Recapping a Tandberg TR-2080

Replacing the 2SC1583 with doubled-up KSC1845's was tricky. Using advice from prior posts, two matched KSC1845's for each 2SC1583, a dab of thermacote, and a short length of 1/4" diameter heat shrink (to hold the two KSC1845's face-to-face), and insulators (small diameter unshrunk heatshrink) for all the legs to avoid shorts.

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The resulting device was difficult to position. I checked the pinouts FIVE TIMES on each device and I'm still unsure if I've gotten it right.
 
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Got it. Can't say I've seen those other transistors cook the board like that but then again this unit appears to have some history to it. The originals should have had little exposed heat sink tabs on them, which seems to prevent that normally.

I'd really want those braced to the board securely, you're going to crack a trace with that much weight on the transistor otherwise and even then it could cause mechanical stress on the solder joints that isn't normally present.

This picture should help with the differential pair layout:

layout.gif

John
 
Got it. Can't say I've seen those other transistors cook the board like that but then again this unit appears to have some history to it. The originals should have had little exposed heat sink tabs on them, which seems to prevent that normally.

Yep, they had dissipation tabs at the top, but that didn't stop the cooking. They were also mounted REALLY close to the PCB, which seemed odd.

Looking at the silkscreening, it appears these remaining six transistors were supposed to have the same heat sink / stands that the driver pairs had. Do yours have those?

I'd really want those braced to the board securely, you're going to crack a trace with that much weight on the transistor otherwise and even then it could cause mechanical stress on the solder joints that isn't normally present.

You make a really good point there. I'll have to think about how I'd mechanically connect or support the new heatsinks.
 
They only provided those u-shaped brackets for the four drivers. Apparently the others weren't in need of them, have to say I have not run into any with the boards cooked like the receiver you're working on. At the most these usually only cook the odd base resistor here or there.

John
 
I will say that I now appreciate _part_ of the Tandberg design. Since I need to diagnose the short, I'll start by disconnecting all the boards fed by the power supply and test just the power supply and components. Since the selector board had its own secondary power supply and the various other boards are all fed via detachable cables, that part is easier. Very little to desolder just to isolate and test...

But the main power supply is _part_ of the main amp board. So THAT bit is trickier.
 
So, I was sent a working TR-2080 that had a distorted channel. I replaced all the electrolytic caps to see if that miraculously would clear the problem... and in the meantime discovered some substituted parts and signs of overheated transistors on the power amp board. So, I replaced everything on the power amp board except the bridge rectifier, output transistors (they were already modern replacements and simple-tested good), the protection relay, and the two inductors. Plugged it all back together... and I'm instantly blowing the main 5A power supply fuse. Could be the oversized power reservoir caps I put in and too much inrush current filling them, could be a solder bridge I created during the work, could be a reversed cap somewhere, could be a bad replacement guess.

So, first step is to isolate the power supply (which is PART of the power amp board) by disconnecting wherever it feeds into the actual power amp then plug it in to see if the over-current is in the restored/upgraded power supply or not.

Looking at the schematic AND the foil diagram, I've located the spots where I'll have to lift a component leg or de-install a transistor to isolate the positive rail. Anyone see a mistake? (The red dots are where I'll disconnect) I'll repeat this plan for the negative rail, disconnect the components or lift their legs and plug it back in.


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which fuse is that? what confuses me is which schematic you use (filename?)
I do not see the fuse where it is in the schematic....
If it switches primary mains power it must be a slow blow fuse I guess.

I guess you need to use a "dbt" or a variac to limit current.

It seems the only difference between TR2075 and the MK2 and 2080 is the TR2075 has separate rectifiers and main smoothing caps, and at the TR2080/2075MK2 the main amps seem connected through wires between the (one here) big rectifier and the smoothing capacitors.

You could opt for putting in some power resistors there to limit current for troubleshooting if you have them, instead of a "DBT" or variac. (Myself I never use the thing called DBT, whether it is useful or not just depends on peoples troubleshooting habits)

It LOOKS to me, if you can separate the power this way by taking away the wires as drawn on the bottom picture you put just above here.
 
It's the fuse on top of the toroidal transformer and it is mains supply. Would be on the back panel for a smaller model but on these it's mounted to the transformer. Not a slow blow in my experience.

I generally just remove the rail voltage leads from the four outputs as a first step in isolating this kind of issue. If it still blows look for issues with the filter caps and rectifier, then solder bridges. Use the DBT throughout until you stop seeing the short.

John
 
It's the fuse on top of the toroidal transformer and it is mains supply. Would be on the back panel for a smaller model but on these it's mounted to the transformer. Not a slow blow in my experience.

I was kinda stunned, too, when I saw "slow blow". I've been using 5A fast blow and I'm ok with that for testing these big changes.

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Well, I isolated the power supply rails from everything else and it didn't blow the 5A fast blow fuse, so the problem isn't the bigger-than-spec power reservoir caps or the bridge rectifier. That's good... but it means I've got to go further into the power amp board to find the problem.

I'll start with testing the output transistors again. They were working fine before I started the recap and they tested ok when I pulled and tested every component. Just in case, I'll pull them and connect each to the DCA75.
 
Never thought I'd say this, but... Rats! All the outputs test good.

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Are the outputs all back in the right places?

I stand corrected on the slow blow aspect - not sure I've ever had to replace that one but I would have sworn it was a fast blow.

John
 
While you're looking, check the base voltages going to the four... if it's more than 0.5-0.6V, the drivers could be in wrong (location or pin-out). Maybe that differential pair you had to fab?

John
 
Doh!

The original FT317A and FT412A had been replaced with MJE15030 and MJE15031 and I replaced THOSE with MJE15032 and MJE15033... perfectly fine replacements... but I swapped them around accidentally: PNP where the NPN should have been and vices versa.

The components TEST fine individually, but they apparently have to be INSTALLED in the right spots on the PCB for the amp to FUNCTION correctly. Doh!
 
Yes!

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I don't like making dumb mistakes, but I sure am happy the issue was just me reversing the PNP/NPN pairs.
 
So when I look at your meter pic, I see -53.22 VDC. Great, now to make sense of that, I roll up to post 109 with the scat chunk in it and see that the rails should be ±52.5V. That seems much better than blown fuses!!
 
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