Valvestate 8080 Troubles

[WMK]

Anyone have any test point data on the output section for one of these things? Anyone had any trouble with drift / runaway of the BDV64 / BDV65 (replacements from MCM electronics)?

Replaced TR7 (shorted), R85, the finals and TR9 (bias) (for good measure).

It works - produces a clean sinewave up to rated output into 8 ohms, but,
when left to sit and idle (with a load and w/o), from a cold start-up, the heatsink (and readings) indicate excessive current through the BDV darlingtons. Before shutting it down at one point a quick emitter resistor drop reading showed a non-stable, ramp up and was at around ~600mV (!) before powering down. Both (R96 & 118) were like this so both finals are drifting similarly.

Although I wheeled around looking for other faulty devices after parts installation, all seems to check out.

Any chance that the finals are counterfeit? Your opinion on me "diddling" the value of the bias resistor (R74 - I believe) as an attempt to stabilize the output? I can take it down into X-over and bring it back up, but what would be a ballpark emitter drop to look for.

It arrived in a box, loose hardware in zip lock baggies and pc board floating around insode the chassis - it obviously sat somewhere for a long time - maybe with a tech who couldn't get it going(?)

Should I throw it off a bridge? :-)

Test point data would be sweet if anyone has it.

Bizarre! It works fine but will self destruct if I leave it in it's current state.

THANK YOU for any comments & assistance!

[Mars Amp Repair]

Well I think you hit the nail on the head with the outputs you received being a very different rating from the ones that were in there. I'm sure someone has more experience, but it sounds plausible. You might check that whatever element they're using for heatsink feedback to the bias circuitry is functioning properly. Schematic is almost illegible...looks like tr9?

I recall having identical issues with one of these before. What's coming to mind was an open resistor somewhere in the driver stage. I know that's not much help. Unfortunately I must have been in a rush & did not make a note of it as I usually do.

As for counterfeit transistors, is your supplier one you've used before & can trust? glen

[Twist]

You may have already done this but unplug the speaker and power up. Check to see if there is DC voltage on the speaker jack (a few millivolts is ok). Check pins 6 and 7 of the Op Amp for DC. I can't tell which IC it is from the schematic, it looks like IC8B or IC9B. Anyway it's at the front end of the power amp. Definately pull one leg of all the diodes and check them for shorts or leakage. Check all low resistance resistors (typically 1K or lower). Many parts may test good in circuit because of paralled components elsewhere on the board (especially transistors with diode shunts). Some transistors may be open but have a 470 ohm resistor close by that fools your diode tester on your meter.
I get a basket case once in a while (it's literally brought to the shop in a clothes basket in pieces).
The repair quote usually goes up about 4 times the usual fee.

[Enzo]

Replace the BDV things with TIP142 and TIP147. (Mouser $1.11) Even Marshall did that after a while. I have never encountered drift or runaway with them, only encountered their destruction by other bad parts still in the system. They are just darlington bipolars. Make sure to insulate rom heat sink.

I have never gotten a bogus transistor from MCM, but of course anything is possible. That BDV would seem like an odd choice to bother to fake though.

It runs hot, which means it is underbiased. I don't think of it as driifting, other than since they are running too hot already, they make themselves run hotter as they sit there cooking. If the MPSA56 is OK, and the bias transistor is actually what it should be, then one of my rules is whenever anything burns, always check the resistors. Are the 220 and 1k2 around TR7 at value? Are the 10 ohm base resistors for the outputs at value? Are the two resistors associated with TR9 at value.

Since both outputs went, don't forget the lower driver, the MPSA06 - can't read the TR # on my print - and its resistors.

TR9 needs to turn on harder, so raising R73 should help. Tack a 1k trimmer in its place. Or maybe a 470 resistor and a 500 ohm trimmer in series. Whatever.

I suppose lowering R74 would have similar effect. Most adjusting bias controls I see are in the R73 position though.

Well, crap. My printed copy is hard to read, even though from the factory. I just downloaded the thing from my dealer site, and it is the same crummy copy.

