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Tandberg TR 2080 No Sound

gslikker said:
My TR2075 was more or less bad but the pulled transistors tested good on the meter.
Click to expand...

Oh boy. So there's no sense in testing transistors for bad ones?

As I said, Q917 definitely looks bad using diode setting to test transistors with my DVM. Q918 on the right channel appears to be ok, but the diode reads were around .707 and .723 vs the .45 - .65 ranges discussed in this post:

http://www.audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/
 
It is easy to detect very bad transistors, but unfortunately, it is less easy to detect "less bad" and good transistors....

BTW 0.7 is quite normal for a small signal diode, what you measure also depends on how much current your voltmeter puts through the diode.
 
Hmmm, not sure what my Klein MM700 is putting through in diode mode. Most transistors I have checked (on this receiver and others) using these tests are in the .6xx range. Only a couple including Q918 have been above .7xx.
 
gslikker said:
Be aware having multiple failures, BECAUSE of that, you do not know the initial cause of the amp to fail..
Click to expand...

Still waiting on component tester to arrive, but here's where things are so far:

Left Channel:

1. One of the left outputs was wrong. Q929: Motorola 2N6029 602, should be a Motorola 2N6030 7840.
2. Both outputs bad. Replaced with MJ15003G/MJ15004G.
3. Both drivers bad. Replaced with MJE15030/MJE15031.
4. One of the Frakos was definitely bad. C905: 0 uF (should be 470/6v). Replaced Frakos (C905/C913).
5. VR931 is bad, replaced it with new Bournes 100 Ohm trimmer.
6. Replaced burned R955 and overheated R943 and R953.
7. Replaced burned D919.
8. Q917 is bad. On list to order from digi-key.

Right Channel:

1. One bad output (Q930). Replaced both with MJ15003G/MJ15004G.
2. One bad driver (Q926). Replaced both with MJE15030/MJE15031.
3. VR932 appears good, but replaced it with new Bournes 100 Ohm trimmer.
4. Replaced Frakos (C906/C914).

Other:

Replaced PS Board Frako (C610)
 
Couple of things in response to the posts since I checked in last:

1) More heat/speed are usually better when desoldering, in my experience. These boards do not usually give me any trouble with desoldering, but I am using a Metcal system which is VERY good at not overheating traces when used properly. For years I only used the MX-500 soldering tool with a Soldapullit vacuum tool, and every once in awhile wick but I find that fresh solder application followed by a quick vacuum with causes less damage than wicking - IME. I added the Talon desoldering tool to that MEtcal system more recently and that pulls solder so fast that trace damage is really unlikely. But, use a low wattage iron and leave it on the wick long enough to heat the joint and it's not surprising you had some trace damage.

2) On your DBT question, what wattage lamp are you using? With a 100W incandescent bulb, you should reach full rail voltage on a 2080 fairly quickly and it should not be causing the DC offset you measured. If you unplug the AC supply harness that connects the transformer to the PA board and the lamp goes dim instantly, you have issues on that board (which your readings suggest is the case anyway). If it stays bright you may also have shorted components somewhere on the +25V rail - I've seen shorted small signal transistors on the RIAA board cause that, for example. Much less common though, and probably you'll find it's just the remaining issues to locate with the PA board.

John
 
Hi John. So with 1) I think I've got it working w my 2 irons now. 2) I am using a 100w incandescent. I'll look into testing with power disconnected to PA board. I'd like to confirm source voltage anyway. What AC voltage should I be seeing at the connector?
 
Hi John, I guess I was looking to identify what Jxxx Connector plugs into the PA Board and provides the Supply voltage for the board. Is there one? And if so, I was looking to check the Supply voltage to verify I'm getting good supply.

I'd also want to be able to disconnect the PA Board from the Supply to see if my DBT still lights up or whether it points to just components on the PA board that are bad.
 
So looking more at schematic, it looks like 3 pin J901 supply's 39V ~ to the PA Board from the Mains Transformer.

