reVox B780 Tuner repair help needed

[YadiYada]

A client brought in his father's REVOX B780 Integrated amp from Sep 1981. Built like a tank here in Switzerland ~40 years ago (https://www.hifiengine.com/hfe_downloads/index.php?revox/revox_b780_b739_service_en_de_fr_imp_scan_5.pdf)
When asked about the issues, he did mention that there was noise when switching the Tape 2 input, the meter buld was shot (and he provided a spare) but he failed to mention that the Tuner was no longer working properly. When Asked he could tell whether his father had mentioned anything about that. As his father his currently on a long trip abroad, it seems we wont find out for a while...

PSU voltages were off a bit so after reading the service manual, I realised that the AC Power selector was on 220V. So after changing the power selector from 220 to 240 (here in CH its more like 235-236) the unstabilised power supply voltages dropped to their nominal values.
I did a final trim on the +32 and after doing that the only fault I find in the supply is a fairly high ripple current on the 6V line and the 4700uF 16V cap running hot... Otherwise amplifier section working pretty well with an auxiliary source.

Back to the FM problem, I noticed that no matter what tuning mode I select, the power-up voltage at P4 on the Synthesizer board is 4.26V and slowly rises towards the supply line voltage (I've measured 31.8V) after 12-15min. Of course as it does, the FM stations are tuned in and out slowly as if in automatic scanning mode but never locks in. The Microcomputer circuitry seems to work fine as the input selectors work as expected and frequency scanning are displaying correctly (but don't seem to be in synch with synthesizer . I'm not a radio expert so any hints about what the problem(s) could be with the FM section would be much appreciated.

P.S.: I am planning to recap the whole thing and removing the Tantalum caps from the audio path but otherwise unit looks pretty solid.
P.P.S.: In a working tuner 87.5MHz should be 4.5+-0V and 107.95MHz should be 24+-0.2V

[Chris56000]

Hi!

I'm not sure what HiFi Engine's manual is like as they blocked my account after downloading one small manual from them, but I can recommend you download the s.m. in two parts from Elektrotanya – it's a beautifully done high quality colour scan with the original top layer pink track tints on the layout drawings and all the data is beautifully clear & readable!

For the F.M synthesizer to lock it needs a local oscillator sample and the i.f. which is sampled to determine the frequency by which the local oscillator is out, usually by internal count–down methods in the PLL chip!

You'll need to pay particular attention to the +32V LT supply feeding the FM synthesizer PCB first, so turn to pages 46 and 47 of the service manual and check the following components on power supply 1.780.110:–

A) C4 for low capacitance;
B) Q4, Q7 and Q8 for defects or leakage;
C) +32V adjustment preset R21 for dud–spots – bin it and fit a new one if you're doubtful about it;
D) –15V from 7915 stabilizer I.C., the error amplifier Q4's emitter is controlled from this line!

The +32V doesn't supply a large current so if you have a bench power supply available, it might be worth your while feeding +32V from this via a 1N4148 diode to J1/26 (violet/purple leads) and chassis (+ lead to J1/26) and then switching the tuner on from cold to see if you can tune in an FM signal straightaway!

I would certainly try this simple test first – if you've not got a bench power supply unit, four PP3/1604 type batteries wired in series with a 1K resistor to give 36V will do – the opamp is rated +44V max. and a few minutes @ 36V while you prove the fault won't do it any harm!

If your +32V supply is correct or repaired as the case may be, next step is to turn your attention to  page 54 of the s.m., and check what's happening with integration stage IC4 (LF356) on the synthesizer panel 1.780.151.81 – the inverting i/p pin 2 is connected to the pulse trains from the "FUP" and "FDN" outputs from the PLL i.c., (Q3 and Q4 are inverting transistors for the pulse–trains from the i.c.), and the non·inverting input pin 3 is biassed to half–supply voltage by R36 and R45 – the output pin of IC4 is a d.c. voltage that should change in response to the tuning controls!

If the supplies and the +2.5V at pin 3 of IC4 are correct but it's output is not responding to the tuning controls correctly, we need to know if you've got an oscilloscope and a counter (or a modern digital 'scope with readouts built in!) as fault–finding the unit after this point really needs one to follow thro' the oscillator and prescaler divider outputs!

Chris Williams

[YadiYada]

Thanks for the quick reply Chris. I do have a quality SM from the folks at HiFi Engine forum (thanks Guys). There is no Q8 on the supply board (maybe a typo?) Unfortunately, I was not able to get to the receiver yet (other projects) but will do in the next days. Keep you posted on what I find.

