Ultra 6020

[BUSHVHF]

OK sounds good so far!!
thanks
laurence

[ekjdm14]

Originally Posted by ms660

Looking at the pics it looks to me like germanium output pair with germanium bias diode, silicon driver, germanium amp, silicon preamp, the latter being before the volume control.

Re: this earlier post, I can now confirm that there are two silicon transistors in this amp. the pre-amp one appears to be an old repair (there's a '70s looking resistor and ceramic cap tacked on underneath this) but the driver, a BC152 appears to be original or at least no other mods to account for it being silicon.

Is it likely that the circuit could have originally used all germanium except for the driver? (bearing in mind the schematic I have calls for all Germanium).

[ms660]

Hard to say, I don't have the schematic, the nearest I can find online is one for the Ultra 6024 which shows silicon preamp and silicon driver with the rest as germanium, which schematic are you working from?

Lawrence.

[ekjdm14]

Sorry I should have mentioned I'm working from Trader sheet 1728 which, as you rightly say, is for the 6024 but matches very closely in most respects. I was under the impression that BC187 (preamp, as specified) and BC107 (driver, as spec'd.) were germanium. (fitted are a 2N 4062 in the preamp, with tacked on resistor below the board, and in the driver position is a BC152)

(My lack of experience showing here, I have located said items online and they are in metal cans which I thought signified that they were Ge? Is this not the case?)

Thanks for your help.

[ms660]

There are lots of silicon types in metal cases.

Lawrence.

[ekjdm14]

As I said previously, learning curve! This is something I've genuinely never heard of before!

Anyway, once the caps are here & installed I shall test what is fitted & hopefully determine where the noise is getting in (still waiting on an oscilloscope I bought a while ago, and a new LCR meter, to arrive) before worrying too much about what is in what case.

[ms660]

Isolate the input tag of the volume control then turn the volume up, that should tell you whether the fault is in the preamp or the output stage.

Lawrence.

[Herald1360]

BC prefix devices are silicon AF types. Germanium with the same sort of prefix would be AC. The case could be anything- various plastic and metal types. The prefix here gives nothing else away about the devices. Look up "pro-electron codes" for more detailed information.

[ekjdm14]

Thanks I will have a look at that now. I wonder though, does that mean certain metal-cased silicon transistors could also be susceptible to tin whiskers in future?

Edit- wow I have to wonder why I never thought to look up this before (aside from not knowing the search term pro-electron codes)! Got some reading to do now, I'm sure transistor numbering will make a lot more sense if I familiarise myself with the codes. Thank you.

['LIVEWIRE?']

Unless anyone knows different, I've never heard of metal-cased silicon (or for that matter the later germanium AF12x) transistors, suffering from the 'Tin Whisker' fault. AFAIK the construction method is different to that of the AC/AF/OC devices, all of which do suffer from the T.W. Syndrome!

[ekjdm14]

OK we have some progress to report on this amp. After replacing the remaining capacitors the hissing was (predictably) still present although the amp sounded a lot more sensitive to a finger touch on the pickup terminals.

From here I started tapping transistors with a screwdriver handle, whereupon the pitch of the hissing changed on tapping the second AF amp (TR2) but also tapping TR5 (AC176 in the output pair) there was some crackling going on through the speaker.

So far, I've drilled & sprayed out the AC176 and it appears to have worked. I wasn't so lucky with the second AF amp though and killed it I decided since I'm effectively experimenting with what works on transistor kit, I would order some transistors as per the spec sheet for the 6024 and replace the one I killed at my own expense plus any others I felt needed.

Now I'm at the stage where the first AF amp is the silicon 2n-something that was already there,
the next AF amp has been replaced (it was an AC156 IIRC, and is now the specified AC128),
the driver I replaced as, although it seemed to function it was a re-based BC152 and the legs were very fatigued, that is now a BC107 as per the sheet. (interestingly, even though it seemed to be working to begin with, the removed BC152 tests as a capacitor across 2 pins now!)

TR4 is the AC128 as original, TR5 is the (drilled and cleaned) AC176 as original. Bias stabilising diode is the original (AC169 transistor tied as a diode).

[ekjdm14]

So now, with all the above complete we have the amplifier functioning & it sounds super sensitive and very quiet with no signal input.

However I have some questions for the real experts regarding bias and an overheating issue. Basically, on first power-up I noted 120mVDC across the speaker (with it connected) which slowly started to rise until I shut off at around 210mVDC.