Is that AC before it hits the Rectifiers?
 
OK, confirmed. With J901 unplugged and meter connected to connector, unit powers up and DBT does not light up. I'm seeing steady 38.6V AC supply.
 
Add another to the bad list. Q913 bad, Q914 good. These BC548B NPN transistors sit in sockets pinned up against the heat sinks next to Outputs Q927/Q928. Bias transistors?

Since this receiver obviously had an overheating moment, I suspect i need to check F901/F902. These are the blue 'fuse' components that are also pinned against the heat sinks next to Outputs Q929/Q928. I can pull these to test them, but what am I looking for? Simple continuity with ohm meter setting? What are these actually called (thermal fuse?) so I can search for replacements if necessary?
 

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The schematic indicates 110 degree C for these thermal cut-off fuses. Do these actually blow when overheated or just 'cut-off' and resume once normal temps return? If I need to replace, am I looking for ones with a Max Temp Limit of 110C? I'm seeing other temperature values on Mouser and Digi-Key like Operating Temperature Range, Holding Temperature and Rated Functioning Temperature.
 
Looks like you answered a lot of your questions already.

Yes, those are bias transistors and likely to be damaged if one or more of the outputs were shorted. The blue parts are thermistors. If you don't break one from handling, I'd ignore - never seen one fail on these receivers and I've serviced dozens that had blown outputs or driver stage issues or both.

Was Q913 shorted? That by itself could have caused the DBT to light up.

John
 
Hi John, I tested Q913 with DVM with Diode mode and where I should see .6-.7 on the 2 tests, one read OL and the other varied and then settled on .002. So I would say it is shorted. I currently have Q913 and Q917 out. I wanted to test a few remaining transistors on both sides before putting in an order for replacements. Hopefully this weekend I can wrap those tests up.

On a side note, with the PA Board unplugged and the receiver on, I noticed a couple of the front lights are burned out (one meter light, one radio dial light). I pulled the meter bulb and these are all fuse shaped and say 12V 3W. Any suggestions on where to find these guys? I tried plugging one of my 8V LED blue bulbs into the meter location (left overs from re-lighting my Marantz 2225) and it did not light up. Do the radio meter lights get power from the PA board that I have unplugged? Neither lights up, but only one looked burned out.
 
Probably, if they fit the clips. Going to be dimmer though, the originals are 12V/250mA festoon lamps (pointed ends instead of fuse-style) and they're still available if you know where to look. You can PM me on that part, if you want - among other things, I sell lamp kits for Tandberg receivers.

These are fed directly off an AC secondary from the power supply. Theoretically LED's would work but personally I think they look hideous and don't see any reason to modify the original design given lamps are still available. Unlike Marantz and some other receiver designs from back then, these do not suffer at all from any heat buildup issues - no plastic to melt or vellum to fade etc.

BTW, it's normal for one of the meter lamps to shut off if you're not using FM - they keep the other one on because it doubles as the power meter but shut off the lamp behind the one that's not doing anything useful if you switch to phono etc.

John
 
Hi John. Thanks. It didn't dawn on me to try selecting FM. So maybe that's all. I'll check tonight. The one that was burnt out that I pulled to inspect was the right Tuning Meter bulb, not the one that doubles as a Power Meter. I'll pull the other. It may be burnt out as well.

I did see the lamp kit on eBay sold in San Diego and thought that might be you :). I'd be happy to buy a set given all the help you given me here! Thanks again.
 
OK, all transistors on PA Board have been checked. In addition to replacing the input and output pairs on both channels, I found the following BC transistors bad on the left channel:

Q913 (BC548B) - available at digi-key
Q915 (BC548C) - available at digi-key
Q917 (BC558C) - obsolete, BC558B available at digi-key

Question are BC558B and BC558C interchangeable equivalents? Digi-key also seems to indicate that BC559C (and BC556B) is a direct substitute. So should I go with BC559C?
 
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