Panasonic RA-6600 repair

First posting: I'm looking for some technical/troubleshooting assistance on repairing a vintage receiver. Maybe this is the right forum to find knowledgeable help, or maybe I am posting in the wrong area? Please forgive me if so!

So I picked up a Panasonic RA-6600 (it's got a built in 8 track recorder!) that has some issues and I would like to get her operational. I have some experience troubleshooting audio equipment, skill with a soldering iron, familiarity with microelectronic component types/functions, and handy at repairing things in general... but still in need of some guidance. I am a younger guy and hoping to find a forum where I can learn about repairing electronics... most specifically vintage audio equipment because it could become a wonderful lifelong hobby for me if I were to get good at it.



As for the Panasonic, the guy who I bought it from said that it was working great (and he seemed genuine) except for the fact that it had blown fuses on the back. He said he thought that someone had bridged the speaker outputs while disconnecting it and therefore replacing the fuses should restore it to operational status. I replaced the fuses with appropriate 2 amp fuses and hooked up a small speaker to test her. When plugged in and turned on, the single lamp which lights the front display did not turn on (filament inside the bulb appears intact). The speaker buzzed and I could see the cone withdraw to its maximum inward position. Searching around online, I found an article saying that behavior of the speaker cone which I observed is a very likely sign of a shorted output transistor from the power amp section, trying to output the full rail voltage into the speaker terminals.



I located what I assume to be the amplifier transistors, mounted onto a big aluminum plate to serve as heatsink. I couldn't directly see the transistors because they are housed inside a protective box (the black box in the image below) but I'm pretty sure these are the ones. I took my multimeter and set it to the "beep" continuity-test setting and touched a probe to each of the leads in various combinations to search for shorts. Interestingly, I noticed that certain locations would produce a momentary beep which then went away in about 1 second. Is this a capacitive effect, or what am I observing there?



At this point I'm looking for possible culprits to investigate which might cause this failure. Or, more broadly speaking, helpful websites or articles that can help teach me how to diagnose problems. Where online can I search to find a circuit schematic of this thing?

When I bought the Panasonic, the guy said hey I've also got this broken Sansui 210 (a pipsqueak) which doesn't power on. You can have it for free if you're into fixing things he said. So that's next after the Panasonic. Thanks in advance for helpful comments or resources!

Welcome to all about circuits.

The black box labelled STK 040 is a modular power amplifier, not a casing housing just output transistors.
If this is shot you may have to build your own power amps to replace. Modern modules may be availble to help.

Another good forum for this stuff is

http://www.vintage-radio.net/forum/index.php

I am not trying to get rid of you, just provide your best chance of repair success.

How old is this amplifier?

studiot said:

Welcome to all about circuits.

The black box labelled STK 040 is a modular power amplifier, not a casing housing just output transistors.
If this is shot you may have to build your own power amps to replace. Modern modules may be availble to help.

Another good forum for this stuff is

http://www.vintage-radio.net/forum/index.php

I am not trying to get rid of you, just provide your best chance of repair success.

Click to expand...

Thank you for the welcome! Helpful to know that about the STK 040... does that mean I could potentially substitute my own power amplifier and beef up the amount of wattage this puppy can wield? Substituting a modern module would be great. What do I need to look into as far as selecting an appropriate one? I really need to find a circuit diagram of this whole box, for starters.... where do you guys go when you need a circuit diagram...?

Thanks for the recommendation on the vintage radio site. I actually took a look at that place but came here instead, thinking that maybe I might have better luck on a site based around electronics in general. Getting these fast replies is promising. I'll have to lurk around here and see what I can pick up.

Re: t06afre, I think the amp is from 1973, so OLD! I was very pleased to see almost no dust on the inside though. It also looks like it may have been re-capped at one point. All those purple caps look quite pristine. That's a good sign. Too bad the power amp section is hosed... time to research how to replace/upgrade her

Ah ha, also:
http://www.datasheetcatalog.org/datasheets2/11/114402_1.pdf

well thats a blast from the past-i remember repairing things like this a very long time ago and ive been in the trade a good 25-30 years now.....the fact your speaker cone moved means there is a large dc offset present on your speaker output leads....you can check this with your meter set to measure voltage,a working output stage shouldnt produce any large amount of dc at your speaker terminals.
The most likely cause is the audio output ic failing,you need to check by replacement and also test any on board fuses for being oc and low value fusible resistors in both supply lines to the output ic for being oc....ill have a look for the service manual for this unit if i still have it

You can get replacement ICs at EBAY(STK040). They are not that expensive. I digged in my junk box and found some electrolytes dated back to 1976. Besides being radial it was kind of hard to tell by just looking at it. If it was that old. I see 72 stamped on the caps. This may or not be the production year. Somehow from maybe from the font used on the plastic. I have hunch they are newer than 1973.

This may be the circuit your amp uses

http://english.electronica-pt.com/db/audio-ic.php?ref=STK&page=12

Dodgydave said:

This may be the circuit your amp uses

http://english.electronica-pt.com/db/audio-ic.php?ref=STK&page=12

Click to expand...

Oh, good find!

