Help me understand this schematic (analog) - Page 1
Forum section: electronics / repair

author: WimWalther
author_rank: Regular Contributor
timestamp: September 09, 2023, 05:16:06 am
content: I'm stuck on repairing this guitar amp, mostly because I don't understand a portion of the circuit. The question relates to the "modulator" circuit, which runs the length of the diagram near the middle-bottom.This modulator is a Leslie speaker effect, so I suppose it works by adding phase / doppler shift to the signal? But presently it seems to have no effect on the sound.One major sticking point is the IC U7 marked PT2201. It has been sanded to remove the markings.. what does it even do? Also, why is there a flip-flop feeding it?Also, I've identified the far, lower right section as an LFO used to trigger the flip-flop, but what is the rectangular section to the left of it that appears to be some sort of bridge?

author: oPossum
author_rank: Super Contributor
timestamp: September 09, 2023, 06:38:25 am
content: Possibly a TDA1022 BBDI think the frequency of the oscillator for the BBD is modulated by the LFO.

author: floobydust
author_rank: Super Contributor
timestamp: September 09, 2023, 07:25:07 am
content: I've worked on real Leslie's and there was great effort made to electronically simulate them, so you don't have to lug the massive cabinets around. They do sound amazing live.The effect is amplitude, frequency response, phase, reverberation- all being modulated as the horn rotates. Musicians like to ramp up and down the speed of the rotating horn speaker, it became a popular effect in the psychedelic era, especially with the Hammond organs. This is why the voltage-controlled oscillator is so complex, it's simulating a motor/turntable (inertia and momentum) dynamics for ramp up and down in speed, as well as having the (angular) duty-cycle of the Y-horn as it sweeps by.I'm not a Polytone expert. It looks like the ("modulator") oscillator ends up as a square wave after the flip flop U10, which goes into U7 as its clock."PT" IC's are polytone part numbers, they seem to be reversed digits of originals...Oscillator U9 PT855 might be a RC4558?Dual op-amp DIP-14 might be RC4739 aka MC1303. The original uA739 or or TBA231A needs comp caps.U7 mystery IC PT2201 most likely TDA1022 but schematic would wrongly have pin 3 input should be pin 5?I see incoming audio after the volume control goes into U6A, Q6 and this mid-filtered audio goes into U7 which does the delay which is then fed into Q8, U8 bandpass filter which is pretty much Polytone's "Leslie" effect audio going into U6B and then into the mixer.I don't see amplitude being modula, OP5 is just switching in the effect based on the pedal.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 05:25:23 am
content: Quote from: oPossum on September 09, 2023, 06:38:25 amBrilliant. Yes, U7 is a TDA2022P, the schematic and what I found with the logic probe jive perfectly with the 1022 datasheet. This is extremely helpful, my thanks are yours!

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 05:33:49 am
content: Quote from: floobydust on September 09, 2023, 07:25:07 amHeh, right.. I haven't checked the others, but you're definitely right per U7.QuoteThat 3 you're seeing may actually be a blotched 5.. but yes, the 1022 input is on pin 5.Thanks for shining more light on this.. your assistance is sincerely appreciated.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 05:40:11 am
content: So am I correct that the 1022 outputs are coming off pins 8 & 12? The schematic shows pin 8 joined to an unmarked pin, which based on the datasheet must be 12?ETA: Yes, that's correct. Example circuit on the Philips datasheet shows the 8/12 pair connecting to pin 2 (Inv. input) of a 741.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 04:49:27 pm
content: So here's what I'm finding on the pins of the 1022. Conditions are: mod. board on bench, running from reg'd +/-14.4V supply, input & output floating, probe set for CMOS due to the 14.4V supplies.(X = no signal)1) hi/lo LF pulse2) X3) X4) hi/lo LF pulse5) hi6) X7) X8 ) hi9) lo10) X11) X12) hi13) lo14) X15) X16) hiAll of this seems sane per the example circuit on the Philips 1022 datasheet. Pulse rate on pins 1&4 varies under control of SPEED knob from about 0.5Hz to 5Hz <

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: September 10, 2023, 09:45:22 pm
content: Hi WimWalther,So where are you now in the repair process ? Do you need some additional help ?Q1) Could you validate that all of the stuff on the top left corner works fine ?Q2) When you feed an input with 1kHz sine 5 mV peak to peak, and using an oscilloscope, do you catch some signal at U7/pin 5 ? And U7/pin 8 ?In the negative, give a second try with 50 mV peak to peak.Q3) The PNP transistor 'Q7' seems to act as a current source. Can you measure its' DC voltage at the base pin ?