I removed and checked the preset (specced as 15-30k on the 6024 sheet) and it comfortably swings from about 12k to over 50k. However. even with it set all the way to the lowest setting I still have around 110mVDC on the speaker at power-on which rises slowly over a couple of minutes to around 150mVDC and I shut off.

The reason I'm shutting off apart from seeing more DC than I would like on the speaker, is that the original AC128 output is getting warmer than I would like rather quickly, none of the others appear to get too warm although the cleaned AC176 does get slightly warm. (This is with the heatsink in place).

The questions I have are-: Firstly, what level of DC would be considered acceptable across this 15R speaker? And how would I go about reducing it further given I'm at the limit of the preset (and technically out of spec)?

Could it be the "repaired" AC176 is causing things to go out of whack/losing gain as it warms up under load?

And could this DC bias issue be the reason behind the AC128 getting warm quickly, as I feel it is?

Thank you if you've managed to stay with me this far, I know it's a long couple of posts! And any help/suggestions would be much appreciated, just let me know if there's any specifics I need to include. Cheers, Dan.

[BUSHVHF]

Sounds like its getting alot better, hope the wizards on here can chip in with suggestions that are causing this overheating and increasing mVDC's to the speaker,
Laurence

[ekjdm14]

Now I've had a think on it, I think I'm leaning more toward the cleaned AC176 being a problem still. Have run it a few more times for short periods and at low volumes it runs OK but anything high enough to fill a room I notice that something is beginning to go leaky and drag down the main DC power (measured at the rectifier).

In fact on 2 occasions whatever is the cause must have shorted since the rail dropped from 24 to around 2v (I've been running it with my hand hovering over the plug!)

(The temperatures seem to be reasonable when running at low volume or no signal, and the 128 shoots up when whatever it is goes leaky/short).

I haven't run long enough to verify what's shorting under load for fear of killing the AC128 but to be honest I do think it's likely the AC176. Obviously expert input appreciated very much, but I'm learning a lot by working through it too.

[mikej56]

I haven't got a circuit diagram for this amplifier so I might not be much help.
I think it is possible to have a low DC voltage across the speaker if the circuit includes a negative feedback loup which is connected directly from the junction of the output cap and the 'live' speaker output i.e at the speaker side of the output cap. The 210mV you measured would produce 2mW in a 15 Ohm speaker so it's not going to damage it.
There is something seriously wrong when the power supply voltage drops to 2 V. Is it possible that you are measuring at the wrong point on the board - with this voltage drop there should be smoke everywhere?
The voltage at the junction of the output transistors and the output cap should be half the supply voltage, have you checked this?
Is the rectifier in the power supply a selenium type?

Mike

[ekjdm14]

Yes I'm measuring directly at the o/p of the selenium bridge rectifier. It's only not smoked because I'm quick on the draw pulling the plug out when whatever it is shorts! I'm sure if I left it plugged in either the rectifier or the AC128 output would emit magic smoke though. It's not permanently shorted, just after playing at a decent volume for a few minutes. The only thing getting hot at the point it goes, is the AC128 output.

I don't think the rectifier would have died if that's what you're thinking of, the player came to me working but suffering from a terrible hiss with no signal applied but would play happily all day apart from that.

I have (from a cold start) -24VDC supply and -12.3 at the output transistor junction.

I'm working from the trader sheet for the 6024 which matches closely, not sure how to post a section up here but it's a free download from radiomuseum site.

Thanks for the info on DC voltage/feedback loop, makes sense now I look at it that way! (to be fair, I did think I was probably being silly worrying about that but it's good to have a "sanity check" at least).

Cheers, Dan.

[mikej56]

Hi Dan,

You said, "it's a free download from radiomuseum site".

Not unless you pay $25 for membership, as far as I can see?

Mike

[ekjdm14]

Strange, I did see reference to paid membership but there's also a few downloads free, I think one per day or 3 per day... Unless I'm somehow a member without my knowledge! I was considering becoming a member but I may just make a smaller donation as the site has been a useful source of info on this player & a couple of my own collection.

I'll go back and try and see how I did it

Edit-: Click on the small pic of a schematic to make it the main pic, then click that & it opens a new window (pop-up) where you enter your email for them to send the PDF to, and verify that you're human. That's all i did.

[mikej56]

https://www.radiomuseum.org/dsp_anmeldungskosten.cfm

[Station X]

RM lets you download a limited amount of information FOC.

Back on topic please.