Heathkit TA-17 - resistor in power stage keeps blowing

[Mark Hammer]

Picked up an old Heathkit TA-17 today for a very nice price. This is a pretty full-featured amp, and back in the day must have impressed the daylights out of people who were embarrassed to have only Volume, Tone, Speed, Depth and 2 input jacks.

I knew it was non-functional at the time of purchase, but figured it would be something fixable. Taking it out of the wooden chassi and giving the board the once over, I immediately spied a 330R resistor in the power amp that was fried. You can spot the resistor here as R313 in the upper right hand corner: http://harmony.demont.net/documents/...hkit_TA_17.gif

I quickly replaced the resistor, plugged it in and fired it up (with speaker, and volume turned completely off). What I got within 5 seconds was another fried 330R resistor.

This was the first attempt at debugging this amp. I need to engage in something more detailed, but I have to confess to being an absolute novice at discrete power amps. Do any of you fine folk have any ideas what could possibly cause the short resistor lifespan, and what sorts of component parameters I should check for to narrow that down?

ADDENDUM: The schematic specifies 2N3772 power transistors. This unit has Sony 2SD88A instead. I don't know if the subbed trannies are the source of the problem, or else reflective of a different problem that blew the trannies some time back and also blows the 330R resistor.

[mozwell]

Unplug the speaker until we have the power amp working correctly.
2SD88A looks to be ok as a replacement, 300V TO-3, NPN, but only 5A collector current, 2N3055 woudl also work. If you are running into 4 ohm loads, i would parallel two output transistors insetad of using just one.

Meaure collector to emitter on all transistors, any shorted, check Base to emitter on diode range, all of them around 0.6V
If you can heatsink them, MJE340 & MJE350 are good for the driver transistors if any of the 40409 / 40410 are blown.
Is R314 (1k 2W) ok. Something is pulling one end of R313 down giving us excess dissipation & blowing it up.
remove the power transistors and power up, do you get approx 40VDC on the output ?

If the amp is fairly old, i would replace the speaker output electrolytic capacitor & the power supply caps, actually just do them all.

[Enzo]

Here, dowwnload a proper copy.
Heathkit Schematic Diagram Archive

Unless the output cap is shorted, you won;t get any DC on the output, and at idle, load impedance won't matter.

If those Sonys are OK they are probably fine. Most any good power transistor would work. I;d probably grab MJ15003 out the drawer and never look back.

If R313 burns up, failure current has to come from somewhere, and I;d wager your Q13 driver is shot. See the two diodes just below it? Make sure they are not open, or more important that the wires to them are intact. If they are shorted, no harm, only thing that does is increase crossover distortion.


The whole power amp is seven transistors, test them all. Four diodes, they are either shorted, open, or OK pretty much.

I see two electrolytics,plus the speaker cap. I'd just replace them.

Check all the resistors for opens, at least any to the right of R313. Each output has a 0.33 ohm. Output bases have a 100 and a 47. And a 15 ohm between Q14 and Q16. And make sure your zobel resistor has not burnt open, R319 over by the speaker jacks.

[Mark Hammer]

Thanks, gentlemen. The caps were already ordered before I posted this thread. I'll attend to the relevant semiconductors tonight or later this week.

The 2W/1k resistor R314 is fine, although having just brought this beast home late last night, I will confess to not having gone over it extensively. R313 stuck out like a sore thumb, but I'll need to check over the rest of it to ascertain that none of the other resistors are simply burnt...rather than having what looks like a black stripe!

[Dave Mac Farlane]

Sensible advice from Enzo - I would also suggest setting up the quiescent current, and having looked at the diagram, this doesn't seem to be quoted.
Oddly enough, in the old Hi Fi days when Sony battled with Bang and Olufsen, the jap stuff always was more robust - a fuse would blow if the dog chewed through the speaker wires, and maybe one o/p tranny - in the B & O it would take out all the o/p stage, including drivers and protection diodes. Heathkit wasn't really rated, but I think their PA was OK.
Quescent is like bias on a valve amp, especially necessary if you're using alternative trannys.
Dave.