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 10:42:08 pm
content: Quote from: timeandfrequency on September 10, 2023, 09:45:22 pmI reinstalled the modulator board, but it's still not working. I'd done a bunch of cleanup of the pcb work, it was substandard. One issue is that I still don't know how & when the modulator effect should operate.QuoteSorry, which stuff? Do you mean the channel switching circuits in upper left of the schematic? Best as I can tell, they aren't even present in the TE model. There's only one footswitch jack and it controls speed & on/off for the modulator.QuoteThis will all require testing. I'll perform it as I'm able - I'm disabled, everything takes like 5x the time & effort it should.Btw, I was also wondering about Q7.. and why R96/97 are dashed-out, almost as if they are optional parts. But you're likely right it's CS, as else I can't see how Q8 is biased linear.

author: oPossum
author_rank: Super Contributor
timestamp: September 10, 2023, 11:20:09 pm
content: Q7/R96/R97 is a simple current source that is used instead of the 47k resistor shown in the datasheet.R96 and R97 are needed - they are not optional.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 10, 2023, 11:54:00 pm
content: Quote from: oPossum on September 10, 2023, 11:20:09 pmDatasheet.. you mean schematic? So you're saying that R98 (47K) is deleted? Why do you think that?QuoteRight, without them the transistor has no base bias current. My question relates to how they're drawn - they're dashed out, like they are optional, or effemoral..Is this effect just a reproduction artifact? It sure seems intentional.So.. backing up.. does anyone know what the purpose of this block is? (Attached). It's like some sort of bridge, with the 4 diodes and mirrored resistor values. Though, there's dissimilarity in the 220R / 270R pair that grounds it.Reminds me of an FM detector.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: September 11, 2023, 04:13:22 am
content: Hello WimWalther,Quote from: WimWalther on September 10, 2023, 10:42:08 pmSure than if the PCB isn't reliable, it might create additional flaws.Quote from: WimWalther on September 10, 2023, 10:42:08 pmYes, I was speaking about the 'channel switching' and 'reverb recovery amp' stuff.But if those features are not present/implemened in your device, this simplifies the schematics.At one point, some input signal should arrive at U6A (full left in the middle) and then head towards U7.As far as possible, try to track the input signal at this location (around Q6).Quote from: WimWalther on September 10, 2023, 10:42:08 pmSo sorry to hear about your predicament. Take as much time you need. Nobody's in a hurry.Quote from: WimWalther on September 10, 2023, 10:42:08 pmI would say that the dotted parts are just a scan artefact and as oPossum underscored, those parts are required.The vertical line just above (which links R63, R60, R61) shows the same printing failure.Quote from: WimWalther on September 10, 2023, 11:54:00 pmI guess that oPossum was talking about 'RL' (47k) that is shown in theTDA1022 datasheetat page 6 & 7.Quote from: WimWalther on September 10, 2023, 11:54:00 pmNot a bad idea, because an RF (AM/FM) detector extracts a baseband (demodulated) signal from another more complex (modulated) signal.So I have another close guess : the weird resistor and diode ladder makes me think about akind of sine converter.I wanted also to figure out what the function of U9A & U9B is. It seems to be a relaxation oscillator so the waveform at U9B/pin7 should be aperiodic exponential signal(charge and discharge voltage of a capacitor).So the block you mentioned could be an 'exponential to sine converter'. But other suggestions are welcome.For such uncommon schematics, a certain answer is not obvious andin situmeasurment and/or spice simulation is not a luxury to confirm the analysis.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 12, 2023, 03:19:48 pm
content: Board is back out, on the bench. I'll try to get some tests done today or tonight.One odd thing. I just happened to notice that the modulator Intensity knob acts like a Volume control - but only for channel 3(?!). It doesn't have full control, but will vary the level by around 3dB.Odd. Now I really wish I had a full signal wiring diagram!You know, Polytone was in biz for like 35 years.. so what's become of their techs, engineers, etc? There's got to be someone out there with a wealth of info on these models.ETA: 400Hz sine input, walking the pins of the TDA1022 (U7) with the scope.. I can't see that any audio signal is reaching the chip. In fact, I'm not seeing any signal after the first opamp. The complementary clock signals from the flip-flop are present on pins 1 & 4, though.That's where I quit last night.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 17, 2023, 02:28:13 pm
content: Spent more time on this last night. I might have made some progress..Look at the Intensity control (R156A) on the left. It's setup as a blend control, with dry signal entering on bottom (ccw) and "modulated" wet signal entering on top (cw). The control then blends between the two signals.Problem is, there is no signal on top, so no modulated signal. There is a switch on the back of R156 (shown) that manually turns the effect on & off. With the switch open, only dry signal appears on the pot. Closed, and the wet signal should appear on top (cw).Note the SPDT switch just left of R156 - this does not appear on my board or anywhere else in the design, at least not that I can find. I'm also unsure of its purpose, it seems to just bypass U6B.Need to find out why there's no wet signal on R156.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 20, 2023, 01:01:17 am
content: As resistant as I am to easy answers, it's really looking like U7 is dead.. as I'd presumed from the start. There appears to be signal on the input (5) and none on the output (. But if I inject signal at 8, it makes it all the way through to the CW / top of the intensity control.Oh how I wish I had a spare U7 to test-by-sub. That would answer all questions.