Hey everyone, thank you for all the input! And glad I could take you on a trip down memory lane, hahahaha

Going off that previous post, I did some searching on the STK-040 and found out the the Sanyo STK-series is vast indeed. Looking at the Datasheet for the STK-040 amplifier, I see that there are two associated models, the STK-041, and STK-043. I was wondering if it would be possible to swap out my STK-040 (rated at 10W, 0.9%THD) in favor of a STK-043 (rated at 20W, 0.5%THD)... Unfortunately they might not be interchangable--let's dig into that:

From the datasheet it looks like they all share the same basic construction and pin-out, however there are 2 resistors which would need to be changed and the circuit schematic is cut off at the bottom of the page. I might be able to use my multimeter and figure out which resistors they are on the PCB which hosts the STK-module. But another constraint is the power supply input needs to be higher as well. It sort of looks like these resistors are actually what's controlling the power supply to the amp, yes/no? If yes, maybe I can get away with swapping a few resistors and boosting my amplifier power from teensy to twice-teensy! Or would I need a whole different power supply, and thusly it's impractical to switch it...?

There's also a ridiculous number of other amps in the STK series, some of them going as high as 140W per channel that I found. However their pin-out is totally different and I'm not sure it'd make sense to try to finagle them in here. Then again, I only half know what I'm talking about! Anyone think it's doable?

Thanks to all. I'm going to await feedback and then order an STK040 off ebay if it isn't feasible to swap up.

An uprated power amp needs an uprated power supply!

studiot said:

An uprated power amp needs an uprated power supply!

Click to expand...

-noted! I ordered the STK040 replacement off eBay today. Only $10 but it's coming from China so it'll be a week or more before I see it probably. I'll update this thread when I get the chance to install. Might be a while, but hopefully this does the trick! Thanks everybody.

Look forward to the pics of the repair.

go well

t06afre said:

How old is this amplifier?

Click to expand...

Has to be late 60's/early 70's if it has an 8-track in it. I bought an Akai 8-track recorder in the late 60's. 8-track went into extinction right after that.

bountyhunter said:

Has to be late 60's/early 70's if it has an 8-track in it. I bought an Akai 8-track recorder in the late 60's. 8-track went into extinction right after that.

Click to expand...

I know, but as an example the major electronics chain-stores in my country still sell surprising many VHS players. This looked kind of high end. So it could have been something that Panasonic made for the 8-track puritans. The reason I asked was because of the electrolytes. Do you have any theory on that. Does it looks recaped or not?

I do think it must have been recapped. It looks super clean on the inside and the caps, visually, just seem newer to me. I searched around to see if there were some older-looking ones and there were, in a hard to reach location underneath another PCB. This thing has got a lot of separate PCBs in there. That might make it "fun" to access the solder locations for the power amp replacement. We'll see.......

Remember to get some heat sink compound for the amplifier change. And clean the heatsink well before applying any new heat sink compound

t06afre said:

I know, but as an example the major electronics chain-stores in my country still sell surprising many VHS players. This looked kind of high end.

Click to expand...

Panasonic was not known for high end audio, mainly consumer mass produced stuff. The use of the "hybrid" power devices was a cheap way to go, high end amps had discrete power stages.

As for the caps, I am not sure. Some probably have been replaced at some point. The only one I can read looks like it may have a "72" date code on the end. I think that is the year of manufacture.

bountyhunter said:

As for the caps, I am not sure. Some probably have been replaced at some point. The only one I can read looks like it may have a "72" date code on the end. I think that is the year of manufacture.

Click to expand...

-good catch, I didn't notice that. I think you may be right with the date code. That wood case has kept things pretty spotless on the inside, maybe they are original.

Re: thermal compound, yes... I've got some Arctic Silver 5 hanging around from computer builds. I'll probably use that as it performs awesome. Thanks!

Alright, update!

So this weekend me and a buddy of mine opened up the RA-6600 and took off the bottom panel. That allows easy access to the solder joints on the modular amplifier. We removed the old one and replaced it with a new one. Unfortunately, during this process, several of the solder pads on the PCB surface came right off, exposing what I assume is bare copper beneath them. The modular amplifier is inserted through holes in the PCB (called vias). There are no contacts on the topside of the PCB since it is soldered from the back/bottom. I'm fairly certain the vias are not plated either. So the only point of electrical contact is the solder pad on the bottom of the PCB, and several of them simply fell off. The solder I'm using is 60/40 Pb/Sn (I'm pretty sure), and it does not want to adhere to the copper underneath, so I think I'm not getting electrical contact there.

On the plus side, when we powered up the amp, a few lights came on, and the signal needle would move up and down as we scanned the radio dial; none of which happened before. So there's an improvement. Unfortunately testing all L/R and A/B speaker outputs produced no sound. So I'm thinking the issue now is just lack of electrical connectivity. How can I get solder to adhere to those exposed copper bits underneath where the pads were? Is there a different type of solder I can use or something? I'll try to post a photo maybe tonight or tomorrow.....

Don't worry, this happens sometimes.
What appears to be copper probably isn't. If this was a green masked PCB then you need to follow where the trace went and if needed you can scrap off some green mask and solder thin jumper wire's to make connections. Or you can follow the
trace to the next pad and connect there.

If I'm making no sense , then why not post a clear picture of the area in question.

You can also use solder wick or braid a s a flexible conductor to epoxy or Krazy glue to the PCB and then make a loop to create the missing pad. If there are connections on both top and bottom, and the PCB via has been lost, you have to solder the top and bottom separately.