[YadiYada]

Allright C4 is good at 510uF. +32V is tunable via the trimpot (although a bit oxidzed).
All caps and pot will be replaced although they are still within or beyond original values.
All diodes checked, transistors checked.
-15 VDC ok (actually -14.90VDC)

+32V supply is fine
Non-inverting input of IC4 is stable at 4.3V. If I put my DMM (hp3478A) at inverting input pin 2, even though the probe is quite high Z, the tuning voltage at P4 is increasing at 2-3 x the rate.

But pin 6 is putting out a slowly increasing DC FROM 4.26V t power on to 31.55VDC after about 7-8 minutes.

What I am missing is what kind of output I should be seeing at LOC when station is tuned. As it never stops scanning (from 87.5MHz to 108MHz) I have the impression that nothing is sent back to the uController to stop increasing the Freq.

I do have Rigol DS1054Z Scope and Rigol DG1022U Function Generator/Counter.

[Chris56000]

Hi!

That's fine – I'll help you look at the Local Oscillator/PLL Circuits tonight after I've had another study of the diagrams!

Chris Williams

[floobydust]

Wow this also does antenna rotator control? I've never seen a tuner done to this level before, quite a masterpiece.

I would look at the THRESHOLD STATION (THSTA) signal, coming from the IF amplifier "T" signal. I think it tells the CPU a strong enough station has been found, as well as THRESHOLD STEREO (THSTE) telling the CPU a strong enough stereo signal has been found. This is my guess after a quick look.
There are trimpots on the meter/de-emph board for calibration, so you could record the position/ohms and then adjust it so THSTA is always asserted and the MCU will think it has a station and hopefully stop scanning. Then you can look for what is wrong with THSTA. Or maybe some other way to force THSTA up to fool the MCU.

[YadiYada]

THSTA and THSTE are responding well only uCTRLR is not reacting... 

Let me try to describe what i observe a bit better.

on Power-on, VCO output (Pin 6) of IC4 on FREQ SYNTH BOARD starts at between 3.66V (lowest observed) and 4.26 (highest observed).
Even if I have manually set freq in the first 5 memory positions (as per troubleshooting section) 87.50, 90.00, 97.00, etc. The FREQ SYNTH board just scans up from 3.66/4.26 until Pin 6 reaches 31.56V (Supply line minus the drop across output Z). That's all it does. Of course as it scans it crosses stations and outputs audio, tuning meter goes up, stereo mode kicks then it goes down again on to the next. No matter what I do, From power up that is the only thing the tune does.

The rest of the Amp is fine, I've put various sources across other inputs. All fine. Its just the FM function that is not working.

Since the input selectors are working and supply voltages are OK, I was assuming that uCTRLR was fine but now I'm starting to doubt it is the problem.

[YadiYada]

Forgot to mention that while the FREQ SYNTH board experiences life on its own  the Frequency display remains at whatever station was selected. If I try to do a frequency scan from the front panel (through the uCTRLR) no matter if I do up or down scans, the FREQ SYNTH board still goes slowly up to his max VCO output before stoping. So it really looks as though there is no comms between the uCTRLR and FREQ SYNTH boards. Although I've checked the harness and at least connectivity to uCTRLR for LOC, THSTA, THSTE, CLK, DATA and DLEN3.

[YadiYada]

Found the issue. Dual gate Mosfet 17853 is toast. Replacing the device or the complete board is in the 50-75USD range which in the end makes no sense since FM is gone by year end. Unfortunately sad since the design was quite advanced.

[floobydust]

Dual-gate MOSFET RCA 17853
I can't find data for it, they usually prefix 406xx or 408xx. It might have been selected. It's a depletion-mode, not enhancement mode mosfet, when you tested.
https://archive.org/details/RcaLinearIntegratedCircuitsAndMosfetsDataBook1982/mode/2up
I think you could find a replacement.

[YadiYada]

Thank you all for the help and guidance. In the end the client did not wish to go ahead and repair the FM section as he lives in the Alps and has pretty weak signal to begin with. He uses the amplifier mostly with a CD player. Till next project... CyA

[didyman]

I have  B780 on my table and I would like to be sure how recap it. My concern are the tantalums. It sports three different kind of tantalums througout: the green ones acting in signal path will be replaced, these are aging (leakage) considerably. The biggest numbers are blue ones, pulled a few, they are in excellent condition, but I'm unsure if they are prone to short or not. These acts everywhere througout in tuner, input, demod and so on PCBs. I suspect these should be replaced if needed to low leakage ones, right? The third type identified is a red one, not yet measured.
What do You think?