Uniden 858 PC-176AB missing (& extra) voltage weirdness

[bgpz]

Dear 858 experts,

Any educated guesses as to what may be happening here? Story time:

So.... I grabbed myself a bargain Robyn SB-510-D (Uniden 858, PC-176AB board) recently as a 'parts only' deal. Then I was pleasantly surprised when the thing fired up, transmitting and receiving as it should....

...well, briefly.

But then, suddenly while doing a brief transmission test on USB, the fuse went 'pop', the lights went out, and ... yeah, that was another surprise, albeit not a pleasant one. After replacing the fuse, there's no TX or RX due to an absence of the 8.80V feed at the collector of TR29 (aka #3 on the SAMS service manual). I am only seeing 0.02V there, but should see 8.80V according to the spec.

In practice, this 8.80V feed is supplied via an underboard wire to TR29. My first thought was the lack of the 8.80V supply (done via R29 to the D44-regulated 9.5V rail, which SAMS calls #2) was because R29 had gone high resistance or cactus. But it would appear not - lifting the underboard wire bridging TR29's collector to where the 8.80V is really coming from (i.e. the C196 / R29 junction at a different part of the board) restores that rail to its listed spec.

Weird, eh?

Although the unit hasn't been fully re-capped yet (meaning it unfortunately still has many of those horrible 10V electrolytic nasties in there, along with those philistine 6.3V tanties) I can't see any old dodgy cap in the TR29-related circuit that could be dragging that rail down. Seems to me the only way TR29's collector is gonna go low/no volts is because something's switching it on.

What could possibly do that - oh, yeah, TR32 could, if pin 1 of the 858 is high, meaning the PLL is unlocked. Which it is (3.8v), and so is.

Time to dig deeper.

Okay, so The Book says to check out TP7, aka pin 5 of IC-5, the rare and unobtanium VCO hybrid. Should read 1.75 - 2.2V. Actually reads 4.75V, same as pins 7 and 8 (both of which measure within spec). Uh oh.

Hang on, what else is found along the path of TP7? Dash and darn - none other than the collector of a dreaded 2SC458 (TR35). Which is also sitting at 4.75V, or roughly double at what it should be sitting.

Looking further along to the 858 PLL chip, I'm getting 4.75V at pins 2 and 3 of it, too, which is a prime example of More Is Not Better (should be 2.28v on each) while the output of the loop filter amp (pin 4) is reading only 0.013v, not the 2.76V as advertised.

Now the thing about pins 2 and 3 of the PLL which is a worry is how those previously-mentioned-but-not-yet-replaced philistine 6.3v tantalum caps are still there.

Or were, rather. They're gone now, replaced with new 25V caps. As are some of the evil 10V electrolytics (C59, c66, C76, C94, C101, and C135). So is TR35, the 2SC458, which has been replaced with a 2SC945.

But it hasn't changed anything. The out-of-lock indication hasn't gone; pin 1 of the 858 is still high; pins 2 and 3 are still at 4.75V, the 8.80V rail is still cactus because PLL lockout, etc.

What say you, 858 Brains Trust? What's the likelihood that the VCO hybrid has got up and gone to Valhalla?

Or am I missing something here that's patently obvious here to you, but which is as sure as golly eluding me?

[lbcomms]

Dear 858 experts,

Any educated guesses as to what may be happening here? Story time:

So.... I grabbed myself a bargain Robyn SB-510-D (Uniden 858, PC-176AB board) recently as a 'parts only' deal. Then I was pleasantly surprised when the thing fired up, transmitting and receiving as it should....

...well, briefly.

Although the unit hasn't been fully re-capped yet (meaning it unfortunately still has many of those horrible 10V electrolytic nasties in there

Big one there is C179, a 2.2uF 25V tag cap, once the radio is confirmed operational (preferably with a lab power supply limited to 1A, a fuse is not a substitute here) it must be replaced with a 35V or higher rated component - a regular modern electrolytic is OK for this one.

If you have a 5A or larger supply (such as a car battery) and that cap dies, it often melts the innards of the mode switch, causing all sorts of apparently unrelated faults as different sections of the innards of the switch start losing contact or start contacting each other. Once that happens, replacement of the switch (2+ hour job!) is the only fix.

This is one of the reasons we insist on a full recapping before repairing these units if the original caps are still there. Some cheapskates demand we only replace "just the bad caps only, I only want it to work so I can sell it on ebay, blah, blah"), in which case we tell them to take it elsewhere

Hopefully that's not what's happened to yours. But if you do get it going, replace ALL the tags and electros before using it as a daily driver radio.

[bgpz]

lbcomms! Thank you for the reply - I read your 'Ye Olde 858 restoration and mods' article from long ago and have much admiration for your work.

Big one there is C179, a 2.2uF 25V tag cap, once the radio is confirmed operational (preferably with a lab power supply limited to 1A, a fuse is not a substitute here) it must be replaced with a 35V or higher rated component - a regular modern electrolytic is OK for this one.

Ah, yes - IIIRC, that's the cap which gets worked pretty hard on SSB (like, over the 25V rating of the OE cap) if the power's been turned up. Luckily I have some new 50V electrolytics of that value, which I can use before ordering in some 2.2 uF 50V tants.

If you have a 5A or larger supply (such as a car battery) and that cap dies, it often melts the innards of the mode switch, causing all sorts of apparently unrelated faults as different sections of the innards of the switch start losing contact or start contacting each other. Once that happens, replacement of the switch (2+ hour job!) is the only fix.

Yeah, replacing that 5-pole triple throw switch is something I'd rather avoid.... (I've had to do something similar before with a Cybernet 02A radio, which didn't have any BRS voltage, because dodgy switch). One odd thing however was how the power switch on this 858 (on the volume control) had gone high resistance, and was essentially 'browning out' the radio after the fuse popped.

That said, the "mode" voltages at the 20 points shown in the Sams service manual are mostly looking good, with the exception of #3, i.e the collector of TR29. Which - near as I can figure - starts crunching that 8.80V rail when told to by TR32, which itself is under orders from pin 1 of the PLL.

This is one of the reasons we insist on a full recapping before repairing these units if the original caps are still there.

100% agree there. The plan is to keep fully re-capping the unit, IF the VCO hybrid isn't dead. All the electrolytics in the PLL / VCO oscillator, plus the two 6.3V tantalums (C140 / C141), have now been replaced, along with TR35 - the old 2SC458 is now a new 2SC945. I've put the Siglent SDS1002 CRT-less CRO onto the PLL, and I'm seeing the correct waveforms, more or less, at the 858 chip. Not unlike these -



Near as I can tell with the Siglent, I'm getting 33Mhz signals out of TR40 (tripler) into one of the gates of FET4 (mixer). (Any recommendations for a good frequency counter? My ancient DSE kit from last century isn't up to the job....)

But the PLL is still in an unlocked condition (3.2V at pin 1) and the 4.75V condition at TP7 persists (i.e. the other gate of FET4), and with me being somewhat unfamiliar with the the failure modes of various components in these particular gadgets - like VCO hybrid chips - I'm a bit hesitant to keep throwing new caps at it if that VCO chip has failed and can't be replaced.

In your experience, would you say the 4.75V level at VCO chip pin 5 / TP7 is an indication that this hybrid chip has failed?

Or is that condition what you'd expect to see if there's another fault which is throwing the PLL into lockout?

Edit: formatting; add image URL