't touched on the amp board.

 

[Ylli]

Definitely do a close physical inspection of the solder connection around the output transistors. Might want to reflow anything that looks in the least bit suspect. This is where most of the heat is generated and that heat can cause things to move around a bit.

 

[MooTheFish]

So I decided to bite the bullet and swap the output transistors between both channels. There are 4 transistors attached to the heatsink on this amplifier board. I desoldered the two outermost ones which were green in colour, and was about to put them back when I noticed that they're actually different transistors. One is a 2SB596 and the other is 2SB595 (on the faulty right channel). Here's a photo of the board, you can see where I've removed the two mismatched transistors from.



My question is, why would they have used two different transistors between channels? Everything looks factory as far as I can tell. And can I swap these two transistors or would that be a bad idea like I think it would be ?

 

[mbz]

Swapping parts between channels can be somewhat risky, end up
with 2 faulty channels. However this is a noise issue rather than
toasted transistors. So your choice...

You would be lucky to find the culprit using this method.

Suggest you build an audio probe. You will need another amplifying
device, many options available, I will assume you have another stereo
amp.

Using an old RCA cable and an old multimeter probe. Snip off one end of
the RCA cable and connect in series a cap anywhere 0.1uf-1uf, at least
100Vdc, needs to be non/bi-polar. An old greencap or ... Connect a length
of wire/old mm probe to the other end of the cap and tape up.

Plug the RCA end of probe into your AUX of spare amp. You may need to
connect the (phono) ground of your spare amp to the RA-402. Power up
the spare amp, select AUX, low volume. You can now probe various points
in the circuit, the audio at that point is siphoned off and amplified
to indicate fault is/is not present. Generally probe the base of transistors
starting at the input to the power amp ie, half way. Then move forward
or back.

Do not probe points of significant Vdc.

That connector at the front of the power amp stage would be a good
starting point or TR402base.

D401,2 is a diode pack, consisting of 3(?) diodes. Some of these packs
have a history of going noisy.

 

[MooTheFish]

Swapping parts between channels can be somewhat risky, end up
with 2 faulty channels. However this is a noise issue rather than
toasted transistors. So your choice...

You would be lucky to find the culprit using this method.

Suggest you build an audio probe. You will need another amplifying
device, many options available, I will assume you have another stereo
amp.

Using an old RCA cable and an old multimeter probe. Snip off one end of
the RCA cable and connect in series a cap anywhere 0.1uf-1uf, at least
100Vdc, needs to be non/bi-polar. An old greencap or ... Connect a length
of wire/old mm probe to the other end of the cap and tape up.

Plug the RCA end of probe into your AUX of spare amp. You may need to
connect the (phono) ground of your spare amp to the RA-402. Power up
the spare amp, select AUX, low volume. You can now probe various points
in the circuit, the audio at that point is siphoned off and amplified
to indicate fault is/is not present. Generally probe the base of transistors
starting at the input to the power amp ie, half way. Then move forward
or back.

Do not probe points of significant Vdc.

That connector at the front of the power amp stage would be a good
starting point or TR402base.

D401,2 is a diode pack, consisting of 3(?) diodes. Some of these packs
have a history of going noisy.

Unfortunately I don't have anything on hand to make an audio probe....BUT I did check the diode packs after reading your response and sure enough when I wiggled d402 with pliers while the unit was on, the right channel noise went crazy. While the solder joint looked fine, I resoldered it and so far the right channel has been stable!

Of course I'll need to run it for a while to make sure but she seems to be running well now. I'd bet replacing all those diodes might well clear up the hiss across both channels as well.

 

[MooTheFish]

So it has been a while since i've used the receiver, and the issue has arisen again. Like before, wiggling the d402 diode pack affects it, so i'm thinking its the pack itself and not just the solder joint like I thought before. However I can't seem to find replacements for these anywhere. Anybody know what I should be looking for \ have any recommendations for replacements?

I assume I would have to remove one and test it to find out the value since I don't have a service manual and they seem to be unlabelled...

 

[Binkman]

Greetings;
Post a pic of that diode pack...

Have you recapped the board? You may also need to replace driver transistors.

 

[mbz]

I assume I would have to remove one and test it to find out the value since I don't have a service manual and they seem to be unlabelled...

Correct

Usefull thread on the subject
https://audiokarma.org/forums/index.php?threads/replacing-the-stv-3h-and-4h-diodes.279530/page-1

Couldn't find the other thread which has comparisons below,


Basically diode test or resistance check if off the scale and replace with diode chain, vaguely recall uf4005 was
suggested as a replacement, suggest do an AK search on STV-2,-3,-4 may find suitable replacement...