I assume you are scoping the output so you would see any RF the thing might be making, yes?

[WMK]

Replace the BDV things with TIP142 and TIP147. (Mouser $1.11) Even Marshall did that after a while. I have never encountered drift or runaway with them, only encountered their destruction by other bad parts still in the system. They are just darlington bipolars. Make sure to insulate rom heat sink.

I have never gotten a bogus transistor from MCM, but of course anything is possible. That BDV would seem like an odd choice to bother to fake though.

It runs hot, which means it is underbiased. I don't think of it as driifting, other than since they are running too hot already, they make themselves run hotter as they sit there cooking. If the MPSA56 is OK, and the bias transistor is actually what it should be, then one of my rules is whenever anything burns, always check the resistors. Are the 220 and 1k2 around TR7 at value? Are the 10 ohm base resistors for the outputs at value? Are the two resistors associated with TR9 at value.

Since both outputs went, don't forget the lower driver, the MPSA06 - can't read the TR # on my print - and its resistors.

TR9 needs to turn on harder, so raising R73 should help. Tack a 1k trimmer in its place. Or maybe a 470 resistor and a 500 ohm trimmer in series. Whatever.

I suppose lowering R74 would have similar effect. Most adjusting bias controls I see are in the R73 position though.

Well, crap. My printed copy is hard to read, even though from the factory. I just downloaded the thing from my dealer site, and it is the same crummy copy.

I assume you are scoping the output so you would see any RF the thing might be making, yes?

Thanks for the comments, Enzo. Guess what? I nailed it last night by lowering R74 from 1.8K to 1.2K - you were right about TR9 needing to conduct more, thus lowering base potential of the finals. I used a decade (never a great idea by itself - with no fixed resistance left in the path - but the switches on mine are make-before-break, so the circuit can't go momentarily open and fry stuff.... oops got off track....) I benched it last night, looked & measured everything that I could think of, and simply could not find anything faulty.
At idle, the finals shut off when I hit 1.4K, so I simply went down two more steps, measured, ran it up & looked at the waveform + measured power.
All was ok, so I planted a 1.2K fixed value in R74 and let it cook on the dummies for at least an hour. Even when the finals & sink are way-hot, the emitter resistors aren't showing any of the conditions that were present before. Cool. Do you supppose that the new TIP29 & BDV64 & 65 were just a bit different spec-wise and that's what caused this?

Anyway, it WORKS!

Thanks.

[WMK]

Thanks for the comments, Enzo. Guess what? I nailed it last night by lowering R74 from 1.8K to 1.2K - you were right about TR9 needing to conduct more, thus lowering base potential of the finals. I used a decade (never a great idea by itself - with no fixed resistance left in the path - but the switches on mine are make-before-break, so the circuit can't go momentarily open and fry stuff.... oops got off track....) I benched it last night, looked & measured everything that I could think of, and simply could not find anything faulty.
At idle, the finals shut off when I hit 1.4K, so I simply went down two more steps, measured, ran it up & looked at the waveform + measured power.
All was ok, so I planted a 1.2K fixed value in R74 and let it cook on the dummies for at least an hour. Even when the finals & sink are way-hot, the emitter resistors aren't showing any of the conditions that were present before. Cool. Do you supppose that the new TIP29 & BDV64 & 65 were just a bit different spec-wise and that's what caused this?

Anyway, it WORKS!

Thanks.

By the way, there's a clean, legible copy of this at schem. heaven
http://www.schematicheaven.com/marsh..._8100_8412.pdf

[Enzo]

That is a much nicer copy than the one on the dealer site or in the service center drawing binder. Thanks.

[sstef]

after 13 years that domain is long gone, but i found the schematic, here is a new link for anyone who needs it

https://stef-dl.secdn.net/stef-vod/p...res=2145916800

https://stef-dl.secdn.net/stef-vod/p...res=2145916800

[Enzo]

Just a notte: if you look at the top of this page in the bar along the top, you see Schematic Hell. CLick there and find most of the contents of the old Schematic Heaven.