UK fixer for desperate hobbyist?

[Fred Bloggs]

Winfield Hill wrote:

Tony Williams <tonyw@ledelec.demon.co.uk> wrote ...

Terry Pinnell terrypinDELETE@dial.pipexTHIS.com> wrote:


Roger, Kevin, John: Unless my PM5134 is truly deceased, kicked the
bucket, passed away, gone to a better place, etc, etc, Tony Williams
will perhaps soon be able to reveal all. Tony - a star - has very
kindly offered to have a go at diagnosing and fixing this beast. I
posted it 8 hours ago, so, God and the UK Post Office ParcelForce
service permitting, all 9 kg of it (including manual and schematics)
should reach him shortly.

It's up on the bench now........

The 1:1 buffer before that looks to be the perp, it has an input of
0.066v and an output of +3.3v. It is down on the under-board, packed
up against the f.panel, and partly underneath a 2-gang pot... damn
near impossible to reach with even the finest probes.

+20v
|
\
/2k37
\
| 140R
+---------/\/\------+
310R |/e |
+--/\/\--| pnp |
| |\c |
| | 12v |
| 0v--||--+---|<|--- -20V |
Vin ->-+ zeners +--> Vout
66mV | 0v--||--+---|>|--- +20v | 3.3v
| | 12v |
| 310R |/c <----- 7.8v |
+--/\/\--| npn |
/ |\e <----- 5.8v |
61mV______/ +---------/\/\------+
| 140R
\
/2k37
\
|
-20v

I can only reach around the npn for measurement.
Voltage results shown alongside each pin.

That 5.8v emitter voltage looks seriously suss, but the resultant
3.3v for Vout suggests that the pnp might be ok (touch wood, because
changing that one would be a full dismantle of everything).

So I'm going to go off and see if that npn can be changed.


Better see if that pnp can be changed. It's supposed to pull the node
in question down and isn't doing so. The npn can only pull up, right?

It's coming out exactly right- with the pnp-e at ~0.77 then
Vout=0.5*(5.8+0.77)=3.3V- so the pnp is working at DC anyway. The 5.8V
at npn-e means 25.8/2.37=11mA through the emitter resistor and 18mA
through the 140R for 30mA total- this may be a symptom of open base lead
and partial short c-e of some kind- it's shot for sure.

 

[Tony Williams]

In article <3e852f41.0307201431.6388d2d2@posting.google.com>,
Winfield Hill <whill@picovolt.com> wrote:

Better see if that pnp can be changed. It's supposed to pull
the node in question down and isn't doing so. The npn can only
pull up, right?

I'll try to redraw it a little more clearly.

+8v --+-- --+-- +20v
| |
310 |/c /
Vin--/\/\--|npn \2k37
66mV |\e Vout /
| | |
| 140 | 140 |
5.8v +--/\/\---+---/\/\--+ (+0.8v?)
| 3.3v | (can't reach it)
| |
\ e\| 310
2k37/ pnp|--/\/\--Vin
\ c/| 66mV
| |
-20v --+-- --+-- -8v

Normally, the npn-e should be -0.8v and the pnp-e at +0.8v.

But there was a wrong 5.8v at the npn-e, and the direction of
the voltage drop to Vout suggested that the source of the
faulty current could only be the npn.

The 5.8v to 3.3v attenuation also indicated that (with a bit
of luck) the pnp was correctly holding it's emitter at +0.8v.

Replacing the npn brought Vout to 0v (88mV actually) and the
thing started working ok.

I hope the power amp gets scanned and discussed. It's an
NE532 op amp driving a discrete complementary V-I converter,
and into a 50 ohm load. The o/p stage runs in Class A (with
125mA Iq) and nicely runs at full output swing up to 20MHz.

--
Tony Williams.

 

[Tony Williams]

In article <90nlhvo2cd97v4vk79rkmlon2abhs6hi6f@4ax.com>,
Spehro Pefhany <speff@interlog.com> wrote:

tonyw@ledelec.demon.co.uk> wrote:
I think there was a lot of luck in it.....

Yeah, sure. ;-)

You don't know exactly how lucky it was Speff..... that
under-board was wired to the f.panel pots in-situ. If it
had been the pnp (or further upstream) then the f.panel
would have had to come off. 25-30 wire links to unsolder,
and passive testing only from then on. It could have taken
the whole of Terry's fortnight.

Yes, I hope so. What are you going to do with the #100 prize? ;-)

Since it is Vienna I think he's going to teach me to Waltz,
a Straus Waltz (with a 't') of course, not a Richard Steven.

--
Tony Williams.

 

[Tony Williams]

In article <3F1B24F8.80904@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:

It's coming out exactly right- with the pnp-e at ~0.77 then
Vout=0.5*(5.8+0.77)=3.3V- so the pnp is working at DC anyway. The
5.8V at npn-e means 25.8/2.37=11mA through the emitter resistor
and 18mA through the 140R for 30mA total- this may be a symptom
of open base lead and partial short c-e of some kind- it's shot
for sure.

Nearly, but not quite, it now appears.

In situ, the b-e and c-b looked like diodes, but c-e looked
something like <100 ohms in both directions.

Just now, out on the bench, c-e looked ok in both directions,
and it now works fine as an emitter follower.

So, maybe it was the transistor, maybe it was some resistive
contamination across c-e (that got shifted when the transistor
was changed). The transistor was in a black plastic carrier
with fingers that clamped around the (live) case.

Doesn't matter though, it's done now, and all working ok.

BTW: It was a BSX20, which is an old Mullard TO-18 NPN Si,
Vcbo= 40v, Vceo= 15v, spec'd for 10mA Ic, and 500MHz fT.

I stuck a ZTX653 in there, which is better all round,
except for a 175MHz fT. The 20MHz square wave looks ok
though.

--
Tony Williams.

 

[Spehro Pefhany]

On Mon, 21 Jul 2003 09:28:41 +0100, the renowned Tony Williams
<tonyw@ledelec.demon.co.uk> wrote:

You don't know exactly how lucky it was Speff..... that
under-board was wired to the f.panel pots in-situ. If it
had been the pnp (or further upstream) then the f.panel
would have had to come off. 25-30 wire links to unsolder,
and passive testing only from then on. It could have taken
the whole of Terry's fortnight.

That sounds like a really miserable setup for repair. Especially since
the output amplifier is bound to get damaged from time to time (the
odd connection to the mains etc.)- those parts should be considered
sacrificial. I wonder if both the 140 ohm resistors are still 140R?
;-)

I like the way they generally do it in oscilloscopes with the
right-angle pots and long extensions on the shafts to get to the front
panel. More expensive, but it sounds like that was not a cheap signal
generator.

Perhaps at the repair rates (#50+/hour?) that Philips would charge,
their "unrepairable" diagnosis was almost justified, if you assume it
had to come apart.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Tony Williams]

In article <hmrnhv41c1qvv8vthkan73010pg0fk0c56@4ax.com>,
Spehro Pefhany <speff@interlog.com> wrote:

That sounds like a really miserable setup for repair.

The modern disease.... cheapest purchase price, paid
for by cost of ownership.

Especially since the output amplifier is bound to get damaged
from time to time (the odd connection to the mains etc.)-

The PA is on the top board, with all comps accessible.

I wonder if both the 140 ohm resistors are still 140R? ;-)

Probing with an ohmeter can be done, because it doesn't
matter if the probe slips. They both measured 130 ohms,
which is outside their 5% tolerance, but still within an
ohm of each other. AFAIR most resistors looked at were much
lower than their marked values.

Perhaps at the repair rates (#50+/hour?) that Philips would
charge, their "unrepairable" diagnosis was almost justified, if
you assume it had to come apart.

Yes the power supplies and PA are accessible, otherwise forget
it. However, the Philips ticket actually says "component
unobtainable".

--
Tony Williams.

 

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <3e852f41.0307201431.6388d2d2@posting.google.com>,
Winfield Hill <whill@picovolt.com> wrote:

Better see if that pnp can be changed. It's supposed to pull
the node in question down and isn't doing so. The npn can only
pull up, right?

I'll try to redraw it a little more clearly.

+8v --+-- --+-- +20v
| |
310 |/c /
Vin--/\/\--|npn \2k37
66mV |\e Vout /
| | |
| 140 | 140 |
5.8v +--/\/\---+---/\/\--+ (+0.8v?)
| 3.3v | (can't reach it)
| |
\ e\| 310
2k37/ pnp|--/\/\--Vin
\ c/| 66mV
| |
-20v --+-- --+-- -8v

Normally, the npn-e should be -0.8v and the pnp-e at +0.8v.

But there was a wrong 5.8v at the npn-e, and the direction of
the voltage drop to Vout suggested that the source of the
faulty current could only be the npn.

The 5.8v to 3.3v attenuation also indicated that (with a bit
of luck) the pnp was correctly holding it's emitter at +0.8v.

Replacing the npn brought Vout to 0v (88mV actually) and the
thing started working ok.

I hope the power amp gets scanned and discussed. It's an
NE532 op amp driving a discrete complementary V-I converter,
and into a 50 ohm load. The o/p stage runs in Class A (with
125mA Iq) and nicely runs at full output swing up to 20MHz.

Brilliant - that's really great, Tony, thanks a bunch! Got back from
Austria (Mayrhofen) yesterday, and just finished downloading email
(1,400 entries, 99% spam) and then a similar number of posts on sed,
to find this delightful surprise.

Sorry about that label. I should have glued it as I usually do,
instead of just cello-taping its edges to save a few minutes. (Sounds
like the Post Office are to be congratulated too!)

If it's still of interest, I'll be happy to scan the appropriate sheet
when I receive the unit and manual back. (Is it obvious which sheet or
section? Seem to recall there was a page headed 'Power Amp'?)

Despite any possible contribution from Lady Luck, I'm darn sure that
*I* wouldn't have been able to fix it. You're a star and, as others
have said, a true Usenet hero!

--
Terry Pinnell
Hobbyist, West Sussex, UK